Running cold?

[c0a8l0v6i8n9]

Temperature gauge shows two lines to the left of 210. Torque scan via OBD shows about 180-185, sometimes 188. I'm getting horrible gas mileage, about 6 city 12 highway if I'm lucky. Does this sound like the thermostat to you or a bad coolant sensor? I also felt the hose coming from the thermostat housing and its hardly warm.

Anything that can cause horrible mileage?

I have spark plugs, fuel filter, and air filter on order.

 

[Matt]

Thermostat. Nuff said.

 

[meerschm]

bad thermostat not letting your engine come up to temp results in computer operating engine to warm it up.

will use too much fuel, and if you are not prompt in fixing, can also ruin your catalytic converter.

 

[Menthol]

Yep, Last year I installed a STANT and now it's 2 ticks left of straight. Reading 186-188, changing it out tomorrow with a Motorad brand. Even with timing the traffic light and using the hills I can barely get 12.5MPG.

 

[c0a8l0v6i8n9]

Thanks guys that's what I figured I'll find out Sunday if that's the fix!

Menthol if you could let me know if that fixes the problem

 

[gmcman]

While you're in there, I would change the coolant temp sensor as well, labor saved.

 

[seanpooh]

And the job would be much easier if you removed the alternator. Less cursing this way though the wheel well.

 

[Menthol]

View attachment 26691
View attachment 26692

Thanks guys that's what I figured I'll find out Sunday if that's the fix!

Menthol if you could let me know if that fixes the problem

Did the rear brakes with new pads/rotors, pulsing now gone. Need new parking brake pads but later, not worried about that.
Cleaned the TB while the battery was disconnected.
Tried to change the 02 sensors but couldn't turn it. I was spaying it for 2 days, not luck. I will take to a muffler shop and ask them to do a back pressure test (is that what it's called) and hand them the new sensor to instal.
Change the STANT thermostat to MOTORAD brand. Improved a lot just barely touching the straight up mark. I'm gettting 195-197 before 186-188. So go ahead and change the thermostat.

PS Don't forget to get yourself a stubby 15mm wrench ratchet type if you can. You will swear a lot less.

 

[c0a8l0v6i8n9]

I replaced the thermostat its now running at 195

I also changed the plugs, new air filter, cleaned the throttle body.

Still getting about the same mileage not too great.

 

[Menthol]

I replaced the thermostat its now running at 195

I also changed the plugs, new air filter, cleaned the throttle body.

Still getting about the same mileage not too great.

What brand did you use?

 

[c0a8l0v6i8n9]

I could only get my hands on a Stant stat.

Put in delco plugs, spectre filter

 

[Matt]

I replaced the thermostat its now running at 195

I also changed the plugs, new air filter, cleaned the throttle body.

Still getting about the same mileage not too great.

You pulled the PCM fuses or disconnected the battery for 30 minutes didn't you?

 

[c0a8l0v6i8n9]

I pulled both pcm fuses before starting the job

 

[Matt]

I pulled both pcm fuses before starting the job

Cool.

Do you have or could you get a scan gauge that reads in real time and have a look at what the forward (or bank 1) O2 sensor is doing?

It should fluctuate fairly quickly between 0.1 and about 0.8 or 0.9. If it is slow, that would indicate that it's on it's way out and should be replaced even though there are no codes pending. The forward O2 sensors do have a bearing on mileage, but we want to diagnose now, not just shotgun parts at it.

 

[c0a8l0v6i8n9]

Cool.

Do you have or could you get a scan gauge that reads in real time and have a look at what the forward (or bank 1) O2 sensor is doing?

It should fluctuate fairly quickly between 0.1 and about 0.8 or 0.9. If it is slow, that would indicate that it's on it's way out and should be replaced even though there are no codes pending. The forward O2 sensors do have a bearing on mileage, but we want to diagnose now, not just shotgun parts at it.

