TBI question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

keg · 12-14-2008, 01:57 PM

1987 4WD, 350, auto, 108k. Added some Seafoam to the gas tank, also poured some down the throttle body.

The engine now has a fast idle and when I push on the accelerator a little bit the engine will bog down, blow black smoke out of the exhaust, strong fuel oder, sometimes the engine will die, if I rev it up seems to run fine, this is in neutral. Drives Ok, idles ok when in gear at a stop although it is a high idle, bogs down a litte when starting out.

Sprayed some throttle body cleaner in the TBI, no change. It ran fine prior to the Seafoam treatment other than a rough idle when cold.

Any ideas as to why this would now have a high idle and apparently running very rich at a low idle? Oxygen sensor? Injectors?

willis0107 · 12-14-2008, 02:49 PM

sounds like you may have some worn out injectors. When the seafoam cleaned them the nozzles are now larger. I would also check your timing though to ensure that it is not off. It sounds like that could be a slight issue as well.

James McClure · 12-14-2008, 03:24 PM

Sounds like the idle air control motor may be stuck. Carefully remove the motor (on the side of the TBI with a 4 wire connector) sprat it with carb cleaner and push the pintle in and pull it out, but not all the way out. Remember where the pintle was before you cleaned it. Push the pintle in a little farther, reinstall it and start eng. It will idle fast. Let it warm up and see if idle comes down. You may have to start eng and shut it down a few times brfore pintle finds it's zero. jim

keg · 01-03-2009, 01:34 PM

Removed the IAC, it would not move, replaced it with a new one, same problem. Rebuilt the throttle body, replaced spark plugs and PCV valve, no help. When I look at the injectors using a timing light they seem to be working OK, not really sure what I should see. I can see gas dripping down the inside walls of the TB. Is there any way to test the injectors? I hate to spend a couple of hundred on new ones if they are not the problem.

Prior to putting the Seafoam in the TBI I flushed the radiator and put in new coolant, but it ran OK after I did this.

keg · 01-03-2009, 05:13 PM

Solved the problem, after lots of research I saw that the coolant temperature sensor can cause problems with the idle and fuel mixture. Checked the resistance on it, wouldn't go below 50k, put a new one in and now it runs fine.

**crossy** · 01-03-2009, 06:07 PM

nice- so where did you find the CTS testing info? thanx , crossy

keg · 01-03-2009, 06:40 PM

The info was posted by ChevyTech in a thread about idle problems.

Coolant sensor approximate resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.

**crossy** · 01-03-2009, 07:28 PM

yeah, he's the man. i remember that now because i had the same issue with a truck i had here once.

12-14-2008, 01:57 PM	#1
keg Registered User Join Date: Jun 2008 Location: Snohomish Wa Posts: 32	TBI question 1987 4WD, 350, auto, 108k. Added some Seafoam to the gas tank, also poured some down the throttle body. The engine now has a fast idle and when I push on the accelerator a little bit the engine will bog down, blow black smoke out of the exhaust, strong fuel oder, sometimes the engine will die, if I rev it up seems to run fine, this is in neutral. Drives Ok, idles ok when in gear at a stop although it is a high idle, bogs down a litte when starting out. Sprayed some throttle body cleaner in the TBI, no change. It ran fine prior to the Seafoam treatment other than a rough idle when cold. Any ideas as to why this would now have a high idle and apparently running very rich at a low idle? Oxygen sensor? Injectors?

12-14-2008, 02:49 PM	#2
willis0107 67lwb Join Date: Sep 2007 Location: Centralia Mo Posts: 180	Re: TBI question sounds like you may have some worn out injectors. When the seafoam cleaned them the nozzles are now larger. I would also check your timing though to ensure that it is not off. It sounds like that could be a slight issue as well.

