Short 'Low Oil Pressure' warning, low MPG...now what?

[Lima Tango]

ALCON,

Help me GMTNation, you're my only hope. I've been searching for days and finally am at the point of needing specific answers.

Problem 1: Calculated ~2-3 MPG lower than it used to be. It is now approx 13/17.5, conservative driver, never speed. Noticed an average of under 18MPG on a 1200 mile road trip recently, mostly 65MPH driving - the same road trip a year ago averaged 21MPG.

Problem 2: I get a quick "Low Oil Pressure" message on the DIC after start up. It only stays on a few seconds and never comes back on the rest of the time the vehicle is running. This doesn't occur every startup, and it isn't consistent to only cold starts, only hot starts, or any other causality I can discern. The oil pressure gauge itself ranges between 50% and 65% or so from startup throughout regular driving (assuming 50% is straight up), forgive me for not remembering the actual units of measurement or hash marks on there. The oil itself is filled fairly high on the dipstick, right about at the top of the proper fill range.

Problem 3: I feel like it has an inconsistent idle; I watched it in Torque, it ranged from 590-660. Every now and then it will do a stumble then surge but this is rare.

Vehicle: '05 TB LT, 4.2L, almost 100K miles, good Michelin Latitude Tour tires, no codes reported in Torque Pro

Background: I've stayed on top of the maintenance since I got the truck at 36k in 2008. New plugs (correct Delcos), removed and cleaned throttle body, plus all the fluids (trans, diff, transfer case, brake, etc), new air filter (Wix) completed last summer at approximately 92k. The issues I'm seeing weren't happening then, they mostly started within the last few months, best I can tell. I also do a bottle of Techron when I think about it. Last one was 3-4K ago. The gas on base is certainly not top-tier quality, so I'm sure that's not helping. Current oil status on DIC reads 40%, but it's in the neighborhood of 5 months/5k old.

I have noticed the upstream O2 seems a bit lazy - mostly ~1Hz variations, but there will be sporadic periods of .25-.5Hz. Ranges from .1-.8 or so. Downstream is pretty flat around .7.

What I've done so far: Ran a bottle of Redline through the last tank. Spent some time looking into it and seemed worth a try. Changed the CPAS and Cam Sensor today; left the battery unhooked for 1 hour or so. No reason to believe they were bad, and even after removal they both looked fine (no oil in connectors, clean screens), but given the 100k milestone it seemed like a good idea to swap them out anyway. Both correct ones from GMPartsEast. It appears something of this lot helped the idle. More like a 615-630 variation now. MPG remains unchanged. I saw on another post about testing the lockup by gently pressing on the brakes while cruising on the highway - the RPMs went up a couple hundred by doing that, which the post seemed to indicate meant it was working correctly.

What I'm going to do: I bought a Delco O2 sensor but was unable to remove the old one. Going to take it to a local mechanic to have him break the old one out for me on Monday. I'm also going to have him do an oil change as I'm about due based on the 5k old oil. Additionally, that will help exclude the oil as a potential cause of the Low Oil Pressure warning.

My actual questions for you all: Is there something else I should have the mechanic do on Monday, or have him specifically check? Is there something I can look at with Torque Pro that will help identify the issue(s)? Is there anything else I should do, check, or replace? Does a 'Low Oil Pressure' warning for a few seconds after startup really mean something is wrong if it doesn't ever come back and the oil pressure gauge looks fine?

Should I do a SeaFoam treatment? If so, is there a good how-to on the process for Seafoam - I've seen that people use it in the vacuum booster or something like that, but I'm not sure how to do this; detailed explanation and pictures would be very helpful.

Hopefully you all can give me some advice. We're about to move to San Antonio and will be doing a very long drive over the course of 10 days (including some Disneyland days) and regaining 3 more MPG that I used to have would be awesome, but I'm more concerned about some problem escalating and stranding us somewhere in the desert.

Please let me know if you have any clarifying questions or can offer any solutions. Thank you for your time in reading my tome. Apologies for the length, but I wanted to give you any and all requisite information.

Cheers,

LT

 

[CaptainXL]

A 10 inch pipe wrench will get the O2 out. Even if you strip it they are made to grip round pipe and self tighten. Just put it on there and wrench away. Don't waste ur money. I would replace this first before speculating.

 

[Lima Tango]

A 10 inch pipe wrench will get the O2 out. Even if you strip it they are made to grip round pipe and self tighten. Just put it on there and wrench away. Don't waste ur money. I would replace this first before speculating.

On the protruding part or the actual nut part? I wasn't able to get anything else on there other than the socket because that exhaust heat shield was in the way. I was able to get the nuts off the heatshield, but wasn't able to figure out how to slide it off, it kept hitting the trans dipstick or whatever else was in the way. I searched for a how to remove it article but didn't see one, so any direction there would be greatly appreciated.

