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Old 07-10-2019, 02:12 PM   #1
no_rice
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This thing is kicking my butt

Normally I'm over in the 67-72 with my 70 GMC Shortbox, but..., I picked up an 89 GMC 5.7 auto, thats been sitting in a garage for the last few years. And getting it running right is giving me fits... it does have gibson headers, and custom exhaust on it, but other then that, motor appears to be reasonably stock.

for the most part its at a point where it fires right up, but it does not want to idle right, and its super rich. the idle air control was not functioning correctly, so it was sucking a ton of air all the time and couldnt slow it down. so got a new one of those. but even after doing an idle reset on it, the iac still does not completely close, even when the truck is good and warm. so it still idles a little high. if i slow the idle down, it will stay that way for a minute or 2, then die down to a stumble and start puffing smoke and running rich. give it a little throttle or speed the idle back up a hair and it cleans out and stays that way, but still idling a bit higher then it should (no tach, so going by ear, but obviously a reasonably fast idle speed) and sucking a little extra air through the iac (atleast i assume at operating temp, in park at idle that thing should be closed off)

when getting it to run, ive already replaced cap, rotor, plugs, and new wires. checked fuel pressure and its good, and some fresh fuel. as well as what appears to be a good cone shaped pattern out of the injectors. checked for vacuum leaks and have found none.

looks like im going to have to replace the harmonic balancer before i can get a good timing check done, because its wiggling all over the place while running. so i assume the rubber inside is shot. but as good as it starts and idles with even a little throttle, i dont see that being to far off. that being said. even when its idling at a good pace and fairly smoothly, a quick whack of the throttle can make it backfire through the throttlebody

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Last edited by no_rice; 07-10-2019 at 02:18 PM.
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Old 07-10-2019, 04:03 PM   #2
sntrym95
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Re: This thing is kicking my butt

Is the harmonic balancer actually wiggling or does it just look that way via a timing light? Did you check/set timing with the timing connector unplugged?

I vote sensor issue, like the temp sensor in the manifold. There are multiple threads on checking that sensor.
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Old 07-10-2019, 05:06 PM   #3
no_rice
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Re: This thing is kicking my butt

it's actually wiggling. cant move it by hand, but it wobbles all over when its running.

and yes ive seen the stuff on the temp sensor. have one of those on the way. figure for 6 bucks what the heck...
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Old 07-11-2019, 02:30 AM   #4
abig84
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Re: This thing is kicking my butt

Ive heard of timing chains jumping a tooth before but have never actually seen it. Almost sounds like a timing issue with it backfiring out of the carb. My 1990 when i got it had a weird running issue to it, was kinda missing and felt like the trans was shifting funny. Anyways started replacing parts and when i swapped computers it ran fine.

If you give up on that truck and sell it I’m sure people would love to have it. Mines about to fall apart soon and will be looking for a replacement within a year or two
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Old 07-11-2019, 09:15 AM   #5
Stormin08
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Re: This thing is kicking my butt

i would look real hard at the timing chain as well...a cousin had a firebird with a 307, ran so poorly that when we pulled the timing cover, the chain had enough slack that it almost wore a hole through the lower portion of the cover.
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Old 07-11-2019, 01:34 PM   #6
no_rice
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Re: This thing is kicking my butt

I obviously cant completely rule out a timing chain. i mean i know it happens on occasion. but it would have to be a rare fluke! this has 108,000 on it. so pretty early for something like that. and it does run pretty smooth at any kind of rpm. but I do appreciate ideas!

as for mention of the computer. i have seen people say that the ecm was having an issue sending whatever signal was needed to control the idle air control correctly. eventually they replaced the ecm and it straightened out. which is also a consideration

as for the truck. it may or may not end up for sale. i bought it on a whim. as another project i didnt really need. lol. and have a long line of other stuff needing my attention. but it would be fun to cruise a bagged truck for awhile. its a 20 footer. no rust to speak of thankfully, but a few dinks and scratches. a simple smoothing and outside repaint would have this thing in amazing shape!
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Old 07-15-2019, 02:29 PM   #7
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It's kicking your but because you are doing what everyone else does here

You are ruling out things that you don't think are possible and are only considering things you do. That's normal but after it kicks your ass, you need to start considering the things you have dismissed so far.

An IAC never does close in normal operation. If so it would shut off.

You're assuming it's pulling air through the IAC but in actuality, it's probably pulling air through another source such as a leak.

Checking and eliminating the chain (and other pieces in the cam system) are easy to eliminate. So why not just take the 5 minutes and get rid of the issue. Remember, it may not be a tooth off but have lots of play.

Why not eliminate compression also? Not what you think the problem is? Yeah everyone says that. Better yet do a leak down test instead.
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Old 07-15-2019, 03:16 PM   #8
95 S_Trucker
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Re: This thing is kicking my butt

Sounds like the gasket between the throttle body and the intake manifold to me. Really common on these trucks. The gasket gets brittle over time. The newest TBI trucks are 24 years old now.
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Old 07-16-2019, 09:54 AM   #9
truckdude239
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Re: This thing is kicking my butt

Quote:
Originally Posted by no_rice View Post
I obviously cant completely rule out a timing chain. i mean i know it happens on occasion. but it would have to be a rare fluke! this has 108,000 on it. so pretty early for something like that. and it does run pretty smooth at any kind of rpm. but I do appreciate ideas!

as for mention of the computer. i have seen people say that the ecm was having an issue sending whatever signal was needed to control the idle air control correctly. eventually they replaced the ecm and it straightened out. which is also a consideration

as for the truck. it may or may not end up for sale. i bought it on a whim. as another project i didnt really need. lol. and have a long line of other stuff needing my attention. but it would be fun to cruise a bagged truck for awhile. its a 20 footer. no rust to speak of thankfully, but a few dinks and scratches. a simple smoothing and outside repaint would have this thing in amazing shape!
NOT REALLY MY BOSS HAS A S10 BLZAER had a reman motor put in and at 20k miles we had to replaced the timing set cause the chain had got loose and jumped time
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