Could it be an intake manifold leak?

[tdamocles]

I've been having a pending misfire p0304 on startup intermittently for a year now and have been slowly trying to fix it. It has never thrown an engine code. The pending code usually goes away. It is a 2007 Trailblazer now with 70000 miles. Over the past year I've tried bottles of Techron, changed plugs, and even moved coil position with the plug change. The misfire stayed on cylinder 4. Afterwards my mechanic checked compression and said it was fine. It never bothered me that much since the pending code always goes away. Recently it has been showing the pending code more often. I've checked out my fuel trims and the short term seems to fluctuate between -3% and 4% but my long term trim hovers between 8% and 13%. When I idle and raise the rpm to 2500rpms it goes back down to 6% long term. I have also noticed that cylinder 4 at this incident(about 20 min idling) misfired between 200-300 times. I've tried the carb cleaner spraying but haven't been able to get the engine to change rpm with the spray. Does this sound like an small intake vacuum leak that is causing a misfire? I never ever got a p0171 code......


By the way..also tried the tsb about the weather stripping at the cowl area.

 

[RayVoy]

Do you remember, when you swapped coils, did you also move the high voltage boot, or did you leave the #4 boot with the #4 plug?

If you left the boot with the plug, there may be a carbon trace on, or in, the boot and the spark is occasionally going to ground.

 

[tdamocles]

Do you remember, when you swapped coils, did you also move the high voltage boot, or did you leave the #4 boot with the #4 plug?

If you left the boot with the plug, there may be a carbon trace on, or in, the boot and the spark is occasionally going to ground.

Completely different plug. All new plugs were inserted at the time. When you say 'Boot' do you mean the rubber at the bottom of the coil assembly which goes over the plug?

 

[tdamocles]

Went ahead and got an intake gasket change today. Problem still exists. Did notice that when I hold rpms above idle there are no misfires.

 

[RayVoy]

When you say 'Boot' do you mean the rubber at the bottom of the coil assembly which goes over the plug?

Yes, the rubber tube that covers the high voltage wire.

 

[tdamocles]

Yes, the rubber tube that covers the high voltage wire.

All plugs were new at the swap. Put all new plugs in and moved #1 coil to #4 cylinder and #4 coil to #1 cylinder. Misfires stayed at #4 cylinder.

 

[Texan]

All plugs were new at the swap. Put all new plugs in and moved #1 coil to #4 cylinder and #4 coil to #1 cylinder.

I am confused. When I changed the plugs in my TB, the coil and boot were all one piece, I think.
When I look at new coils on Rock Auto, it appears to me that the "boot" is included.

 

[RayVoy]

All plugs were new at the swap. Put all new plugs in and moved #1 coil to #4 cylinder and #4 coil to #1 cylinder. Misfires stayed at #4 cylinder.

I know, I'm asking about the rubber tubing that covers the wire spring between the plug and the coil.

If you moved it with the coil, then all is good; if you did not move it and you left it with the plug, try moving it, or buying a new on.

 

[tdamocles]

I am confused. When I changed the plugs in my TB, the coil and boot were all one piece, I think.
When I look at new coils on Rock Auto, it appears to me that the "boot" is included.

Confused also....mine were one piece also. Never took it apart. Moved coils as one piece assembly. I didn't think they came apart.

 

[RayVoy]

Never took it apart.

Ok, perfect, it got moved to #1 (those "boots" can go bad and new ones are available).

If I remember, you tried spraying some carb cleaner over the intake manifold some people have good luck using propane (as in a propane torch).

 

[RayVoy]

I am confused. When I changed the plugs in my TB, the coil and boot were all one piece, I think.
When I look at new coils on Rock Auto, it appears to me that the "boot" is included.

I guess we answered about the same time. The "boot" is a separate piece and can be purchased as a part from GM. Sometimes what appears as a bad coil is really a burnt boot and the problem can be fixed with a new boot. It is not unlike replacing a sparkplug wire.

 

[tdamocles]

Ok, perfect, it got moved to #1 (those "boots" can go bad and new ones are available).

If I remember, you tried spraying some carb cleaner over the intake manifold some people have good luck using propane (as in a propane torch).

Doesn't matter now.....intake gasket changed today with no change and money wasted. Any ideas?

 

[Texan]

Doesn't matter now.....intake gasket changed today with no change and money wasted. Any ideas?

What type of scanner are you using? I never hook up my scanner unless a CEL comes on.
Did you say that the miss stayed at #4? Got my curiously aroused.

