Buick 3800 V6 - stumbles, lack of power, need direction

2002 Buick Regal LS, 3800 V6, normally aspiratedSymptom:Starts and runs. Lacks power and stumbles under anything but the gentlest of acceleration. Will not rev over 3500 rpm.I'm stumped, having worked my way through low fuel rail pressure and a hole in the intake plenum.Right now, I'm puzzled by the O2 sensor readings. The

Can you graph your 02 sensors, map sensor (and maf if this car has it) and rpm on a wide open throttle run through first or second gear and post the results? This will tell us if it's a fuel or air problem

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I'll get that data in the morning. The idle is a bit high (1000-1100 rpm), and the MAF is reading around 9 g/s.

The high idle is setting a P0507 (iirc), but I think that's but a symptom. The IAC is showing 0 counts.

Sounds like another problem that needs to be addressed your iac count should never be zero (or maf that high at idle) you either have a huge vacuum leak or a stuck/misadjusted throttle plate

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At 1000rpm that maf reading is normal. Iac is where I would start remove the two areas that hold it in and look into the iac opening for soot/carbon buildup clean it out and brush off iac with a tooth brush then reinstall and see if it improves. The other thing to look at is back pressure for exhaust which you will need to get a pressure gauge that reads 0 to 20 psi and rig something up to put in the upstream O2 sensor hole and start car from cold and check reading at idle and a few snaps of throttle should be no more than 2_4 psi if it's more then a blocked cat would be suspect

Attached are three graphs.

One with engine speed (RPM) + IAC + MAF + MAP

One with engine speed (RPM) + O2 Sensor 1 + O2 Sensor 2

One with engine speed (RPM) + STFT + LTFT

Note that the top engine speed is the highest I can get. It'll get up to 3500, stumble, and drop to 3000.

On the IAC, I pulled it out. It was clean from when I cleaned the throttle body. I noticed a bit of carbon on the spring below the pintle, so I made sure to remove that.

I then connected it back to the harness, turned on the car without starting the ignition, and used the scan tool to change the idle speed via the IAC. I can feel the IAC solenoid engaging, but I can't detect any movement at all on the pintle. I figure this will be a small movement, but I can't see anything move. I also notice that the IAC gets slightly warm when doing this, but I'm not sure if that is normal.

I also notice that the IAC count was actually moving this time (see the graphs above).

Finally, I'll note that while running this test, the exhaust manifold was getting hot enough to glow faintly. I turned off the engine once I saw that.

I've smoke tested the intake side, and don't see anything. I'm still learning, but can you help me understand how a big vacuum leak on the intake would cause fuel trims to go negative (indicating rich)? I figured they'd go sharply positive to cope with the extra air by adding more fuel.

Vac leak will not cause a rich condition and from what I am seeing the O2 sensors are working like they should. I would say if the iac was not replaced before to replace it as it looks like it's the issue. How ever I would check fuel pressure just to make sure it's not a fuel problem

GeekPriest wrote: Finally, I'll note that while running this test, the exhaust manifold was getting hot enough to glow faintly. I turned off the engine once I saw that.

That and the downstream O2 sensor are both pointing toward the engine running lean. I'm wondering if part of the problem is a lying upstream O2 sensor??

How does one validate that upstream sensor's report? The scanner data looks perfectly normal to me, but I'd love to confirm it.

If the IAC count is truly 0, then plugging the port in front of the throttle plate with your finger should not change the idle speed.

slow down guys that is not a normal looking wide open throttle run. notice low MAF and sharp rise and flatline MAP vacuum that does'nt build vacuum again till throttle is released this is exactly what I look for to know if a exhaust pressure test is warranted pair that with rich exhaust during wide open throttle says this is not a fuel issue this is a engine that cant move air. a exhaust pressure test is next best test but if you dont have access to exhaust back pressure tester than you can disconnect and remove the front 02 sensor (relieving some of the back pressure) and the car should run much better if it is indeed a plugged exhaust. the idle is not related to the bogging and is another problem of its own. plugging the port with your finger is an excellent suggestion to see if the IAC is dead and stuck open (this will need to be done with MAF disconnected).

Ben wrote: slow down guys that is not a normal looking wide open throttle run. notice low MAF and sharp rise and flatline MAP vacuum that does'nt build vacuum again till throttle is released this is exactly what I look for to know if a exhaust pressure test is warranted pair that with rich exhaust during wide open throttle says this is not a fuel issue this is a engine that cant move air. a exhaust pressure test is next best test but if you dont have access to exhaust back pressure tester than you can disconnect and remove the front 02 sensor (relieving some of the back pressure) and the car should run much better if it is indeed a plugged exhaust. the idle is not related to the bogging and is another problem of its own. plugging the port with your finger is an excellent suggestion to see if the IAC is dead and stuck open (this will need to be done with MAF disconnected).

The $14 Harbor Freight gauge shows nearly 3 psi at the front O2 sensor port. With the gauge removed and O2 sensor out (i.e., open hole in the pipe), the car would rev a bit higher -- to 4000 rpm -- although it wouldn't enter closed loop due to the missing O2 sensor.

I guess the cat is clogged.

Any recommendations on affordable but decent catalytic converters for this car?

Also, I assume there's an underlying cause to this, possibly the other conditions I've fixed along the way (low fuel rail pressure, leaking intake manifold). How do I ensure things are good before putting on another cat?

Is thay 3 psi at idle or at 3k rpms?

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Doh. That's 3 psi at idle. When I saw that, I didn't look to see whether revving would help. I can take another reading in the morning as the gauge port is still in the car while I let the exhaust manifold cool.

I figured that was idle as bad as the numbers were I usually see over 20 psi at 3k

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I'll definitely do a second reading in the morning at higher than idle.

I did a second reading. It's still 3 psi at idle, but jumps quickly to 10 psi when pulling the throttle cable just a little. The meter is pegged at that point, but I think this is now the proof we're looking for.

Yep that confirms it is a plugged cat. Magna flow makes a good aftermarket universal cat that will need welded in other wise Amazon has walker brand cats that are not overpriced. Next would be to test O2 sensors and look at fuel injector on times for each bank. At idle they should be around 3.7 ms on time.