Tom Rathjen

I am continuing to experience problems with very rough idle at start-up on my 1988 944 (automatic). Based on advice from an early forum posting, I replaced the idle stabilizer. That did not change the behavior.

When starting the car, the idle is very rough (almost dies) until the engine has been rev'd to 2-3000 RPM for 30 seconds to a minute. Then, it runs fine. This behavior is worse on cold starts, but it also occurs even when starting the engine that has already been driven and warmed up (but turned off for any longer than 15 minutes or so....like a stop at the grocery store).

Also, note that I have followed the instructions for adjusting the idle as posted on Clarks 944 Garage Manual site.

And: it seems to me that the idle stabilizer valve is always open, even when hot. I say this because there is a drop in RPM when I disable the ISV by jumpering the B&C connectors on the test port...at any temperature. (Which also tells me that the valve itself is functioning.) So...could it be that a temp sensor or something is bad, and telling the DME to keep the ISV open all the time?

Any suggestions?

Thank you.

Spidey944

Have you checked for any vacuum leaks? This can cause an unstable or rough idle at times, then when the engine gets some heat in it and components expand, the leaks get smaller making the idle more stable. Just a thought.

It could also be leaking fuel injectors causing your rough idle on the initial start up issues. Just a few things to check.

knfeparty

It sounds like once your 02 sensor is warmed up the car runs fine. You should check the closed-loop components, then, which I'm pretty sure is just the valve your replaced, the air bypass screw on the throttle body (although I doubt this is the root, consider getting a throttle body rebuild kit; most of those orings are probably shot and it's a fun little project with the right hex wrenches), and the idle mixture screw on the afm.

Of course, none of those are really supposed to be adjusted by a DIY'er, but I did get some good results by bypassing my ISV and tinkering with the air bypass screw a bit (I had to reset it because I had replaced the oring, which threw the settings out of whack). I would just check to see if they've been messed with by a PO or something; if not I wouldn't touch.

82-T/A

Hey guys, not to hijack this thread, but where is the idle stabilizer? And would a 1984 / 944 NA have it?


Thanks!

Wipeout

An early (through 85.1) doesn't have an ISV, it has an Auxiliary Air Valve - they're both located in the same place, directly underneath the intake manifold.

Legoland951

Tom, you didn't happen to go to South High School, did you? As for the idle problem, it could be a leaky injector flooding a cylinder and once you run it for a little while, it cleans up. A broken diaphram in the fuel pressure regulator leaking fuel can also cause hard starting and rough idle as the car will run rich at idle/low throttle.

BTW, when you adjusted the idle per Clark's garage, did you adjust the idle mixture screw or did you adjust the idle speed?

JimV8

Perhaps the engine mounts are shot.

Tom Rathjen

OK, the most interesting response was the question about whether or not I went to South High. As a matter of fact, I did. Torrance South High, that is. Class of '83. Whoever asked that question (Legoland951?), send me an email at rathshop2@comcast.net. Or visit myspace.com/iceman944. I'm curious who you are.

To answer the other technical questions:

- I don't believe it is a motor mount issue. When I say "rough", I mean a drastic drop in RPM, and often a full stall if I don't rev it enough.

- Regarding potential fuel pressure regulator bad diaphram...I believe I replaced the fuel regulator when I had to replace a leaking fuel regulator. I can't remember for sure. But I have more than one lying around, so I can try that easily enough. I don't think that's it, though.

- Leaking fuel injector is an idea, but I don't think that is the case. It stays very rough for longer than I would expect it to take a flooded cylinder to clear. Also, I have found (by doing something stupid) that the fuel system stays pressurized after shut down, which wouldn't happen if the pressure bled through a leaking injector.

- Regarding which idle screw I adjusted when following the Clark procedure....I adjusted the "idle speed adjustment screw". I'm not familiar with the idle mixture screw. Where is that?

- Regarding potential vacuum leaks...I've checked to the best of my ability, and I don't believe there are any. But, maybe I am missing something. Where would be a good place to look for leaks?

- I'm afraid you lost me on the "air bypass screw" and "afm". I don't know what/where those are.

- O2 sensor. That's a good idea. Is there a way to test that? Are there any temp sensors that are part of the "closed loop" system for idle control?

Thanks!

U-30AT

Have you replaced water temp sensor ($16 part) ? From what i've gathered it tells the ICV the engine temp and how/when to adjust idle. If it is bad the ICV doesn't know the temp. Correct me if i'm wrong guys.

Small head gasket leak? Check the plugs for trace coolant?

Legoland951

Tom, there are many areas where a vacuum leak can occur. Some of the most common places are the lines/hoses under the intake manifold. There are some large preformed hoses running from the bottom of the intake manifold to the idle stabilizer valve where cracks are hard to see. I have even seen a couple instances where the intake manifold to head gasket was damaged by overtorqued intake bolts or wrong gasket orientation (3 goes in one way 1 reverse) causing vacuum leaks. I use carburator cleaner with a straw to spray around the suspected leaking areas. If you hit the leak, the idle will increase to almost normal.

The air bypass screw aka idle mixture screw is under a cap (unless tampered with previously) on the AFM (air flow meter). Normally, it does not need adjustment but people do anyways to compensate for vacuum leaks they cannot detect.

O2 sensor does not make that much difference at idle to be a problem. You can disconnect the O2 sensor and will not cause stumbling or misfire whatsoever. The engine temperature sensor mentioned in the last post could be a problem though so you may want to check that along with vacuum leaks.