 

[tdamocles]

What type of scanner are you using? I never hook up my scanner unless a CEL comes on.
Did you say that the miss stayed at #4? Got my curiously aroused.

It's an UltraGuage. Can display 35 Tb sensors and will tell you of a pending code. This pending code is never enough to trip the CEL but it is telling me there will be a problem soon. You can view short term and long term fuel trims. I used torque app on android to view misfire counts. The miss did stay on #4.

The misfire history is telling me there has been 2500 misfires on #4 but not sure since when? All other cylinders are showing about 5% or less of 2500.

 

[Texan]

It's an UltraGuage. Can display 35 Tb sensors and will tell you of a pending code. This pending code is never enough to trip the CEL but it is telling me there will be a problem soon. You can view short term and long term fuel trims. I used torque app on android to view misfire counts. The miss did stay on #4.

The misfire history is telling me there has been 2500 misfires on #4 but not sure since when? All other cylinders are showing about 5% or less of 2500.

My daughter tells me that I am behind times. I do not have a smart phone (plain cell phone). She may be correct.
Does UltraGuage run on a notebook (PC)? I see the torque app mention on this site, but have no idea what it does.
I have been using computers since the punch card days. I have never paid any attention to my short and long term
fuel trims. When the weather improves I will look and see if mine look anything like what you show. At least we learned
that the I6 has plug wires.

 

[tdamocles]

My daughter tells me that I am behind times. I do not have a smart phone (plain cell phone). She may be correct.
Does UltraGuage run on a notebook (PC)? I see the torque app mention on this site, but have no idea what it does.
I have been using computers since the punch card days. I have never paid any attention to my short and long term
fuel trims. When the weather improves I will look and see if mine look anything like what you show. At least we learned
that the I6 has plug wires.

Ultragauge is just a sensor reader that can display vital info real time. It is connected to the obd2 port all the time. Ultragauge has it' s own website....take a look. The torque app is very interesting and can read many more pids but needs a bluetooth connection to work.

 

[lint]

If you moved the coil packs and the boots, maybe the plug connector to the pack or a bad wire going to number 4, not a fun job chasing wire problems, . I did see some videos on YouTube of them chasing a misfire in the wiring , I think it was a 2005 trailblazer I don't know whats wrong ,just something to try.

 

[bobdec]

Are your LT trims still reading 8-13% high. If so you have a problem. The LT trims are what the PCM uses for correction during fueling calculations, and the ST trims are the actual O2 interpretation of the burn. ST's will vary a few % either way. What LT's are saying is your PCM is adding 8-13% more fuel to the fueling calculation to obtain ST trims within specification. Or basically at one time the ST trims started to read lean and to correct the PCM started adding fuel (incrementing LT's) to correct the lean condition. You are running in a grey area as PCM correction (adding or pulling) over 10% fuel can sometimes give flaky problems. Could be bad O2 (pre CAT), vacuum leak, exhaust manifold leak, a weak injector or spark (no fire reads as lean). Or rarely a valve train problem. Do LT trims get better at higher RPMs, a small vac leak has less significant effect at high air flow.

 

[tdamocles]

Are your LT trims still reading 8-13% high. If so you have a problem. The LT trims are what the PCM uses for correction during fueling calculations, and the ST trims are the actual O2 interpretation of the burn. ST's will vary a few % either way. What LT's are saying is your PCM is adding 8-13% more fuel to the fueling calculation to obtain ST trims within specification. Or basically at one time the ST trims started to read lean and to correct the PCM started adding fuel (incrementing LT's) to correct the lean condition. You are running in a grey area as PCM correction (adding or pulling) over 10% fuel can sometimes give flaky problems. Could be bad O2 (pre CAT), vacuum leak, exhaust manifold leak, a weak injector or spark (no fire reads as lean). Or rarely a valve train problem. Do LT trims get better at higher RPMs, a small vac leak has less significant effect at high air flow.

LT trims back down to 4-6% at higher rpms. Yesterday the intake gaskets were replaced and the trims haven't change. Cylinder 4 throws the pending code. I did notice that when I rev the motor above idle the misfires disappear according to the scanner. Clogged injector?