I do have the torque app for my phone. I fluctuates quickly from 0.1 to 0.85. If im at idle, it fluctuates between the same voltage, just not as fast.

 

[Matt]

I do have the torque app for my phone. I fluctuates quickly from 0.1 to 0.85. If im at idle, it fluctuates between the same voltage, just not as fast.

OK. That pretty much rules out the O2 sensor...but as we're still running winter fuel, that would be a contributing factor with lower mpg's.

As to what's next...I'm really not sure.

 

[c0a8l0v6i8n9]

Yea I changed out the original fuel filter today. 61k on the filter.

I'm just going to wait till it gets warm and hope for the best.

Thanks for your help.

 

[c0a8l0v6i8n9]

Sorry to bring this thread back.

I'm still getting bad mileage. Torque app tells me my truck is running at 195.8 on the highway. One tick left from 210. Is this still too cold?

If I am driving at 55 mph and put it in 3rd gear, the instant mileage stays the same around 14.7 mpg. Shouldn't it drop?

I also did a 0-60 on the torque app and it was 9.471. It seems too slow, not too sure.

I also recently changed the upstream o2 sensor. I was getting decent mileage on the highway around 19-20 mpg, but after replacing the sensor, and pulling the PCM fuses, I am now down to 14-15 mpg highway.

I am getting the feeling I have a partially clogged cat, having the slow acceleration and bad mpg.

Any input?

 

[CaptainXL]

I am now down to 14-15 mpg highway.

I am getting the feeling I have a partially clogged cat, having the slow acceleration and bad mpg.

Any input?

Check your exhaust back pressure using a gauge. But I'm thinking the new 02 sensor isn't working right since the old one was. You used an ACDelco 02? If you unplug the 02 do you still have poor acceleration? If not then the cat is more than likely OK.

Excessive fuel consumption could also be stuck injectors, dirty air filter, faulty fuel pressure regulator, transmission issues, brakes binding or bad coolant temp sensor or thermostat. I would replacd the CPAS and havd compression checked too.

 

[c0a8l0v6i8n9]

Check your exhaust back pressure using a gauge. But I'm thinking the new 02 sensor isn't working right since the old one was. You used an ACDelco 02? If you unplug the 02 do you still have poor acceleration? If not then the cat is more than likely OK.

Excessive fuel consumption could also be stuck injectors, dirty air filter, faulty fuel pressure regulator, transmission issues, brakes binding or bad coolant temp sensor or thermostat. I would replacd the CPAS and havd compression checked too.

I was going to find a place to do a back pressure test. I replaced the o2 with an ACDelco. Injector pulses are all normal, transmission shifts good, no dragging on the brakes, new thermostat. I recently took out the CPAS and it was very clean.

 

[CaptainXL]

I was going to find a place to do a back pressure test. I replaced the o2 with an ACDelco. Injector pulses are all normal, transmission shifts good, no dragging on the brakes, new thermostat. I recently took out the CPAS and it was very clean.

Sounds like bad fuel pressure regulator. You did say Acceleration is poor. Perhaps a hose wasn't connected last time the 02 was replaced. Check simple stuff like vacuum as well. I would check fuel trims as well. You can actually do an injector balance test using the LTFT. Not sure what you mean by looking at injector pulses. If you mean the injector on time as seen on your scan tool...that won't tell you if the injectors are leaking. Also curious as to the very clean CPAS. On a 2003 it would be dirty unless it was previously replaced.

 

[c0a8l0v6i8n9]

Sounds like bad fuel pressure regulator. You did say Acceleration is poor. Perhaps a hose wasn't connected last time the 02 was replaced. Check simple stuff like vacuum as well. I would check fuel trims as well. You can actually do an injector balance test using the LTFT. Not sure what you mean by looking at injector pulses. If you mean the injector on time as seen on your scan tool...that won't tell you if the injectors are leaking. Also curious as to the very clean CPAS. On a 2003 it would be dirty unless it was previously replaced.