12-14-2008, 03:24 PM	#3
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: TBI question Sounds like the idle air control motor may be stuck. Carefully remove the motor (on the side of the TBI with a 4 wire connector) sprat it with carb cleaner and push the pintle in and pull it out, but not all the way out. Remember where the pintle was before you cleaned it. Push the pintle in a little farther, reinstall it and start eng. It will idle fast. Let it warm up and see if idle comes down. You may have to start eng and shut it down a few times brfore pintle finds it's zero. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

01-03-2009, 01:34 PM	#4
keg Registered User Join Date: Jun 2008 Location: Snohomish Wa Posts: 32	Re: TBI question Removed the IAC, it would not move, replaced it with a new one, same problem. Rebuilt the throttle body, replaced spark plugs and PCV valve, no help. When I look at the injectors using a timing light they seem to be working OK, not really sure what I should see. I can see gas dripping down the inside walls of the TB. Is there any way to test the injectors? I hate to spend a couple of hundred on new ones if they are not the problem. Prior to putting the Seafoam in the TBI I flushed the radiator and put in new coolant, but it ran OK after I did this. Last edited by keg; 01-03-2009 at 01:36 PM. Reason: spelling

01-03-2009, 05:13 PM	#5
keg Registered User Join Date: Jun 2008 Location: Snohomish Wa Posts: 32	Re: TBI question Solved the problem, after lots of research I saw that the coolant temperature sensor can cause problems with the idle and fuel mixture. Checked the resistance on it, wouldn't go below 50k, put a new one in and now it runs fine.

01-03-2009, 06:07 PM	#6
crossy Oh,you can't buy that new Join Date: Apr 2000 Location: Pennsville,N.J. 08070, USA Posts: 2,039	Re: TBI question nice- so where did you find the CTS testing info? thanx , crossy __________________ "been there, done that, ruined the T-shirt". 2007 LBZ GMC Sierra, ECSB 2006 LM7 RCSB Silverado 98 Vortec project. 94,3500,6.5L 4L80 81 SWB GMC POS finally gone 73 Pinto, stock, w/CragarSS's-eww LOTS of Cummins trucks.

01-03-2009, 06:40 PM	#7
keg Registered User Join Date: Jun 2008 Location: Snohomish Wa Posts: 32	Re: TBI question The info was posted by ChevyTech in a thread about idle problems. Coolant sensor approximate resistance specifications: 177 ohms @ 212 deg. F. or 100 deg. C. 241 ohms @ 194 deg. F. or 90 deg. C. 332 ohms @ 176 deg. F. or 80 deg. C. 467 ohms @ 158 deg. F. or 70 deg. C. 667 ohms @ 140 deg. F. or 60 deg. C. 973 ohms @ 122 deg. F. or 50 deg. C. 1188 ohms @ 113 deg. F. or45 deg. C. 1459 ohms @ 104 deg. F. or 40 deg. C. 1802 ohms @ 95 deg. F. or 35 deg. C. 2238 ohms @ 86 deg. F. or 30 deg. C. 2796 ohms @ 77 deg. F. or 25 deg. C. 3520 ohms @ 68 deg. F. or 20 deg. C. 4450 ohms @ 59 deg. F. or 15 deg. C. 5670 ohms @ 50 deg. F. or 10 deg. C. 7280 ohms @ 41 deg. F. or 5 deg. C. 9420 ohms @ 32 deg. F. or 0 deg. C. 12300 ohms @ 23 deg. F. or -5 deg. C. 16180 ohms @ 14 deg. F. or -10 deg. C. 21450 ohms @ 5 deg. F. or -15 deg. C. 28680 ohms @ -4 deg. F. or -20 deg. C. 52700 ohms @ -22 deg. F. or -30 deg. C. 100700 ohms @ -40 deg. F. or - 40 deg. C.

01-03-2009, 07:28 PM	#8
crossy Oh,you can't buy that new Join Date: Apr 2000 Location: Pennsville,N.J. 08070, USA Posts: 2,039	Re: TBI question yeah, he's the man. i remember that now because i had the same issue with a truck i had here once. __________________ "been there, done that, ruined the T-shirt". 2007 LBZ GMC Sierra, ECSB 2006 LM7 RCSB Silverado 98 Vortec project. 94,3500,6.5L 4L80 81 SWB GMC POS finally gone 73 Pinto, stock, w/CragarSS's-eww LOTS of Cummins trucks.



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