Calling knowledgeable guys to analyze live data

[TylerW]

Hey guys:

I have a 2000 Q45 that's been having an ongoing drivability issue that I am attempting to resolve. I posted before and got very little response, but I figure most people dislike trying to answer those type of questions.

So, this time I am bringing DATA with me, lol. I ordered a blue OBDII adapter and then bought the Torque Pro app to get live data capabilities. I put it on my tablet and really like it so far.

Anyway, today I set up several parameters to view and went for a 20-mile loop that consisted of urban driving, a steep climb up a grade, closed-throttle descent, 65-mph cruise, and a couple WOT pulls. I know this app has graphing and logging capabilities, but I took the old-school approach and set up my camera in front of the tablet and did it that way. That's probably better anyway because I narrated and captioned the conditions, and you can also hear some of the engine conditions, so to speak.

I put the video on Youtube. The gauges you will from top left to right are:

Top Row-
Ignition Advance MAF flow rate LTFT Bank 1 LTFT bank 2

Bottom row-

Intake air temp STFT Bank 2 STFT bank 1 intake vacuum

Here's what I'm dealing with, condensed:

The engine is all over down on power. I also get a low-rpm (1200-1800) surge and lack of throttle response. Apply some gas, it acts like you're giving it air but no fuel. At the same time it might surge like you are pushing and letting off the accelerator incrementally. Hold your foot still and the rpm's eventually climb and you start getting more response. It will also randomly nose over briefly after a gear change. Weird, huh? The car is well-maintained with about 188k miles.

I swapped MAF's a couple times, did all the filters, did seafoam through the fuel, not one thing made any difference. The best part is there are no MAF codes, rich or lean codes, O2 codes or misfire codes. The engine idles smooth as glass and fuel consumption is normal. It has never stalled except once it did a start and stall.

Here's the video. Please note how timing is being pulled way back at certain intervals and how the vacuum reading tends to take drastic drops without a corresponding drastic application of throttle. The LTFT's are not consistent bank to bank either. Thanks for looking.

https://www.youtube.com/watch?v=9cNz_kCZqnw

[Skibane]

I swapped MAF's a couple times, did all the filters, did seafoam through the fuel, not one thing made any difference.

Has the fuel pump ever been replaced?

Might be worthwhile to look at the fuel pressure, particularly while driving.

[3Q Jay]

like data!
Didn't watch the whole video.
but.
MAF reading at 2:21 there is a HUGE step function, and It didnt 'sound' like you changed throttle position at that point.
STFT and LTFT may be interesting down the road. But they are not off far enough to be your primary concern.
intake temp seems quite high to me. but let's focus on the MAF first.
Plot engine RPM, fuel inj duty cycle, throttle position, and MAF. timing too. coolant temp if you have room. where is the vacuum reading coming from? the ECU? I didn't think the manifold pressure sensor to ECU was that accurate?
the fuel pump pressure while running (old school, with a Tee tube and an analog mechanical pressure gauge) would also be useful, i agree. but might be hard to rig up and drive....

[TylerW]

Thank you for the advice.

Before I went any further, I took your advice to check the fuel pressure. I had been meaning to do this but procrastinated because of having to tap into the fuel line and buy $46 worth of EFI fuel line. But, I did it and got some interesting results. This is measured just past the fuel filter:

Turn key to ON, engine off: the system pressurizes to 34psi, then drops back to zero when the pump clicks off.

Engine warm and idling: 32psi

2500 steady rpm: 33psi

2500 rpm steady, then wing the pedal briefly to the floor: drops to 25psi, then recovers

I also saw a couple weird spikes to 50 psi briefly, immediately followed by a drop to 25 and back to 32psi when I winged the gas hard several times.

And finally:

The pressure drops to zero immediately upon turning off the ignition. Totally to zero, every time. That tells me the regulator/check valve/supply line at the pump is leaking or failed of the fuel pressure regulator at the engine has failed.

EDIT: I blocked the line between the gauge and the engine, and the pressure goes to 30 psi immediately key on engine off and then drops only incrementally. When I popped off the vice-grips it dropped to zero immediately.

I'm glad for your thoughts, thanks again.

[3Q Jay]

fuel pressure regulator sounds fishy. it is vac operated. when you 'blip' the throttle, it is supposed to increase pressure (loss of vac temporarily causes regulator to not return fuel to the tank).
similar with KOEO (key on engine off). no vac, should pump up and stay there.....

[J30tChumpCar]

Random thoughts from a random mechanic

Fuel pressure under load when misbehaving is what you need to be looking at, did I read right where you deadheaded the fuel pump with vice grips clamping off flow to the engine and it has 30 PSI? If so, that is way low not to mention "wing the pedal briefly to the floor: drops to 25psi, then recovers" is a bad fuel pump or clogged filter.

... saying that, your fuel pump sounds like its the issue.