DME Closing ICV during warmup??? Help please.
 

Hi Team, 1985 carrera here.

I have been chasing a low rpm idle (600), while the car is warming up, before 194f.

Once it gets close to 194f the rpms start coming back up and then idles fine.

I shut the engine off after pulling the ICV plug off, while the car was idling at 600 rpm. This was when it was just at the first white temperature mark. Looked at the ICV and it was completely shut.

I also did another test by lowering the base idle once warm, to 700 rpm, and the ICV wouldnt raise the rpm to 800.

It seems when I plug the test port to set base idle, the ICV will return to mid point, but doesnt seem to want to open further. That being said, at initial cold start, the car will idle up to 1100 rpm for first 45 seconds. Not sure if this is done by the IAC or other means.

Any help on this? Should I be checking the DME transistors that controls the IAC, or is this merely perhaps a fueling issue? I have checked CHT, AFR when hot is set to 14.2, I have tested and replaced the IAC, new o2 sensor and fully rebuilt the intake system (no air leaks, vaccum is now at 18 inch). Thanks

Subscribing to the thread...

86 with similar issues with similar checks...

I am now working my way through the DME connector, testing against the Bentley manual. So far I have found that the Altitude corrector is broken. It has been cutting fuel by 6%. I live next to the beach, so I just jumpered the plug. Dont fancy paying $350 for a new one. I dont think this will fix the issue though, not sure why DME would drop rpm due to the injector pulse being reduced for high altitude.

Read post #20 here; http://forums.pelicanparts.com/porsche-911-technical-forum/360766-bench-testing-carrera-idle-control-valve.html

Thanks Dave, I have completed this and can confirm my ICV is fully functioning. What I did find interesting was this post:

"With engine at idle you can measure DC voltage between the center pin and the 2 outer pins and you should see 3.0 to 3.5vdc on each side. If both sides don't read in this range you could have a bad drive transistor in the DME or a wiring issue. Of course I assume the valve is good.

Note that the valve MUST be connected and functioning when you perform this voltage reading test. This means you MUST have a way to access the pins with the valve plugged into the harness. Easiest way to do that is pull back the rubber boot on the harness so you can expose the wires in the connector. Or use 3 test leads to connect the harness to the valve."

Rather than me pulling transistors off the board, this will at least tell me if the DME controller for ICV is in good order, without removing transistors.

Ok, just adding to my issue, started the car and as I guessed the altitude fix did nothing to remedy the low idle. Did the same thing lowering idle eventually to 600.

I tested the ICV with car ignition on and instead of getting 3 to 3.5V, I got 5.3 to 5.8V. So I guess I will pull the transistors off the board and test them to figure out what is up.......

EDIT: I started the car and tested voltage at idle. one sat at 3.5V the other at 9V. I also bridged the valve and got 4.0V and 9V. So looks like I need to open the DME and try pull transistors off...

Why are you doing this??? You did the test in post #5, and it indicates the DME ECM transistor drivers are OK.

I did the test in post 5 yes, but once plugging the connector back in, it always took the rpm down to 600rpm. My base idle was set to 800rpm. So even though the DME transistors could still move the ICV, they always move to shut. If I applied load by turning all accessories on, my idle would just lower. Everything pointed to a DME that could close the ICV but not open it.

So I took the transistors out last night and tested them. As they are darlington, their pin layout is different to a normal transistor. its 1.Base 2.Collector 3.Emitter (Took me a bit to realise)

They passed all test but this one -

"Hook the positive lead from the multimeter to the to the EMITTER (E) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor.

For an good NPN transistor, you should see “OL” (Over Limit)."

Instead of OL I got 1.7V. And this was the same for both transistors.

Happy to be told I am wrong, as I really want to get this sorted, but from searching multiple test methods, they all point to this failure.

So as transistors are only $2, ill go grab some.

For anyone elses sake, from my research these should all work - TIP120 TIP31 NTE263 BDX53 BD649. Some google research can find others replacing their DME ICV transistors with these with good results.

So it did not pass the referenced test!

Typically, it's the valve itself that damages the drivers, or an over-voltaging alternator.
If you're becoming an electronics engineer, remember that a darlington transistor has two B-E junctions, i.e. VBE= 1.5V.

Thanks, that's good to know, wasn't sure is something else on the DME board may be doing it. My ICV does test within range (44 ohm and 22 on each side). So perhaps it was a previous owners icv that did the damage.

Hopefully, you didn't forget to check the idle switch pin 2 on the DME ECM connector. It should be at ground at idle.