Hi All,

I've been chasing gremlins with my car for a very long time, ever since I made a bunch of upgrades including 1000cc COBB injectors. So I decided to drive around with a fuel pressure tester(again) to see what the fuel pressure is doing when the car feels off.

I have a brand new 3.8 FPR. My fuel pressure is right around 52-55psi(3.8 Bar) non boosting. When boost goes up, the FP goes up the corresponding amount. In other words, if I am at 1 Bar of boost, the fuel pressure goes up by 1 Bar to around 68-69 psi. The Porsche FPR is a 1:1 ratio so this is normal. It is to counter act the boost trying to push back the fuel.

The issue I have been having is typically when I am simply driving along and apply throttle quickly, let say 1/8 to 1/2 throttle. The power comes on nicely and strongly for a second or so then the is a significant drop off even though the boost has not dropped nor has my foot moved on the pedal. What I did notice is that the FP goes right to 69psi as it should since the boost went up by 1 Bar(14.5psi) for a second or so and then drops off to +/-63psi or so(seems to vary some what). This drop off in FP happens exactly at the same time as the drop in power/torque. However, the boost has not dropped off(still at 1 bar or more), just the FP. My AFR also change when this happens.

Of note is that this is the 3rd new FPR I have tried with the same results.

MY AFR is initially at +/-12 when the initial hit comes but once the FP drops, the AFR often goes into the 10s's (no drop in boost). Needless to say the car does not pull well when that happens.

One would think dropping FP would cause a lean condition not a rich one. Every time my car stumbles or feels off, the AFR go into the 10's. after being in the 12's during the initial boost hit or from the 14.7 under normal driving. I'm thinking maybe the drop in FP triggers the DME to richen up thus causing the AFR's in the 10's ?

At WOT my AFR are in the 12's. During normal driving, they are spot on at 14.7

No Boost leaks, no codes ever, no fuel leaks, new everything: Plugs, coils, IPD, 997 Throttle body, Throttle positioning sensor, intake pipping, fuel filter, boost sensor, N75's, intercoolers, billet Turbos, wheel sensors, brake & clutch switches, DV's, clutch and so on.

Thinking: Bad Fuel Pump? Clogged return fuel line? Obstruction in fuel rail? Bad DME?


Karl

 

Obstructed return line would raise pressure. A drop is lack of flow, fuel pump or its wiring - check the wiring. I've seen a few reports of bad connections causing connectors to fry and that's easy and cheap to check. Can't explain why when pressure drops it goes rich, what's timing doing? Knock?

 

I'm not so sure the fuel pressure going up under boost is normal. Porsche only gives out 2 fuel pressure callouts one with the engine off (3.8 +/- 0.2 bar) and the other with the engine running at idle (3.3 +/- 0.2 bar). One would think if the pressure went up under boost it would offer the numbers.

The symptoms suggest a bad fuel supply problem, a problem with the lines, the FPR and its plumbing, not a bad fuel pump or bad DME. Still you can't rule out a problem with the fuel pump all that in tank plumbing.

I assume the fuel filter is ok? A modified engine can require more fuel flow and the stock filter may not be up to the task. These have been known to start to come apart under some conditions.

 

Of course it goes up under boost at a 1:1 ratio. At 1.0 bar of boost for example, you will have 4.8 bar fuel pressure. This is can easily be seen on all my logs. Fuel system 101...

 

+1 what mods are done to the car? still a stock pump?is the pump too small for the power you are making?

 

Correct, I am seeing fuel pressure rising to an equal amount as the boost but only initially. After a second or so after a healthy dose of boost, the FP goes from 4.8-5 bar down to somewhere between 3.8 to4.2 bar even though the boost has not dropped. That's when the power drops and the AFR's go weird and the car stumbles , typically the AFR are rich at that point which makes no sense to me since the FP went down. One would think that with full boost with decreasing FP would cause a lean condition not a rich one.

 

My mods are pretty commonly used with a stock pump. Certainly worth checking though. I hate to buy a new pump just as a test....pretty expensive test.

 

Fuel filter is new. I too think my issues are fuel related somehow especially since I never ever get any codes and also since just about everything on the car has been replaced except for the fuel pump. The only thing that make no sense is that when the car runs weird, I am seeing rich AFR not lean ones. I would think a bad fuel supply or fuel pump would cause a lean condition not a rich one.

 

That doesnt make sense because if you start loosing fuel pressure under boost your car will be going lean, not rich. I had the same thing happen last year. This was before I had fuel pressure alarms set up so I didn't know it. Under boost car started dropping boost on the top end and going lean, AFRs on logs were 1.2 instead of 0.8. The culprit was a Walbro 450 that started going bad. When fuel got hot, the pump started dropping output up top. I could feel misfires which I thought was detonation but when I looked at the logs there was very little knock but my injector duty cycles were maxed out. Good thing these ECUs are good a protecting the engine by retarding timing, reducing boost, and richening the fuel mixture to the greatest extent possible. A new fuel pump fixed the issue and gave me a chance to really look at the fuel system in detail where we made a bunch of upgrades over what I was using before.

 

My theory is that the ECU senses the FP drop and richens everything up as a precaution.

One important thing I forgot to mention is that these issues do not happen at WOT. At WOT the car pulls fine. If I do pull at half throttle, the car has issues and seems confused. Another reason why I am not sure it is a bad fuel pump. At WOT, my AFR are high 12's. At a 1/2 throttle pull, the AFR's will be in the 10's. Again makes no sense.

 

that would make me think its in the tune.BTW afr's in the high 12s is a little lean for my taste on a boosted motor.How is the charging system and battery? low voltage will cause the pump to run at less capacity .just something to check.

 

Check the tune for sure. I hope to finally get my car back this coming weekend and should have a stock pump available when I do, I'd be happy to let you test with it.

 

Wow, that's very nice of you...I just might take you up on your offer.

Thanks again.

 

My car was dying and leaving me on the side of the road for ten and twenty minutes before the pump would allow me to get moving again, nothing more terrifying! I bought an eBay pump somewhat cheap after finally catching the car acting up with a pressure tester attached. The pump worked great and didn't cost a mint and has now been replaced with dual Walbro and a complete fuel system. I'd be happy to help you sort this out if I can, nothing worse than having a fine running car become unreliable and less fun! Hopefully I will have my car back this weekend so feel free to ping me about it afterwards