Rough Idle, not throwing any codes, 97 C3500

[blueridge725]

Cleaned up the intake, etc, and put the truck back together. Other than the cleaning, didn't change anything, so didn't expect it to run any different.

 

And it didn't. I pulled a couple of plug wires, some seemed to make it stutter for a second, some didn't make any difference. They are pretty old, probably time for a tuneup.

 

It did throw the check engine light on the first time I started it. But then it disappeared later. I had forgotten my code reader, but I scanned it later in the afternoon (had a code stored even though the check engine light was off). The code was for low voltage to throttle positioning sensor. I cleared it and it never came back. It was hard to start the first time, cranks up easy since.

 

Didn't find any vaccuum readings in my code reader. Must not be fancy enough. The only vaccuum type lines I see are one or two leading to what I believe is the emissions canister.

 

The PCV has a good bit of vacuum on that line. Truck didn't run much different with that big vacuum sucking when i pulled the line off it. When i reconnected it, it practically died, then loped back to life.

 

Idling at 700 rpms pretty steady.

 

Then I cranked it for about the third time (I had to restart it to rehook some of the spark plug boots without getting shocked). So after this restart, the engine idled smooth as can be. I mean, i couldn't have asked for a smoother idle. I let it run like that several minutes (my brother even took a video). Then I revved the engine a little and when it returned to idle, rough as it ever was.

 

It was nice to see it run smooth again, even if just for a minute. Restored my faith a little, even though it was also frustrating. I hate to just start throwing parts at a problem, but without a code, I don't see much other choice. At least I feel pretty confident that the spark plug wires need replaced, a couple of the boots had cracks in the rubber.

[Jsdirt]

Don't ever yank plug wires and let the engine run very long without grounding them to something - that's a good way to fry a coil. If you could see (with a scope) the amount of built up energy stuck in the coil with no place to go, you'd be amazed. Never seemed to be much of an issue back in the day, but these days, it's easier than ever to cook one.

 

I yank the wires off while it's running, while holding a GROUNDED test light (NOT an LED one) close to the wire opening (much closer than my fingers) - electricity always takes the path of least resistance, and will jump into the test light. It's a good way to check for spark / spark quality, and do a cylinder drop test without a scan tool. Be aware that you can still get lit up if there's a break in the plug wire - like I said, path of least resistance.

 

Another thing I want to mention is, just because it's storing a code doesn't mean that the code tells you exactly what the problem is. In fact, far from it. I had to learn this the hard way over the years. Sometimes something COMPLETELY unrelated will fail, and it's effect will trigger a code for a totally different system. This is where experience comes into play. If automotive diagnostics were simple, everyone and their brother would do it. I've been doing it for years, and let me tell you, it's a friggin pain in the ass today! More difficult than it's ever been in the history of the automobile. Guys get slid every day in this industry that have been turning wrenches for 3 or 4 decades, or more! There are a few professional automotive forums that are loaded with guys yanking their hair out, trying to get a customer's car out the door, and not have it come back 10 minutes later. VERY difficult to do these days. Sometimes you win, and sometimes you lose. Just the way it is now. Very little is set in stone anymore. SO many variables it will make your brain hurt!

 

That said, I think you're on the right track with the plugs and wires. Sounds like you may have temporarily "fixed" a broken plug wire by removing, and then reinstalling it. No engine hurts from new wires and plugs (and cap & rotor if equipped).

 

Oh, and if you're going to find a vacuum reading on a scan tool, it would be under the MAP sensor PID. If the vehicle isn't equipped with one, the PID won't be there. Best way to get vac readings is with a gauge tapped directly into the intake manifold.

 

Persistence pays off! Keep at it - you'll get it eventually.