Truckpdate 

"Future Heap Owner" (aperiodic) 08/26/2018 at 12:27 • Filed to: Minor victories

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(Title has a silent ‘p’)

I finally had a reason to take the truck out after replacing the knock sensor a few days ago, and it ran just fine, so I think that’s solved.

The issue I’m focusing on now is that after it’s warmed up and the idle revs drop to ~700, it starts to idle roughly. It seems like it misses a step, the revs drop halfway to zero, then it catches back up. Often the stutters will be clustered a few seconds apart, but then it could fine for 10-30 seconds. Once it stalled itself (all the other times it’s stalled it’s been my fault).

It seems like a cylinder or two might be messed up? I haven’t really gotten around to inspecting inside the engine, so giving the plugs and distributor cap a look over is my next step.

Spamfeller Loves Nazi Clicks > Future Heap Owner 08/26/2018 at 12:41

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Sounds more likely you’ve got a vacuum problem or the idle air control motor’s toast. Especially if it’s idling clean before it warms up.

Sovande > Future Heap Owner 08/26/2018 at 12:43

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Check the IAC valve. They can be cleaned, sometimes. 

Future Heap Owner > Spamfeller Loves Nazi Clicks 08/26/2018 at 12:54

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Vacuum leak was the next thing to look for after the plugs and cap (that ordering mostly because finding a vacuum leak sounds hard), but I didn’t know about the IAC motor. Thanks!

Know any way to diagnose the motor? I’ll check the Haynes manual, but, you know...

dogisbadob > Future Heap Owner 08/26/2018 at 13:05

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LS swap time yo :p

Sovande > Future Heap Owner 08/26/2018 at 13:15

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In my experience (with Volvos and Hondas) you can pull the motor after removing associated hoses, wires, etc and check the valve to see that it moves freely. It’s will have a shaft and an opening somewhere that exposes a gear looking thingy (a technical term). Rotate the shaft to see that it easily moves. If not, spray it with carb cleaner or similar and clean the “gear” until it spins with ease. You can put power to it as well to make sure that it operates. Could be that when it warms, the shaft binds on whatever goop (technical) is on it, which won’t allow it to open to allow for more or less air.

You can also get the truck up to temp and spray carb cleared on all the vacuum hoses and listen for a change in idle. If you get a change in idle that's the likely spot for a vacuum leak. 

Spamfeller Loves Nazi Clicks > Future Heap Owner 08/26/2018 at 13:28

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For GMs of this vintage, you’re gonna need an Tech2 to actually test the IAC. Which obviously, sucks. Don’t attempt to manipulate anything manually. It’s inaccessible for a reason. Forcing it will break it. These are IACV not IACM as well. Dunno why I said motor - insufficient coffee I guess. But it’s a valve here.

Honestly, my advice here is to check for vacuum leaks, then go over the throttle body before just chucking an IACV at it. If you can do OBD operations and test it, that’d be much better. Because for the 5.3, it’s a $100 part.

edit: fucking hell, I called a Tech2 OBD-II. Somebody bring me more coffee.

If only EssExTee could be so grossly incandescent > Future Heap Owner 08/26/2018 at 14:14

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As others have said, vacuum leak or a bad idle air valve. Depending on what setup those engines have it could also be a bad crank vent valve. If the  oil separator is gummed up it can present the same symptoms as a vacuum leak.

Nibby > Spamfeller Loves Nazi Clicks 08/26/2018 at 14:20

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this problem sounds like it really... sucks

HammerheadFistpunch > Future Heap Owner 08/26/2018 at 14:38

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Vortec or tbi?

Future Heap Owner > HammerheadFistpunch 08/26/2018 at 14:46

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TBI ( 1992)

HammerheadFistpunch > Future Heap Owner 08/26/2018 at 14:47

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Ah sorry then I’m no help

Urambo Tauro > Future Heap Owner 08/26/2018 at 22:29

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Kind of a surging idle , then? I had a similar issue in my truck that I kept finding possib le causes for. Each time I found something, I would say AHA that’s gotta be it . B ut even though several items were in need of attention, the surging kept coming back. It was a while before I finally managed to put a stop to it.

I started out with the simple stuff, trying to make sure that I was diagnosing it properly. I found some bad grounds, fixed them, truck still su rged. Found some loose vacuum connections, fixed those, truck still surged. I kept finding more and more stuff that definitely needed attention. Before you knew it, I cleaned the EGR, replaced the IAC valve, changed the timing chain, replaced the intake gaskets, rebuilt the TBI, replaced the coil, and replaced the distributor.

Not sure where, but s omewhere along the line I slacked off on my quest to diagnose it properly, and just started throwing parts at it instead . The worst example of this was when I replaced my brand new distributor with a much more expensive Delphi one. Wouldn’t ya know it- that’s what fixed it.

Future Heap Owner > Urambo Tauro 08/27/2018 at 02:15

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It seems like the opposite of a surging idle: the needle always drops first, then climbs back up to where it was.