P0171System Lean Cosworth Supercharger

[sly]

I am at a loss for what to do next. I am getting a CEL with code P0171 with no other associated codes. Lean on Bank 1. I checked for vacuum leaks by removing and plugging vacuum lines leading to the throttle body and watching the short term fuel trims. Expecting that if I plugged a leak, the fuel trims would go negative to compensate for the positive long term trims.

Long term fuel trims are hovering about 30%. They drop some when driving but never really go to 0. I took some propane around the engine to every spot I could find, checking for a vacuum leak. This had no effect of fuel trims. They stayed 30% long term and around 0% short term.

I checked for an exhaust leak. Thinking that if the O2 sensor was pulling in air, if I pressurized the exhaust while it was idling, it would blow back against the leak. I placed my hands over both exhaust outlets while it was idling and let the exhaust pressurize. I checked the short term fuel trim log and it had no effect. I did hit a rock in the road a few weeks ago. It left a small dent on the hemholtz resonator but no hole. I always keep the skid plate on the car. I removed it to look for damage around the header and saw none. There are no black marks indicating a leak anywhere I can find on the exhaust.

I have removed and retightened all the hose clamps on the intake hose. I sprayed the intake hose and air box with propane and it had no effect on short term fuel trims. Again, if I had a leak, sucking in propane should add fuel to to engine and the ECU should cut the fuel trims back to compensate. No amount of propane seemed to have any effect.

The exhaust smells perfectly fine. In fact it has no odor at all. The car is running mostly ok except that it gets a very occasional miss when going uphill in 6th gear under light load. I am getting some spark knock at high rpms and high load. This may be tune related.

I am at a loss for what to do next. I can't find any intake or exhaust leak. ECU is maintaining about 14.70 AFR but with about 30% positive long term fuel trims. Mileage is about average. I'm getting around 30mpg.

Still thinking, maybe I have something wrong with the MAF sensor. I removed it and looked at it. It still looks brand new. No dust, oil or dirt on it. I did not bother trying to clean it. Car has only 36,000 miles on it. Perhaps I have an 02 sensor problem or a failing injector. However the car idles perfectly and the exhaust has no smell at all. I have a CEL with P0171, high long term fuel trims but no other obvious problems.

https://datazap.me/u/sly/log-1542132743?log=0&data=7

[Update] Solved: http://www.ft86club.com/forums/showp...1&postcount=21

[sly]

@Matt@Cosworth has suggested I may have a leak. He has personally tweaked the tune and according to a log I took a month ago, fuel trims were normal. Long term was close to 0 and short term varied but averaged 0 as well. So I don't suspect the tune is causing a problem with fuel trims.

I also wanted to add that I have changed out the factory fuel pump with a high volume pump. Matt says fuel pressures look normal. I had been getting good fuel trims with this pump when I took the log a month ago. So I don't suspect the fuel pump.

Mods include Cosworth midpipe and axle back exhaust. Basically everything after the factory header is a Cosworth exhaust. I still have the factory header. I have added the Forester oil cooler and, of course, the Cosworth supercharger with intake manifold.

Could a mechanical problem with the engine cause this? I may try the propane again to look for a leak. Maybe replace the O2 and AFR sensors (although I hate random parts swapping).

I also tried plugging the PCV valve temporarily and the intake port for it on the throttle body. No effect on fuel trims. Pumping the brake peddle makes the fuel trims go more positive which I would expect as it does introduce a vacuum leak. But the trims reduce back to 30% when I stop pumping the peddle.

[sly]

I have not yet taken off the air cleaner to inspect the air filter. I have also not cleaned the MAF, but I'm hesitant to do this because it looks completely new. I may try getting some smoke. Maybe the propane was not concentrated enough to affect the short term trims (assuming I have an intake leak). I also plan to take a stethoscope to the injectors to be sure they are all firing. Things to do... :sad:

[gtengr]

Just from searching around it seems some NA folks have had this code too. One poster said the dealer fixed it by replacing the MAF. Do you know any locals who might let you do a quick MAF swap to rule it out?

[MCTeeJ]

The spark knock and occasional misses at load in higher gear suggests maybe a failing injector. Now I said failing and not failed, so that could explain why it idles fine but drivability is suffering.

When I had a direct injector go bad the car idled fine and drove ok, but when I got on it the car would miss for a little bit and then clear up in the higher rpm. The car would be one point lean compared to target (target being 12.3 and the car only would hit 13.5 or so) and my idling fuel trims were about 14%. Do one test pull while logging and see if your AFR is matching target.

EDIT: I looked at your log, were you at full throttle when you were hitting 5-6k rpm? your AFR is waaaaay lean.

[weederr33]

Wow this is no fun at all. I'm sorry you're having so many issues. Someone local (who bough my previous Cosworth FR-S) had some similar issues. He tore it apart till settling on swapping out the DI seals. That ended up fixing his issues. However, it could also be your MAF.

I'd say nuke it all and change the seals (or at least check them), and the MAF too. But I hope you figure it all out. :/

[MCTeeJ]

Originally Posted by weederr33

Wow this is no fun at all. I'm sorry you're having so many issues. Someone local (who bough my previous Cosworth FR-S) had some similar issues. He tore it apart till settling on swapping out the DI seals. That ended up fixing his issues. However, it could also be your MAF.

I'd say nuke it all and change the seals (or at least check them), and the MAF too. But I hope you figure it all out. :/

This but flow test your injectors before re using them.

