RXcetera

Hey guys, I'm hoping someone on here has experienced this problem before and might have some insight on fixing it. I realize there are many idle related problems with the FD but no threads or write-ups have been able to help up to now.

Car is stock with a highflow cat and downpipe. This problem started as soon as I started the car after winter storage.

Symptoms: The idle is rough. Cold starts have always been kinda rough but now it's affecting normal idle. It's cycling between 700 and 720 rpm with no electrical load. Sometimes the idle will stabilize at 720 rpm and be smooth for a few seconds, then it starts cycling again. 700-720-700-720-700-720... braap, braap, braap.

When I turn the lights or blower on 3, it goes up to 800 rpm as normal but has to hunt around a little before holding 800 rpm and after a few seconds I'm dealing with 800-820-800-820 again.

The only time idle is stable is when the AC is on. Then it holds steady around 900 rpm.

I've tried setting the idle as per the FSM but that hasn't helped. Strangely enough, when I do the TEN/GRND, the idle tends to stabilize... I've set the idle on the car in the past and never had a problem. There's no codes in the ECU.

I've checked compression, plugs and coils/wires and everything is fine. I've checked for vacuum leaks and haven't found any. In fact, if I remove the TB nipple to induce a vacuum leak, the idle tends to smoothen out. It's a stock FD so it's possible there's a vacuum leak somewhere but it would have to be tiny since my idle settings at 720rpm are basically the same as they were last year when the car worked fine.

PCV is ok.

I've cleaned out both primary and secondary throttle body and butterflies.

I randomly checked the TPS last year and it was ok.

Just replaced the O2 sensor.

Only thing I can come up with is possibly leaky injector? (although the car starts, drives fine). Can the ISC become sloppy like this? And why would it hold idle for awhile and then lose it again?

Anyone have any ideas??

rossc

I would help you, except I have the same problem and haven't found a solution.

Does your car miss occasionally at idle? That seems to be the cause of my RPM variation. Will hold steady then get a slight miss, dropping the RPM by 10-20.

What are your vacuum readings? I'm hitting 15-16 at roughly 700-750rpm and sometimes I'm getting 13's or 14's.

I would say our solution lies somewhere in a very small and hard to detect vacuum leak. Definately in either a small crack/hole in a vacuum hose or I'm thinking UIM or LIM gaskets. Having sat for winter you could have developed a leak after a hose got brittle or the gaskets became dry and cracked.

RXcetera

Exactly, it's a miss which shakes the engine and drops the idle a few rpm. Vacuum readings are a steady 18 at 720. It drops to 16 when the cooling fans come on but the car has always done that.

I've tried spraying carb cleaner around the solenoids and UIM/LIM gaskets and haven't turned up anything but it's impossible to get access to everything when it's stock.

Some days the miss is bad enough to drive you crazy, other times it's slight. It's consistently worse when my lights/blower is on and it's trying to hold 800rpm.

rossc

Yeah that's exact same problem I have.

Last year before storage my vacuum reading was 10 and the problem was horrible. The car would miss and you could litterally watch the engine shake the entire car. Sometimes the car would hang up idle at 1100rpm. I figured the engine was dead, except that the car pulled like a sonofa while driving.

This year I adjusted the screw under the throttle body (by adjusted I mean turned in about 15 times) and now the problem is alot better, but not perfect. No more idle hang up either. The occasional miss still shakes the engine but its barely noticeable inside the car now. Just that tiny bounce in idle rpm.

I remember a little oil buildup in my throttle body. Maybe the ISC fills up with a bit of oil and makes it sloppy. I might remove mine and clean it.

I hope someone has a solution for this.

RXcetera

Wow, that is exactly the same problem.

Like you, I had the 1100rpm hang up ever since I got a rebuilt and finally fixed it last fall when I turned in the air bleed screw and set the idle properly.

I'm going to pull the ISC tonight and test it but I cant see how it would cause the idle to miss like that. Maybe running the car with it unplugged will shed some light...

RXcetera

Ok, just tried unplugging the ISC and running the car... and the problem is still there even with the ISC taken out of the equation. Once I manually raised the idle to 750rpm, it still had the same misfire and when I tried turning on the lights, it only got worse.

Any ideas anyone?

wstrohm

I have a similar problem, except the idle is stable at either 720 rpm or about 1100 rpm, changing only after the car has been driven and then stopped at a light, etc. Have tweaked all the normal things, and no effect. The slight miss is worse at the higher rpm.

Don't know if this has anything to do with it, but did you know there is a deliberate vacuum leak in the FD's engine? If you look very closely at this vacuum hose diagram, you can see it. At the drawing of the turbo, look at the Y-pipe right at the junction of the "Y." There are two horizontally-spaced hose attach points. Those are picking up the output of the turbos, either at atmospheric pressure, or boosted. Now follow the nearer line from the tap point back to the intake manifold where the other end attaches. That is a vacuum leak, built into the stock engine! I found that when I did my vacuum hose replacement job; couldn't believe it, but hadn't enough self-confidence to plug up a stock hose connection. There might be a restrictor in there, but whatever, that is definitely a designed-in vacuum leak.

RXcetera

I fixed the 1100rpm hang-up by making sure the primary throttle butterfly was only very slightly open. Anything more than a 1/4 turn on the throttle stop (once it's touched the arm) would cause the hang-up.

So I pulled the ISC, cleaned it out and tested resistance. Everything checks out. It's back on the car and the rough idle continues.

I spent some more time looking for vacuum leaks but couldnt find anything. I've started the car by accident in the past with a nipple cap off etc and I remember how it felt to have a vacuum leak... this doesnt feel like that.

The car idles smoothly for 5 seconds or so... then it starts to stumble 700-720-700-720 for another 5 seconds, then it clears up again. And it's always worse with the lights/blower on.

I dont get it.