Stumped: Car won’t run with MAF connected – Buick 3800 V6

[GeekPriest]

2002 Buick Regal LS – 3800 V6, normally aspirated

Current problem: Car starts, then stalls within 2 seconds. If I disconnect the MAF, the car will stay running, but very poorly, and with a bit of throttle applied. Unplugging the MAF sends fuel trims sky high (LTFT 40%, STFT 20%).

Fuel pressure at the rail is good (52 psi). The fact that it will run at all without MAF suggests that injectors and ignition are both working at some level.

However, I ran the scope on the MAF, and it looks good to me, although that might not be the case. I’ll attach a screenshot in a moment.

Next steps?

[GeekPriest]

I’ve been working on this car for some time addressing its issues.

Already fixed:
1. Low fuel pressure at the rail. After ruling out other causes (leaking injectors, etc.) I replaced the fuel pressure regulator with a Delco one. It holds proper pressure now.

2. Intake manifold lower and upper gaskets replaced, after observing a little coolant dripping from the near the throttle body.

3. Intake plenum (upper manifold) replaced after the EGR gases melted a hole in it. (It took me a while to see the hole. When this was going on, long term fuel trim skyrocketed as the unmetered air caused a super lean condition and lots of heat. I turned off the car when I noticed the exhaust manifold starting to glow red. I believe this problem is now resolved.) This is why it would only run with MAF disconnected.

Current complaints:
1. Car idles high at 1000-1200 rpm.

2. Engine won’t rev over 3500 rpm. At 3500, it stumbles markedly and refuses to go higher.

3. Fuel trims at idle and higher revs are off. Long-term fuel trim was at -20%, but has since improved to around -15%. Short-term trims are mostly around 0-5%. I saw it occasionally touch 7%, even with LTFT at -14-15%. (Note: the long-term trims are all negative, indicating a rich condition, if my understanding is correct.)

4. Misfires show up across several cylinders at different revs. Scanner will report maybe half a dozen, then they’ll drop back to 0.

Other observations:
1. Crankshaft and camshaft sensors look to be fine (18x and 3x signals from the crank, plus the 1x on the cam) on the scope.

2. I’m unsure whether the MAF sensor readings I’m getting are good on the scope.

3. The secondary ignition waveforms don’t look great to me, but switching the ICM and coils with used ones (in my one act of parts changer desperation) made no change.

4. There is a steady flow of exhaust gases from the tailpipe.

So, does anyone have any suggestions for direction to go here? I’m about to give up and take it to a shop, but I’d like the personal victory of figuring out what’s wrong with this car.

Thanks from a new member, if not a newbie under the hood.

[Author]

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