bonus12

Hi, this is for a 944 turbo. I'm hoping to get some guidance before I start any work. Basically the engine seems to miss a cylinder or two on cold starts. It huffs and puffs and reads vacuum below 10 mm Hg. If I tap the gas pedal, it dies. After about 30 seconds it finally starts idling normally and it starts running like a top. I'm suspecting oil is getting past the valve seals or guides. Need some confirmation or advice.

-No smoke out the tailpipe but it can be stinky after acceleration
-No coolant / oil mixing and no coolant loss (head gasket replaced recently)
-Vacuum at warm idle is normal at about 17 mm Hg -- coasting in gear it's 22 mm Hg
-I don't think there is any new or major vacuum leak (but I know AOS seals are old)

Any way to confirm the valve seals are shot? Any help appreciated! Thanks

Dan Martinic

This sounds like classic coolant temp sensor, no?

Gage

While symptom is active, listen to each of the injectors with a scope or long screwdriver pressed to your ear. All 4 should have a solid clicking.

bonus12

I thought this would last longer than 30 sec?

Dan Martinic

Did you time it? Maybe your 30s is longer... or it just warms up quick enough; I'm guessing it's not too cold in N. California? Anyway, quick & easy enough to check the coolant sensor.

If you say after 30s it "runs like a top", this has to be warm-up related; anything else would show up somehow when warm.. poorer mileage.. hesitation.. something

bonus12

Thanks... now I'm just trying to find it

bonus12

Hi, I'm still trying to solve this problem. I replaced the coolant temp sensor. No change after that. The issue has not gotten worse. It still idles rough, like a pulsing or missing a cylinder, for about the first 30 seconds when the engine is fully cold. Vacuum pressure will also pulse back and forth from 8 to 10 mmHg. Bluish (oil) smoke will also pump out the tailpipe if i rev the engine while cold. There are no loud noises like an exhaust leak. Just some huffing and puffing. My narrowband Air/fuel gauge shows around 13.5:1 ratio.

After the 30-ish seconds, it runs very well.

Injectors are working fine and had them cleaned about about 20k miles ago.
I confirmed the ISV is functioning. I also resealed it and checked it out a few months ago before this problem started.
I just tested my fuel pressure at the gauge and it's just fine. The 20-minute leakdown hardly dropped at all though.
I have zero coolant loss. I do lose some oil, but not from any major external leak.

Is it possible the head gasket is letting a bit of oil slip by while the engine is cooling? I replaced the HG 3 years ago.

Are my valve seals busted?
I guess my next step is to do a cylinder leakdown test or look inside the cylinders.

Any ideas?
Thanks in advance

Dan Martinic

Hopefully, someone with more expertise will chime in here. When I had a cold-engine running issue--hesitation--I solved it by servicing the AFM.

Checking compression & leakdown is always a good idea

PS.. next time, test the sensor before replacing: it's very easy with a pot of water, a stick, and some string

bonus12

if you mean servicing the wiper arms in there, I did do that recently.

gargamel647

Mine did this and changing the blow off valve fixed it.

divil

Are you sure that your idle stabilizer (ISV) is working? One way to check would be to disable it on a cold start and see if things get any worse. If they don't, then it's probably not working. Clark's Garage has the details on disabling it - it's part of the procedure for setting the idle speed.

I've had a problem like this for as long as I've had my car and I've never been able to get to the bottom of it. (In my case it's not he ISV). It has persisted through tons of maintenance, including replacement of all vacuum lines, intake pressure testing, sensor replacement, different DMEs, different AFMs etc. In my case, there are 2 distinct stages to the problem as the car warms up: when it's very cold, it sounds like it's running on 2 cylinders at idle. During that time, any throttle at all will kill it. But when it gets a little warmer, it settles into a stable idle. In this second phase, if I crack the throttle, just a little, it starts to misfire and stumble again. But if I give it a decent bit of throttle, it revs normally. Then eventually after a few minutes of warming up, everything works normally. I've never been able to figure out why a little throttle makes it worse, but more makes it ok. I've had theories for this, but nothing has ever panned out.

Is that happening in your case?

User 41221

Out of curiosity, who cleaned your injectors? I agree that you should start by making sure that all four are "clicking" consistently at start up so you can at least eliminate that as a problem. I know that injectors can stick when cold until they warm up a bit so that's a good place to start. After that, advice provided above is solid, I'd suggest eliminating one thing at a time.

Can you specifically identify which cylinders are *not* firing initially? That might also be a good clue to help you track the problem down.

User 41221

Out of curiosity, who cleaned your injectors? I agree that you should start by making sure that all four are "clicking" consistently at start up so you can at least eliminate that as a problem. I know that injectors can stick when cold until they warm up a bit so that's a good place to start. After that, advice provided above is solid, I'd suggest eliminating one thing at a time.

Can you specifically identify which cylinders are *not* firing initially, or is it random? That might also be a good clue to help you track the problem down.

Dan Martinic

Check the choke. Oh wait... not a VW Beetle lol

But seriously.... that's some issue... would drive me nuts all the years! All that air at cold start...

bonus12

Yes, exactly the same, unique symptoms. This problem seems to be correlated with time/engine cooling after parking. It does it worst after sitting overnight for me, for example. That misfiring doesn't seem random because it has a regular rate correlated with RPM. And interestingly, it's a non-threatening problem. But you'll be interested to hear what I found.

I realized I left my FQS at position 1 (+3.1% fuel) . I'm confused because I have adjusted this before to fix this same problem. 2 days ago, I found out that switching it to position 2 (-3.1% fuel) fixed the problem. But I got some detonation on hard acceleration (up hwy hill in 3rd). I put it back at position 0 and all seems good. Immediately before adjusting the FQS, I tested the fuel pressure. (Pressure was normal, by the way). So maybe I got some air out of the rail when I tested the pressure?? One or the other thing fixed the problem.

So my question is, divil, do you think you could be running really rich at startup?

I resealed my ISV a couple months ago, tested it's performance, and cleaned the contacts. My injectors were cleaned by Mr. Injector a while ago and getting them back after cleaning noticeably improved performance. They are all clicking, but I haven't tested yet if they don't click within that first 30 seconds.

Thanks everybody for the help.