Troubleshooting lean conditions under boost - Boost Forum

Forum Post / Reply
You must log in before you can post or reply to messages.
Troubleshooting lean conditions under boost
Sunday, December 09, 2012 12:19 PM
I'm looking for some ideas on how to troubleshoot a problem where my wideband guage is indicating lean conditions under boost. I appreciate any help and if you need any more information let me know and I'll try to get it. So here's the details:

I have an 04 Cav with the GMSC intercooled, rebuilt engine, 60# injectors and some other mods. I also have used HPT to develop a tune based off of a stock ECM. All of this I did years ago when it was my daily driver. It's been sitting most of the time for the last 3 years give or take, taking it out sometimes to drive.

At the time I quit driving it, it was running fine to the best of my knowledge. Recently, I have been thinking of getting it back on the road and re-doing the exhaust. Since the gas was old (but low), I first tried draining some but it looked not too bad, so I left the rest in and added some new 93 octane. I also changed the battery, oil, and aired up the tires, that was about it. So then I took it out for some test drives and it seemed to drive just fine most of the time. The AFR's from the wideband seemed to be where they should be...except when I get on it and quickly go into boost.

When I do that, the AFR's start out around 12.5 and get worse as I go, 12.7, 12.9, 13.1, 13.3, 13.5..try not to let it go past that obviously. I have also tried slowly getting into the pedal so it's not all at once and it does make a difference in the AFR's even under boost. It will start out around 11 then more slowly get leaner.

So of course this is something I want to fix so I don't blow things up, but trying to find the best ways to diagnose.

I have HPT and the Torque Android app to help scan items, along with the wideband. I also have a fuel pressure gauge I can hook up to the schrader valve (so not a gauge that is inside the car by the wideband). So I was trying to think if there's a way I could use these to help find the problem.

I have tried doing scans and noticed that when it goes lean like that there doesn't appear to be any misfire corresponding with it, or at least not consistently if any. After a bit it does show some knock retard values of around 6-8. At idle I have seen some fairly significant misfire from a cylinder or two but appears to be only at idle (and may have been only the first few times I drove it recently, but not positive).

When the intercooler is going the IAT's do go down and not as much knock, but doesn't affect the AFR's (as expected).

I've also tried adding some Seafoam in case there was some buildup in the injectors or somewhere that was causing them to not flow right, but so far it doesn't seem to have made a difference.

I have also hooked up my fuel pressure gauge and would rev it some while the car was sitting to see if the pressure dropped off. It seemed to at first bump up some then jump back down a little possibly to slightly lower than idle, so some change, but not huge. I had thought about trying to thread the gauge under the car hood towards the windshield wipers so I could see it while going down the road, but not sure if that will work or be visible/safe.

I don't think the fuel pump's output should be a problem since I had the same setup not give any problems before. Also I don't seem to get hesitation so didn't think it would be the fuel filter either.

One problem I did have a few years ago was that it was going really lean (even more so than now) when I would go into boost. Then I found out the problem was the aftermarket Aeromotive AFPR I had on. It had been working, but then something must have went wrong with the internal mechanism. When I hooked up the fuel pressure gauge and revved it up the pressure would drop a significant amount. So I put the stock FPR back on and it ran like a champ.

So I've been wondering if maybe the stock FPR also has went bad and I should try changing it, but haven't done it yet because I wanted to see if there were any other things I could try/log with HPT/Torque first.

Another thing I've been wondering about is if there could be an issue with the plugs or injectors. As I mentioned it didn't seem like there was any consistent misfire when it went lean or knocked, but could the plugs still be causing a problem even without misfiring? I haven't pulled them yet, but could do that. I'm running NGK Iridium's 2 steps colder than stock. As for the injectors they are 62.8# and had been working fine before, but is there a good way to check if one of them has some kind of problem now, or maybe not opening all the way or something?

As for the tune, it was also working fine before. Just to make sure though I tried reflashing the car's ECM based on the tune I had stored on my laptop in case somehow it got corrupted from the car sitting so long, but still no luck. So then I tried editing the VE tables to make it richer under high rpm's and that still didn't fix the lean condition. So I'm thinking it's not something with the tune. For reference, here's a link from 2007 to a thread I posted of how I did my tune and how to tune using a stock ECM and fake the 2.5 bar MAP to get around the problem of the eco gm s/c reflash being locked to HPT:

Something else I was thinking about is what if I accidentally bumped one of the wires to a sensor or injector too hard and now it's not working right. I would think I would probably have noticed more significant problems though unless the computer was doing enough compensating when out of boost. For something like the MAP sensor I could see what the readings are through hpt (and last time I looked i think they seemed ok from what I can remember), but if it was to an injector I'm not sure how the best way to tell would be, since I would think the signal going to the injector from the computer would be the same regardless of if the signal actually got to the injector correctly. Are there any things I should check with wiring and what would be the best way to go about doing that? I do have hptuners and torque to log items, but not sure if that will give me everything I need.

Another thing I was wondering about (but may be a longshot) is that a problem I did have from before that I never fixed was that the exhaust manifold had a leak (yeah no header yet). I wasn't sure if that got worse over time and driving that it could contribute to a problem like this, or even just throw off the reading from the wideband? Any ideas here or ways to test the possibility without actually replacing the exhaust manifold? Maybe one way would be to temporarily disable closed loop then see how the fuel trims do, but not sure that would work either since even if the values did go way in one direction it could just be it's because the fuel trims haven't been used under boost so now they're naturally adjusting (when closed loop is enabled). Or if the leak was affecting the sensor's reading it would probably affect the stock O2 sensor reading as well. So not sure what a good way to test if the leak is causing a problem, any ideas?