Truck only has 63k so maybe that's why it is so clean. I'll test fuel pressure and test the regulator. All vacuum lines are connected. Fuel trims are -5 to +5 seem normal. Some things I notice is engine load seems to be high sometimes at 100% on the highway doing 60 also idle moves around from 600 to 630 rpms doesn't stay at a steady number seems to have a lope as well.

 

[CaptainXL]

Truck only has 63k so maybe that's why it is so clean. I'll test fuel pressure and test the regulator. All vacuum lines are connected. Fuel trims are -5 to +5 seem normal. Some things I notice is engine load seems to be high sometimes at 100% on the highway doing 60 also idle moves around from 600 to 630 rpms doesn't stay at a steady number seems to have a lope as well.

I don't see any problems with any of those numbers. What's the vacuum reading from the scan tool at idle?

In park, will the engine rev to 4k?

 

[c0a8l0v6i8n9]

I don't see any problems with any of those numbers. What's the vacuum reading from the scan tool at idle?

In park, will the engine rev to 4k?

Vacuum is 22.0-22.3

I didn't rev it today. I'm pretty sure its between 3 and 4k

 

[CaptainXL]

Vacuum is 22.0-22.3

I didn't rev it today. I'm pretty sure its between 3 and 4k

If it revs to near 4k in park then your cat is not clogged.

 

[gmcman]

So when you changed the upstream sensor and lost almost 5 MPG that says alot. When you said it changes quickly are the voltages changing at 1Hz or faster? Being at idle doesn't matter.

Check your vacuum again with a gauge, what is the reading and is the needle steady? There is a port at the front of the intake covered by a plug, port faces towards the pass side and is near the intake resonator. You can pull the EVAP hose from the TB and use that port but if that hose has a leak then it won't show. You could try both.

When you changed the plugs did you grease the tips with dieletric grease? There is specific grease for the boots (not needed) and specific grease for the coil leads to the tip.

195 deg at cruise is good. The gauge will hover between straight up and about 1 tick to the left. Thermostat opening is 195 so you're fine there.

Pull CPAS harness and check for oil in the connector.

What's your tire pressure? Low tires will bury MPG's

Are you allowing excessive warm-up time? Greater than 10 min? Five minutes is plenty when very cold if you are conserving MPG's

Fan clutch: when cold can you stop it with a rag? (A/C off) Runs partially engaged when starting(be careful) A fan that won't stop will drink fuel as well.

When all is checked, I would pull the battery cable and wait about 30 min. Allow TB relearn and drive a half tank of fuel then fill back up.

Still you were getting good mileage before the O2 swap so I would start there.

 

[CaptainXL]

All good info. Also need to throw in another thing that seemed to be overlooked.

How is the mileage being checked? It's been known that the DIC can be a little funny presenting mileage. I would use the fill method to get a more accurate figure.

I did notice that gmcman said to put dielectric grease on the spring tip and plug tip interface. I wouldn't do that. Could cause a decrease in plug performance. I just use the grease on the boot to keep water out and to ease installation. The spring tension is more important as it needs to positively contact the tip of the plug.

 

[CaptainXL]

Thermostat opening is 195 so you're fine there.

Went over this a few months ago. There is actually no exact opening temp for our thermostats. Many aftermarket tstats open at different temps. The engine is designed to run within a certain range. According to the service manual normal operating temp for the engine is 190-221F. The stant thermostat I just purchased is a 190F which should in practice put the cylinder head temp somewhere around the 195F perceived mark as the CTS sees it.

 

[gmcman]

Went over this a few months ago. There is actually no exact opening temp for our thermostats. Many aftermarket tstats open at different temps. The engine is designed to run within a certain range. According to the service manual normal operating temp for the engine is 190-221F. The stant thermostat I just purchased is a 190F which should in practice put the cylinder head temp somewhere around the 195F perceived mark as the CTS sees it.

You are correct, the Stant is 190.

