Rough Idle/No start

Asked by Dec 23, 2015 at 06:35 PM about the 1996 GMC Sierra 1500 C1500 SL Standard Cab LB

Question type: Maintenance & Repair

Okay guys, so my truck has been sitting since May
of this year, and it broke down while I was driving
about 200 miles from home. So obviously, it was
running well and I didn't think there were any signs
that it might be about to give out. But after I
stopped at a store, I drove back down the road and
stopped at a red light. When I went to take off, my
truck was dead(couldn't hear it die because I had
the music up). Wouldn't start. Had to push it and let
it sit in a parking lot. Fast forward a day, I get back
out there to see what it could be, and it starts, and
sounds fine. So it starts going down the highway,
and it's severe loss of power, the speedometer is
reading 80 when it's doing about 45, and then dies.
Bring it home the next day, it won't start. But over
the months since then, a lot of things have been
replaced, which I'm about to list:
Fuel pump, fuel filter, ignition switch, ignition control
module, spark plugs, spark plug wires, and
probably a couple of other things I can't remember,
but if it's mentioned as a possible solution, I'll
remember if that was replaced or not. I uploaded a
video to YouTube so maybe you can better
understand the way it's acting. It currently starts
after replacing the aforementioned parts, half the
time it stays at a severely rough idle with backfire,
or it will have the same severity of idle and then die.
I'm currently at a loss of things to do/replace, as I
obviously have no clue what could be causing the
problem. Here's the link to the YouTube video:
https://m.youtube.com/watch?v=hBCRTGAxPrw
Please tell me what you think it might be, I'm trying
to get it running as soon as possible.

5 Answers

170

I and a couple thousand other people have the same problem..... Some have said that the guaranteed fix is to replace the fuel pump, or fuel filter, or ignition module, EGR valve, ECM/PCM, distributor, or one of the multitude of sensors and relays that are interconnected with the ECM..... I'm betting on either a dead short, or a faulty grounding wire somewhere in the wiring harness, or instrument panel causing a drop in voltage to the ICM or IAC valve when you put it in gear..... I've gotten mine where when it warms up it'll idle like straight off the showroom floor, but has no pulling power when you try to drive it..... Acts like the transmission is going out..... Speedometer goes crazy..... Heater fan comes on by itself..... Sometimes try to start it up and fires right up, but have to baby the accelerator to keep it running long enough to warm up the engine..... Other times the starter drags briefly, then kicks in..... Exactly like one does, or use to do, when there was a bad ground connection to either the engine or the chassis, which is easy to fix.... It's the "hidden" faulty grounds that are a double headache to attempt to diagnose, cause it could be as simple as a bad ground connection from the instrument panel to chassis..... Or a bad diode in one of the circuits within the instrument panel..... Even a loose connection on the fuse block.

Other things that were replaced: Crankshaft position sensor Cam sensor

170

Your truck is about six years different from mine and has quite a few more sensors and several more control appliances. So there is going to be some difference between the two....The engine in my truck is virtually new....took out the old 302 that was in it, because of the same problem that you've described....And replaced it with a rebuilt 350 block from a reliable and experienced rebuilder.....Moved everything over to the 350 block, pulled the 302 and there was "at least" an inch of oil caked inside the valve covers, and pretty much everywhere else in the old block....I figured from the start that I would need to replace just about all of the sensors, changed all sensors except the knock sensor, rebuilt the TBI, new distributor cap and rotor, plugs, wires, IAC, ICM....Though not actually new, the engine is virtually new.....I guessed at a dead short or loose ground somewhere causing the Engine control module to send wrong signals......I guessed this because gremlins were acting up as if on a timer..... Bought a new multimeter and tested everything I could possible think of to test....Found several shorts and pulled the fuse to the circuit....still acted up without the fuse.... Pulled the fuse on every circuit "except" the hot all the time fuses....Did the same thing, started, idle, stall, idle, stall, idle, shut off, like it was on a timer...Waiting on a signal from "somewhere" that it wasn't getting....When I got around to changing the ignition control module to see if "that" was the problem, The old ICM ohmed out correctly, but while I was changing it I noticed an oily film within the cap and on top of the old ICM....Long story shortened....I replaced the old distributor that we,...a friend who only "thinks" he's a mechanic,...It was also covered with a thick coat of oil, and had to be thoroughly cleaned before we put it in the new block,.....with a new distributor which just so happened to include new cap and rotor, and ICM....Shoulda bought that in the first place instead of the new cap and rotor, then the ICM, then the distributor which had all three....Oh, the distributor pick up coil also ohmed out good.....Dropped the new distributor in today, and there's still a small gremlin in there somewhere, probably not quite in time yet... It was a double B to get the distributor back in right to get it to start, still wanted to try and stall, but if it did shut off, could start it back up and it would usually idle good...I'll have to borrow a timing light tomorrow,...Had to buy a distributor clamp wrench, cause the odds are that you ain't getting to that bolt with anything but....Had the rear wheels up on blocks, and truck bed raised eight inches off the frame to check, and if need be replace the fuel pump...Any time I got the truck running, before, when I shut the ignition off, the pump continued to run for anywhere from 15 to 30 seconds, which ain't right,...But the fuel pump ohmed out good....Got off the ramps and drove around the block, no stall, trans still tried to act up just a little when trying to change from first....Around the block, hooked up my seat belt and hit the highway... 50, 60, 70, going up a fairly steep grade, and the trans stopped slipping in the process....The actual culprit in this instance was, I believe, the distributor gasket, which was somewhat distorted on one side, and cracked at least half the width of the material,....which allowed a small amount of oil to constantly come out from around the gasket to get sucked up into the distributor cap through the vent holes in the bottom of the distributor plate.....Check under the distributor cap and see if you can detect a small oily film, or dried oil stains on the plate....you "MIGHT" have a simple problem of a cracked, broken or distorted distributor gasket....I figured out how to replace the distributor by researching a few utube videos, and by cruising around these questions.....These questions are one of the primary reasons I even considered the new distributor, cause I hate doing that, cause I really don't know exactly what I'm doing. Good luck on your truck.

170

Oh, the fuel pump no longer over runs after turning the ignition off.....It did at first, after I changed the distributor, After the first start I forgot to pay attention...Then shut it off and listened to hear the pump continue to run for about ten seconds.....Started it up again, shut it off and maybe a couple seconds overrun...Since then I haven't noticed any fuel pump over run.

745

Truck is old/truck has been sitting up/.if you were 78 and been off the shelf for a good smount of time you would not run to good either.common sence does apply.go back to the basics.filter/you have bad gas/water in gas.drain it.clean the inside of the carb with cleanser/a spray.get it started let it run 20 min.back on the roaf

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