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Username Post: 1988 4.3 with high idle        (Topic#366414)
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-15-21 02:19 PM - Post#2827151    

The idle is not always high but most of the time. Sometimes 1200 to 1400 sometimes higher than that.
I removed the IAC and carefully clean the tip and inside the throttle body. Still had the same problem.
I went online and found numerous videos showing how to reset the initial idle. I did that and it appeared to temporarily work and by temporarily I mean 5 minutes of driving and it was back to normal. The videos I mentioned show unplugging the IAC plug and adjusting the screw on the throttle body to 650 to 750 RPM, then disconnecting the battery for 5 minutes and then plugging everything back in.
Anyway that's what I did and it worked for a few minutes.
Okay this idle air control valve might have 200,000 miles plus on it so I just replaced it and it also idled high.
I went through the aforementioned process a second time and immediately upon restarting the idol was quite high. Rereading a few things that I had seen there was mention that it might need to be driven a little bit for the computer to relearn. So I drove for a few miles in town, and I'm coming to a stop sign the idle came down to a beautifully normal RPM. Hey drove it up over a hill in town and came to a stop sign and again a nice perfect idle. After a half dozen or so stops and starts it kept coming back down to a good idle so I thought I had the problem cured.
I stopped at a mini Mart grabbed a six pack of beer and when I restarted the engine the high idle was back again and it hasn't gone back to normal yet.
My question now is do I need to address the throttle position sensor. I read about it but I've never messed with one before.
Should I check that next or do you think there's another problem?
I just replaced the intake gaskets and noticed a few bad vacuum lines so there are no vacuum leaks.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 


Shepherd 
Frequent Contributor
Posts: 2331

Loc: Lake George, NY
Reg: 11-11-15
09-15-21 05:01 PM - Post#2827161    
    In response to ranman

Anytime you mess with the base idle setting you are changing the tps setting. There is a procedure for both these adjustments. Gotta get these set properly before going any further. Plenty of info on the how to on the net.



 
CowboyTrukr 
"10th Year" Silver Supporting Member
Posts: 4697
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
09-15-21 07:58 PM - Post#2827169    
    In response to Shepherd

Do you know if your TPS is good? I bought some off brand one once as a replacement for a failing TPS that actually threw a trouble code.

The replacement left me with my idle stuck at 2K. Replaced it with a Delco and it went back to normal, 650.

Greg

‘01 Silverado 2500HD 8.1/Allison 1000 Xcab/LB
'00 Explorer XLT 4.0 SOHC V6 Auto
'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500 SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-15-21 09:58 PM - Post#2827170    
    In response to Shepherd

This is new territory to me. Give me a quadrajet. Let me rebush the throttle shaft and rebuild it. This computer crap messes with my mind. ????
The truck is a 93 and someone put an 88 engine and transmission in it. There were silicone gaskets on The rocker covers and a one-piece gasket on the oil pan. Did they do that in 1988?
Oil pressure is 42 lb at 60 MPH and 55 lb at cold idle. I'm thinking during the engine and transmission swap they may have installed a new oil pump.
When I took the idle air control valve off I swear it was original. I'm talking 265,000 miles original.
Anyway from where I'm at now after replacing the IAC valve the only thing I can think of is the throttle position sensor. I haven't taken a close look at it or removed it so I don't know how old it might be or who made it. Heck could that be original too, GM was pretty good at longevity.
Just you mentioning that anytime that idle adjustments are messed with the throttle positioning sensor needs to be messed with...... makes perfect sense. I'm learning.
I don't think I'm going to be able to mess with this until after the weekend because the weather is going to turn crappy and between now and then I'm going to be working. I'll report back after I have a chance to check that out.
Good Old Scotty has some good things to say.

https://youtu.be/cPkk6md3Sy0

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


Edited by ranman on 09-15-21 11:33 PM. Reason for edit: No reason given.

 
Mercedes 
Contributor
Posts: 359

Reg: 07-09-18
09-16-21 04:31 AM - Post#2827181    
    In response to ranman

Those old GM fuel injection systems are very simple and reliable. A high idle can also be brought on by vacuum leaks which is highly likely given the age of you vehicle and components.

My advice is to buy a rebuild kit for the TBI and check all of the vacuum hoses. Also, make sure that the PCV is working and not leaking outside air.