[steve99]

Originally Posted by sly

I am at a loss for what to do next. I am getting a CEL with code P0171 with no other associated codes. Lean on Bank 1. I checked for vacuum leaks by removing and plugging vacuum lines leading to the throttle body and watching the short term fuel trims. Expecting that if I plugged a leak, the fuel trims would go negative to compensate for the positive long term trims.

Long term fuel trims are hovering about 30%. They drop some when driving but never really go to 0. I took some propane around the engine to every spot I could find, checking for a vacuum leak. This had no effect of fuel trims. They stayed 30% long term and around 0% short term.

I checked for an exhaust leak. Thinking that if the O2 sensor was pulling in air, if I pressurized the exhaust while it was idling, it would blow back against the leak. I placed my hands over both exhaust outlets while it was idling and let the exhaust pressurize. I checked the short term fuel trim log and it had no effect. I did hit a rock in the road a few weeks ago. It left a small dent on the hemholtz resonator but no hole. I always keep the skid plate on the car. I removed it to look for damage around the header and saw none. There are no black marks indicating a leak anywhere I can find on the exhaust.

I have removed and retightened all the hose clamps on the intake hose. I sprayed the intake hose and air box with propane and it had no effect on short term fuel trims. Again, if I had a leak, sucking in propane should add fuel to to engine and the ECU should cut the fuel trims back to compensate. No amount of propane seemed to have any effect.

The exhaust smells perfectly fine. In fact it has no odor at all. The car is running mostly ok except that it gets a very occasional miss when going uphill in 6th gear under light load. I am getting some spark knock at high rpms and high load. This may be tune related.

I am at a loss for what to do next. I can't find any intake or exhaust leak. ECU is maintaining about 14.70 AFR but with about 30% positive long term fuel trims. Mileage is about average. I'm getting around 30mpg.

Still thinking, maybe I have something wrong with the MAF sensor. I removed it and looked at it. It still looks brand new. No dust, oil or dirt on it. I did not bother trying to clean it. Car has only 36,000 miles on it. Perhaps I have an 02 sensor problem or a failing injector. However the car idles perfectly and the exhaust has no smell at all. I have a CEL with P0171, high long term fuel trims but no other obvious problems.

https://datazap.me/u/sly/log-1542132743?log=0&data=7

looks like small exhaust leak before or near first 02 sensor

ltft is high at low rpm and gets better as rpm rises

ltft is high positive, this is often caused by 02 sensor seeing fresh air from leak between exhasut pulses. ECU then get signal its lean so it add fuel via ltft.

as rpm rises pressure in exhaust increases freash air no longer gets in hence ltft returns to normal at higher rpm

manifold pressure at idle is 0.3 bar ie 0.7 bar below atmospheric so look like intake doesnt have leaks

look at header welds for cracks
look at block to header flanges/gaskets for leaks, it will only be small leak

[sly]

Thank you very much for the replies. I have ordered a new MAF in the hopes that will make a difference. I played around with it last night. I took apart the intake hose and air cleaner box to look for any deformities or holes. Everything looked good. When I put it back together I forgot to plug in the MAF. Started the car and let it idle. Noticed the fuel trims were at 0. But the idle was high. Got a CEL and realized I left the MAF unplugged.


Plugged in the MAF and cleared the code. Restarted and immediately got a MAF low flow ECU code. Reset the ECU and tried again. Let it idle and the trims eventually crept back up to where they were at about 30%.


Is it normal for fuel trims to return to 0 with the MAF unplugged? I assume the car was basically in Open Loop mode and was relying off the map. Running map fuel curves, the ECU did not need to trim the fuel requirements. But I did get a CEL.


So based upon the advice here, I ordered a new MAF. I will also look harder this weekend for exhaust leaks. I did hit a rock a few weeks ago that left a small dent in my resonator downstream of the header. Maybe the jarring force was enough to crack the stock header. Hard part is getting those heat shields off.


Thanks for everything. I'll update once I find something out.

[weederr33]

Sounds like a good time to get a header!

[sly]

Originally Posted by weederr33

Sounds like a good time to get a header!

In case I have to go this route, any suggestions? Stick with OEM or go with something different? I'm interested in quiet and reliability. The car is already plenty fast. Faster would be nice but not at the extent of being able to have a conversation inside the cabin.

[weederr33]

Originally Posted by sly

In case I have to go this route, any suggestions? Stick with OEM or go with something different? I'm interested in quiet and reliability. The car is already plenty fast. Faster would be nice but not at the extent of being able to have a conversation inside the cabin.

I'd say aftermarket uncatted. But if you don't want to keep the noise up, you could always put the OEM catback back on or find a quieter one. Of course that's looking at it to get the most out of your set up since the catback doesn't provide as much as an aftermarket header would. If you don't want to ditch their exhaust, finding another OEM would be easy I'm sure.

[sly]

Replaced the MAF and there was no difference on the fuel trims. Still at 30%. Started the car and took a log of it idling with my hands covering the tail pipes. Let it pressurize to push back against any possible leak. Noticed short term trims dropped 7%. Took a second log and it did it again. Short term dropped some. So now I need to find the exhaust leak. Probably at the header as was already suggested. At least it's, hopefully, not the DI seals.

[sly]

Took the header off. Heat shields are crazy. I can find no sign of a leak. No burn marks, no cracks and the gaskets looked good. I pressurized the header some and did not see any leaks either.

Would failed DI seals really cause this problem? Car idles fine. No shudder at all. The hesitation I described is only slight and only under certain circumstances. I can't even get it to do it all the time.