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Discussion Starter · #1 ·
I have an 88 S10 with a 4.3 and it seems when driving the truck each time I let off the gas pedal this slow to get back down to a normal idle speed.
I like carbureted vehicles when you let off the gas pedal it instantly drops down to an idle. With his truck if you rev the engine it goes up and then slowly comes back down but rarely gets to a normal idle.
I went online and found a handful of videos showing how to clean the idle air control valve and then reset the idle.
That sort of worked but then went back to a higher than normal idol which is always had.
I said heck with the cleaning and bought a brand new idle air control valve. It idled high also, so I went through the routine again.
This video shows the routine I've seen in a handful of other videos. The only thing missing from this video is that it was mentioned in other videos, after setting the idle speed screw, to remove the battery cable for 5 minutes and then plug the idle air control plug back in place before starting the vehicle.
Numerous places show this same sequence.
So the new valve was installed and the idle was quite high. It's been mentioned to drive the vehicle around a little bit and the new pintel should adjust itself to normal. I drove it around about a mile and when I came to a stop the idle was around 650 to 700 and I thought all was good. I drove it around town, up over hill down the other side to a stop sign and it came back down to a good idle again. Every time I came to a stop sign the idol went back to normal. Cool I thought this sucker is fixed!
Pulled into a mini mart to get a six pack of beer and go home and when I turn the truck on the idle was back up around 1500 and wouldn't come back down. It's been that way for the last few days and sometimes the idle is even higher than that.
This video mentions checking and adjusting with the throttle position sensor. I haven't done anything with that.

Is that my problem?
If so, do you guys have a procedure for checking and adjusting the throttle position sensor?
 

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There is no adjustment for the TPS. It can bind and cause a high idle. Try a new one.
Messing with factory set base idle screw just helps folks who don't understand the idle system on a TBI. As a dealer tech we were not allowed to touch it. That's why they have the anti tampering cap. You might have a vacuum leak. Put a gauge on it. The base gaskets are known to leak. Or the TB maybe in need of a rebuild. Remote possibility is a faulty ECM. Rare, but can happen.
Do a search on here for high idle. I know I posted the IAC install procedure less than 2 weeks ago.
 

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Discussion Starter · #4 ·
Thanks for the response. There are no vacuum leaks. Throttle body was removed, cleaned and a new gasket installed. New IAC was installed and the problem was worse than when I cleaned the old one.
Intake gaskets have been replaced and new spark plugs installed. During that process I also took the distributor part and cleaned a bunch of carbon out of it. The upper part of the shaft was getting a slightly blue tint to it because of the carbon build up. I cleaned the shaft polish up the bronze bushings in the upper and lower back and put a dab of moly lube on the upper and lower parts of the shaft.
The cap and rotor on good condition. With a vacuum gauge attached, I think it runs around 22 inches and is rock steady without any movement or wiggling.
I may have the timing set at about 2°
All of this seem to start about the time I replaced the base gasket on the throttle body. It had a pretty good size piece missing from it, and by believe that piece had been missing for a long long time.
I have owned the truck for at least 5 years and it has always run the same. The idle is always a little bit high but I just accepted that.
I understand what you're saying about not touching that screw on the throttle body. GM told you not to and I absolutely get that. My dad had a fear of carburetors because a mechanic messed with his and dad always said the truck never ran the same again. Dad did automotive/mechanical work including replacing a rear main rope seal in an old 51 Chevy. I figure if you turn an idle screw one turn one direction and then turn it one turn back to where it started from, then it should run exactly the same. . I rebuilt numerous quadra Jets and modified them with different metering rods and jets. I even bought a bushing kit and replace the throttle shaft bushings in a number of carburetors.
Just letting you know that I don't only a novice at the throttle body so this is something I'm learning about.
It can't be total magic removing that plug to make an adjustment. The factory set it at a certain point and at least in theory it should be possible to send it back at that same point.
From everything I've learned on the internet and read on the internet guys have isolated the IAC from the equation and adjusted the basic idle speed to 650 to 750 RPM and then connected things back up.
I would love to hear what General motors says about this. Not that don't ever touch it play with it thing..... I'd like to know how they determine how many turns out it's supposed to be.
It would be very interesting to know how General motors set these things to begin with. Logic would tell me that they set the base idle with the electronics disengaged and that adjusting that screw should set the idle to 650 or 700 RPM..... and then let the electronics take over from that baseline mechanical setting.
Regardless, the plugs been removed and the screw has been messed with so that's where I'm at. I don't think the screw is more than half or 3/4 of a turn away from its original setting that's a very most.
Last night I checked these position sensor in voltages and it's plug voltages. First time I've done that.
With the throttle in the idle position, the blue wire read .745 volts.
.5 volts to .9 volts is considered ok from different sources that I've seen.
At wide open throttle it only got up to 3.97 volts. I understand it should be 4.5 to 5 volts, this is a little under what it should be.
I checked the voltage on the plug at the gray wire and it was 5.02 volts and it should read five so I think I'm okay with that one.
Connecting the black wire to the positive terminal of the battery I got 12.2 volts so that looks good too.
I think it needs to be replaced regardless.
I'm not familiar with the map sensor and how this might fit into the equation. It looks quite old too and the filter is a relatively stiff piece of foam. Maybe it's supposed to be that way?

