I bought this 2000 GMC Jimmy. When the problem began it would run great at times and act as though nothing was wrong. Occasionally, it would randomly spit and sputter, then die and not restart for a bit. This has happened driving 70 on the interstate and 35 on a street in town many, many, many times. No warning. So, codes were TPS and MAF sensors. Changed both of those, twice. No change. Took it to the Chevy dealership and had them diagnose it. The said the computer needed reflashed. Did that, no change. Found another computer, had it flashed, no change. New coils, plugs, wires, timing chain, distributor, fuel pump, fuel regulator, fuel filter, cam sensor. No change. 3 professional mechanics, 4 if you include the Chevy dealership, have given up. What is left to change? The problem changed just a bit after installing the distributor. Now it acts like you hit a rev limiter once you hit 2500 rpm. At this point I'm just throwing parts at the problem and I'm running out of ideas and the mechanics have thrown in the towel. Do any of you have ideas on what the issue might be? Thanks in advance for your help and/or ideas.

 

I'm not the parts changing guy, I'm the analyze, identify and repair guy. I also live for impossible to solve problems. If that appeals to you I will do my best to help you get your truck fixed. A few things are your friend here, but require some detailed work:

1) Identify any remaining clues to narrow down the problem
2) Measure key parameters while you are having the problem
3) Eliminate things that are not the problem until you have your suspect
4) Keep an open mind until the info sends us to the problem. I know that somewhere in the world Uncle Fred's truck ran like crap because of a clogged converter but that may not apply here and is easy enough to rule out with a back pressure gauge and an IR temp gun and not by replacing the converter.

There is a technique in computer programming called a Simpsons approximation to converge on a value which was developed to increase efficiency. Simply stated you keep cutting the number of possibilities in half until you find the value. That's way better than walking through all of the possibilities in alphabetical order until you stumble over the issue. Its faster and cheaper with a difficult problem like this. If the fuel system can be ruled out as an example, your work is greatly reduced. That's way better than replacing the fuel pump, injectors, regulator, grounds, relays, ..... and possibly not even solving the problem. This can be many things: fuel delivery (ex: injectors), electrical (ex: ignition switch), ignition (ex: ICM), mechanical (ex: cat converter), air (ex: unmetered air, vacuum leak), etc. Another guide for tough problems is Occams Razor (of all the possibilities, its usually the simpler explanation). To get started:

1) Do you have any codes, pending codes or freeze frame data now after all of this work?
2) Have you done a compression test with leak down?
3) Was your cat properly tested (back pressure and temps)?
4) What do you know about when this does or does not happen? warm vs cold. Before or after transitioning to closed loop. Under heavy vs light load. Raining vs sunny outside.
5) Do you have a scanner capable of live data and graphing? We have guidance here to do that on the cheap.
6) Do you have a fuel pressure gauge?

First thing I would do after the above (you might tell me something that changes my mind on where to start):

1) Inspect the engine compartment and look for loose or broken/chaffed wires or connectors, a loose air intake with particular attention to unmetered air after the MAF, cracked or loose vacuum hoses (does your ac blend door work well?).
2) Travel with a way to measure spark at the plugs. When the truck dies and won't start look to see if you have 1" of strong blue spark at the plug(s). Look at this when the truck is running great to get a baseline.
3) Measure fuel pressure according to the sticky in this forum. If you pass that test then travel with a fuel pressure gauge. When the truck dies and won't start. Check key on fuel pressure and pressure while cranking. If it wont die but runs like crap at times, measure fuel pressure then. You can tape the fuel pressure gauge to the windshield or measure with a remote sensor.
4) When the truck stumbles does system voltage change with a meter on system voltage, a cigarette lighter plugin meter or at the very least the gauge meter?
5) The results from this should literally cut your problems in half unless you have a 5 sigma problem (really unlikely)

If you have a capable scanner and you are still engaged on this and have not yet found the problem then check:

1) Key sensor values at key on, engine off and make sure that they agree with the ambient conditions. MAP, ECT, IAT. TPS output through the full range of accel pedal movement to see if the output is proper and smooth
2) What are your fuel trims now and what do they look like when the truck runs like crap? Do you go lean or rich? When you try full acceleration do your O2 sensors peg high (go rich)?
3) What does a graph of your O2 sensors look like? Are the voltage swings ok, is the response rate within range
4) Does your MAF output make sense at idle and under gear shifting at full load (you need an online volumetric efficiency calculator)?

BTW, when you have a code for a sensor you don't know what's actually wrong yet. Is the sensor faulty, is the wiring and connectors to that sensor faulty, is the ECM faulty or is the system producing an out of range condition that the sensor is correctly measuring. Also, all repairs should be confirmed. There are bad fuel pumps right out of the box or shortly thereafter for instance.

If you like to watch you tube videos, prioritize anything from Scanner Danner or South Bend Auto. Those two guys are sharp. The first is more academic, the second more the every man/women.

There's more but this a roadmap if your up for the journey. You don't need to do all of this right away but lets see what you already know and what is the easiest for you to do first. It would be a great start just to find out that its fuel delivery or spark and then pursue the limited number of possibilities from there. Ask any additional questions and we can drill down on any of this. I'll stick with you as long as you want.

f you want to swap parts you can get a lot of suggestions here. I'll even throw in my best guess if that's what you want because I want to see you fix your truck but I prefer to help you with proper analysis.

George

 

I should add:

In reading your original post again, look under your old distr cap and see if there is any arching. Also if you can measure cam retard, what is the value?

