1999 Silverado Stumbles

These problems can be hard to find when it does not code. If you can get access to a real time OBD2 scanner and read live data, you will likely be able to nail it down. Their is a chance that you have a cam position sensor that is getting flakey and providing bad timing info at time but not bad enough to code.

----------------- The SnoMan

I'll amen that (good scanner). I'm having ongoing problems with my Battery Warning light (intermittly) on for no good reason. I had it to a GM dealer service center on the road and nothing was found using the scanner they used, However later (like weeks) I had someone with the official GM ($3500 +) scan tool and he found the problem right away, even without a Service Engine light being set. Turns out the "L" wire circuit from the alternator has a problem, and today thanks to a monitor wire I ran to inside the cab and reading with my Fluke DVM, it is a bad "L" wire circuit inside the new (Auto Zone) alternator causing the problem. BG

I would also try changing out the plug wires as well. They can cause some pretty stupid idling/running problems at certain rpm ranges. My Grand am was doing a similar thing at about 1900 - 2200 rpm. and the plug wires were the problem. I can't say for sure that changing them will cure the problem, but considering the age of the vehicle and the cost, it might be well worth it.

I'll second on the plug wires... My truck (same year and motor) was doing the same thing, new plugs and wires fixed it.

Adair

A while ago I had a erratic idle problem on my 2000 K3500 with a 5.7. From time to time it would deleop a eratic idle in that it would vary a few hundred RPM quickly for and then slowy clear after 15 to 20 seconds when engine was hot and you first came to a stop.. It would not code and dealer said it was fine (it was under exstended warranty at the time). It turned out that it was a flay crank position sensor because by using a real time OBD2 data reader I discovered that the engine timing was swinging wildly at times causing the problem..The scanner I used cost me a little over 100 bucks on Ebay.

----------------- The SnoMan

My '00 Silverado was doing the same thing. The scanner showed odd fuel adjustments to "Bank 2" compared to relatively stable adjustments on "Bank 1"... turned out to be a bad spark plug wire. It literally fell apart in my hand when I was removing it. David

I replaced the wires at about 180,000 miles... but I think the "stumble" was present for a long, long time before it got bad enough for me to starting really looking for the cause. I put new plugs in it about 20,000 miles before that and the "stumble" went away for a little while and then came back. The plug wires seemed "normal" at the time and I sprayed them down with silicone then, too. My guess would be 140,000 to 150,000 miles when I first noticed it acting goofy? David