I finally got my car assembled after about 8 years of partially-assembled ownership and am trying to get it to run correctly. I am having a surging issue in that when I start it, the car idles smoothly until it gets warm. I don't know if the problem begins when the auto-warm-up high-idle mode ends, or if it's when the fan comes on, or if it's triggered by something else... But it always does it and when it begins, it revs between perhaps 1500 and 2500 or so, on about a 1-2 second frequency. (Opening the throttle-body idle bleed screw speeds up the frequency of the surging.)

My engine is a bit of an oddball because I am running a S4 TurboII rotating assembly. In addition, the exhaust ports are mildly ported, I have removed the double-throttle plates and actuator, and I have a run-of-the-mill DP. EGR is disabled (non-Cali '93). Everything else is stock, including the air pump. I thought I could get away with running this conglomeration on the stock ECU because I figured the ECU wouldn't care much if the compression was a bit lower than what it is supposed to be. And the other mods don't seem to constitute a violation of the 3-mod rule. What do you guys think?

-James

 

It's a T2 rotating assembly (8.5:1 rotors?) but FD every thing else?

When you turn the air adjusting screw you need to have the engine in diagnostic/initial set mode. that means jumper "TEN" and "GND" in the diagnostic box. See the fuel and emissions control section of the service manual. It also wants you to verify the ignition timing... I suppose it should be fine given the design of the FD crank angle sensor but I guess you could check it. More importantly though you should check the TPS.

 

Thanks for the reply.

Yup, everything is stock FD stuff except for what I listed. I will check out that section of the manual and the TPS adjustment procedure. I have read on here about many people havnig surging issues due to the TPS.

 

OK, I have checked the voltages on the TPS.

The bottom wire checked as being within spec, throttle closed and open.

The 2nd-to-top wire (green w/ red stripe), read somewhere around 0.5V regardless of what I did with the throttle. Moving the throttle didn't seem to have any effect on the reading.

What does that mean? Bad TPS sensor, I assume?

-James

 

Another data point:

It seems that if the battery is drained fully, it erases some sort of memory in the ECU, and this causes the car to run properly for a much much longer time before it starts to surge. The first time I started it after building the engine, it ran for 10 minutes or so before starting to surge. Ever since then it has started to surge (I now believe) after the fast idle cam comes down. Today when I started the car, the battery had been pretty much totally drained from sitting for a very long time. I had to put the charger on it for a while to get it to start. And again, the car ran for a "long time", 10 minutes or so, before it began to surge. I thought I was having a "Eureka moment", because I started the car with the charger connected (thought I had discovered it was a grounding issue) but I now realize that it took longer to begin surging because it had been drained so far.

The car seems to idle awesome before it starts F-ing up and surging.

 

That wire goes the the 3F ecu connector, closed throttle it should read 0.75 to 1.25 and fuly open should read 4.8 to 5.0. So yes I would say something is wrong.

edit - to rule out its a bad TPS adjust it ever so slightly and see if that number ever changes, if it does not I would say its a bad TPS. I THINK I paid like 120 for mine new. or 180, I forget.

 

OK, I've replaced the TPS with an extra one I had sitting around, and I've checked the voltages on the two pertinent wires at closed and WOT...They are correct. The car doesn't surge any more but it "idles" at 3k. All the adjuster screws on the throttle body are set such that the plates are completely closed, as is the idle bleed screw. Checked the throttle cable...tension is not holding the the plates open. When you first start the car it hovers around 2500 but then swells up to 3k after about a minute or so. Then it just stays there no matter what.

I know it will be suggested there is a vacuum leak but I thought I was really very thorough when I put the engine together to avoid that.

Wondering if it could be the IAC?

 

Ensure your cold start lever is adjusted properly. It sits on the back side of the TB. You'll see as a 3/8 coolant line comes up to it. As the coolant warms, it separates from the throttle lowering the idle. It could be stuck or mis-adjusted.

I don't think a vacuum leak would cause that high of an idle. A small leak would push it to 1500 or so, and a large leak would simply make the car die.

 

Thanks for the suggestion, but the tab on the front side of the top throttle-plate pair is touching the stop screw as it is supposed to, so unless I don't understand something, I don't think the top plates are hanging open.

 

Hmmmm..... IAC is suspect then. Vacuum leak still a possibility. I just don't think it would pull 3k without getting air from somewhere.

Did you try some starter fluid around the engine to check for leaks?
How does the car drive?

 

I can't drive it because it's not tagged and the brakes need to be bled (system was empty because I re-plumbed it.) I'm messing with the IAC now. If I remember right, I can't just remove it with the stock ECU without throwing a code, right? I've got another one to swap out if needed. I will also check to see if it revs up when spraying starting fluid / carb cleaner around... Thanks.

 

OK, I sprayed some brake cleaner (hopefully that's flammable enough) around the LIM and TB and it didn't do anything...Working on the IAC next.

 

WOOoooohoooo.

OK, I replaced the IAC with my spare one and that was the final solution.

To re-cap: At the beginning of the thread, I had a bad TPS and bad IAC. Both of those now are replaced and the car is more-or-less doing what it's supposed to do. The IAC valve was stuck open (I was so happy to see that it was bad when I removed it!) I still have a little bit of a high idle (maybe 900 or so) but I'll take it. It also runs just slightly lumpy at idle, or I should say it has an intermittent miss or stumble, but I'll attribute that to the fact that the gas in it is 1 and a half years old. Hopefully good gas will make everything hunky-dory.

Thanks.