 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-16-21 06:44 AM - Post#2827183    
    In response to Mercedes

Thanks Mercedes. I just replaced the intake gaskets. A week and a half ago. During the process, I replace some old cracked hoses. I've sprayed around the throttle body base with carb cleaner and the same with the intake manifold.
I followed the procedure to reset the base idle and I was able to get the idle down low. The idle remained normal for less than half an hour, but I did get it to go where it was supposed to be. Then it decided to go back to high idle again. A couple of months ago, I found out that the base gasket was leaking on a throttle body. It was brittle and there was pieces missing. I took the throttle body off and gave it a thorough cleaning.
The PVC valve was replaced at the same time the manifold gasket was replaced.




~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-16-21 06:49 AM - Post#2827184    
    In response to ranman

Oh and greg, you asked about the IAC. It was made by Standard.
I don't think I'm going to know anything until I'm able to check the throttle position sensor. If that checks out fine I'll have to keep looking. Since I did mess with the screw, that might be part of the problem and I'll have to adjust the sensor..... Or possibly replace the sensor if there's a possibility that that can go bad.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
Mercedes 
Contributor
Posts: 359

Reg: 07-09-18
09-16-21 08:29 AM - Post#2827193    
    In response to ranman

I have an older Snap On MT2500 that would connect to your vehicle and read all of the data streams. There are quite a bit of data parameters on an 88 model GM. I'd be curious to see if the computer sees the RPMs that high and it it sees the TPS within range for idle.



 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-16-21 10:45 AM - Post#2827198    
    In response to Mercedes

You mean that the computer thinks that the idle is okay?
Only thing I can do is do the paper clip trick and read the blinks. It hasn't thrown any codes.
I don't believe the tachometer on the vehicle reads correctly and I have an old tune-up gauge but even on the low scale it doesn't give me information I really trust. I just drove into town stopped at a store and came back out and turned it on. The fast idle jumped up to 2700 during the fast idle. And then drop down to 1800. On a flat surface I suppose I could be traveling at 15 miles per hour without my foot on the gas pedal.
Yesterday even after being warmed up the idle became closer to normal for a short period of time.
This morning when I initially started the truck the idol was pretty high so I took off the air cleaner and unplug the throttle position sensor. The engine RPM dropped and immediately rebounded to where it was before. If I plugged it back in slowly it would also drop for a fraction of a second and jump right back up.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
65_Impala 
Very Senior Member
Posts: 4818

Reg: 12-29-02
09-16-21 10:56 AM - Post#2827199    
    In response to ranman

It sounds like the IAC is sticking or just not functional.

An easy way to check the IAC motor is to blip the throttle a relatively slow on and snap off. The rpm should kind of hang higher for a second before returning to the previous lower idle rpm. As you open the throttle the IAC opens with it so you can also hold open the throttle with the breather off and you should hear the IAC passage suck more air as you do so.



 
CowboyTrukr 
"10th Year" Silver Supporting Member
Posts: 4697
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
09-16-21 12:46 PM - Post#2827202    
    In response to ranman

Actually I asked about your TPS.

Greg

‘01 Silverado 2500HD 8.1/Allison 1000 Xcab/LB
'00 Explorer XLT 4.0 SOHC V6 Auto
'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500 SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
Mercedes 
Contributor
Posts: 359

Reg: 07-09-18
09-16-21 01:42 PM - Post#2827207    
    In response to ranman

Pulling codes is fine, if any are there. However, I think that you will need to look at the data stream to get more clues. There is a ton of data in those older GM systems and I can almost be assured that the answer is right there.

PM me and we can discuss further.



 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-16-21 06:13 PM - Post#2827229    
    In response to CowboyTrukr

Sorry about that Greg, I've got too many abbreviations stuck in my head.
Haven't even had a chance to test the throttle position sensor. It might be original to the truck, who knows.
I just looked as close as I could and at the very bottom it has CTS or GTS stamped on the lower most part of the sensor.
Near the top there are some numbers. It almost looks like C1198 that might mean March 11th 1998?
I don't know if the original one said GM or Delco on them but it doesn't have that visible on the face of it.
I'm resetting the idle screw again, according to this.
I turned it one half turn to lower the idle speed during this process. It will still idle when I put it in gear.
https://itstillruns.com/reset-baseline-idle-g m-tbi...
The guy in the article didn't mention leaving the battery disconnected for 5 minutes. I've had a disconnected for about 15 while I'm looking things over so I guess time to plug things back in and and see what happens.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-16-21 06:40 PM - Post#2827233    
    In response to ranman

I just checked the blue wire on a throttle position sensor with two different meters one said .751 and the other one said .74
Houston we have a problem.
... But I don't know if I have a torx nut that correct size at the moment and it's starting to get dark.
The meters top out at
.381 and .389
I haven't checked the yellow wire yet but I don't know if it's worth the bother because I think reading on the blue wire is supposed to go up to 5 volts and it doesn't.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-16-21 07:04 PM - Post#2827236    
    In response to ranman

I said the meter tops out at 381 that's with the throttle wide open and it should be at 5 volts I think.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
Mercedes 
Contributor
Posts: 359

Reg: 07-09-18
09-17-21 02:48 AM - Post#2827245    
    In response to ranman

I believe the way that that system works is that with the throttle blades closed, you get what is called "minimum airflow". That's a factory setting so if it has been messed with, you have a new issue to resolve.

