Idle surging after warm

[steveo123456]

My 90 Cab has a habit of surging at idle from 1200 to 200 RPM after it warms up. It comes real close to stalling but never does. It only does it after running for about about 10 minutes then, after a few minutes it returns to a normal idle until I shut it down. This is real annoying. Any ideas what may be causing this? I'm thinking a vacumn leak that goes away after the temp increases.

[BananaClip]

does it idle at 1200 once it stabilizes, or at ~850?
does it stabilize while your AC is on?
I'm having a somewhat similar problem with mine. it surges from ~800-1300 a few minutes after starting, and idles at 1200 when warm. When I turn on the AC or heater, though, it stabilizes and goes right to 850.

sorry i couldn't be of more help i guess

[steveo123456]

The surging is really pronounced. It goes from 200, comes real close to stalling and then shoots back up to 1200. When it stabilizes. it's at around 900. I have not turned on the AC when it is doing it.

[944cabby]

Could be your idle speed valve needs cleaning as it gets gummed up and causes the idle to hunt.My 89 cab was doing it about 2 months back.I couldn't get at it to fully remove it as it's situated under the intake manifold but i managed to remove one of the hoses attatched to it and spray a good amount of carb cleaner through it.Mine was surging and dropping between 750and 1400rpm.Problem solved after using the carb cleaner.

[EA425]

I second 944cabby's idea of the idle control valve being dirty or defective.

If not, another thought is to make sure that the idle switch on the throttle valve is working properly.

[blown 944]

sounds like a vacuum leak, that is pronounced once the cold start enrichment gets leaned down. Of course I m thinking of earlier systems so Ivery well may be off on this but symptoms are the same.

[EA425]

hey blown944, I just noticed your sig has "83 944 with eaton m90"

Not to be OT, but could you post some info about this? I'm intrigued! How does it run? Where did u mount it? What pullies did u use? Is it intercooled? So many questions....

[Kevin0323]

I would like to see that too. You should start a new thread and post some pics.

[gtroth]

Yeah, I'd probably suspect the idle stabilization valve too (it was for me). It's supposed to open a little more when the engine's cold to raise the idle some (to 1000~1100rpm?). You might also check the operation of your TPS. When mine wasin't making contact, my idle bounced all over the place.

[dangerous]

I had a similar prob and replaced the ISV. never had it again. I've heard they can be cleaned, as 944cabby mentions, definately worth a try. man that was annoying!!!

[944cabby]

How much did it cost you to replace the ISV "dangerous"?When i looked into it they're well over £100 here in the uk ($150?).

[rb101]

they are on ebay all the time $20-30

[Lawrence Coppari]

Are these new ones for 20-30 dollars?

[blown 944]

let me get it fully tuned and I will let you know how it is all working out. It is in it's infant stages right now. I have it mounted and had some temporary tubing done. It was a real PIA getting the tubing around back for the intake and it looked messy with the tubing all so scrunched in front not to metion getting it low enough for the hood to cl;ose. I did it all temp amd ran it out easy not boosting over 10psi under part load and it seemed like it was going to run good. Like I said is is just starting to come together.
I decided to pull the intake and FAB some runners and a box to get more room not to mention the ease of having more room for the intake tubing and placement of the TB and AFM. I saw a Huntly set up and am making it similiar to that using the stock intake (cut) and some runner tubing.

I did do some work ahead of time getting the 42# injectors to run (quite well in fact) N/A by lowering the FP to about 25#'s vac and 29 not vac. I am tuning using a computec EGT and and watching the o2 voltage. I got it to run at about 1400 WOT N/A and got smooth idle. The A/F was about .7 (rich) on the top end but feels good. With a RRFPR it should tune in good. With the blowert on the EGT's got up to about 1600 without getting crazy on the throttle so I ran it easy. With the RRFPR it shoud be able to tune to about 1580-1650 at about 15psi I figure.
Right now it is apart getting intake fabbed up.
And no I am not running an I/C I will most likely run meth injection using a SAAB boost switch and separate FP for it.
Thanks for the interest I will put some more info up when I get it to the point where I feel it is worth mentioning. I like this car (got it at an auction) it is a test mule of sorts if all works out I will get a newrer model in a little better condition to keep. But who knows I just got back into messing with cars after a couple of years off.

[EA425]

Sounds really cool! Thanks for all the info on your progress. I have great interest in what you're doing, especially since I have an old M90 that I'd like to install as well, but I've done nothing so far. Sounds like you're taking a nice cautious approach, paying close attention to the EGT, boost, and A/F ratio. Too bad wideband O2 meters are so expensive! as that might be helpful too. Please keep us posted!

[dangerous]

Originally posted by 944cabby
How much did it cost you to replace the ISV "dangerous"?When i looked into it they're well over £100 here in the uk ($150?).

I think I paid somewhere around $60-$70. I would have to double check my records though. It was back in '96-'97 when I lived in Germany. I don't remember if I had ordered from the US or if it was locally purchased.

$150 in the UK? Ouch....that stings.

[Lawrence Coppari]

I replaced ISV recently. Before doing it I took the old one apart. Here is why mine was not working properly.

[rb101]

used on ebay of course