xeps

Have had the 964 for several years now, and it has always been a bit rough at idle.

It usually hovers around the right spot (~880 rpm) but will dip down to ~800rpm for about 1 second out of every 10, complete with a slight drop in oil pressure and the car shaking slightly.

I have a light weight flywheel and have generally just been thankful that it doesn't ever stall (clutch in or out) but have started wondering if the slight idle hunt is normal.

Is your idle steady?

DaveK

Mine is rock steady.

Maybe your dips coincide with the compressor engaging?

Makmov

Rock solid now.

It took getting rid of the K&N thing some call an airfilter and put a Mahle in, removed and cleaned the Idle Control Valve, O2 sensor, and reset the DME.

Smooth and doesn't budge warm or cold now.

xeps

The idle drop is very consistent, but I haven't been able to be able to determine what could cause it.

Will happen anytime I am at a complete stop - doesn't matter if clutch is depressed, brakes are in use, etc..

Has persisted through cleaning of the intake and ISV, replacing fuel filter and pump, valve adjustment, replacing of dizzy caps, rotors, spark plugs, coils..

No change in the idle behaviour at all.

Steve Weiner-Rennsport Systems

Howe old are the plug wires????

xeps

They are older than I care to admit - on the list to replace over the winter slumber, which will be rather soon

Although I intend to replace them anyway, they seemed an unlikely source of this particular problem. Wouldn't they more likely cause misfires or generally intermittent issues?

crg53

In reference to Steves post, mine used to do the same as yours, idle would dip, oil pressure drop, then back up again. Last winter I changed caps, rotors, plug wires and plugs, and now the idle is rock steady at 840 RPM.

Makmov

I will probably be changing out my wire set soon too.

It just got a band new dist complete, cap, rotors and plugs.

Before putting in the new O2 I would blip the throttle and it would drop down to 450-500 and come back up.

Now it just settles at 840 with dropping down and coming back up.

rarebear

I have the same on my 90 964 C2 it has an RS clutch with LFW and it idles very low (2nd line on the rev counter about 650 rpm as long as the DME has not be programmed with RS values this will stay a problem with LFW P's

Andy Roe

Rock steady idle with RS flywheel. No different than when it had the LUK.

c2dweller

Are happy there aren't any intake air leaks? When I got my car it was terrible, I tightened what I could see and now its much better - although not perfect. I plan to clean and remove the whole lot over winter to get it perfect.

I believe a good test is to spray 'easy start' along the joins and look for an increase in revs.

xeps

I'm wondering about this - I had the intake manifold powdercoated earlier this year, and introduced an intake leak that made the car not idle. Ended up trying the starter fluid test, found the leak and fixed it.

I'm reasonably confident that everything is pretty air tight now, but is there anywhere I should be looking other than the area immediately around the throttle body, vacuum reservoir, etc?

c2dweller

I tried this test myself this evening and couldn't observe any real difference. I could observe a small difference, but put this down to spray getting into the normal intake route.

I did try another test I've been meaning to do for a while and this gave interesting results. I turned the garage lights off and, using a water spray bottle, I sprayed some water into the engine bay near the distributors. Without any ambient light I could see very small hairline arcs across the distributor cap in a number of places. I can only assume the cables are breaking down and causing an alternative path for the high voltage. This would certainly lead to a weak missfire IMO.

It seems we both might be suffering from the same problem, so it may be worth you trying the same.