Effect of HPFP timing on idle fuel trims
Is there a consensus to the effect of the timing of high pressure fuel pump in relation to instant fuel rail pressure, and as a consequence, fuel trims? It's easy for a HPFP inlet solenoid to maintain an acceptable average rail pressure, but does instant pressure really matter that much? I have long read about the effect of cam timing causing fuel pressure issues on VR6 motors, but have not yet had to deal with it outside of obvious failures. This vehicle has a 2.0T instead of the VR6. I understand that it is desirable to have to injection event happen at the top of the pump stroke, hence every DI engine I have seen using a pump driven 4 cam lobes for a 4 cylinder, or 3 lobes at crank speed for a 6 cylinder. What does a mistimed pump do to idle fuel trims though?
The vehicle came to us with a CEL and codes for p053F fuel pressure to low at startup, p0088 fuel pressure to high, as well as misfire codes. No codes returned, but I could get it to set a p0171 lean code after extended idling. Looking through the scan data, the only problem that was immediately apparent were elevated fuel trims, around 25% short term, adapting to 32% long term. (ST and LT seem to have different weights or max ranges on this vehicle), and a slightly elevated cam adaptation of 5 degrees. Fuel trims decrease to around 15% at 2000 rpms. I believe they were even lower under load, but I'd have to rescan it to double check. Low side fuel pressure desired and actual on scanner match with an analog gauge. Crankcase vacuum seems normal, smoke testing shows no obvious intake or crankcase leaks. Blocking the breather ports, purge valve and brake vacuum pump inlet cause no decrease in fuel trims. Before the oxygen sensor is operational and fuel trims are active, the engine has a rough idle and misfire consistent with lean running. I suspected possible high alcohol fuel, but this is a flex fuel vehicle. Alcohol content on scanner was 8%. I did an fuel water separation test which came out to ~10% alcohol. The fuel tank was topped off with non-ethanol gasoline, and now the alcohol content on the scanner is reading 3%. I have no reason to suspect an issue with the ethanol calculations. Desired and actual air mass readings on scanner match almost perfectly.
The boss insisted on replacing the breather based on common failures, with no difference. In an attempt check the fuel rail pressure sensor, I disconnected the HPFP solenoid connector, expecting a rail pressure to match the low pressure sensor, as done on the earlier FSI motors. On this motor, the rail pressure maxed out at ~160 bar, with the internal HPFP relief valve singing. I wasn't expecting this and let the boss know my confusion. He gambled on replacing the HPFP, but of course nothing changed. The customer isn't paying for his guesses so I don't feel bad about it in this case.
My feeling is that there is an intake or crankcase leak that I am unable to find. I am pretty sure that I have blocked off every possible intake path that a leak could be present. Now that I mentioned it, I have to find the heater valve to make sure there is no leak there, but I don't believe it is isolated by any check valve, so it should have shown up on the smoke test. I always watch the flowmeter and pay attention to the pressure decay. I don't think I missed anything, but am somewhat doubting.
We are subscribed to Autologic tech support so we send them a help request. After speaking to the tech there, he suggested that every VW/Audi with the p0088 code was almost surely caused by stretched timing chain resulting in incorrect HPFP timing and incorrect fuel pressures. I mentioned the measured correct pressures, and the worrisome fuel trims, and the high, but not extreme camshaft adaptations, but he insisted that it was surely HPFP timing that caused the p0088, and would be responsible for the high fuel trims also.
I was doubtful as I've never experienced this on TSI motors. As I said I've read about this issue on Vr6, but I've seen plenty of TSI's in various stages of chain stretch and jumping teeth, and don't remember having an issue of high fuel trims with no other symptom.
The timing chain and tensioner are of the original styles, and I'd recommend replacing them to avoid failure anyways, but I left the job this afternoon after checking the timing, and while the tensioner is extended almost the maximum amount, the cams are well withing the 124–126 mm and 61–64 mm measurements. I am certainly not expecting a change in fuel trims after replacing these timing components.
What am I missing???
You’ll see a change in the trims Alan. Every p0088 I’ve had on the 2.0 has been from timing chain stretch. Replace the chain and tensioner and you’ll be fine.
What mileage are you seeing these happening at?
I hope you are right. I should know sometime today.
Grab a can of propane or brake cleaner and spray around watching the fuel trims. Spray inside the transmission bell housing to check for rear main seal vacuum leak. Sometimes smoking is not showing all leaks. Good luck.
I will do that if it's not resolved. I have had good luck in the past with a smoke tester, not necessarily relying on visual smoke, but watching the flow gauge, and pressure decay after the smoker is turned off. Also I blocked off the breather ports to isolate any potential crankcase leak, with no change in fuel trims.