 

[Chickenhawk]

One tick to the left is still too cold. The thermostat starts opening at 195 and should keep temps around 200 to 210.

If you changed to a good thermostat, I would suspect the coolant temp sensor. It sits in the coolant stream and functions using an electromechanical circuit, so it stands to reason that they can get lazy after many years in the coolant stream.

This is why we recommend changing both at the same time. If the thermostat is bad, the engine runs cool and starts enriching the mixture when it doesn't need it. The cat runs on a VERY narrow fuel/air ratio and the cat will gradually clog.

If the coolant temp sensor is bad, then the engine is a proper temp but the computer THINKS it is running too cool. The results are exactly the same.

Before you start monkeying around with less-common causes, check out the simplest and likeliest: the coolant temp sensor and the partially clogged cat. Temps should not be 195 when warm; that is the approximate temp at which the thermostat starts to open.

 

[CaptainXL]

One tick to the left is still too cold. The thermostat starts opening at 195 and should keep temps around 200 to 210.

According to my scan tool my temps are 195-197 when the engine is fully hot. Under load on a hot day and they get up around 210. This is with the ACDelco thermostat and a new coolant temp sensor.

According to the service manual this is normal. The range is 190 - 220F. One should have no reason to believe what someone says the dash gauge should show. A visual inspection like that just isn't in the service manual.

There are multiple different thermostats with different spring rates and opening temps. There are 185, 190, 192, 195F tstats, etc... These thermostats do not all open at 195F. I have boil tested three brands and they all open differently.

If you changed to a good thermostat, I would suspect the coolant temp sensor. It sits in the coolant stream and functions using an electromechanical circuit, so it stands to reason that they can get lazy after many years in the coolant stream.

Even new thermostats can go bad. I have read about some people with ACDelco's which lost the gasket shortly after installing.

I agree that the coolant temp sensor should be changed at the same time as the thermostat. However the coolant temp sensor can be tested outside the vehicle in some boiling water and a lookup table of resistance values. I have done this myself with my old temp sensor and it tested good but I decided to replace anyway.

195... is the approximate temp at which the thermostat starts to open.

Well I just kindly disagree with this last part. I would hope my Stant thermostat would start to open at 190F as indicated. Same for other brands and temp ratings. You have to keep in mind that even though a thermostat opens at a certain temp, you need to take into consideration the flow of coolant and proper evacuation of the heat from the engine to the radiator. If your towing I would think a 190F thermostat would be better. I have even seen people put in sub-190F thermostats and the coolant temp sensor is still shown 190+. According to GM this is absolutely normal and expected by the PCM.

Other than taking a scan tool reading I can tell you when the thermostat is past due to be changed. It just so happens that if the engine doesn't heat up to a certain temp within a certain timeframe the PCM will increase RPM's to try and warm the engine. So if you have been driving a while and you notice the RPM's up around 1000 when stopped for a while then that's a good sign the thermostat is bad.

So in summary, my take on the whole matter is that we should not be seeing clogged cats from thermostats which keep the temp between 190-220F. From what I have seen it seems that tstats that are providing sub-180 temps is when problems due to enrichment start to take place.

 

[gmcman]

One tick to the left is still too cold. The thermostat starts opening at 195 and should keep temps around 200 to 210.

If you changed to a good thermostat, I would suspect the coolant temp sensor. It sits in the coolant stream and functions using an electromechanical circuit, so it stands to reason that they can get lazy after many years in the coolant stream.

This is why we recommend changing both at the same time. If the thermostat is bad, the engine runs cool and starts enriching the mixture when it doesn't need it. The cat runs on a VERY narrow fuel/air ratio and the cat will gradually clog.

If the coolant temp sensor is bad, then the engine is a proper temp but the computer THINKS it is running too cool. The results are exactly the same.

Before you start monkeying around with less-common causes, check out the simplest and likeliest: the coolant temp sensor and the partially clogged cat. Temps should not be 195 when warm; that is the approximate temp at which the thermostat starts to open.