The computer knows the position of the throttle by the TPS. It gives a voltage range based upon where it is in the range. The distributor signal feeds the RPM. But, if the TPS is out of range, I believe that the engine computer will rely upon the TPS to regulate the engine speed. I'm going to guess that your TPS is reading an idle position even though the engine speed is high.

The scan tool is the only way I know to diagnose this properly.



 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-17-21 08:11 AM - Post#2827258    
    In response to Mercedes

The scan tool may be the ideal way to do it, but I think I'm just going to have to go seat the pants. I discovered a few months ago that I had a major gasket leak because the carburetor base gasket had become brittle and started breaking apart. Even with a big piece of the gasket missing the engine seemed to run fine. Even at that point it had a higher than normal idle. I guess the computer and sensors did their beat to compensate. I never sensed a problem because of that vacuum leak.i discovered it by accident.

Here's what happened last night.
I checked the voltage of the throttle position sensor, then I put a paper clip across A&B in the scan port or whatever that's called.
I turned ignition to the arm position for about a minute just to be sure then disconnected the IAC plug and start of the engine.
The idle was about normal. I turned it half turn clothes to reduce the idle a little bit more.
I turn the engine off, then disconnected the battery cable and the paper clip, then left it that way for 10 minutes.
Then I plugged the IAC back in, reconnected the battery cable and start the engine up.
The idle was nice and low and I took it for a 10 minute drive. I got back home and it still came down to a low idle and it seems everything has been corrected.
...but it did the same thing after I put in the new IAC. The idle was fine for a very short period and then became high again and has stayed that way.
I guess I'll have to wait and see what happens this time after driving it to work and back home today.
The idle screw is very close to where it was before I started messing with it. It's not like it's a turn in or out from what originally was from the factory.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-17-21 06:34 PM - Post#2827289    
    In response to ranman

Okay here's my last bit of info until I replace the product position sensor.
With engine hot the blue wire had .745 volts in the idle position and 3.97 volts at wide open throttle.
Doesn't appear to be too far out of whack, but I think I just need to replace it and see what changes, if any happen.
Connecting to the gray wire sensor unplugged I got 5.02 volts so that looks normal.
Connecting the black wire to the positive battery terminal I get 12.2 volts so that looks good too.

After backing out the throttle screw 1/2 turn last night via the procedure as in one of the videos I posted yesterday, I got the idle down to what I would guess is around 650. After driving around for 15 or 20 minutes and coming back home it stayed at about that level.
Today when I started the engine the idle was back up a little bit but not nearly as high as what it was before. I'm guessing 1000 RPM. That's probably down a good 300 RPM or more from a high idle but I used to have. At times it seem to idle would stick around 14 to 1600. If I got up to 27 miles per hour on a level surface and took my foot off the gas pedal that would continue at that speed.
After work tonight,and driving around and warmed, up I would guess the idle might be around 1000 now. I have to guess because I don't trust the stock guage, and the tune-up guage that I have has two scales on it, a high and low. It jumps around more than I'd prefer.
Anyhow, the idle is something that's livable right now. Way below the super high idle speeds I was getting yesterday and the day before.
I'll report back after I replace the throttle position sensor.


~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-19-21 06:56 PM - Post#2827451    
    In response to ranman

