bradthebold

I am having a hell of a time with my idle – I have never been able to set it on this car. It idles around 810-820rpm. I have tried to set the idle following Lindsey’s instructions – setting the hard stop, setting the TPS position, and jumping the diagnostic port. When I adjust the idle, it will go up a little for a few seconds then settle back down. I can unscrew the idle screw 4+ turns and it will still idle around 810rpm after it settles down.



I have a Vitesse MAF and PB. Bypassing the piggyback made no difference. John said his tune/hardware will make no difference to any of the idle valve control. I have replaced the TPS, idle control valve, DME temp sensor, rebuilt my throttle body, boost leak tested. I have traced the wires from the idle control plug to the DME, verified 12V at the idle plug, traced pin B in the diagnostic plug to the KLR and from the KLR to the DME, verified 5V going to 0V between B and C on the diagnostic plug going WOT, verified the idle valve buzzes with the key on.



When I jump the diagnostic port, it doesn’t affect the idle in any way. I forgot and left it in and it caused some bucking/jumping AFRs when driving though. Sometimes with a cold start it will idle closer to 900, but only sometimes. Generally, when I start it or push the clutch in, in drops down to 500-600 RPM and has to recover. If I unplug the idle valve connector or the TPS connector, I can adjust the idle above 1000rpm. When I plug them back in, the idle drops to 810rpm.

I am guessing it’s something bad in the DME or KLR, but I have no idea what else to do.

Humboldtgrin

Just a thought is your A/C button on and the A/C compressor not working? Don’t know if that would send a signal to the DME and effect curb idle. I know the A/C does effect curb idle. Maybe disconnect the heater-A/C controller and rule that out? Also are your intake manifold gasket installed correctly? One gasket is opposite of the others on an 8v head. Also double check your idle switch on your TPS making sure there is a signal and the switch is working. It could “click” at idle but still not work. It happens.

bradthebold

The A/C button and compressor work fine. Having the A/C on does make the issue worse though. If it's hot out with the A/C on, it won't start without throttle sometimes. The rpms drop too low to recover after letting off the starter. With the A/C on it takes longer to recover after pushing the clutch in, etc.

I replaced the intake manifold gaskets. #1 gets flipped the other way. Above idle, there's no issues. Car runs fine, AFRs are good.

I tested the TPS and signal to the DME prior to replacing the TPS and everything looked good. There is a significant, notable difference in the idle just giving it enough throttle to click the TPS. I went and double checked and it's 3ohm closed and infinite after the click.

Droops83

Have you checked that the actual idle bypass port in the throttle body is clear? It could be plugged with carbon. Remove the adjustment screw and check if you can blow through it.

bradthebold

I just checked, it is wide open and very easy to blow through. I thoroughly cleaned the throttle body when I replaced the seals.

Adjusting the idle screw does make the idle go up for a few seconds, it just settles back down to the same point, regardless of adjustment.

Droops83

In that case, it sounds like the ICV is doing something. Maybe some of the wiring is shorted? It would be worth unplugging the ICV itself to confirm this.

EDIT: I just re-read your first post and what happens when you unplug the ICV. I'm out of over-the-internet ideas, I'd have to see the car myself. Sorry!

Tom M'Guinn

In your first post you say you can adjust the idle if you unplug the ISV and TPS. Do you have to unplug both to be able to adjust the idle? Since you replaced the ISV, it sounds like the DME might be closing the ISV all the time. I'd pull the hoses off the ISV and plug them up tight, then see if you can adjust the idle at all with the throttle screw. If so, then this might help:

https://rennlist.com/forums/944-turb...n-the-dme.html

bradthebold

Either one separately can be unplugged and the idle will stay where adjusted. Once they're plugged back in, it drops back to its normal. I did not try with both unplugged simultaneously.

It acted the same way with the leaking stock ISV. I guess I can just pull the hose off the intake manifold and plug both sides to test without removing the manifold at least. I am going to guess it will work though, since that eliminates the ISV similar to unplugging it.

Otherwise hopefully replacing those resistors will fix it, which is more what it seems like, I had not seen that before. Thanks. The picture links are dead, but I should be able to figure out the where they are coming off pins 33/34.


Edit: Here's a working pic from Doug in a different thread showing the ISV transistors. https://rennlist.com/forums/924-931-...n-circuit.html

Tom M'Guinn

Right, you can just pull the hose off between the intake runners and block the port and hose. If the idle can then be adjusted, I'd suspect the DME driver or the harness/connector. Before changing transistors on the DME, I'd try a spare DME and/or monitor the ISV signal with an o-scope, just to be sure.

bradthebold

Well, I unhooked it at the intercooler pipe because it was easier to get to and blocked both sides. It would barely, barely idle though and die after ~10secs. Even adjusting the idle screw way open, it was idling well under 800rpm.

Droops83

Did you do the above test on a cold or warm engine? With the ICV passages blocked, it will be difficult for a cold engine to stay idling, so you have to keep your foot in to till it warms up.

Even still, plenty of 944 owners have eliminated the ICV completely (I did so myself on my Spec 944 project car that I sold and it idled well when warm), and adjusting the idle bypass screw all the way in the throttle body should at least allow enough air to allow the engine to idle while warm. If the above is happening on a warm engine, something else is restricting airflow, unless there is a vacuum leak large enough to affect idle (though you say AFRs are OK).

bradthebold

It was after I got home and the car was completely warm. Even cracking the throttle blade barely helped, it wouldn't rev. It was not happy trying to run like that. Maybe my sealing method leaked a lot, but it didn't seem like it. I recently boost leak test and I just get a couple little bubbles with soap at an IM fitting at 20psi, otherwise it looks good and is silent.

I'll have to try again and my sure the block offs are sealed well.

Droops83

The other thing to remember is that the Bosch idle control valve is normally open slightly at rest. When the ICV is disabled by jumping the connector under the hood, this does not force the valve all the way closed (only full voltage to the ICV does that), but it merely disables idle control and the valve reverts to its "at rest" state, and there is still some air bypassing the throttle body via the ICV.

In other words, blocking off the ports might be a good test to completer eliminate the ICV's influence on the idle, but there will be even less air going through the engine than would be during a "normal" base idle adjustment.

What are the AFR readings during this test? Not wanting to rev sounds like a vacuum leak.

Humboldtgrin

Have you checked to make sure you exhaust isn’t plugged at all? I melted a catylitic converter before and my engine just didn’t run well when it couldn’t breath. Maybe a calapst down pipe? Cracked intake hose somewhere?

bradthebold

So I retested and redesigned my plugs so they wouldn't leak and I could rev it at least, but it was about the same otherwise. It wouldn't run with less than a turn of idle screw at least. With the screw all the way out, it still only would have a loping idle around 650rpm. AFRs varied, at first it sat at 14.7, but then it went lean messing with it. Average reading was ~16. Revving to 1200rpm went into the 17s. I am leaving the ISV electrically plugged in during this.

Exhaust should be fine. It's a 3" SFR turboback with no cat. Intake is the Vitesse MAF and the piping and couplers looked ok. With it hooked up normally, AFRs are good - 14.7 idle and cruise to ~11.5-11.8 full boost.

I have boost leak tested a bunch of times at the intercooler inlet. Unless it's leaking between the intercooler and MAF, I'm sure there's no boost leaks. I replaced and sealed a bunch of leaking things.

I think I'll replace the transistors at least. They're only $6 and I don't have an oscilloscope or spare DME to test.