Hey guys, can't quite seem to find my rough idle problem. Here is the situation though. Car runs great except when idling at operating temp, when its idling around 600 rpm's. It feels like it has a miss, but it doesn't occur like a dead miss (it's random). I have checked to see if I have fire, and it appears that all cylinders are firing correctly. I pulled the retaining clip and removed injector power on all the cylinders, and doesn't seem to make any difference in idle quality, maybe slightly affecting cylinder 7. Fuel pressure is a little low I think around 38 lbs at idle. So it seems to me that it is air related or fuel related. No compression check has been run, but don't feel like that is the problem.

I was planning on doing some mods at the end of summer, head work, cam, and injectors. Gonna pull the throttle body off, and unsure if I would be wasting my money on buying a 52 or 58 mm throttle body? I don't plan on doing anything too wild, something comparable to a hotcam or slightly bigger lift. Would I be just as well off cleaning the stocker and putting it back on to verify that that isn't my problem? Would I be getting a code if the IAC valve where bad? Also, any tips on the problem would be helpful. It has been recommended in another post I put new O2 sensors on it. That a good place to start? Also, fyi, no codes or any other running problems that I know of except at idle described.

P.S. I have the capabilities to datalog, but I am unsure of the values that I should be seeing or even posting for that matter. Any help would be great, and thanks in advance.

 

Did you measure fuel pressure with the vacuum compensation line connected (38psi would be normal, or slightly higher than normal), of with the vacuum line disconnected (41-47psi, with 43.5psi preferred)?

There are no diagnostics that check the condition of the IAC valve.

Here is info on what you are looking for in a data log:

http://www.injuneer.com/ScanMast.html

I can review data logs if you post them to an FTP site. But it might take a while, because I've got quite a few waiting already.

I'd say a 52mm TB would probably be more than adequate for what you have planned.

 

Sorry, should have clarified. It was still connected.......that leads me to an FPR problem then right? I put a new one on it about 3 month ago, Delphi brand if I recall correctly. Had any problems with these?

I read your article, thumbs up by the way. Very helpful. I do recall, at idle, MAF was reading around 8 gps. This is within spec of what you said on your site. So I'm assuming the steps I need to take are to clean the passages on the IAC, and if that doesn't help then it's most likely fuel related. Am I correct saying that?

Also, any particular brand throttle body you would recommend? I can buy a summit brand fairly cheap in comparison to other products, but I assume it's like everything else. You get what you pay for.

P.S. I feel like there is enough data on your site that I can figure out what my problem is. But, I will PM you if I need you to look at it, or if something seems strange.

 

Not sure why you are focusing on the idle air control valve. If its holding the programmed RPM, it's working.

Delphi is the old GM AC Delco brand, which was spun off many years ago to raise cash, but has recently been reaquired, at least in part, by GM. In effect, its factory.

The cheap TB's are ususally rip-off copies of a better brand that have been made in China. The quality is often questionable, with regard to the casting. It's probably more of a political issue than a tech issue. I prefer "made in the USA" parts, but they are getting harder to find.

 

I'm not sure either honestly......ha......I just feel like I'm back to square one now is all, and am just trying to find anything that will be my problem....I plan on hooking the computer up to it tomorrow, maybe I can find it then.