 

[bobdec]

Can you update your profile with year and model. Older 4200's had cylinder sleeve problems.. Could be an injector, but a bad injector could tend to get worse or not clear at higher RPM's, plus they look like PITA to get at on this engine. Under the intake manifold.. Have you been able to inspect the #4 plug and compare it against another cylinder. Since you're not throwing a DTC code the problem is not " that bad" yet. Here is a rather long article on misfires . Tech Feature: Detecting Misfires in OBD II Engines: Underhood Service . As it states a misfire (slight difference in RPM for only a few degrees of the crank cycle) says something is wrong during/after that cylinder fired, but does not pinpoint the actual problem. I'm tending to go with a lean condition causing the misfire as your LT trims at idle are on the lean side. Could be #4 is just the weakest and showing up first. At normal warmed up idle what are is your LT trims (mine LT is 3.1%, ST is -0.8 to 1.6) , What is your vacuum (MAP) (mine is 32 KPA, 4.5 PSI if using Torque App..ugg), and what is TPS% (mine is 13.3% at 600 RPM, in N ,no a/c on). I'm thinking if you get trims into line just Maybe misfires will clear. That just an opinion and I'm open up to any other Ideas others may have..

 

[tdamocles]

It is a 2007. ST of -3 to 4. LT as high as 14.2%. Vacuum at 4.4PSI.

 

[Texan]

It is a 2007. ST of -3 to 4. LT as high as 14.2%. Vacuum at 4.4PSI.

If you would update your profile with year(2007), model(TB), Engine(I6), 2 wheel drive?, it would make it
easy for us to remember what we are looking for. I looked at the websites for Ultragauge and Torque app.
and will comment on them tomorrow. The article reference above is a must read. I have a Autoxray 4000
that I bought back in 2005. They were bought by SPX(OTC).

 

[IllogicTC]

I'm going to just copy-paste some information from easterncatalytic.com here. Apparently double-digit fuel trims are usually indicative of a problem.

From the website:

Diagnosing with Fuel Trims

Fuel trims can help you zero in on the problem, especially when there are no other trouble codes present. Knowing whether a vehicle is running too rich or too lean will help narrow down your diagnosis. Fuel trims that differ greatly from one cylinder bank to the other will also point you in the right direction. Always evaluate fuel trims at idle and at 2500 RPM.

Running too rich – High negative fuel trim corrections can be caused by MAF sensor problems, high fuel pressure, leaking fuel pressure regulator diaphragm, faulty evaporative emissions components, leaking injectors, defective O2 sensors, exhaust leaks/pinholes before the O2 sensor, coolant temp sensor problems, and base engine issues such as low compression and incorrect camshaft timing.

Running too lean – High positive fuel corrections can be traced to MAF and O2 sensor faults, vacuum leaks from intake gaskets/hoses, unmetered air (intake snorkel leak), clogged or dirty fuel injectors, fuel delivery issues, and exhaust restrictions such as a clogged catalytic converter.

Diagnostic Tip:

For a suspected vacuum leak, note the fuel trims at idle and increase engine speed to 2500 RPM and hold. If the STFT immediately decreases and moves to acceptable levels and the LTFT slowly starts to come back down, you have a vacuum leak. After the repair, reset the KAM and start the vehicle. Monitor the fuel trims to make sure they are within the normal ranges. It could take up to 10 miles of driving for an accurate LTFT reading.


You can read more about fuel trims here: What are fuel trims all about?

And from O'Reilly Auto, on misfires:

There are four main causes of engine misfires - a loss of spark, an unbalanced air/fuel ratio, a faulty fuel injector and a loss of compression. The loss of a spark could mean that the spark plug needs to be replaced, which is a relatively simple process. It may also be caused by a faulty spark plug wires. In either case, if the spark plugs or wires are failing in one cylinder, they will need to be replaced. When one spark plug or wire goes bad, there's a good chance the others are not far behind. It is a good idea to replace all of the others when replacing one of these components.

If the mixture of fuel and air is too lean - meaning there is more air than fuel - then it will be unable to ignite, even if the spark plugs are working. There is also a possibility that the mixture is too rich - too much gasoline - even though this tends to be far less common than a lean mixture. A gasoline-heavy ratio of air and fuel can occur if the fuel injector leaks, but this tends to occur in all cylinders, not just one, if it happens.

An improper mixture can be caused by leaks in the fuel lines, a weak fuel pump or a faulty fuel injector. It is important to check the fuel injector on the problematic cylinder. It should spray a fine mist of fuel, but if there are drips, it may indicate that it is clogged, dirty or otherwise damaged.

Low compression can also contribute to engine misfires. Pressure in the combustion chamber is just as important to a healthy cylinder as the fuel mixture and the spark that ignites it. If a chamber is losing compression, it means that the gasoline and air are leaving the cylinder before they can be ignited. This can be caused by leaky vacuum hoses, a busted exhaust valve or a blown head gasket. When two adjacent cylinders are misfiring, chances are that the gasket between them is cracked or damaged.