Last night I went through the adjustment sequence that I've seen in numerous places on the internet. All of them do exactly the same process there's some people don't include details like when to remove the paper clip from the AB terminal.
My idle has been quite high anywhere from 1200 to maybe 1800.
When I redid that idle procedure last night, I gave the screw one half turn to close the throttle plates a little more then it was before. One more half turn and the engine would have died, so anyway I simply decrease the idle screw one half turn and that brought me to a nice little idle I'm guessing around 650.
At that point I'll turn the engine off removed the battery terminal then remove the paper clip and plug back in the idle air control plug. I didn't reconnect the battery for well over 10 minutes.
I restarted the engine and the truck had a nice low idle and I drove it around for 20 minutes and it kept coming back to that same nice little idle.
This morning when I started the truck to go to work the added a slightly higher maybe around 900 ish. Hydro 6 mi to work and it seemed like it was closer to 1,000 RPM when it came to an idol. After work I drove to the store and then home. The idol was still around 1,000 or maybe 1,100 not way high like it was before. Well the engine was hot that is when I checked the throttle position sensor wires and plug for voltages.
If driving tomorrow, the idle jumps way back up too crazy levels like 15 and 1800 then I will have learned something and we'll have to borrow someone's 50 caliber machine gun. 😁
So I believe the first thing I need to do is replace the TPS and see what happens. I'm not sure when I get around to that but it'll be at least a few days.
If you have any more feedback from the information that I've given you, go ahead and give it. The ride I'm on right now is a total learning experience
If the idle remains near a thousand RPM out of gear it doesn't start going buggy on me again then I could at least live with this but I still want to replace that throttle position sensor.
If something changes, I will have learned something.
If nothing changes, I will have learned something.
If you have any feedback about the map sensor and how this falls into the equation I appreciate that.
I do appreciate your critique. 👍
 

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Better to over explain what you did than leave out important details. Nice to read a post that's been proof read. Hydro??
Sounds like you're on the right track.
You mention the idle being 1K when not in gear. IIRC it's normally set in gear if it's an automatic.
Some of the high idle might simply be that era computer. My 89 4.3, that I bought new, wasn't real bad, but did fluctuate more than I would have liked. Cold starts was the worst and here in the frozen tundra I mean COLD starts. There were times I'd wait close to a minute for it to settle down, not wanting to drop it in reverse with everything still real cold and it idling high. I'm sure if the trans could talk it would've said thank you. Once fully warmed it was pretty good for the most part.
 

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Discussion Starter · #7 ·
Mine goes from high idle to regular idle in 17 to 18 seconds. I can't stand that. Even when the truck is fully warmed up it goes through that same freaking sequence. With a carburetor you bash the pedal and the car revs up and immediately drops right back down. If you do that with these throttle bodies, they rev up, the gradually start slowing down and just when you think it's all the way down it keeps slowly creeping down until it's happy. I've just never gotten used to the way these things behave.
I will have to find a better tachometer from a friend and check my idle speed while it's in gear. Maybe that will make some better notes to compare.
Thank you again, I really appreciate your input.
 

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Discussion Starter · #8 ·
......A funny thing happened on the Way to the forum.....
The engine in this pickup is a 4.3 it's a 700R4 out of a 1988 pick up.
What I've known since I bought the truck that it was a 2.8 with a 5-speed.
A friend told me the dash didn't look right for a 93 and this is the end was changed maybe the dash was changed also. I found out that it wasn't changed. The pickup has a VIN code R which is a 2.8 liter L4.
Something just hit me today. Even though I've had this truck for a number of years now it's the first V6 I've ever owned and I just assumed everything sounded the way it did because it was a V6.
I remember just before Christmas I bought a few cheap toys and one of them was a little RPM meter.
I've always thought the tech was incorrect but never checked it. I have an automotive tune up gauge with the needle bounces all over at lower RPMs so I can't trust either one of them.
Anyhow I got some white out and painted a little white spot on the engine pulley and check the engine having my brother move the throttle to different RPMs on the tachometer.
With the engine held steady at 1500 RPM my little handheld meter showed 1,052 rpm.
At 2000 RPM my meter read 1300.
At 2500 RPM my meter read 1700.
At 3000 RPM my meter read 2100
If you do a little division between a 6 cylinder and 4 cylinder, you come out with .666666
Multiply .666 times the truck's tachometer readings.....
..... And I think I found at least most of my problem. 🙊
The engine still does not always throttle all the way down but it's quite livable. Tonight after driving around for an hour and coming home I rechecked the idle speed and it was 521 in gear and 635 out of gear. Purring like a little kitten.
It's what I checked it this morning it was probably a good 100 RPM higher than that.
I might still change the throttle position sensor because it only goes up 3.89 volts and not 4.5 to 5 as it should. At idle the reading is within tolerances.
I don't suppose there's a way to recalibrate the tachometer in the truck to realize that it doesn't have before cylinder engine anymore?
I might have to buy an aftermarket tach.
I would never have thought that the truck actually thought it still had a four-cylinder engine in it. 🥴
 

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1988 the 2.8 is a 6 cylinder.
4 banger that year was a 2.5L
Tach shouldn't care which V6 it has.
Wonder if you might have a 2.8 chip in the ECM? Since both the 2.8 and 4.3 were TBI the person who swapped in the 4.3 might not of known that he should change the chip. Years ago folks weren't as tech savvy. It contains the tune for the specific engine, but the 2.8 TBI and 4.3 TBI were similar enough that it probably could run with either chip. Just not at it's best if it has the wrong one.
If you pull the little cover off the ECM there will be a broadcast code on the chip. Let us know what it is.
 

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Discussion Starter · #10 ·
Well, if it's a six cylinder that doesn't explain why my tag is off so much.
The VIN number is an R. So I don't know why the RPM is off exactly the same amount as if it was a four-cylinder.
How hard is it to get to the ECM to look for that number? I never ventured to guess where it was located.
 
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