1991 TBI Idle Loping - S-10 Forum
 
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post #1 of 15 Old 10-13-2015, 11:47 PM Thread Starter
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1991 TBI Idle Loping

Idle is going up and down really bad. Drops down to about 400-500 comes back up to where it should be 800-900. If I step on the gas it doesn't help at all.

I drive the truck about 200 miles a week. Every now and then this would happen while I'm cruising but never got this bad. Injectors appear to be spraying good. She's not throwing any codes. I've replaced the fuel filter twice hoping to get lucky lol.

I went ahead and tried pulling the IAC, PCV, and EGR valves, cleaned them, stuck them back in. Nothing

This is what I'm dealing with: https://www.youtube.com/watch?v=FRoTQ6OAsxg

I'd really appreciate the help guys thanks.

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post #2 of 15 Old 10-14-2015, 01:24 AM
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Re: 1991 TBI Idle Loping

Of the three parts you've pulled, PVC and EGR wouldn't impact idle like you describe. My guess would be the IAC.

I've had other vehicles, mainly Acuras, do the same thing...what I call porpoising...idle going up and down. On these vehicles, the problem disappeared once I replaced/cleaned the IAC.
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post #3 of 15 Old 10-14-2015, 08:09 AM Thread Starter
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Re: 1991 TBI Idle Loping

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Originally Posted by 57 Handyman View Post
Of the three parts you've pulled, PVC and EGR wouldn't impact idle like you describe. My guess would be the IAC.

I've had other vehicles, mainly Acuras, do the same thing...what I call porpoising...idle going up and down. On these vehicles, the problem disappeared once I replaced/cleaned the IAC.
Alright thanks for your advice.

Do they usually just go out like that?

Why isn't the CPU throwing a code?
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post #4 of 15 Old 10-14-2015, 05:28 PM Thread Starter
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Re: 1991 TBI Idle Loping

I replaced the IAC Valve with a new one. First time started right up and rpms shot up to 2000 rpm. I turned off, tried again, same thing. I did some quick research and tried unplugging the battery to reset the ecm. Plugged it back in, started her up, Went back to porpoising up and down.

I removed the new one, put the old one back on. I start it up rpm shoots up to 1400 and stays there for about 20 seconds then goes back to porpoising up and down. Any thoughts?
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post #5 of 15 Old 10-15-2015, 04:10 PM Thread Starter
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Re: 1991 TBI Idle Loping

I tried new TPS sensor. Didn't do anything.

Cmon guys this truck runs great. Help me get past this stupid hiccup.

Anything else I could try? Fuel pump relay?
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post #6 of 15 Old 10-15-2015, 04:26 PM
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Re: 1991 TBI Idle Loping

Did you clean the seat in the throttle body?Also,check your plug connector and wiring.
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post #7 of 15 Old 10-15-2015, 04:32 PM Thread Starter
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Re: 1991 TBI Idle Loping

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Did you clean the seat in the throttle body?Also,check your plug connector and wiring.
What part of the throttle body is the seat?

All plug connectors and wiring seems to be in good shape.
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post #8 of 15 Old 10-15-2015, 04:42 PM
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Re: 1991 TBI Idle Loping

The seat is where the tapered end on the plunger shaft seals in the throttle body.Spray carb cleaner on a small rag on the end of a screwdriver and use it to clean the seat.After reinstalling the IAC valve,wiggle and tug on the wires at the plug and see if anything changes.
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post #9 of 15 Old 10-15-2015, 04:56 PM Thread Starter
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Re: 1991 TBI Idle Loping

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Originally Posted by jrd1962 View Post
The seat is where the tapered end on the plunger shaft seals in the throttle body.Spray carb cleaner on a small rag on the end of a screwdriver and use it to clean the seat.After reinstalling the IAC valve,wiggle and tug on the wires at the plug and see if anything changes.
What's a plunger shaft? Its on the IAC?
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post #10 of 15 Old 10-15-2015, 10:55 PM Thread Starter
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Re: 1991 TBI Idle Loping

Tried new fuel regulator. Still idling up and down. HELP
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post #11 of 15 Old 11-20-2015, 03:11 PM
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Re: 1991 TBI Idle Loping

Boulky12,

Have you solved your problem? If yes, what did you do? If not, then here's a different suggestion.

These TBI vehicles have a bunch of sensors which feed the ECM and could result in your situation. You've fiddled with two (IAC and TPS) which many of us would first suspect to be the culprit. At some point, you need to abandon remove and replace approach because it gets expensive.

My next suggestion is rebuild your TBI. I've done it and, for someone good with taking apart and putting things back together, it really isn't that difficult. I say rebuild rather than get a reman because, like carbs, they aren't exactly the same and, many times, are NOT interchangeable. A rebuild kit from NAPA will cost around $50 or so.

So, get a couple cans of carb cleaner, your tools, a couple of brushes, and a pan and have at it. It shouldn't take more than an hour. Be careful with the both fuel lines when reinstalling...you don't want to strip the threads.

Hopefully, rebuilding your TBI will solve the problem. If not, we'll discuss some other things to do.
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post #12 of 15 Old 11-22-2015, 07:23 PM
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Re: 1991 TBI Idle Loping

Is the timing set right?

I've had this problem, and for the life of me at the moment, I can't remember what the fix was.

Here are a couple of links that may help:

http://api.viglink.com/api/click?for...TBISystems.pdf

and

http://api.viglink.com/api/click?for...el_Control.pdf

1985 GMC S-15 Gypsy ext cab.
3.4l, Edelbrock intake, a 4.3l throttle body controlled by an '88 TBI ECM. 700R4 with 3.73 rear. Real time tuning using a CF-30 Toughbook with a Moates Ostrich 2 emulator.
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post #13 of 15 Old 11-23-2015, 11:48 AM
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Wow, so many wasteful siggestions.

When's the last time you did a full tune up? What's the fuel pressure at while running at idle and under load (not just a free rev)?
What's your base timing set at?

Most of the time problems like this can be solved with a simple tune up and check/reset of systems.
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post #14 of 15 Old 11-23-2015, 09:50 PM
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Re: 1991 TBI Idle Loping

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Originally Posted by The_Raven View Post
Wow, so many wasteful siggestions.

When's the last time you did a full tune up? What's the fuel pressure at while running at idle and under load (not just a free rev)?
What's your base timing set at?

Most of the time problems like this can be solved with a simple tune up and check/reset of systems.
Yeah, I know what you mean.

Nice thing is though that with either TunerPro or WinAlDL, I can plug the laptop in and see what everything is doing at once.

1985 GMC S-15 Gypsy ext cab.
3.4l, Edelbrock intake, a 4.3l throttle body controlled by an '88 TBI ECM. 700R4 with 3.73 rear. Real time tuning using a CF-30 Toughbook with a Moates Ostrich 2 emulator.
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post #15 of 15 Old 11-24-2015, 01:10 AM
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Re: 1991 TBI Idle Loping

When i had my S10 i had the same issue as you until i changed the temp sensor that goes to the ECM. Also the O2 could cause such issue as well.

As suggested above either do a full tune up or do some diagnosing.
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