Sounds like the HP Pump solenoid is the Normally closed type since rail pressure goes to max with solenoid unplugged KOER. Have you compared scan data rail pressure actual vs desired at different rpm's? Scope solenoid current vs ignition synch for 720 reference. Number of current ramps should match the number of cam lobes that drive the HP Pump. Look for consistent solenoid pintle bumps. Any…
Thanks. Scan data for desired and actual matches very closely at all rpms and loads.
Ok. I meant to add the rail pressure can also be a separate scope trace. You will see the rail pressure go up and down in time with each HP Pump solenoid command/pump stroke. Each ramp should be equal in amplitude and rate of change. Amplitude should be the same as a known good for that engine and condition.
I am not well versed on this engine. From what you have described, it leads me to believe you have multiple issues. I am curious as to when your previous codes set. Where they immediately after startup? A poor, surging idle will wreak havoc on the pcms ability to control fuel pressure. As Mihail stated, I would be looking for a crankcase leak. A rotational seal can leak vacuum without leaking…
I'll have to review the freeze frame data. The only time I can get it to run poorly, is after clearing codes (which also clears fuel adaptations), causing it to stumble until it enters close loop shortly after. These do have problems with rear main seals leaking vacuum, as well as the front crank seal occasionally. In the past I have been able to narrow down on crankcase leaks by blocking the…
While I agree that the timing chain could be the issue, we have seen a lot of carboned up intake valves cause lean codes on these engines. We will use an inspection camera to try and see down into the intake before we remove it. what is the mileage?
I agree with Jim Gorman. Have seen the same fuel trim problems caused by carbon on the GMC 4.3L engines in our fleet as well. Drove me nuts the first time.
Unfortunately, replacing the timing chain, tensioner and rails did nothing but bring the cam adaptation down to a reasonable 0.2 degrees. Fuel trims remain high. I did find a basic setting option to disable HPFP operation, and was able to see the high pressure sensor was reading 3 bar higher than the low pressure sensor. With the high pressure sensor removed from the rail but electrically…
Hi Alan that sucks :(. I thought For sure your trims would come down after the chain. 0nly time I’ve had the p0088 is from chain stretch. Now Your long term are yet still way too high. You already have a new breather on and said you have no other leaks. The only leaks I’ve seen are either breather or rear main. Are you sure the rear main isn’t leaking ?
I can't positively say the rear main isn't leaking, however the test I have used in the past on TSI motors is to block the breather hose going to the intake manifold and watch the fuel trims. Any crankcase leak is irrelevant with that hose blocked, and immediately apparent in the fuel trim change. In this case, blocking the breather port does nothing. I blocked the secondary breather port at the…
Hope you are clearing adaptations to bring the trims down to zero after each repair. This is extremely important! I would double check for vacuum leaks, especially the rear and front seals area squirting some fuel and looking at the o2 sensors response. Lastly, I remember fixing a bmw with changing the MAF sensor for lean codes at idle. Driving this car had perfect trims and was only going lean…
Thanks, yes I clear codes to reset adaptations often when checking things over on vw/audi. We did have a known good MAF to try on this vehicle, and it made no difference.
Thanks everyone for their thoughts. Unfortunately (fortunately?) after replacing the rail pressure sensor, the long term fuel trims once warmed all the way up, are hovering around 23%. After many test drives, and a couple hours of idling, it has yet to set any codes. Management has decided to let the customer take it. I will dig back into it if / when the CEL comes back on. Thanks again.
Gosh this one sounds frustrating. I too have seen high rail pressure faults from a stretched chain, but fuel trims were still normal. Nice test for checking the rail sensor! One of the most common air leaks I have seen on this engine is the rear crank seal. In the past, I have smoke-tested these and found no leaks, but then removed the little black inspection cover in the bottom of the…
Thanks, yeah we've done quite a few rear main seals because of crankcase leaks, but It's never given me any issue to verify a crankcase leak is present. Sometimes it's difficult to say if it's front or rear seal, as it seems the smoke never makes it out clearly. Carb cleaner sprayed around the seal seems to make it apparent though in past cases.
Good point. Relying on the smoke machine alone can steer you wrong. Can’t wait for you to post the fix.
have you tried to run Basic settings test (107) with engine at operating temp and see if it comes back ok?
I have not, but I will try that if I get a chance. My understanding is that it disables any adaptations and long term trims, and looks to see if short term is within the 25% limit?
Hi Alan I do believe that your theory about the chain being stretched is correct. The fact that the Total fuel trims improve as engine RPM and load increase supports this theory. I was trying to think of a few ways to prove it and this is what i came up with. VE testing on turbo charged vehicles is very difficult, so I have resorted to using the absolute load PID as a guide in this case…