A Funny Thing Happened On The Way To The Forum.......
My truck originally had a 2.8 L4 engine with a 5-speed manual. A 4.3 V6 and a 700R4 transmission was installed at some point in time.
The 4.3 is a 1988 engine.
The truck itself is a 1993.
Anyhow I forgot that I had purchased a handheld tachometer just before Christmas last year. It was cheap and seemed like a fun toy to have.
Ever since I've had this truck I thought that the tachometer was inaccurate.
My automotive tune-up gauge has a needle with jumps around a lot and the two gauges don't agree with each other.
Anyhow, remembering that I have this digital tachometer I went outside this morning and painted a white stripe on the crankshaft pulley and started the engine up. I had my brother hold the throttle at different RPMs and here are the numbers I came up with.
The larger numbers are the trucks tachometer.
The smaller numbers are the RPMs read off the crankshaft pulley.
1500/1052
2000/1300
2500/1700
3000/2100
I divided 4 by 6 and came up with .666666
If you multiply the truck's tachometer readings on the left by .666666 they pretty much equal the readings I got with my handheld meter reading off the crank pulley.
The truck still thinks it as a four-cylinder engine in it. ????
I don't know why that didn't occur to me at some other point in time.
I've never owned a V6 before and I installed electric fan a long time ago for the Dodge intrepid and so how the heck am I supposed to know what a V6 RPMs sound like?
Anyway it appears that the RPMs sound like .666666%
The idol still doesn't always drop down to an idle, but the idol is totally livable.
The real RPM at one point today what I returned home was 521 injured and 635 out of gear. Most of the time the idle is higher than that and I was surprised it dropped down this low but it proves like a kitten and has no issues either in that low in gear.
Since the idol does not drop down to 635 on a regular basis, it still has some idle problems but it doesn't have the super high idle problems it was having before. Those were sometimes in the 1400 to 170 range.
My throttle position sensor registered around 7 something volts idle and that was within range. Is that wide open throttle it was less than 4 volts and it should be 4.5 to 5 volts. I think I still need to replace the throttle position sensor but I don't know how much of the problem that might be causing.
In driving around town the last 2 days the idle is totally livable, especially so since I know my truck is not a 4 cylinder and the tachometer is not reading correctly for this 4.8 V6.
When I get around to changing the throttle position sensor will give an update.
I don't suppose there's any way to modify the way the tachometer in my pickup works to make it behave correctly with a V6?
All the comments here are very much appreciated.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
CowboyTrukr 
"10th Year" Silver Supporting Member
Posts: 4697
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
09-19-21 08:34 PM - Post#2827457    
    In response to ranman

There was a post on here from back in the day. It has been lost due to the forum management software

I’m sorry that the link is broken. I forget if it came from Chevy355 or Chevytech. Can’t recall.

Greg

‘01 Silverado 2500HD 8.1/Allison 1000 Xcab/LB
'00 Explorer XLT 4.0 SOHC V6 Auto
'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500 SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-19-21 09:42 PM - Post#2827458    
    In response to CowboyTrukr

To change the tach?
Just discovering this today I haven't even researched it. I'm just relieved I'm not doing 3000 RPM at 60 MPH with an overdrive. More like 2100. Me likes knowing that.

~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 
65_Impala 
Very Senior Member
Posts: 4818

Reg: 12-29-02
09-20-21 05:55 AM - Post#2827466    
    In response to ranman

Now you have to question if the PCM was updated for the V6 of if the PCM thinks the rpm is higher too. That's when live scanner is so handy.



 
CowboyTrukr 
"10th Year" Silver Supporting Member
Posts: 4697
CowboyTrukr
Loc: Salt Lake City
Reg: 06-20-09
09-20-21 06:22 AM - Post#2827468    
    In response to 65_Impala

Very good point, 65.

My ‘95 tach was reading 1/3 higher than it should have after I put the 5.7 in. I did update the PCM to the proper one for the V8. Here’s what it looked like with the factory one and an aftermarket one set for the V8.



Attachment: 887C436D-8C04-42D1-8B95-B5278CAE20BB.jpeg (980.89 KB) 2 View(s)




Greg

‘01 Silverado 2500HD 8.1/Allison 1000 Xcab/LB
'00 Explorer XLT 4.0 SOHC V6 Auto
'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500 SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke


 
ranman 
"2nd Year" Silver Supporting Member
Posts: 10863
ranman
Loc: The cold wet and windy O...
Reg: 03-10-01
09-20-21 07:19 AM - Post#2827476    
    In response to 65_Impala

It wouldn't surprise me if the guy that put the engine and transmission in this swapped computers also. I'm just trying to use logic here. One would think a person was swapping an engine and transmission, a big concern would be, will this work with the old computer. The engine shows no codes.
I was mistakenly thinking the 2.8 was a four cylinder and that was the reason the tach was off. Since both the 2.8 and 4.3 are both sixes, that throws my theory out the window. It's just odd that the reduction in RPM between the handheld tach and the one in the pickup is exactly the same percentage as the percent change in cylinders from a 6 to a 4 cylinder..... I thought I was on to something here. ????
There is an additional space on the dash to add a small tach if I wanted to. For now the tachometer being inaccurate doesn't affect the driving so it's not really an issue. When I bought this truck I believe I had 220,000 miles on it and now I've got 265,000.
Since discovering that this is the original dash then I don't have any clue what the true mileage of the motor itself is. I'm going to doubt that they rolled back the odometer when they switched engines.


~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~
If plants ruled the world, would they regulate oxygen?


 


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