Tried to post this yesterday but it somehow ended up in the 991 vendor classifieds. Thought I'd start a new thread and quit mucking up the cobb accessport thread as this clearly isn't an accessport issue.


I'm still fighting the lean issue with a stock 996 turbo. Car has a Cobb Accessport installed with the stage0 stock map and is just being used for datalogging at this point. Car had seemed to drive fine overall since i bought it 2 years ago at 32k miles, albeit a little sluggish or inconsistent when going WOT, now its at 69k miles. Only reason I found the lean issue was logging when I tried to add a stage1 flash for a stock car. I'm still thinking it's something on the mechanical side of the fueling. For the most part I see AFR's in the 15-16 range, but with some higher spikes of boost in 1st gear MAF unplugged it dipped to 11's.

So and this point I've replaced a handful of things without really any improvement in the following order.

-spark plugs
-FPR
-fuel filter
-swapped fuel pump for a walbro 465
-MAP sensor
-Throttlebody Oring (slight leak found during boost leak testing)
-vacuum line from plenum to FPR

Currently really the only differences made been unplugging the MAF sensor which seems to allow higher boost as hi as 9psi, and seems a little richer, plugged in I'm hitting only 4psi max.

Since installing the walbro the base fuel pressure seems to have bumped from about 60psi to about 63psi.

So here's the wierd part, it doesn't seem like the old FPR or the replacement are basically having any affect on fuel pressure. Hooking up my current mityvac direct to the port on the plenum that feeds the fpr I see about 20in vacuum being produced at idle but that line attached to the fpr does nothing. When I hook the mityvac to the fpr and add up to 25in of vacuum it will only pull out about .5 psi of fuel pressure and then raises .5 psi when I release the vacuum.

So not quite sure what to look at next. Some sort of issue with the lines or rails even. I have a new mityvac on the way that also does pressure but I'm doubtful that will show much difference testing the fpr with adding pressure. Thinking maybe I need to look at the fuel rail that contains the fpr, maybe a defect in the rail keeping the fpr from being able to adjust the fuel pressure.

 

The one thing you forgot to replace is the maf sensor. Clean it or replace it. Do not get on your car anymore or you will have melted pistons and broken rods.

 

What happens to the fuel pressure when you add pressure to the fpr with the mityvac? Simulate what it would be like in a boost scenario.
Have you tried a new MAF?

 

My understanding is that the boost pressure should increase the fuel pressure through the fpr.

 

My mityvac doesn't have the option for pressure, waiting on delivery of a newer one that does to see if pressure to the fpr behaves any different than vacuum.


Was looking back at my notes and pics from when I swapped the oem pump to the walbro. At that point I had the y-pipe off and I had access to the FPR line so I pulled it off and jumpered the fuel pump to the battery and 15in of vacuum from the mityvac to the fpr caused fuel pressure to drop from 57psi to 53psi. So at that point the fpr did seem to be adjusting pressure a little more.


I have not tried a new MAF yet since the MAF reading seemed within reason in the logs when compared to someone else's for same rpm/boost, and unplugging didn't resolve the issue. But I'm certainly open to trying that. Was a little worried when I started reading about knockoff Bosch sensors and continued trying to rule out other things.

 

One other thought I've been having is could an issue with the scavenging system failing and bleeding off more fuel pressure from the pump then it should, in turn affecting the FPR's ability to control pressure?




Thinking about trying to test bypassing that whole Y-pipe by connecting the pump direct to the feed line and either prefill the surge tank by hand or submersing the pump in fuel in some other way just to see if that allows the FPR to start making bigger changes to fuel pressure.

 

That's been my biggest concern, at this point its just been short tests going around the block after making changes trying to see if the issue has been corrected.


Saw a lot of knock and leanness as soon as I installed the Accessport and stage1 map and started logging. As soon as I saw the first logs I reverted back to the stage0 which eliminated most of the knock counts and have been trying to troubleshoot the lean issue ever since. I'm pretty sure this condition has been affecting the car for the last 37k miles since I've owned it, and who knows how long it was there prior to when I purchased the car 2 years, maybe since it was new.


Assuming the knock control has been working overtime to save the engine for lots of miles, hoping there hasn't been any damage over time or during the short time I had the stage1 Map installed.

 

Have you checked for leaks between the MAF and turbo inlets?

 

That would not cause him to run 15.0 afr at wot. He has a fuel issue or maf problem.

When i had a intake leak i was still running 11.8afr but my LTFT were high negatives.



Relace maf sensor those things are annoying.

 

Pulling a vacuum on the FPR won't do much as you saw...it's the base pressure of the FPR. When boost pressure is referenced at the fpr port it should respond 1:1 assuming they perform like 1:1 regulators. I also would not expect to see lower AFRs in 1st gear at that load you stated...4psi so nothing to worry about there in my opinion. However, when you unplug the MAF and see more boost, more fueling..then leads me (as stated above as well) that something is goofy with the MAF possibly.

 

03EvoIII, can you elaborate more on the FPR not responding to vacuum? I was under the assumption I should see it drop under vacuum which is why since mine doesn't seem to be affected currently by adding vacuum that I'm facing some sort of fuel issue on the mechanical side of things.


Factory service manual states approximately 55psi pump on engine off and 48psi engine idling which I assume would mean I should at least see around a 7psi difference. My past experience on other cars pulling the vacuum line off the fpr while idling would return pressure back to around stated base pressure.

 

Thanks for the link, I'm going to try and rule out any possible mechanical issues with the in tank scavaging system this weekend, and likely try to temporarily bypass that system all together sending 100% of the pump flow directly into the feed line. If I don't come up with anything there I'll get a new MAF on order Monday to try and eliminate that as a possibility. Worst case I end up with a spare MAF as it sounds like sooner or later I'll like end up needing a replacement.

 

FPR adjusts pressure up and down to keep pressure at the injectors constant by lowering and raising pressure on the rail. When the engine is under vacuum, fuel is pulled into the motor more easily, so the sensor line (using vacuum) decreases pressure to compensate for the vacuum. Under boost, the fuel has a harder time getting into the engine, the sensing line (now boost pressure) causes the fpr to raise pressure to compensate for the extra pressure in the cylinders.
So pressure on your fpr sensor hose should raise fuel manifold pressure by the amount of boost your running.

 

^^^ what he said, it should be rising and lowering based upon manifold vac and pressure. This is a new fpr, new bosch? You've changed pump so that shouldn't be it. To me it sounds like lack of fuel not an issue with commanded fuel due to a bad MAF but I could certainly be wrong! IMO it's not the siphon tubes. Putting pressure to that fpr will tell the story I think.

 

Not sure I understand as this behaviour is close to normal. At light load/cruise, you'll see it right around 14.7. It swings a bit and some it into the 15s.

And medium load, or as boost builds, you'll see it richen. Anywhere between 14.7 - 11. More load/boost, will drive it richer, progressively into the 11.

Full throttle/max boost should see it right into the 11s.


This sounds fine too. Just to clarify, you're pulling vaccuum to the FPR with the mityvac? Try to pressurize it. FPR (fuel pressure RISER) has much more influence on fuel pressure in the positive direction.

Post a log and that'll help us understand the problem better.