I recently picked up a 2000 Jimmy. Starts right up, and idles smooth and quiet. The problem is when I go to move/drive it, it'll fall flat on it's face. It'll spit and sputter really bad, sometimes to the point that it won't even move an inch. If hold it wot for a few seconds, it clears up and drives ok. Once it sits, sometimes after 10-15min, sometimes it takes longer, it does it all over again. It was throwing a P1345 code, so I swapped the crankshaft position sensor with one from my 99 Blazer, and that got rid of the code, but it still ran bad.

The coil wire was arching bad, and there was an arch at one of the points in the cap. I put in new NGK wires, new ACDelco plugs, and the distributor and camshaft position sensor from the Blazer. After changing those parts it threw a P1351 code, so I swapped the ICM with one from the Blazer, and that took care of the code, but it still ran the same, and a little bit later it was throwing a P0135 code.
The long trim was 7.8% during open loop. Once the system went closed loop, the short trim was 0-2% on avg. lowest was -1.6%, and the highest was 2.3%. The long trim was 19.5%. These number are from bank 1. Bank 2 was within 1% for both short and long trim.

Today I changed the upstream O2, and all of the vacuum lines under the hood, with the exception of the one to the actuator under the battery. Still ran the same, and the fuel trim was pretty much the same. Short trim was -.8-3.1%, and the long trim was 22.7-23.4%

I tested the fuel pressure at the schrader valve, and got the following results:
62psi initial prime
61psi after 5min
60psi after 10min
57psi after 15min
55psi after 20min
52psi after 25min

Tonight when I got home, I pulled the egr from my 99, and swapped it. It ran noticeably worse at idle, and the short trim during open loop was 50-52%. After going into closed loop, it ran better, but still the same as before. Have to stomp on the gas for a moment before it's drivable. The short trim during closed loop was 28.1-29.7%, and the long trim was 25%

Those trim values seem excessively high, and so far nothing has made any difference in the way the truck runs. Anything else I can check? The 99 has a rod knock, so any parts I need can be pulled from it. Maybe the canister purge valve?

 

Look at the MAF numbers while it's idling, driving normally, and stomping on it, I had a bad MAF on mine that never threw a code. It just wouldn't read over about 60 grams/sec (or whatever the airflow unit of measurement is), no matter how much air was going through it. It would run ok under light throttle, but fell flat on its face when I put my foot in it.

 

Thanks. I just took a video of some live data, and I'll upload it shortly.

The egr I put on was no good, so I cleaned the old one with carb cleaner and a brush, followed by a run through the ultrasonic. Now the open loop trim is back to 7.8%.

When the truck 1st goes into a closed loop, the trims are consistent with each other and look good, but within a minute or under, the long trim jumps back up to around 20%. Also multiple warning lights in the I/M monitor of my scanner. They're for CAT, EVA, O2S, HTR, and EGR.

 

Here's the live data from a few minutes ago, I'll also add that when I took it to the parts store this morning, there was no stutter or hesitation. The truck ran great. In fact, when I took off, I gave it plenty of throttle since that's what I normally have to do at 1st, and it left rubber on the road. Once I got home, and went back out 30 or so minutes later, it was back to running horrible again. I took a can of carb cleaner, and sprayed all the vacuum lines, and all around the intake, and there was no changing in rpm.

 

Fuel trims are a measure of how different the fuel delivery is from theoretical stored table valves based on various sensors like MAF, ECT, ECT once the system goes into closed loop and starts calculating fuel delivery based on O2 sensor values. You always look at fuel trims as short term plus long term added together on a given bank, being mindful of sign. New trucks should be around a few percent total. Older trucks should be less than 10 absolute value. Greater then 25 generates a code. Long term is a time based average and st is instantaneous. Fuel trims only have diagnostic value in closed loop. Positive trims are adding fuel to compensate for a perceived lean condition. The converse is also true. Trims that are out of bounds can be the result of an actual system failure such as high positive trims being caused by an unmetered air leak or they can be because of a failing sensor giving erroneous readings. Lastly and least likely is a failed pcm.

Report your fuel trims as stated above and get your live data ready to look at sensor values such as what Tom suggested. If Tom didn’t nail it then we move on to other possibilities.


George

 

The sensors you’ll need to read are:

MAF, MAP, ECT, TPS, O2 sensors, and CMP retard if you can.

Also I forgot to ask, does it run better, worst or the same when your scanner shows the transition to closed loop as it warms up?


George

 

Fuel trims in the video were closed loop. So should I include them in the live data along with the maf readings when I take it for a drive?

 

In your video, I don’t know what fuel trims in synch means. Again add st and lt together on a given bank. You have +20ish numbers on both banks which means that the truck is adding fuel for a perceived lean condition.

Does it run better or worse in open loop when the O2 sensors are not involved?


GEORGE

 

Most of the testing that you’ll need to do will be with the truck in park. There are a few exceptions if we don’t find the problem in your driveway like upstream O2 sensors under full load but that’s getting ahead of where we are at this point.


George

 

I was told that the trims need to be close to the same, but I found after that video was taken that that's not the case.

Haven't tried with the O2's unplugged. Sometimes it runs great, most times not. I just took 3 videos of the stft, ltft, map, and maf at idle, driving 30-40mph, and 55/wot. Not sure if they'll be useful as the truck decided to run fine during the drive.

The videos are uploading now. I'll post them when done.