The only reason I posted the questions first prior to the O2 sensor being replaced is I presumed that even a bad O2 sensor wouldn't possibly generate a 'Low Oil Pressure' message, so it seemed like there was at least some other problem at hand.

 

[CaptainXL]

I'm not sure how I could make it easier for ya. Just remove the tranny dipstick tube holdown bolt and swing it out of the way. Then remove the heat shield and bend it a little to get it to go down and back then bring it forward. Oh, there is the ac line holddown as well on the passenger side of the engine that needs to come off. Just take your time and you will get it out. A shop is gonna charge over $200 to do this.

 

[Lima Tango]

I'm not sure how I could make it easier for ya. Just remove the tranny dipstick tube holdown bolt and swing it out of the way. Then remove the heat shield and bend it a little to get it to go down and back then bring it forward. Oh, there is the ac line holddown as well on the passenger side of the engine that needs to come off. Just take your time and you will get it out. A shop is gonna charge over $200 to do this.

Copy, I'll give it a shot, thanks!

 

[CaptainXL]

Your welcome

 

[Lima Tango]

O2 sensor changed out - much more active movement now in Torque, 2-3Hz instead of .5-1Hz like before. Tested mileage on same mostly highway route in similar conditions: 16.2mpg before 19.6mpg now and I even had the AC on for the after test but did not have it on for the lower mpg before test. Huge improvement. Obviously, the lazy 98k mile sensor was the culprit in reduced mpg.

Have not seen the low oil pressure warning since the oil change, but it wasn't consistent before so I will need to monitor it for awhile longer before confidently concluding that item.

Strange thing though, she still feels like there is a slight miss or shudder. I can't quite explain it, it isnt that perceptible, it isn't violent or anything, but it definitely doesn't feel like she's running like a top. I was hoping the o2 sensor, CPAS, and cam sensor replacements would have made her run perfect. Other ideas? If you don't recall, she had new delco plugs and a complete throttle body clean last summer 8k miles ago. Left the battery unhooked overnight after ths new sensors went in. Input is appreciated.

Thanks

LT

 

[triz]

No codes is a good thing. Did you check the gaping of the plugs though when you did the install?

 

[Lima Tango]

No codes is a good thing. Did you check the gaping of the plugs though when you did the install?

Correct, no codes pulled via Torque. I read on here that the plugs should already be gapped to spec, so I just eyeballed them next to each other to see if any one looked different than the others and they all looked good.

 

[triz]

Yes they should be gaped but I have had times where they have been off.

 

[CaptainXL]

Yes they should be gaped but I have had times where they have been off.

Negative. Iridium plugs come gapped from the factory and should be correct. Just hold a feeler gauge up next to them. If they are off then they need to be returned for a replacement. Its much easier to replace them than to try and gap them. Iridium is very brittle and one small slip will ruin the plug as well as ruin any chance of getting a replacement.

 

[tblazerdude]

any reason you didn't do both O2 sensors?

 

[CaptainXL]

Strange thing though, she still feels like there is a slight miss or shudder. I can't quite explain it,

Well here is the scoop. The reason O2 sensors get slow is due to soot from too rich a mixture or just age. Usually a combination of the two. If replacing the O2 helped mask part of the problem I would start looking at others that contributed to the problem. Namely, incomplete combustion issues such as weak or intermittent spark or dirty /stuck fuel injectors. Without doing a fuel pressure balance test you will need to go the cheap route. Run some top tier gas and some fuel line drier through it for about a month. Also run a bottle of Techron Concentrate Plus and BG 44K. If symptoms get better then it's fuel related.

 

[Lima Tango]

Had to move cross-country, so I haven't had a lot of time to dive back into this. Everything seemed pretty good for a couple months, no low oil pressure warnings or anything, but the other day she died right as I was starting to pull out of a parking lot and flashed low oil pressure for several seconds. Mileage is fairly poor (~14) but I'm thinking that's attributable to the mostly city San Antonio driving and the 100+ degree weather. Still no codes. I did try a bottle of Techron injector cleaner with no real effect.

Is there some thorough fuel system/injector cleaning service I can have done to get these things as good as new? I feel like the over the counter cleaners just aren't doing the trick. I'm not confident that this is necessarily the problem, but if the O2, CPS, and CPAS are all new, what else would be causing generally reduced drivability without throwing codes? Any more advice would be appreciated.

 

[djthumper]

Catalytic converter could be the issue if it is partially clogged. That is only because you changed everything else in the chain. Also have you done a motor flush and changed the oil?

 

[Lima Tango]

Catalytic converter could be the issue if it is partially clogged. That is only because you changed everything else in the chain. Also have you done a motor flush and changed the oil?

I had a backpressure test done on the cat and the shop said it was fine. Assuming no human error (pretty big assumption) then I'm disinclined to think it's that.

