89 gmc performance
Printed From: Unofficial Allis
Category: Other Topics
Forum Name: Shops, Barns, Varmints, and Trucks
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=151881
Printed Date: 22 Aug 2025 at 7:07am
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Topic: 89 gmc performance
Posted By: bobkyllo
Subject: 89 gmc performance
Date Posted: 07 Jul 2018 at 3:53am
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So customer dropped of his 89 gmc 2 wheel drive pickup with the 350. It run but very poorly. It almost seemed to be flooding it's self. Well I figured it might be the coolant sensor. So I let it sit until I had a little time to go screw with it which was about 2 hours. Well that pickup would barely run now it was bad enough you couldn't put it in gear or it would die. Well I decided to peak under the cap. Woof that looked terrible. So new cap, rotor, and ignition module. Put it together, reached in and hit the key and it fires right up. It runs but not well. It has a miss to it but hey it runs on its own except if you try and give it any gas it will fall on its face again like it's flooding. I run out of time to screw with it. So today I bust out my multi meter and do some poking around. The coolant sensor does test good. Injectors test well. Where shall I go from here. A guy in town whom I trust said the fuel regulator. Is there something I'm over looking here. Now that it runs it runs a bit rough. It will run at idle on its own kind of. It's more of a sloppy idle where it's always hunting for a good idle speed. It revs up a bit then it will start slowing down until the computer kicks in and speeds it back up. If I try to speed up the motor it starts to cough and stumble like it's flooding. |
Replies:
Posted By: jaybmiller
Date Posted: 07 Jul 2018 at 5:51am
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honestly that's what Goggle is for... I'd try '1989 GMC rough running' as keywords,see the 1,000s of hits that come back. two things though 1) replaced all the plugs ? 2) replaced all the fuel filters ? maybe a bad fuel pump ? OK, that's 3.... Jay ------------- 3 D-14s,A-C forklift, B-112 Kubota BX23S lil' TOOT( The Other Orange Tractor) Never burn your bridges, unless you can walk on water |
Posted By: fixer1958
Date Posted: 07 Jul 2018 at 7:06am
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Map sensor, EGR stuck open? |
Posted By: bobkyllo
Date Posted: 07 Jul 2018 at 8:06am
| I should have mentioned that I had the egr off and it moves good. |
Posted By: fixer1958
Date Posted: 07 Jul 2018 at 11:08am
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If the fuel regulator diaphragm is leaking/torn I would think you would have some fuel dribbling out of the injectors after you shut it off. I think fuel pressure for TBI is 7-15 psi. The 'hunting at idle' means something is wacking the PCM and trying to find a happy medium. You could down the throat of the throttle body and see if it sucked in the throttle body gasket. They were bad for that but doesn't generally cause the problem you describe. Check the vacuum to the map sensor, if is leaking or no vacuum the PCM thinks it is at full load. Same with the EGR solenoid if it's bad. About all I got.
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Posted By: SteveM C/IL
Date Posted: 07 Jul 2018 at 3:19pm
| an O2 sensor can't cause that kinda trouble can it? |
Posted By: jaybmiller
Date Posted: 07 Jul 2018 at 3:23pm
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Is the 'engine light' ON? If so, get the scan code.....it'll 'point' NOT telll you where the problem is, maybe,hopefully... ------------- 3 D-14s,A-C forklift, B-112 Kubota BX23S lil' TOOT( The Other Orange Tractor) Never burn your bridges, unless you can walk on water |
Posted By: Hubert (Ga)engine7
Date Posted: 07 Jul 2018 at 5:26pm
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Throttle position sensor? ------------- Just an old country boy saved by the grace of God. |
Posted By: Dakota Dave
Date Posted: 07 Jul 2018 at 6:42pm
| My 88 suburban ran horrible when I bought it. It would toss a o2 code but nothing else. I changed the coolant temp sensor that helped some.follow all the plastic vacuum lines and replace any that are suspect. Tha TBI is abigimprovement over a carb but the OBD1 dosent help when trouble shooting. It's kinda stupid and soft faults reset themselves without keeping a record of it |
Posted By: DMiller
Date Posted: 07 Jul 2018 at 6:51pm
| Take a squirt bottle with water, spray around Throttle body base and intake check for vacuum leaks. Use water not gas and not carb spray, old lesson from twenty years ago plus. |
Posted By: shameless dude
Date Posted: 07 Jul 2018 at 7:15pm
| what you decribed sounded like one of my vehicles years ago....was a parcial flat cam |
Posted By: truckerfarmer
Date Posted: 07 Jul 2018 at 9:19pm
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Have you checked the timing? ------------- Looking at the past to see the future. '53 WD, '53 WD45, WD snap coupler field cultivator, #53 plow,'53 HD5B dozer Duct tape.... Can't fix stupidity. But will muffle the sound of it! |
Posted By: AllisFreak MN
Date Posted: 08 Jul 2018 at 8:05am
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I second HubertGA's suggestion, throttle position sensor. ------------- '49 A-C WD, '51 A-C WD, '63 A-C D17 Series III, 1968 A-C One-Seventy, '82 A-C 6060, '75 A-C 7040, A-C #3 sickle mower, 2 A-C 701 wagons, '78 Gleaner M2 |
Posted By: nella(Pa)
Date Posted: 08 Jul 2018 at 9:15am
| My 77 chevy pickup with a new 350 in it couldn't pull an average hill without down shifting. A mechanic friend told me to take the battery ground cable that went to the alternator bracket and put it on the bolt that holds the bracket on the engine block for a better ground. I couldn't believe it but that solved my problem. That taught me how important it is to keep all electrical connections clean! |
Posted By: bobkyllo
Date Posted: 08 Jul 2018 at 9:42am
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I have not checked the timing for one simple reason. He said this come on all of a sudden. Which to me I would lean to a bad sensor. Maybe I'm thinking wrong about that. Tomorrow I'll build a jumper harness so I can check the throttle position sensor. I did try a different map sensor that I had sitting on the shelf. It was used and I hope I didn't keep an old junk sensor. But any way that didn't help. |
Posted By: Thad in AR.
Date Posted: 08 Jul 2018 at 12:17pm
My first thought as well. They were notorious for that |
shameless dude wrote:Posted By: john(MI)
Date Posted: 08 Jul 2018 at 3:17pm
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Take the Neg cable off the battery for awhile. When you reconnect it that should reset any faults. And like was mentioned,a good ground to the body and the engine. ------------- D14, D17, 5020, 612H, CASE 446 |
Posted By: Steve M Ind
Date Posted: 09 Jul 2018 at 8:18am
| Cracked magnet on the distributor shaft will cause to run bad. Pull the cap back off and check real close with a flashlight for a crack on the magnet usually at a rivet that holds it together. only fix is to replace the shaft as the magnet is made part of it. |
Posted By: Ray54
Date Posted: 09 Jul 2018 at 10:58am
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OH the good old days,flat cams,the early carbon wires that the insulation would crack on or the carbon itself would break. All good fun. Good luck in this case luck can save a lot of time and parts.
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Posted By: bobkyllo
Date Posted: 10 Jul 2018 at 12:42am
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Well guys... I tested the throttle sensor and found that good. Then changed the fuel regulator. I did get it to run on its own but still very very poorly. As I was screwing with it this whistling noise kept bugging me. It sounded like a massive vacuum leak. So I went and got some brake cleaner. I know I know it's against your guys advice. I started to spray around the base of the injection module and the intake. By gosh I found my problem. Got a gasket coming in the morning. I'll let you know how it turns out. |