Simonka

On starting the 944S I get high idle (± 2000 rpm) slowly rising to ± 3000 rpm after about 30 seconds.
Driving is no problem, but power seems limited when kicking the pedal to the floor.
I would think the problem is the idle control valve. If I disconnect it after start, the idle drops to around 1000 rpm.
What I don't understand: the idle control valve should be open by default and closing when engine warms up / making more rpm. Why would the rpm drop if I disconnect it (it should be fully open then) ?

Tested:
- vacuum leeks: ok
- idle control valve: does move when connecting to a battery
- fuel pressure: ok

Replaced:
- engine temperature sensor
- potentiometer inside the air flow meter
- fuel pump + filter

Porsche 944S 1988 - engine 2.5 L M44/40 I4 16v 190HP

walfreyydo

To me it sounds like either the ICV is sticking open (open more than it should it) or the throttle plate is not fully closed/closing. Ideally, with the throttle plate closed (idle), the opening/closing of the ICV is what controls engine RPM (in general at idle, more air=higher rpm). The idle control valve shouldnt necessarily be fully open at idle, it needs to be open enough to let in enough air to maintain the set RPM. This is based on engine temperature. The signal for the ICV is variable (not binary) so the DME sends a variable voltage to control how much the ICV opens.

They commonly build up with gunk and can be sticky. When you tested it, is it moving cleanly and opening/closing fully? It should "snap" fully open/closed when putting 12V onto it. A good cleaning with brake/carb cleaner can usually fix this issue, making sure it moves freely and quickly when 12V is applied.

The other possible issue is your throttle cable is over-tightened allowing the throttle plate to be partially open when starting allowing too much air to bypass in conjunction to the air already bypassing through the ICV. Plugging the ICV in this case is bringing it back into balance due to the air bypassing the throttle plate. Check that the throttle plate is fully closed with foot off the accelerator. If not, adjust the throttle cable or the stop on the throttle body.

Those are two possibilities that I can think of. The last possibility is a vacuum leak which you may have overlooked, but to me this is less likely. Somehow air is getting into the engine causing the idle to be too high - either through the throttle, the ICV or a vacuum leak. Also confirm you replaced the DME temp sensor (blue top) and not the gauge temp sensor (deeper under the intake manifold, near the knock sensors)

Simonka

Thank you, Walfreyydo for the reply.
It is the engine DME temp sensor (blue connector) that I've replaced. Also tested the resistance on the old one in hot water and this was ok. So not broken.
The throttle plate is also ok. Stop screw is almost completly out. No tension on the cable. Clear closing click when releasing tension on the gas cable.
When I tested the ICV I took it out and could see it moving with a clear click when putting 12V on it. I cleaned it, but maybe not enough then. I am just not looking forward to removing the intake again to access it.

Dan Martinic

This is interesting: I didn't realize you can easily replace the finely-calibrated insides of the AFM?

Unplug the O2 sensor during testing / adjusting / diagnosing your idle & running issues.. you want to get it running right without it first

Simonka

I don't think there is an O2 sensor on my 944S.
Concerning the AFM: always willing to share some pictures from while replacing it. You can replace the worn out potentiometer without removing the calibrated parts.
This didn't change a thing to the high idle problem though...

Dan Martinic

Yes please start a new thread on replacing that AFM part.. I'm sure it will be of interest

If the car came with a catalytic, it must have an O2 sensor, no?

Good luck with the high idle!

Simonka

Indeed, but mine has no catalytic converter as far as I'm aware of. So no O2 sensor.
I will start a thread about repairing the AFM.

Simonka

Problem solved. I replaced the idle control valve and runs great again. So an ICV that seems to work fine when testing, can still cause high idle and low power problems.

walfreyydo

Did you inspect the old ICV? My guess it was likely full of gunk and stuck open. A good cleaning with brake cleaner/carb cleaner would likely free it up.

When you say the old one "seemed to work fine", what was that based on? Did you do a voltage test and did the valve move cleanly and quickly (it should "snap" open/shut with 12V)?

Im trying to understand why the old one wasnt working, and how you confirmed it seemed to work fine previously. Usually these fail due to gunk buildup from the AOS sending oily vapor back into the intake, so Im curious what condition your old one was in. Perhaps the issue was electrical or related to a failed solenoid? Yours was clearly stuck open, just trying to understand why and if it was related to gunk or something else.

I outlined this in my original reply but you never elaborated on what testing you did on the old one or what you found.

Simonka

I am wondering about the same thing. This was an unsolved problem for years!
I took out the ICV and tested it by putting shortly 12V on it. There was a clear click while snapping open. I had cleaned it by spraying a lot of brake cleaner inside.
The old one was closed when disconnected and looked exactly the same on the inside as the new one (The angle of the opening I mean).
What I also did while replacing the ICV was clean out the small vacuum lines under and around the intake by spraying brake cleaner through. Also the intake manifold looked greasy, so i blew air and cleaner through the vacuum connection point to be sure it wasn't clogged up. Also found a small crack in the T-connector near the oil fill cap, but I think I teared it by flipping over the intake manifold. I hadn't noticed it before. I've put some silicone on the crack to repair it...
So yes, there is a small chance that the problem was something else and I am now the proud owner of two working ICFs, but I don't really see how the above would cause high idle.

sefeing

Having a similar problem here, and your experience is leading me towards likey replacing the ICV as well - also an '88, also an S 16V model. Shop cleaned out the ICV, but now the car stalls out when the ICV is connected, and run at a higher 1100-1200 idle once the ICV is removed from the equation. I can hear the valve move when 12V is applied to it, but that doesn't mean that it's functioning correctly. These parts got EXPENSIVE. Betting it's a combo of vac leak + ICV, but we'll see.

How's pulling the manifold on the 16V's?

Simonka

Think I have payed around 70 euros for the ICV at maf-shop.com (Germany).
Pulling the manifold is easy and I am not experienced. I should have some pictures somewhere. I'll check.