olaugh

I have a very rough idle on my 968 with it searching from 900 to 1200 rpm, shaking and near stall on both start up and after warm up. At higher rpms it runs good. I replaced both the o2 sensor and TPS throttle switch with a new bosch, but alas neither seemed to do the trick. I have no check engine lights besides the air bag warning. I am new at this and not set up for major work but would like to do as much as I can before taking it in. Any advice on next steps would be great.

I do have a couple of things I want to confirm. Does a replaced TPS require any adjustment? Unlike the 944 I believe the DME automatically detects idle position right?

I also noticed the crankshaft position sensor cable connector end is sketchy. The connector end probably cracked and had been removed with the 3 wires siliconed together with leads sticking out. It appears to be working but I will replace if this in any way could be contributing to the problem.

Thanks!

RajDatta

It could be your idle stabilizer which can gunk up. Does the car cycle where it almost shuts off and then the idle increases?
You should also fix any known electrical issues like the speed and ref sensor connectors.

olaugh

Thats correct, it cycles up to 1200 or so rpm and then down sometimes stalling out and then back up again and repeat.

By idle stabilizer i assume you are referring to idle control valve correct? i will hit that next to clean and test.

RajDatta

Yes, idle control valve, under the intake manifold. Also check for vacuum leaks or brittle vacuum connectors. There are a few vacuum lines that connect to the intake manifold, one to the throttle body, one in the back to air/oil separator. Check to make sure they are connected and not leaking.

msojka

Is red line 75w90GL5 synthetic hypoid lsd oil suitable for Porsche 968 manual transmission ?

olaugh

So I got my ICV off and although I dont think its every been replaced and likely never cleaned, it wasn't crazy dirty. I cleaned it with throttle cleaner and tested it on a 9v battery. Motor works and it definitely open and closes. However once I disconnect negative on either side the valve automatically rotates back to somewhere off of middle between open and closed. Is this normal? I thought that it should remain fully open or closed until receiving a call on the opposite negative side.

Also I get a very clear assertive clicking sound on closing but no discernible click when opening. I am wondering if the motor is weak and leading to rough up and down idle? Time for a new IVC?

thomasmryan

I would look for gross intake leaks or excess fuel. Make sure all vacuum lines are attached according to the underhood label and airtight. These cars are getting old so paper gaskets and rubber bits should be replaced at some point.

if you have the idle control motor off, changing the intake gasket and AOS seals are just ten fasteners deep. You might swirl some rtv on the rear of the upper balance shaft housing. A little more than twenty bucks.

Injector seals can also harden with time and heat. It is economical to have them serviced/cleaned for not much more than purchasing the seals.

Probably not your particular issue but smell the vacuum control line to the fuel pressure regulator.



​​​​​​​I will bet $5 on the intake gasket.