[Gery B]

1998 "W" S-10 4.3L 5spd RWD
144k Miles

I'm out of ideas with that TB, read a lot about them but the topic never had the solution at the end...
Idling fine when the truck is not moving. (has a slight misfire only at idle and some backfire at back shifting but had it before the problem started) One day I decided to clean the TB (with ign on, depressed gas pedal) then the other day, the problem started. So when the engine is warmed up and the truck is moving, the idle goes up to 2500-3000 or even more just randomly. With cold engine it acts good 70% of the time. Idle goes back to normal if I stop the truck. I tried everything and it just getting worse every day. TPS shows good percentages no drops on graph, new IAC valve (reset to closed position), no vacuum leaks, fuel pressure 60 with eng off, 50-55 running (didn't do a drop test). Also revs up too high at start up, no matter what.
I'd be thankful for any kind of help, ideas, or suggestions.
My guess would be the fuel injector but this time I have to make sure.
Thank you!

 

[bent wrench]

I'd guess there is a problem with the wiring for the IAC, the IAC, or the computer.

Or it's getting air from some other place, blown PVC valve, bad brake booster, bad manifold gaskets, etc.

 

[Gery B]

Thanks, I will look at these. I think the only thing I didn’t check is the pcv system. Iac should be fine because i was monitoring it but I’ll take an other look.

 

[Jayhawke]

using a can of carb cleaner- spray slowly around base of TB and along vac line. when the idle changes- you found your leak.

 

[Gery B]

tried, I have no leaks. Manifold pressure good and steady.

 

[Joe Cool]

So the idle stays elevated until stopped? When my TPS went out, it would idle high randomly even when stopped, so I would agree it sounds like that part is okay. Honestly, if you have no leaks and the computer is causing the RPM to drop when stopped, that sounds like a bad sensor versus a mechanical issue (leak). Even so, what condition are the vacuum lines?

 

[Gery B]

Just found the problem this morning. I reset my iac again before starting it up and everything was normal. The iac valve doesn’t return to closed position and I just don’t know why... it is a new, free moving valve. Does that mean that I have a bad computer or is it a fixable problem?! I’d check on it if someone could help me with the parameters that I should read...

 

[Joe Cool]

When you say free moving valve, do you mean you can press or pull the pintle and it moves? I don't know if the pintle is supposed to move freely, but my OEM IAC doesn't move when pressed. It could be possible your new part may be bad. As far as what the readings should be, sorry I can't help with those but I think they may have been posted on the forum in the past.

 

[Gery B]

I’m not pulling pushing it, I was playing with the ignition and the connector. Definitely they posted it before but I’m sure I can’t find it been looking/reading for days...

 

[Joe Cool]

Got it. I'm thinking they've been posted, too, but after doing some searching I didn't see any numbers either. I see where others have had similar issues, both with manual and automatic transmissions, but they don't seem to post the solution. Sorry I can't help more.

 

[Gery B]

yeah I know... I have found so many posts with the same problem but no solution. I read all of them.

just a little update...
as soon as the ECT reaches 135-140˚ this high idle kicks in. Below that temperature I'd say it acts acceptable.
With A/C even worse...
One more thing, since I replaced the water pump the temp won't go higher then 165˚ idk if it can play a role in this... I guess I have an stuck-open thermostat.

 

[Gery B]

A little update, just like every day today I’ve found these

If someone could take a look at the grounding wire where it goes, I’d really appreciate it. The big connector looks like never been plugged in and the third one... I see on other cars every day...
If someone could take a look at the grounding wire where it goes, I’d really appreciate it. The big connector looks like never been plugged in and the third one... I see on other cars every day...

 

[Rhotpursuit]

In the first pic, I'd bet it screws to the hood. Look for the tell tale star marks on the hood. Not sure about the second pic...not a good view of the last connector.

 

[Gery B]

I have to make sure but couldn’t find anything... so where the engine is grounded If this one goes to the hood?

 

[Rhotpursuit]

Negative cable from the battery goes to the front of the block, there are grounds on the back of both heads, on each side of the core support...engine side, and another on the inside rear of the frame towards the back. I'm sure there are more...

 

[Gery B]

Thank you

 

[Jayhawke]

i think Rhot is correct- ground for hood - little connector look to be a light connector- underhood light maybe. not sure on the larger one. need larger area pix for reference to see what options you have/have not.

 

[Gery B]

Some closer pics
Coming out of the harness that is right on the top of the brake booster. 9 pin

 

[Gery B]

Idk what is going on but if I start the car with ac and leave it on, it acts good, as soon as I turn it off, does the funky thing.

 

[Gery B]

Just ordered a new computer... lets see...