jerome951

I'm trying to diagnose an occasional poor idle problem and am looking for suggestions. Here's some background.

Last fall installed the Iceshark negative cables, which required pulling the exising negative cable between the alternator and starter out from under the intake manifold. After doing this, I had an occasional rough idle. Sometimes the car will barely idle (almost dies), but runs fine off-idle and sometimes idles fine. No smoke from the tailpipe when idling poorly. The rough idle will not cure itself while the car is running, only sometimes after shutdown.

So, I thought I must have cracked a vacuum line pulling the negative cable through. Removed the manifold and replaced the vacuum lines w/ the Lindsey kit (didn't find any lines that were cracked). After reassembly, I ran the car for ~10 minutes and the idle was stable again. Thought I had fixed it. This was ~Dec.

Over the weekend, ran the car again to bleed the coolant system (car hasn't been started since Dec.). First time I started it and let it run ~10 minutes and idle was smooth. Shut if off for a few minutes to add more coolant, and when I restarted the rough idle was back.

TPS seems to be set properly. Haven't adjusted it recently. AFM was disconnected and reconnected at the same time the vacuum hoses were replaced.

Here are what I'm thinking could be the problem:
- Idle control valve (though I think when this goes out the idle simply oscillates, but could be wrong)
- cracked temp sender wire from engine block: These wires come off the block about where I had to pull the negative cable through. If engine were warm but it's not sending a signal, I would think it would run really rich, but I'm not seeing black smoke from the exhaust)

Any other suggestions???
I thought of replacing the idle control valve, but at $140 I don't want to just blindly throw (expensive) parts at the problem.

As an aside, I installed Lindsey's solid motor mounts over the winter and it's really eerie not seeing the engine vibrate when it's running.

hosrom_951

Are the ref sensors and o2 sensors cracked?(look at the bracket for the manifold, the sensor covers would sometime scrack exposing the wires and could have effect (the o2 sensor wires maybe comming loose).

When you took off the ISV, did you try cleaning out the bottom end? there was a good thread from rennlister Jeremy on sealing the ISV...

If ithe idle starts ever since you 'messed' around the rear engine for the battery cables, you may have a loose connection back there.....

jerome951

Thanks Hussam, I'll check these things when I get home today.

I didn't clean the ISV when I had the manifold off. I didn't know it could be cleaned. I'll do this if I have to take the manifold off again.

I'm leaning against a bad connection on the back of the engine or O2 sensor as this should cause a problem off-idle, too. As smooth as the car runs above idle I'm thinking it is something associated w/ the idle circuit.

Porsche-O-Phile

Some other things to check:

All the "junk" under the intake. Those hoses are subjected to a lot of heat and tend to get brittle and crack.

The idle stabilizer (you mentioned this). Is there any difference when you jumper it "offline" using the diagnostic plug?

Is the fuel possibly skunked? You mentioned you had the car sitting a while. Not likely, but possible.

Fuel filter?

Air filter?

Idle set screw backed out or leaking?

jerome951

Hmm, didn't know the ISV could be bypassed. Found the procedure on Clark's website so I'll try that later.

The bad gas is possible (I did put Sta-bil in before sitting back in Oct.), but I would think it would idle rough all the time if the gas were bad. I'll smell the tank, though, to check.

Air filter looked fine when I put it back in (only ~5k miles on it). Fuel filter has ~20kmiles and would likely be a problem at higher flow rates (i.e. above idle).

hosrom_951

POP: He already replaced those old hoses with new ones.........that elimates this one and the problem 'went' but came back.........

jerome951

I didn't check the underside of the hoses into and out of the ISV very carefully, but they were fine from what I could see from the top. If bypassing the ISV shows some improvement I'll go back in and replace them. Those clamp-on fittings scared me away..

FAST44

Sta-Bil makes my lawn mower run like crap until I put a fresh tank of fuel in...

JDeitz951

Check out these achived posts (thanks to Claus Groth):

http://members.rennlist.com/jerseyshore944/F_IDLE.doc

http://members.rennlist.com/jerseyshore944/F_AFS.doc

jerome951

Thanks Dietz, I'll check them out.

The Sta-bil is a good thought, though the rough idle problem started before I put it up for the winter. This is why I tackled the vacuum hose replacement anyway.

Peckster

Now why would you have to bleed the coolant system after it's been sitting? That doesn't make any sense. But I wonder if you're loosing coolant, and we all know what that can be, especially if accompanied by a rough idle just after start-up.

jerome951

I replaced the distilled water in the system over the winter and had to rebleed (normal winter maintenance). Didn't notice any coolant (water) loss last fall when the rough idle problem started.

My car is a full-time track car, so I have a pretty extensive winter maintenance list to reduce maintenance and repairs during the driving season. Since I use distilled water and Water Wetter, I change it each winter.

jerome951

Finally had a chance to try to diagnose my poor idle.

It will occasionally hold idle, sometimes drops very low as if the engine were about to die.

I jumpered the ISV both when the idle was good and when it was bad. Here is what I found-
smooth idle - disabling ISV caused poor idle
poor idle - disabling ISV had no effect

So, I'm guessing the ISV is bad. I think I'll just replace it rather than attempt to clean it (too much work to get the intake manifold off to risk the cleaning not working).

Anyone have a differing opinion before I order the part? Anyone know the best prices for an ISV?

Thanks,
Jerome

hosrom_951

A brand new ISV runs at ~$150

When you disabled the ISV, was it done when the car was warm/cold? the ISV is activated when the car is cold and when the a/c is on........

One tip though, do you remember when installing the ISV where the engraved arrow was pointing?

jerome951

As far as I know the ISV is the original in the car. I haven't replaced it.

The car had idled a few minutes before I tried to disable the ISV. It ran fine while it was cold, which may rule out the ISV.

My AC doesn't work (no freon and no relay) so I can't check that.