Hey guys, been chasing a rough idle on a 2000 GMC Jimmy since early summer. Replaced everything could think of in engine. "Rough Idle" remains. It was idling rough all the time, except in park, neutral and reverse. Now that winter is here (live in Texas) I discover the rough idle does not appear till vehicle has warmed up. What would cause a noticeable vibration, only in drive, 1, or 2, and only when vehicle is warm? I say warm, 40 out and roughness appears after only 2 minutes of driving it. Not sure how fast or what temp components would reach after only a couple of minutes of driving. Also, never paid attention to it, done it since I bought it, but this vehicle does not propel itself forward at idle with foot off the break at all. Have to give it gas. First vehicle I have seen do that. Could these problems be the Torque Converter failing me? Thanks for the input.

 

Doubtful that the torque converter would cause what you're experiencing. Is the SES light on? If so, have you retrieved the DTC's?

 

No lights are on. Not sure what DTC stands for, assuming diagnostic code of sorts, and no I have not. No reader. I'm just baffled thinking what would cause a rough vibration that increases with heat. Starts as soft shutter but increases to a very noticeable lope after 15 minutes or so. Does not get any worse after it has reached a certain point though. 0-2 minutes=perfect, 2-10 minutes=light, 10-15 minutes=heavy, 15+= almost missing cylinder feeling. Again, this is only in drive, stopped, with brake applied. Once going all is good. Thanks for response.

 

Yes, DTC=Diagnostic Trouble Code, good job!
It's odd that the SES light is not illuminated. When the engine is first started, the Powertrain Control Module, (PCM) operates in "open loop" which means that certain sensors that control fuel mixture are ignored and the PCM uses a pre-programmed set of adjustments. When the PCM switches to "closed loop" all sensors are used for fuel control. It sounds like this is when your problem arises. As the engine warms up, the fuel mixture is leaned out. It's possible that a fuel injector nozzle is partially restricted not allowing good fuel flow once the PCM leans the mixture. Without the benefit of the PCM to help out with what's causing the misfire, it will probably have to be checked out on a capable scan tool to accurately pinpoint the cause. In the meantime you might try a bottle or two of injector cleaner, cross your fingers and see what happens. A check of the fuel pressure and leakdown wouldn't be a bad idea either.

 

Thanks Captn. I do appreciate it. I can get a fuel pressure reading. The way the PCM works makes sense, as how you describe it so well. But would the PCM change the mixture just from park, neutral, drive and reverse. If I may probe further, I am guessing the PCM open loop mode is time based, as it is using this predetermined time to wait for the engine to warm before responding to inputs. The reason I ask, is not long ago, it was 105+ outside every day. This rough vibration was there once the vehicle was started immediately. Now that its cold, its taking a couple of minutes. If PCM were time based then it would not seem to be a component that would fault the PCMs computing. Does that make sense. Chevron Techron in every oil change.

 

This will show you how to test fuel pressure and leakdown: https://blazerforum.com/forum/showthread.php?t=45941 All tests are done with the engine off, GM does not publish any "engine running" fuel pressure specs, there are far too many variables.

The PCM has several different "cells" to choose from ranging from engine temp, engine load, transmission gear, engine RPM, throttle position, vehicle speed, etc etc. It's not a "timed" occurance, the PCM processes data from various sensors to determine when to go into closed loop. Makes perfect sense that the problem was different with higher ambient temperature: higher ambient temp requires shorter warm up period.

If you have access to a scan tool, that would help. Compare the Inlet Air Temp, (IAT) to the Engine Coolant Temp, (ECT) after the vehicle has set for 8 or more hours, and before you start it. The two sensors should be within a degree or two of your actual ambient temperature.


Chevron Techron...good stuff! You might try a double dose and check fuel pressure.

 

LOL. Double dose. I did actually about 3 months ago. I started to type that but deleted it thinking, eh, not important information. I will follow through with your recommendations, as they are certainly better than mine (dropping tranny and replacing torque converter) Thanks for all the help.

 

Proper diagnosis saves time, money and grief. Any questions, just ask. Post back your results.

 

There is the possibility that an injector, or poppet nozzle, is restricted to the point where the injector cleaner won't clean it out. An injector balance test will determine without a doubt if that's the problem. If that's the case, you might consider the upgraded injector assembly. It relocates the injectors to the intake ports and does away with the poppet nozzles. The 4.3L "X" engine comes with the upgrade from the factory.

 

Psalms 109:8, yes sir. The BBE version most certainly