NOT another idling thread

[JustJay]

Hello everyone,

New here. Just posted an intro and here is the problem I am having that I have not found on here. Or anywhere for that matter. 

1993 R33 GTST with no real performance mods done yet. All my reading here has a garage full of parts to install but having a running/idle problem.

Car starts and runs great. Did all the standard stuff, filters, fluids, coil packs, plugs, brakes, etc. Idles fine and normal during warm and cold starts.

The issue is when driving. When coming to a stop at a set of lights and I put it in neutral or just push in the clutch the rpms drop to around 1500 rpm and sit there until the car comes to a complete stop. Then they drop back to normal. Doesn’t matter if brake easy or hard either. 

I’ve cleaned and tested MAF. Cleaned IAC Valve, replaced vacuum hoses with silicone. Checked and rechecked clamps/hoses/piping throughout engine bay. 

Anyone got any ideas?

Thanks!

 

Edited July 29, 2019 by JustJay

[GTSBoy]

It should hold at around 1100 rpm under those circumstances, so it is doing mostly the right thing, just a bit too high. I would suggest that you have an air leak somewhere and the idle is simply too high and the ECU is dropping timing (at proper, stationary idle) to get the revs down - but it can't/won't do that with a non-zero speed signal. You could put it on the dyno and put a Consult unit or other scan tool on the ECU and see what the timing is in the ECU while it coasts down, then at a stationary idle. I wouldn't be surprised if it was 0-5° stationary.

[JustJay]

Thanks. Kind of figured it must be a leak somewhere but I can’t seem to find it. 

So at non-zero speed elevated RPM’s are normal. 1100 vs 700?  It’s been a while since I’ve had a manual but maybe it was so gradual I didn’t know whereas 1500 rpm is quite noticeable.

i used to get constant high rpm until replacing a split vac line. Now only as described. Do these symptoms suggest a particular area to focus on? 

[GTSBoy]

9 hours ago, JustJay said:

Do these symptoms suggest a particular area to focus on?

Nope. Leak must be pretty small though to be able to be compensated by timing (assuming that's what's happening).