One tick to the left isn't too cold, depends on how you look at it. Anything further definitely, but we can agree the gauge isn't 100%. When mine is around 205-207 it's almost straight up.

I agree you definitely should have changed the coolant sensor when you had everything out. Of course your scan showed 180 ish before the stat change and now it shows 195 so that's a good thing.

 

[Chickenhawk]

Cats run on narrower ratios than ever before. I stand by my comments.

195 is a bit too cool when warm. Whether it is running at 195 or the PCM thinks it is running at 195 is irrelevant. There is only one coolant temp sensor and both the PCM and the gauge (as well as scan tools) get their readings from this one sensor.

As for the only time cats can get clogged is if the idle is sitting at 1000 instead of 650, you forget I am from Canada. If your idle is ever this high when warm, you have a serious problem. Cats will start to get clogged a lot sooner than this. It is a progression: running a bit too rich may take many months or years to clog a cat but it will eventually. Running really rich will clog it in weeks. Running it with a serious misfire and a flashing engine light will clog it in minutes.

 

[AtlWrk]

According to my scan tool my temps are 195-197 when the engine is fully hot. Under load on a hot day and they get up around 210. This is with the ACDelco thermostat and a new coolant temp sensor.

According to the service manual this is normal. The range is 190 - 220F.
{...}
So in summary, my take on the whole matter is that we should not be seeing clogged cats from thermostats which keep the temp between 190-220F. From what I have seen it seems that tstats that are providing sub-180 temps is when problems due to enrichment start to take place.

Exactly. 195 is perfectly normal.

As others have mentioned my attention would be on the new 02 sensor and/or a clogged cat from running in the 180s for who knows how long--since you did mention dogged acceleration. Did unscrewing the 02 sensor have a noticeable difference on acceleration? It's going to be subjective unless the cat is very clogged which doesn't sound like your case. A backpressure test is the way to be objective. Do you have the old sensor that you could swap back in for a week or so to rule out the new one?

 

[Sparky]

The gauge is a good indicator BUT not all the gauges are exactly the same. Mine always has run 1 tick to the left since I got the truck, but does that mean it is actually running too cool? No. There would be no way I could be getting an average of 19 mpg per tank out of my truck if it was running too cool as the computer would be dumping fuel trying to warm it up. Apparently, my needle wasn't put on the shaft exactly right.

If you're having mpg issues and the needle is over by a tick, then I'd take that as a good sign things are too cool. But if everything has been normal and it is one tick left then it is probably just a slightly off gauge. It isn't like these gauges are the most accurate things anyway - 100 to 210 from low to middle point, then 210 to 260 from mid to high? What kind of wonky scale is that? definitely not linear, that's for sure.

The only real way to know exactly what temperature the computer is seeing is to read the sensor data with a scan tool.

 

[gmcman]

Cats run on narrower ratios than ever before. I stand by my comments.

195 is a bit too cool when warm. Whether it is running at 195 or the PCM thinks it is running at 195 is irrelevant. There is only one coolant temp sensor and both the PCM and the gauge (as well as scan tools) get their readings from this one sensor.
.

His stat opens at 190 and his coolant is at 195. How is 195 too cool on a motor designed to run at 195.

He's not on the cusp of going cold, temp is being regulated properly as I see it.

 

[MrFrodo]

The gauge is a good indicator BUT not all the gauges are exactly the same. Mine always has run 1 tick to the left since I got the truck, but does that mean it is actually running too cool? No. There would be no way I could be getting an average of 19 mpg per tank out of my truck if it was running too cool as the computer would be dumping fuel trying to warm it up. Apparently, my needle wasn't put on the shaft exactly right.

