missing/rough running
 

My Z has developed an issue with a slight miss. I know step one is to pull the codes and I'll do that tonight, but I'm posting while everything is fresh. It was running fine on the drive into work last week but then when I started it to go home, it was clearly not happy. Definitely missing on at least one cylinder, and there was a noticeable hitch in the idle. This happened in either normal or full power mode, and at times the idle or very light throttle seems almost normal.

Use a very light throttle and it will accelerate smoothly all the way up. In full power mode with a light throttle, you might not know anything is wrong. However anything more than crack the throttle, and the computer cuts the engine power and it almost sounds like an old Beetle. So it knows something is wrong. Try to rev the engine at idle and it's like an old carb that needs a good cleaning.

I didn't have a chance to look into it over the weekend, so took a quick look this afternoon. When I opened the hood first thing I noticed was the intake boot had popped off the air filter assembly. Well duh. So I put it back on and made sure the clamp was tight. First thought was the computer had put the car in limp mode because of the boot. So I disconnected the battery for a while, then reconnected and drove it for a few miles hoping it would simply relearn. Nope, still not right. So tonight I'll pull the codes and start following the procedure in the WSM, but if you guys have any pointers please let me know. Thanks!

Re: missing/rough running
 

Did the SES light come on? could be something as simple as the map vacuum line off. Just a guess could be an injector, coil or plug wire issue.

You can start by isolating the cylinder by pulling 1 plug wire at a time.

Re: missing/rough running
 

No lights at all. If it wasn't for the slight vibration at idle, you wouldn't know anything was wrong until you started moving. These engines seem to tolerate a miss pretty damn well.

I'll be pulling plug wires too, and then plugs. Hard to believe it might have fouled a plug but anything is possible and it doesn't have to be a digital problem.

Re: missing/rough running
 

Ok I couldn't resist going out and checking a couple things. Vacuum line on the MAP sensor looks fine, but two very weird things. The manual throttle plunger/bladder thing right above the battery does absolutely nothing now. Push it in and nothing, engine does not rev. That's a vacuum-controlled item right? The normal mechanical throttle cable works fine.

Even weirder...my vacuum pump, which has never worked at all since I got the car, is now running!!! WTF?

Re: missing/rough running
 

block the line to the pump with your finger etc and see if it stops once a vacuum is built up. This may be somewhat unrelated though as vacuum leaks under the plenum usually result in a high idle and not as much as a rough running engine.

If the car runs better after it has cooled off overnight, that's very likely the injector(s) failing. Do you know if the previous owner ever replaced them?

After that, my diagnosis would be ignition, fuel, and then ECM or other electric oddities.

Re: missing/rough running
 

This is for your cruise control and is vacuum operated. It does have a cable to the throttle body and should operate the throttle when squeezed.

Re: missing/rough running
 

Interesting. Before this happened, the cruise control was fully operational and I used it all the time, worked perfectly. Haven't tried it since this problem occurred. Is it possible I had a vacuum line blockage which made the pump think everything was fine, but now it's cleared which allowed the pump to come on and now I'm seeing vacuum issues? I'll try the book over the intake in a few minutes.

Re: missing/rough running
 

Got any scan software loaded? That would tell all, I bet...

Is 2' vac system integrated into main vacuum plumbing? Or is it dedicated? Solely to secondaries?

Re: missing/rough running
 

A little more clarity and I think I'm homing in on it. Engine not throwing any codes at all, and vacuum holds 20" for several minutes, pump is cycling like it should. Cruise also verified working perfectly, even though that plunger still doesn't affect the idle.

So I disconnected the IAC and started pulling plug wires. When I got to #4 there was no change in the idle and it's the only one that didn't arc/spark as I pulled it. So I pulled that plug and I swear it wasn't tight. The plug looked very good, no residue, and when I reinstalled it the idle was immediately better. I actually thought I had fixed it, but a quick test drive and it's still missing badly. I ran out of daylight, but tomorrow I'll pull all the plugs and just make sure they are all seated/torqued properly. Then it looks like I have something to figure out with #4.

Re: missing/rough running
 

the dual mass tends to eat up vibrations.