 

[tdamocles]

If you would update your profile with year(2007), model(TB), Engine(I6), 2 wheel drive?, it would make it
easy for us to remember what we are looking for. I looked at the websites for Ultragauge and Torque app.
and will comment on them tomorrow. The article reference above is a must read. I have a Autoxray 4000
that I bought back in 2005. They were bought by SPX(OTC).

I was using tapatalk and I couldn't change the profile.

 

[tdamocles]

When the pcm adds fuels does it do by changing Pulse width of individual injectors? When I see fuel trim of 14% is it a global addition of fuel to injectors or just to the affected cylinder?

 

[bobdec]

Adding fuel is global, with only one O2 sensor the PCM does not isolate a lean condition to a specific cylinder. It's technically possible with a scope, or sometimes by using an IR thermometer at the cylinder exhaust ports or header primaries to find a hot or cold firing condition. Fueling for the 2007 is calculated using two systems Speed Density Mode using MAP sensor and RPM at low RPMs and MAF Mode using MAS sensor at RPM's over 3600. However the PCM still compares results of both and if one is showing funny stuff it can correct by using one of or a combination of both modes. Your MAP (vacuum reading of 31 Kpa (4.4 PSI using Torque App) looks good, I was thing bad MAP readings would mess up the SD fueling calculation. And possible bad MAS readings used for MAF mode may somehow be involved. What is you air flow at 600 RPM it should be between 4.3-4.8 grams/second. If it's lower you have un-metered air entering the intake, AKA: a vacuum leak. Since you replaced intake manifold gasket, have you checked the small HVAC port cap on intake that is not used on SWB. Since LT trims get better with more air I can't get away from a vac leak.. A 14% error needs to be fixed.. I hate shotgunning and throwing parts at a problem while still isolating, I really can't recommend anything at this point.

 

[tdamocles]

At idle.

 

[tdamocles]

At 1765rpms the misfires go away.

 

[tdamocles]

By the way my mechanic put a new cap on that nipple and zip tied it. I did notice in the 20F degree temps outside my car cannot achieve 180 degrees on a regular basis. Not sure if that matters or not.

 

[tdamocles]

Cylinder 4

Cylinder 2.....wierd...iridium electrode worn on this cylinder after 15,000 miles(AC delco plugs). I thought iridiums were 100,000 mile plugs...I call BS.

 

[Texan]

I am reminded of a old Snoopy cartoon, Lucy said to Lenius(sp) about the metric system, "give them a inch and they will take a meter.

First comment, if you look for P0128 through these threads, you will find that the operating temp. need to be 200f +/-.
I very seldom have need to use my scanner, so I have to get out the instruction books to relearn what to do.
I drove my TB today with amb temp around 40f.
The No. 1 O2 sensor recorded these parameters.
Rich-Lean Thrshd 0.345 v
Lean-Rich Thrshd 0.550 v
Low Volt Switch Time 0.250 v
High Volt Switch Time 0.500 v
Rich-Lean Switch Time 0.052 s
Lean-Rich Switch Time 0.028 s
I assume that you are not using E-15 gasoline.

 

[tdamocles]

No e15. Have never got a p0128 dtc. Not sure how that would cause only cylinder 4 to misfire all the time at idle but not enough to throw a code.

 

[bobdec]

#4 does not look burnt white, not sure about the ware on #2. The only thing I see is your air flow at 594 RPM with TPS at 13.7% is 4.2 Grams/sec and my '07 runs 4.5-4.6 AFGS that's a 10% difference. It's saying you have 10% less air passing the TB so the PCM would be providing 10% less fuel via the injectors to your engine. However since the O2 saw a lean burn (via the ST trims) it upped the LT trims and is adding an extra 10-14% fuel to fix the error so your ST trims look better. Could be MAS reading 10% low or your getting 10% air from some other place (vac leak)
Here's my dilemma, I'm under the impression that our '07 does not use MAF readings at low RPM, but there is very little info on how our P12 PCM actually works, GM only used P12's in '06 & 07 so they are not real popular with the tuners. It it were my car I would unplug the MAS to get it out of the picture. That's your decision. To disconnect it you have to actually pull it out of the intake pipe just after the filter. Remove the two 7/32 screws, then snap off the grey plastic connector lock, then unplug it. You don't have to re install the MAS and can leave the hole open as it won't effect anything. This is key you have to reset the PCM, by pulling the fuses for a min or so. That will reset the LT trims to 0% . When you start it up w/o the MAS plugged in it will run crappy, maybe stall and require restart, but if you can keep it running for about 10 seconds till the PCM switches to speed density mode it will smooth out and run like normal. SES light will come on, ignore it. You will need to let it idle for about 5-10 mins and then look at the LT trims if they are still at 10-14% that says the 4.2 AFGS reading is correct and the problem is not the MAS. By the way that's my last real shotgun idea, a bad O2 should read the same at all RPM's. same with a bad injector. And since you are getting misfires on other cyls the problem looks like fueling, but a bad fuel pump, or clogged filter usually does not fail at the lowest loads (idle) and run OK at high demands. I've see a MAS have flat spots and they are not accurate at low air flow.