I had an oil change done and it helped resolve a lot of the 'low oil pressure' warnings (the oil level was fine, so it wasn't just that it was low), but it was not a complete flush. I can certainly have that done, the truck's over 100k now and hasn't ever had an oil system flush.

 

[djthumper]

I had a backpressure test done on the cat and the shop said it was fine. Assuming no human error (pretty big assumption) then I'm disinclined to think it's that.

I had an oil change done and it helped resolve a lot of the 'low oil pressure' warnings (the oil level was fine, so it wasn't just that it was low), but it was not a complete flush. I can certainly have that done, the truck's over 100k now and hasn't ever had an oil system flush.

I would do a motor flush and then change the oil. Something could be clogged and not allowing enough oil someplace. If they still have a auto hobby shop on base I would just go there and do it myself. There is a lot of knowledge there from off-duty transportation guys to the retirees. Which base are you at now?

 

[triz]

Had to move cross-country, so I haven't had a lot of time to dive back into this. Everything seemed pretty good for a couple months, no low oil pressure warnings or anything, but the other day she died right as I was starting to pull out of a parking lot and flashed low oil pressure for several seconds. Mileage is fairly poor (~14) but I'm thinking that's attributable to the mostly city San Antonio driving and the 100+ degree weather. Still no codes. I did try a bottle of Techron injector cleaner with no real effect.

Is there some thorough fuel system/injector cleaning service I can have done to get these things as good as new? I feel like the over the counter cleaners just aren't doing the trick. I'm not confident that this is necessarily the problem, but if the O2, CPS, and CPAS are all new, what else would be causing generally reduced drivability without throwing codes? Any more advice would be appreciated.

For the fuel injectors. I would send them to witchhunters. They have a good rep on my other board. Ill try and answer some of your other questions when I get home since Im typing on my phone. Fluids you cant go wrong with any of the Amsoil. They carry everything from Powerfoam to Flush if you so choose. Have you tried doing a compression test?

Here is a link for witch hunters. http://witchhunter.com/

Clean your throttle body and MAF. Also check your CPAS sensor right above the power steering pump. I've had where it got oiled up so bad it was causing misfires.

 

[gmcman]

What oil filter are you using? From your posts I would think maybe a Fram, but that's not knocking Fram. I don't know if these vehicles have a low-pressure shutdown but I know some older GM models wouldn't fire the plugs unless a minimum oil pressure was reached.

Check the oil pressure electrical connection at the oil filter housing, could be failing, cake with crud, but the next thing you need to do is an oil pressure test with a quality gauge, cold and hot.

What oil are you using? You stated you had the oil changed, what shop and do you know for sure what oil they are using?

I would get a new AC PF61E filter, and 7 quarts of Valvoline Max Life, and one of the below listed oils, preferably the Pennzoil just because it's Dexos approved.

The slight stumble is an issue but your oil pressure needs to be figured out asap.

 

[Lima Tango]

What oil filter are you using? From your posts I would think maybe a Fram, but that's not knocking Fram. I don't know if these vehicles have a low-pressure shutdown but I know some older GM models wouldn't fire the plugs unless a minimum oil pressure was reached.

Check the oil pressure electrical connection at the oil filter housing, could be failing, cake with crud, but the next thing you need to do is an oil pressure test with a quality gauge, cold and hot.

What oil are you using? You stated you had the oil changed, what shop and do you know for sure what oil they are using?

I would get a new AC PF61E filter, and 7 quarts of Valvoline Max Life, and one of the below listed oils, preferably the Pennzoil just because it's Dexos approved.

The slight stumble is an issue but your oil pressure needs to be figured out asap.

Unfortunately, I'm really not sure on brands, but I'd expect just the basic Fram filter and cheap conventional oil - I was in the middle of the move and had to drop it off at a local shop to see if an oil change helped with my 'low oil pressure' issue without doing my usual due diligence. I'm still a few thousand miles early, but I'm planning on getting a fresh change anyway, so I will use your suggestions to ensure I'm getting the good stuff and will see what I can do about checking out the oil pressure connection and test gauge.

 

[Lima Tango]

I would do a motor flush and then change the oil. Something could be clogged and not allowing enough oil someplace. If they still have a auto hobby shop on base I would just go there and do it myself. There is a lot of knowledge there from off-duty transportation guys to the retirees. Which base are you at now?

I'm at Fort Sam Houston for a year.

With the whole budget cuts nonsense the auto hobby shops and other luxuries have taken a hard hit, so they aren't the useful option they used to be. Still, worth checking out the post one here to see what they have to offer.

 

[Tiggerr]

I have read in other threads and other places that the oil pressure switch (not really a sensor on these but a switch the gauge isn't true gauge) has a habit of flaking out.
Could try replacing it.

 

[gmcman]

Does the motor make a rapping or knocking sound after you start it until the oil pressure build up or is it fairly quiet?

