Recently I was having problems with my '99 SE that were baffling me. The car would run fine at speed, but was having a very rough idle. The symptoms continued to get worse until eventually the car would almost always die when idling. I checked all the usual culprits such as plugs, fuel filter, EGR cleaning, vacuum leaks, etc. but I continued to have the problem. Eventually while digging through old threads here I came across someone who mentioned they had an issue with a gasket.

I wasn't excited about having to pull the intake manifold, but eventually I needed uninstall my MEVI anyway (for sale if you're interested ) so I went ahead and took the plunge. It turns out that my PCV valve got clogged and the intake manifold was sucked in thereby creating a HUGE vacuum leak. I've never heard of this before, so I wanted to make a separate post for those who may encounter the issue in the future. The gasket isn't that expensive, and the manifold R&R isn't even that bad, but I never would have thought to check for such a thing.

If you are having idle problems and hear a vacuum leak but can't seem to trace it, try spraying some WD40 or Throttle body cleaner onto your intake manifold. If everything is sealed correctly the fluid will run down and pool near the gasket at which point you can wipe it off. However, if you have the same issue that I did then the fluid will be sucked into the manifold along with all of the unmetered air, hence the idle problems. You can see how mangle the gasket was between the fifth and sixth runner in the photo below. This occurred WITH the manifold installed and bolted down, so I'm really surprised the gasket was able to move so much.


I did run a high pressure fuel pump for about 60k, so I'm sure that didn't help the clogging of my PCV valve. I wouldn't expect this to be a regular occurrence even on mildly modified cars, but for those running high pressure fuel pumps and/or nitrous/forced induction with variable intake manifolds keep this in mind.
-hype

 

so tried to see if there was a leak, sprayed WD-40 and then water. didn't affect the engine at all. sprayed it around the #6 cylinder. not sure if i should still try and get a new gasket. and if it was a small leak, maybe it wouldn't be affected by the water spray.

i am getting a rough idle as well as code 0306 (misfire #6). my original problem is sputtering and "engine hiccups" when cruising 60-70 km/h.

 

When you're quoting codes you mean P0306 right (6 long blinks, 3 short blinks)? I got P0306 "Cylinder #6 Misfire", and after doing some searching here that's how I figured out to do the WD40 trick. I could definitely hear a vacuum leak, and I tried replacing lines and such to no avail. If your gasket was leaking you would hear the leak for sure, and your car would idle terribly because of all the unmetered air that is getting to the engine.

I did have hesitation while accelerating on the highway as well, but the problem was most notable at idle.
-hype

 

so now im not sure where to go now. not really sure what else i can do.
tried coils, injectors, swapping them. it didn't follow. i just keep getting #6 misfire.
i heard wiring harness, could be faulty. but i didnt think that would cause misfires.
im lost...

 

It could very well cause misfires, yes. So have you not checked the harness yet?

 

to be honest pmohr, im not even close to guys in car knowledge. i wouldnt even know where to start to see if that was the problem. i would just have to ask my mechanic.