If you're having mpg issues and the needle is over by a tick, then I'd take that as a good sign things are too cool. But if everything has been normal and it is one tick left then it is probably just a slightly off gauge. It isn't like these gauges are the most accurate things anyway - 100 to 210 from low to middle point, then 210 to 260 from mid to high? What kind of wonky scale is that? definitely not linear, that's for sure.

The only real way to know exactly what temperature the computer is seeing is to read the sensor data with a scan tool.

Yeah mine always shows 1 tick to the left too. Before I had my thermostat replaced it was showing 3 ticks to the left. I have a Scangauge II, but I haven't figured out how to program it yet. Anyone have any tips for programming it specific to our 4.2L engines?

 

[gmcman]

The gauge is a good indicator BUT not all the gauges are exactly the same. Mine always has run 1 tick to the left since I got the truck, but does that mean it is actually running too cool? No. There would be no way I could be getting an average of 19 mpg per tank out of my truck if it was running too cool as the computer would be dumping fuel trying to warm it up. Apparently, my needle wasn't put on the shaft exactly right.

If you're having mpg issues and the needle is over by a tick, then I'd take that as a good sign things are too cool. But if everything has been normal and it is one tick left then it is probably just a slightly off gauge. It isn't like these gauges are the most accurate things anyway - 100 to 210 from low to middle point, then 210 to 260 from mid to high? What kind of wonky scale is that? definitely not linear, that's for sure.

I agree it can depend on how the needle was mounted, but I also believe these gauges are fairly accurate. For the sake of this thread...can we also agree that there is a constant varying voltage coming from the temp sensor and the gauge moves accordingly? We all drive these almost daily and the needle reads the same when hot, it's not some arbitrary voltage it's an actual thermistor and changes voltage with temp changes. Sending a changing voltage to the ECM then sending a voltage to the cluster.

When the coolant temp rises, the gauge rises,.... not cold, warm, hot, but incrementally. The thermostat according to the service manual is normal when opening from 188-206 deg....195 is fine. One tick to the left is fine, 2 or more is not good. 210 deg is straight up, I don't want my gauge reading 210 when my coolant is 195, I prefer to know what the temp is, especially when it starts getting too hot.

Back to the quote, the odd display is not linear for sure, it's setup like a half-sweep gauge and this is used widely. Helps to monitor changes like when the coolant starts to rise past normal, easier to monitor the smaller increments.

 

[Sparky]

I do believe the gauge inaccuracy is more the needle placement than the gauge itself. You have a good point on the scale and the half-sweep thing.

 

[gmcman]

One tick to the left is still too cold. The thermostat starts opening at 195 and should keep temps around 200 to 210.

If you changed to a good thermostat, I would suspect the coolant temp sensor. It sits in the coolant stream and functions using an electromechanical circuit, so it stands to reason that they can get lazy after many years in the coolant stream.

This is why we recommend changing both at the same time. If the thermostat is bad, the engine runs cool and starts enriching the mixture when it doesn't need it. The cat runs on a VERY narrow fuel/air ratio and the cat will gradually clog.

If the coolant temp sensor is bad, then the engine is a proper temp but the computer THINKS it is running too cool. The results are exactly the same.

Before you start monkeying around with less-common causes, check out the simplest and likeliest: the coolant temp sensor and the partially clogged cat. Temps should not be 195 when warm; that is the approximate temp at which the thermostat starts to open.

What you are saying about the temp sensor is correct, however his scan is showing 195 being read by the ECM.

I have never seen an engine that is too cold at 195 deg, please show me this engine, it surely isn't an LL8.

195 deg is a normal temp for this motor. When mine is 1 tick to the left, like others, it's at or very near 195. When mine was 2 ticks to the left, it was in the low to mid 180's and was too cold.

If the sensor is reading 195 and the coolant is flowing at 180 then you have an issue with the sensor, this is why the melt test is used. Generally the sensor reads low when bad due to higher resistance in the thermistor.

This is taken from the service manual, if anyone thinks 195 deg is too cold and wants to start changing thermostats then that's on you and you need to take it up with GM.