Have you inspected the TB recently and when was the last cleaning?

Did you perform a TB relearn when you cleaned it last and when you unhooked the battery?

Stumble at low RPM with stalling could be a dirty TB, also throwing the low oil pressure light.

You stated the pressure problem lessened after the change so I would check those areas first just to cover some bases, hopefully it's not a restricted oil pickup or oil pump showing excessive clearance.

Do you regularly have a shop change the oil or DIY? Reason I ask is when you change your oil, you know that 5W-30 is going in. Regardless of how many miles are left on the current oil, I would change it now. Oil starvation issues are serious and the above listed is far, far cheaper than another engine.

There are two Harbor Freight's near you, and a Sears, HF has an oil pressure gauge if you can't fix the problem with another oil change TB cleaning.

 

[gmcman]

I have read in other threads and other places that the oil pressure switch (not really a sensor on these but a switch the gauge isn't true gauge) has a habit of flaking out.
Could try replacing it.

Yeah if he has crud around the connection it's a sign.

 

[Lima Tango]

Does the motor make a rapping or knocking sound after you start it until the oil pressure build up or is it fairly quiet?

Have you inspected the TB recently and when was the last cleaning?

Did you perform a TB relearn when you cleaned it last and when you unhooked the battery?

Stumble at low RPM with stalling could be a dirty TB, also throwing the low oil pressure light.

You stated the pressure problem lessened after the change so I would check those areas first just to cover some bases, hopefully it's not a restricted oil pickup or oil pump showing excessive clearance.

Do you regularly have a shop change the oil or DIY? Reason I ask is when you change your oil, you know that 5W-30 is going in. Regardless of how many miles are left on the current oil, I would change it now. Oil starvation issues are serious and the above listed is far, far cheaper than another engine.

There are two Harbor Freight's near you, and a Sears, HF has an oil pressure gauge if you can't fix the problem with another oil change TB cleaning.

No rapping, knocking, or ticking, or similar noises. That's the weird thing - it doesn't SOUND like it's not getting oil to the lifters and you'd think it would. It doesn't ever do the low oil pressure warning except at idle, and usually only right after startup or in conjunction with a rare stall.

I pulled off and cleaned the TB thoroughly a year ago, ~12k miles. I've been told it's a 30k procedure, so I haven't done it since. I did leave the battery unhooked for awhile at the time, but I'm not sure what a TB relearn is so I'm guessing I didn't do it. I'll have to search for that process to figure out what to do. I did inspect and partially clean it from the front side recently, looked fine and didn't have much buildup on the front side of the butterfly.

I basically never DIY the oil changes, so there is definitely a risk of them putting some wrong viscosity in there.

Good call on the HF oil pressure gauge to gain more information. I'll get fresh oil and filter in there right away though.

 

[Wooluf1952]

No rapping, knocking, or ticking, or similar noises. That's the weird thing - it doesn't SOUND like it's not getting oil to the lifters and you'd think it would. It doesn't ever do the low oil pressure warning except at idle, and usually only right after startup or in conjunction with a rare stall.

I pulled off and cleaned the TB thoroughly a year ago, ~12k miles. I've been told it's a 30k procedure, so I haven't done it since. I did leave the battery unhooked for awhile at the time, but I'm not sure what a TB relearn is so I'm guessing I didn't do it. I'll have to search for that process to figure out what to do. I did inspect and partially clean it from the front side recently, looked fine and didn't have much buildup on the front side of the butterfly.

I basically never DIY the oil changes, so there is definitely a risk of them putting some wrong viscosity in there.

Good call on the HF oil pressure gauge to gain more information. I'll get fresh oil and filter in there right away though.

That is the relearn. Disconnecting the battery resets the PCM to factory settings.

 

[djthumper]

I'm at Fort Sam Houston for a year.

With the whole budget cuts nonsense the auto hobby shops and other luxuries have taken a hard hit, so they aren't the useful option they used to be. Still, worth checking out the post one here to see what they have to offer.

Yeah, They shut the one down here at Nellis and sold everything off.

I would start with the motor flush and oil change and go from there. I just have a feeling there is a minor clog.

 

[gmcman]

I would pull the TB again and ensure it's clean and nothing around the edges of the butterfly.

TB relearn: Elapsed time is not set in stone but this works for me to allow cleaning up...

Unhook NEG battery cable Remove harness from TB Perform cleaning Attach TB harness After 30 min of having the battery unhooked(about the time to clean the TB), reattach NEG terminal. Turn key to ON with all the dash lights on. DO NOT START AND DO NOT TOUCH GAS PEDAL TB will perform series of noises during this time. Leave key ON for about 5 minutes. Do not turn key off. Start engine DO NOT TOUCH GAS PEDAL let idle for 10 min.

 

[triz]

Are you stalling?