Any thoughts or ideas on how to find out what's going on and troubleshoot this issue are greatly appreciated. Also if you need any more details on anything just let me know and I'll do what I can to get those for you. It's been a while since I was doing the mods and tuning heavily on this car so I've probably forgotten a few things, but hopefully talking on here will help spark some ideas.

Thanks a lot

Re: Troubleshooting lean conditions under boost
Sunday, December 09, 2012 2:09 PM
seems like you have a handle on troubleshooting lol

im gonna go with you just need to get all that old gas out of it honestly.

It's nice to be injected but I love being blown.
Re: Troubleshooting lean conditions under boost
Sunday, December 09, 2012 6:52 PM
Since it sat for so long I would start with a fuel filter, plugs.

Also what wideband? It might need calibrated after sitting soo long.

FU Tuning

Re: Troubleshooting lean conditions under boost
Sunday, December 09, 2012 8:29 PM
I might have missed it in reading your post but are you on a stock fuel pump still? And how many miles are on it?.

Re: Troubleshooting lean conditions under boost
Monday, December 10, 2012 8:31 PM
Thanks for the replies.

It's the AEM wideband.I think I read that one is auto-calibrated.

I was thinking changing the plugs might be a good idea, too. Do you know if that could contribute to it running lean, too?

At one point I did have to replace the fuel pump with a stock replacement, that's probably been around100k miles or so ago. I think the filter may be a little newer but can't remember for sure. Do you remember what the idle fuel pressure should be?

Any other thoughts on what to look for?

Re: Troubleshooting lean conditions under boost
Tuesday, December 11, 2012 3:08 AM
I would get a new o2 sensor for your wideband at least. Then make sure you have fresh gas in it. Fuel filter would be next on my list. If all else fails, upgrade your fuel pump and see if you have issues still.

Re: Troubleshooting lean conditions under boost
Tuesday, December 11, 2012 4:42 AM
^ I would agree...

Fuel filter for sure

Instead of just buying a new wideband o2 sensor test it first..

From your explanation though it does sound like its bad.
Re: Troubleshooting lean conditions under boost
Tuesday, December 11, 2012 2:31 PM
Ya, Agree with built... except get a new wideband sensor

Also the AEM is not auto calibrating, there is no such thing.

Re: Troubleshooting lean conditions under boost
Tuesday, December 11, 2012 2:47 PM
Philly, thanks for the youtube link, I'll try that.

Also, it seems like a lot of responses are saying to try changing the fuel filter. I think that makes a lot of sense because it's probably time anyway, but just wondering, would a bad filter make it consistently go lean or would it be more sporadic? Either way sounds like something I should do.

Jason, sorry I was in a hurry and typed the wrong thing. You're right, it's not auto-calibrating. What I had seen was that it is individually calibrated so you don't have to re-calibrate. I was reading from page 3 of this link:
Re: Troubleshooting lean conditions under boost
Tuesday, December 11, 2012 6:23 PM
Clogged fuel filter will 100% make it go lean...prolly would make sense that it would go lean when demand is the highest.

But on the flip side...on my sportbike it would go lean as soon as the secondary injectors kicked in....ended up being a fuel pump that was on its way out. Had enough pressure and flow to keep the primary injectors happy but once the secondaries kicked in it couldn't keep up to the demand.

Swapped pumps and was like new again.
Re: Troubleshooting lean conditions under boost
Wednesday, December 12, 2012 11:16 AM
interesting (sorry to go off topic, OP). i bought a used aem aego wideband and i was not aware it needed calibrated. seems pretty on or and least close to me considering all the changes ive been doing lately.


It's nice to be injected but I love being blown.

Re: Troubleshooting lean conditions under boost
Wednesday, December 12, 2012 2:55 PM
The aem doesnt need to be calibrated.

The test I showed was to test if the sensor was bad.

My innovate though I got to do a fresh air calibration basically every oil change.
Re: Troubleshooting lean conditions under boost
Thursday, December 13, 2012 10:17 AM
John 02 - that is not calibrating AFR on page 3 that is what is called label resistance compensation. This is a trim resistor that limits the current to the IP cell in the sensor much like a ASIC chip.

Philly - I have seen that video before and that is a very bad way to test the sensor, that is a Bosch sensor and AEM or Stephan is testing, what is wrong with that picture? This does not test full function of the sensor. Nor does it test the heater function which would pass this test and fail anyway..

Re: Troubleshooting lean conditions under boost
Thursday, January 24, 2013 3:27 PM
also excuse me if I have missed anything. you said you switched back to the stock FPR? Which intake manifold are you using? I am running the factory FPR on my car, with the TTR intake manifold, so there was some guess work as to redoing all of the accessory vacuum routing. my car would also lean out under boost, and the cause was that my engine was boosting my FPR causing fuel flow to slow down under higher rpms. Even with a check valve inline, it would still lean out under boost. Not as bad, but still not something I was prepared to have in my daily driver. My solution was to run the vacuum hose off of the FPR to the pre-turbo intake, so it was getting constant vacuum, and increasing as the rpms get higher. problem solved. For me anyway.

Turbocharged '04 Cavalier
Re: Troubleshooting lean conditions under boost
Thursday, January 24, 2013 3:31 PM
but I would definitely double check all of your vacuum line routings because I know the factory setup will result in some things getting boosted that should only get constant vacuum

Turbocharged '04 Cavalier
Forum Post / Reply
You must log in before you can post or reply to messages.


Start New Topic Advanced Search