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General 1g 2.0 NT - Idle surge when coasting with clutch in or in neutral

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knochgoon24

DSM Wiseman
6,135
92
Jan 29, 2008
Troy, Michigan
Hi all.

I have a 91 2.0 NT MT. I'm driving myself nuts trying to fix this idle issue. It happens both while the car is rolling in gear with the clutch pressed in, and when rolling with the clutch out with the car in neutral. It has an idle surge from 1200-1500 rpm. Worse when cold, and less so but still noticeable when up to temp.

Also, the car needs the BISS to be screwed in all the way to hold a proper idle. Idle comes up high when first started, might surge once or twice, the settles into a nice smooth idle.

I've tried--
1. Adjusting the idle stop switch and TPS to spec.
2. Rebuilding thr throttle body with new seals.
3. New (used) ECU.
4. Multiple ISCs that all ohm out just fine (4 of them).
5. New injector seals
6. New PCV valve
7. Blocked the PCV valve.
8. 2g TPS, just swapped and set like a 1g TPS.

(At this point, the timing belt snapped. I installed a new (used) head, new TB gaskets, and checked the IM for visible cracks.)

9. Timing checked and set to spec.
10. 2g TPS with a pin added and jumped to replace the 1g idle switch.
11. Different throttle cable tensions (super slack to nearly pulling it open tight).

At this point, the only thing left to try is a new (used) FIAV, or a FIAV blockoff.

Car is almost bone stock. Cheap eBay header.No AC. PS disconnected (leaks).

Do these sound like symptoms of a bad FIAV?
 
Also forgot to mention that it isn't throwing any codes. It runs great except for the idle surge while rolling. Immediately stops when the vehicle speed drops to zero. Starts back up as soon as I let the car start rolling (ex. downhill roll).
 
Block off the fiav (turn it all the way in)
If it's the FIAV, that'll fix it. Buttt...most fiav symptoms are surging while warming up, and at operating temp the car idles without issue.

Obviously.... since you said the BISS is bottomed out, check really well for boost leaks. Reset the isc, biss, and base timing to bring the system back into adjustment, sometimes the isc just won't play well if any of the other 2 are off.

The isc can also pass its resistivity test, and be malfunctioning still. In my experience, the original brown iscs tend to fit that bill. They're crap, and if you have the resources, nab a black cased revised isc.
 
I could block the FIAV, but I also still have a spare that I could try swapping on to the car. It's probably going to have to happen at some point here. I feel like blocking the FIAV is a band-aid fix. The car is basically stock otherwise. I shouldn't have to block the FIAV to get the idle to work right.

With it being an NT, I feel like I've checked everywhere for vacuum leaks. I've even replaced all the emissions hoses with new stuff. Forgot to mention that. I've also tried capping all the emissions hoses at the TB. No change there either. And it acts the same now as it did before the head swap. The only hose I haven't checked is the brake booster.

I finally bit the bullet and checked eBay for a Palm and cable. Should be here tomorrow.

I just find it really weird that it settles down to a rock solid idle as soon as the car comes to a stop. I feel like that's a big clue. I thought that I got the base timing set correctly (used a timing light), but maybe it's still too far advanced. I could always try adjusting the CAS just a bit more retarded to see if that helps. Maybe my eyeballs were out of calibration that day?
 
Sometimes....I play with the base timing just to get a good idle, without the proper light. It's not ideal..... but it usually fixes the issue. Might as well try it and see how it goes!
 
So I'm still messing with this problem. I got the Palm logger, and it confirms that the ECU is controlling the ISC. I'm able to turn the BISS and watch the ISC count change. I've also tried adjusting the TPS based on these readings, and that hasn't helped either.

I messed with the timing a bit but eventually set it back to base timing.

Swapped another FIAV on... still had the same problem. No noticeable improvement. Swapped ECUs (have 3 total now) and that didn't help. Swapped the whole TB as well, just in case. No change.

I was hesitant to completely block the FIAV. I did the "free FIAV block" method, where I removed the freeze plug and screwed down the valve. I closed it down to less than half the original opening. It helped significantly, but it still surges some while coasting with the clutch in when cold. I'll eventually get around to removing the TB to block the FIAV completely, but I haven't done that yet. I think that might solve it completely, but it seems like a band-aid fix.

I don't think the car is even trying to control the idle on cold coasting with the clutch pushed in. I can watch the ISC count spike high (open) until I nearly stop. At that point, the ISC count starts to drop and eventually settles around the value of about 30-40 required to keep the car idling smoothly. I can watch engine temps, and the problem goes away after the engine hits 175F.
 
Pull the isc and watch it while cycling the key. There is no feedback of isc position to the ecu, so just because the logger shows it moving doesn't mean it is. If the car idles normally while sitting it's not the faiv. IIRC on the turbo codes the ecu does not control idle speed while coasting. It just sets the iac to a certain position. So if there is to much extra bypass air it will try and idle to high and hit the idle fuel cut.
 
I've had quite a few failed ISC motors that said they had proper ohms between the pins when tested. Bastard is right, if you don't see the ISC range when the key is put to on then your ISC is bad or the drivers on the ecu are bad..... but this sounds more like bad ISC.
 
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