hawk911

help plz; what will a compression check tell me about my rough starting?

springer3

Rough starting is one of the first symptoms of leakage on one or more cylinders. Cylinder leakage will affect cold starting long before it affects power or warm idle.

Leakage affects both suction and compression strokes. A leaking cylinder does not develop its share of the intake system vacuum. Even the healthy cylinders are affected, because then suck through the same intake system. Cylinder leakage affects the intake, because the leaky cylinders don't suck as much air through the flow meter on the suction stroke (they suck air in through the leaks, but the air flow meter does not measure the leakage). On compression when cranking cold, the cylinder pressure may not get high enough for the cylinder to fire. Spark plugs are more likely to foul on low cylinders, because the plugs don't get hot enough to burn off the carbon and oil deposits.

More than likely you will get good news, but check the compression before spending money on parts. It is cheap insurance against spending a lot of money needlessly.

hawk911

how much is reasonable for a compression check?

JasonAndreas

Hawk,
Your O2 sensor is ignored during the first two to three minutes of running while cold so its unlikely to be the cause. Have you tried the usual suspects; jumper/swap the DME relay? checked and cleaned the ISV?

hawk911

I have swapped the DME relay with no change, but I have not cleaned the ISV lately. My MAF looked fine when we swapped the suspension, so I doubt that is at fault.

springer3

It would probably add an hour to a spark plug changeover. I would guess an extra $60 if tune up was under way, and perhaps 3X that if you did it stand-alone. You probably want to pull the plugs to see if carbon fouling is causing or contributing to the starting problems.

hawk911

I have the tune-up scheduled for spring, but I was going to do it, not the shop. DIY tools for this?

there was a thread on Pelican that a guy found a leak in his intake manifold. Tell me if you think this might be worth investigating...

http://forums.pelicanparts.com/showt...hreadid=256543

springer3

Definately check for vacuum leaks. Plastic and rubber get brittle with age, and these cars are not new any more. A vacuum leak causes lean mixture and can cause rough running. Good news is that vacuum leaks are cheap to fix. Always look for cheap problems first. You might find one that fixes the problem, and avoid fixing an expensive problem before finding the real one.

hawk911

so what is he talking about? anyone got a picture?

Heirsh

We've got some little plastic trumpet like things that sit on the heads that the runners connect to fromt he throttle body. He's talking an actual gasket below them mating to the head (I think). I never tore into an older 911, so I don't know this for sure.

Ours are different in that they have square cross section silicone (or maybe viton, I dunno) rings that seal the plastic trumpets to the heads. I rebuilt all of that on my car looking for vacuum leaks and while it needed a lot of cleaning, everything looked ok (except the injector o-ring sealing area). Well, the rubber sealing pieces on top of those trumpets were old and needed replacing. I do have some pictures of these things, but I dont have access to them at the moment.

edit:

Oh and I had this problem slightly lately until I fixed my seemingly last vacuum leak. It was the hose to the fuel pressure regulator. It caused a vacuum leak, but amazingly enough, caused rich running a good bit of the time. Not to mention some other pesky problems that drove me crazy.

hawk911

is this trial and error? spray an area with starter fluid as he did, and see where it disappears?

Smokin

Wouldn't it be a good idea to make sure that your plug wires aren't arcing prior to doing this procedure? Isn't starting fluid pure ether? <very explosive>

frankvan

Just curious, would low compression and/or vacuum leaks correct themselves after pedalling the throttle for 30 seconds? (expansion of materials?) My attempt at finding vacuum leaks was fruitless as the engine was already idling smooth as I sprayed around and there was no changes in idle (darn, I was hoping for the quick fix!)

My car idles rough or not at all when it is below freezing and at that it resolves within 30-60 seconds (before engine has a chance to warm up even a little) and then idles smooth. When above freezing it fires up smoothly with no extra throttle input.

Does anyone know of a link that shows the 964 DME pin numbers so I can test the CHT? The only one I could find on the forums was for a 944 and it only has two rows of pins (mine has three rows...)

dfinnegan

I've attached a doc file with the DME Pin and CHT Test pages from the shop manual.

Cheers,
Dave

Edit: Well, at least I *tried* to attach a doc. PM me with your email address and I'll mail it to you.