So I have a 1989 GXL and I am running into idle problems.

The idle fluctuates between 1200 rpm and 1800 rpm at idle. I got a multimeter and adjusted the idle screw until the resistance was between 1.00 at rest and ~5.00 at WOT. Car was warmed up completely and I measured it seconds after I shut off the car (engine was still ticking ) I repeated this several times and the idle never went lower or got any smoother. When I unplugged the TPS the car idled somewhat normal at about 1000rpm.

Acceleration is smooth and the car runs great accept for the idle. This is frustrating me to no end.

My question is what am I doing (or not doing)?
Is there a way to adjust the idle speed aside from the TPS on a S5? (possibly MAF)
What are the causes for this problem?

 

Idle issues are usually a pain, i had a similar issue for my '88 FC T2, I tried the same approach as you but to little resolution. I now live in LA and i found a shop called Lucky Seven Racing, their info is all on their website and if you need any tech support they are allways available during operating hours.
Let me know if they gave any good info.

 

It's been a while since I've had an FC but this is on the TPS adjustment, I assume?


The car could be trying to compensate for a vacuum leak and its affect on the idle which I don't think it can do without the TPS input.


Yes. there is a screw on the UIM near the throttle body. A search should find good instructions on this but I think a bouncing idle is more than just an adjustment issue.

I would look for vacuum leaks first, I have heard many people suggest using carb cleaner for this but I use an unlit propane torch that I modified by cutting off the flared end and installing a long vacuum hose. I use the hose to route the gas all around the UIM to check for leaks.

 

I failed to mention earlier that it might also be a sticking ISC motor. I have heard that some people take them off and spray some carb cleaner through them. I haven't done this and when you look into them there is a little accordion looking boot in there sealing the components so I don't know if that is effective.

 

I've had success with Ether(Quick start/Starting fluid).
I knew I had a vacuum leak and when I sprayed the Starter fluid near "the leak" the engine would rev up.That is what happens,when you have a vac leak.
I found a Vacuum hose off.That was after I ripped the upper manifold off the engine..again..I had missed a spot!.
Oh,Idea:
I met a guy named: Zmame.(on Forum) look at his posts,he had a problem with his s5 Vert.it was doing the same thing.

 

https://www.rx7club.com/canadian-for...my-rx7-957266/
If this is the one you are talking about its different, his car would only idle at 500rpm while my idle fluctuates around 1500rpm. I cant get mine to go down. I adjusted my tps using the resistance method and everything turned out ok.

I will see about looking into that vac leak later any particular spot to look for on the engine?

 

This works well but I have never scrutinized it from a safety standpoint if something was to ignite the gas. Also, I have heard that propane is heavy and can settle in areas.

I know the use of propane for vac leak checking is talked about in other car forums but not necessarily via hacking off the end of a torch. It could be advisable to use a different setup. I don't have much experience with this kind of stuff so I just don't know.

Something made me think again of this recently and I realized I had posted about it several times.

 

A lot of people only focus on the vacuum leak and tps. The air flow meter could be the problem also. matter of fact, a lot of people overlook it.

 

there are two adjustments that you should be using, the TPS and the idle speed screw.

the idle speed screw is on top of the intake, and should be covered by a little rubber cover.

the TPS is the one you are adjusting, with the voltmeter.

so you should try adjusting the idle speed with the idle speed screw.

if that doesn't work, then there is something else wrong, like the thermowax being stuck

 

Wow I forgot about this it's almost 2years old. I fixed this a while back. I had a couple of vacuum lines that had gotten broken/disconnected. It's all good now.

 

Lol at least you updated us. Better late than never!

Before I read your post I was just gonna say to make sure the fast idle cam at the back of the TB is disengaging when the engine warms up. This will ensure the throttle plates are closed when you go adjust the idle and TPS. Also, for idle, you want to jumper the set connector (I believe a two pin plug near the leading coil).