George

 

George,
Thanks for the response. I appreciate you taking the time. I am not the type of person to throw parts at a problem nor is my friend, but here we are unfortunately. I changed the parts that my scanner showed initially, the MAF and the TPS. I reset the CEL and took it for a drive. About 2 miles into the drive, the light was back on and I was stranded for a bit. Knowing this wasn't an issue for me to fix, off it went to the mechanic and another, and another, until they have "thrown in the towel'. This has been going on since September. Below is what I do know. I'll gather the other information.

• Without fail, when the road is wet the problem will occur. This lead me to think its electrical (just a guess). This is the only time it actually dies. It has a consistent miss the rest of the time.
• There is no time frame on it happening that neither he nor I have noticed. It might be 5 minutes or an hour.
• I initially thought the converter might be the issue, so a new converter was put on. No change.
• Sprayed carb cleaner everywhere looking for a vacuum leak, but found nothing.
• Changed the coil, twice. No change.
• Plugs and wires, No change.
• Fuel pressure was 5lbs low when the problem occurred (according to the current mechanic). Changed pump, still low. Changed regulator, don't know the current pressure. I'll find out actual numbers.
• New distributor was installed and it began acting as though it was out of time. It was probably off a tooth. New chain installed and distributor and timing rechecked, still acting like it's out of time. It acts like a rev limiter is set at 2500 rpm.

As soon as I can get actual data you spoke about in this post, I will share. Thanks again for your help.
Phillip

 

No judgement on your work so far, your working hard at this and I want to help. I put my philosophy forward to see of you want to work with me. I admire your perseverance . See comments below:

George,
Thanks for the response. I appreciate you taking the time. I am not the type of person to throw parts at a problem nor is my friend, but here we are unfortunately. I changed the parts that my scanner showed initially, the MAF and the TPS. I reset the CEL and took it for a drive. About 2 miles into the drive, the light was back on and I was stranded for a bit. Knowing this wasn't an issue for me to fix, off it went to the mechanic and another, and another, until they have "thrown in the towel'. This has been going on since September. Below is what I do know. I'll gather the other information.

• Without fail, when the road is wet the problem will occur. This lead me to think its electrical (just a guess). This is the only time it actually dies. It has a consistent miss the rest of the time.

OK good another clue. In the dark mist water around all of the ignition and look for arching. CMP retard is important here as is looking for carbon tracks under the distr cap. If your stranded add drying out the distr cap and reinstalling. I agree that this suggests electrical or ignition.

• There is no time frame on it happening that neither he nor I have noticed. It might be 5 minutes or an hour.

This tells us that it doesn't change when it warms up or transitions to closed loop fuel control. It tells us for instance that it does not get better when the O2 sensors take control of fuel delivery or its not happening when components heat up that can fail when hotter like the icm or coil.

• I initially thought the converter might be the issue, so a new converter was put on. No change.
• Sprayed carb cleaner everywhere looking for a vacuum leak, but found nothing.

That's a good test. Fuel trims are good for many things and as an additional test to rule out vacuum leaks. If you are lean at idle and significantly less lean at 2500 then you may have a vacuum leak. If those things are not true then you can probably rule out a vacuum leak and move on.

• Changed the coil, twice. No change.
• Plugs and wires, No change.
• Fuel pressure was 5lbs low when the problem occurred (according to the current mechanic). Changed pump, still low. Changed regulator, don't know the current pressure. I'll find out actual numbers.

If you still have low pressure you could have leaking injectors or tubing in the upper intake, or of course an unsuccessful repair. Even so however, seeing pressure during the problem is best because that would really tell us if that's your major problem. If all of us 65 year old guys got a full body scan we would get to hear about all kinds of stuff that is worn out or marginal. Doesn't mean that all that stuff is making us feel bad.

• New distributor was installed and it began acting as though it was out of time. It was probably off a tooth. New chain installed and distributor and timing rechecked, still acting like it's out of time. It acts like a rev limiter is set at 2500 rpm.

Again we need a measure of your ignition timing. If CMP retard is hosed or you have a timing code then that would be a good clue(s).

As soon as I can get actual data you spoke about in this post, I will share. Thanks again for your help.

My pleasure, lets get your truck in top shape with minimal cost and time.

Phillip

 

Also, have you statically checked distr timing: #1 TDC and distr rotor point to the "6" on the housing?


George

 

I have been thinking about your problem, considering the new stuff you sent me. While there is a large list of what this can be the two most likely are fuel delivery and ignition and figuring out if its one of those 2 reduces this problem significantly. If all of this is overwhelming here is a priority list. If not then test in whatever order matches your capability and/or makes the most sense to you:

1) Engine compartment inspection
2) Codes, freeze frame data (remaining clues)
3) CMP retard value (timing issues?)
4) Distr cap inspection (timing issues?)
5) Water misting (ignition arching and misfires)
6) Fuel pressure
7) Fuel trims at idle and 2500 rpm (rich or lean, vacuum leaks?)
8) Fuel trims under a load when the truck acts "rev limited". Do you go lean or rich? (fuel delivery, ignition, sensor, ...?)
9) Misfire counter when you are "rev limited" (all cylinders, some cylinders or no cylinders?). This is new.

If you can't do CMP retard yet then statically check timing and move down the list until you can. Les is one of our moderators here and he has done good work assembling low cost solutions for code reading, live data and graphing. If your going to tackle this yourself that's pretty much required capability. In the olden days only expensive GM test gear or Snap On type scanners were needed but today you can do almost everything for under $150 and most of it for under $35 or so. As I have mentioned before, all of the things that I have suggested have value but getting existing codes and data while the truck is running like crap are the most useful.


George