98 Silverado RPMs randomly start jumping around, running horribly

tahoe.mkIII · Sep 23, 2019

Might want to check the ignition switch, had a similar issue with the 2500. The key was worn out and would not engage the switch sometimes. Had to push the key in quite hard to get it to engage the switch. Occasionally it would disengage while driving, all the gauges will die/drop to the off position and the trans will go into limp mode. I had to pull over, shut it off, and slam the key into the ignition to get it to contact again. New ignition cylinder and switch and it's all good now.

Also possible ignition control module, I haven't replaced one of those in a long time, but I recall the old 88 had some similar issues that were solved by replacing the module.

stutaeng · Sep 23, 2019

Do you have any codes? I would start there. I would bet you have a code stored on your system. Any auto parts store can pull code for you.

I just changed the camshaft position sensor on my 4.3 on my NBS 99 Silverado. Mine acted like it was about to die and my tachometer would die intermittently and then come back on. It never died on me, however.

I did have a code (P0340) about camshaft sensor, so pretty easy to diagnose, and problem solved.

I believe both crank and cam position sensors can be checked without pulling them out, but you need voltmeter and crank the engine by hand with key on.

https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-the-crank-sensor-1
https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-the-cam-sensor-1

cptcrash80 · Sep 26, 2019

Well, replaced the crank position sensor Tues night. Started up fine, drove it for a little bit and it died twice. Last night I was gonna test the battery at full load (blower, radio and hazards on) at idle. Ran for about 3 minutes then died. Wouldn't start back up. Put my spark tester on and now I'm not getting any spark :flame:

east302 · Sep 26, 2019

cptcrash80 said:
Put my spark tester on and now I'm not getting any spark

Sucks, but at least you have something to work with now. Here’s a nice write-up on ignition diagnostics for a ‘cranks but won’t start.’

https://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/ignition-misfire-diagnostic-tests-3

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cptcrash80 · Oct 14, 2019

Sorry I haven't replied in a bit, been stupid busy. The good news is the truck is running like a champ now. Turns out the battery had a bad cell. I could fully charge it but it would steadily drop voltage anytime a load was placed on it. Swapped it out and everything has been fine since.

Oh well, live and learn. Thanks for the suggestions and help everyone.

Scott Martin · Oct 23, 2019

cptcrash80 said:
So within the last couple months my 98 Silverado 2500 (350 Vortec) started having an intermittent issue after driving it throughout the day. It will startup and run fine first thing in the morning but the more I drive it during the day the worse it runs. The RPMs will start bouncing up and down and it will act like it isn't getting enough fuel. The only code it has thrown was codes for O2 sensors (which I replaced last year). I also just replaced the exhaust flange gaskets and that didn't make a difference. I have also replaced the fuel filter and spider harness.

Testing the alternator from a cold start I was getting 14.6 volts. The truck has right under 289,000 miles on it so I'm trying to figure out what could be causing this kind of issue without it being consistent. I haven't had any codes come up in over a year until just the other day and once I replaced the exhaust gaskets and cleared the code it hasn't come back.

Thanks in advance for any ideas.

Two things.. First try running some premium gas in it and I would highly recommend changing the engine coolant sensor!

98 Silverado RPMs randomly start jumping around, running horribly

tahoe.mkIII

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