By the way P0128 comes on with coolant below 74* C about 165* F, running at 175 should not cause problems..emissions like it better over 194*F but in reality our '06-07's actually have a bit more timing advance 0-2* a hair below 194* F. And we use Intake Valve Temp instead of coolant temp for some of the controls...

 

[tdamocles]

Will try but not now. It is snowing outside. If the trims are not high after the mas pull than does that mean it is suspect?

Also, can't i just disconnect the connector at the mas Instead of pulling it completely? When you say vacuum leak do you mean between the mas and throttle body?

Also, my tps% is also higher than yours. Could this be adding fuel also?

 

[bobdec]

Q1: If the trims are not high after the mas pull than does that mean it is suspect?
A1: Yes if trims are normal w/o MAS then problem is the MAS is sending low air flow info to the PCM.

Q2: Also, can't i just disconnect the connector at the mas Instead of pulling it completely?
A2:; Absolutly, however removing to the darn light grey connector lock underneath the connector using only feel is not easy. Rather than break something I removed mine so I could see what it looked like first, before pulling on it. Leaving it out leaves a hole in the intake hose, but that's not effecting anything w/o the MAS connected, if MAS was connected and not installed in that hole effects would be very evident.

Q3: When you say vacuum leak do you mean between the mas and throttle body?
A3: YES, but any leak 'after' the MAS including the intake manifold will let in 'UN-Metered Air'. That's air the PCM does not see and thus does not add fuel. For every gram of air entering passing the MAS the PCM adds 1/14.7 grams of fuel. That maintains an AFR or 14.7 parts air to one part fuel. (all those numbers are for 100% gasoline). Now if 14% of the air entering the intake is entering from after the MAS (leak before TB, or intake manifold then the PCM does not add enough fuel and engine runs lean. When you disconnect the MAS and engine reverts to 100% Speed Density Mode the PCM then reads MAP (vacuum) and RPM, and goes to a Volume Efficiency table (VE) in the tune and calculates fueling from that table. If I'm getting to detailed let me know.. I sometimes get long winded.

Q4: Also, my tps% is also higher than yours. Could this be adding fuel also?
A4: I saw that also but since (13.7-13.3)/13.3 is only 0.03 or 3% it's not enough to cause what we are seeing, could be just mechanical TB bore/blade/cleaning differences, TP sensor, engine drag (water pump, alternator, torque converter, fan, oil) .

 

[tdamocles]

This is with the MAS disconnected. No misfires!

Here is the kicker..... I drove home 8 miles with it connected after disconnecting it and the trims were great. I stopped and then let it idle for 5 and the LTT was 3.91%! Do I have a wonky sensor?

 

[IllogicTC]

This is with the MAS disconnected. No misfires!

Here is the kicker..... I drove home 8 miles with it connected and the trims were great. I stopped and then let it idle for 5 and the LTT was 3.91%! Do I have a wonky sensor?

If falling back to MAP provides such an improvement in performance, I would absolutely start with a new sensor.

 

[Texan]

I am not familiar with late model MAS (Mass Air Flow Sensor).
In the old days you could carefully clean them with carb spray cleaner.

Could have a bad connection.

 

[tdamocles]

I am not familiar with late model MAS (Mass Air Flow Sensor).
In the old days you could carefully clean them with carb spray cleaner.

Maybe a month ago I sprayed it with mass air cleaner.... No change then....

 

[tdamocles]

I think I'm almost at the end of this journey, I think.....thanks to all the guys that commented and special thanks to bobdec that got me to disconnect the mas. I will be off to the parts store tomorrow to get the sensor.

Bobdec, are you a gm auto tech? You seem very knowledgeable about GM.