Well it's been months since I did the Hughes plate swap, and ever since I reassembled the car I've had random misfire, felt especially strong at low RPMs. I suspect that perhaps the intake manifold gasket didn't seal, and want to verify first by identifying a vacuum leak.

How can I identify an intake manifold vacuum leak?

 

You can do the air pressure test that is documented in the intake plenum leak TSB. This will tell you if you have a leak or not, but not necessarily where. The other option is to get the colored dye from a dealership and run that through the system like you would injector cleaner. You have to have the special light to see the dye, but you will be able to tell exactly where the leak is.

 

Take a good look at the injector plugs and wiggle them all around at idle. This is the most common place for a miss fire I see on these after people do the plenum fix.

 

I had a leaky injector after the plenum becuase I didn't oil the o-rings and ended up cutting one. I have since replaced it, but still a random misfire.

Can you send me the Plenum Leak TSB?

 

My intake manifold gasket leaked and I never had a misfire. I would check you electrical connections. Also, if you had a leaky intake manifold gasket, you would hear an audible vacuum coming from it.

 

ok got it, nevermind on the TSB. I still need to go get a vacuum gauge.

 

You could also do the shade tree way and cautiously spray some starting fluid in the area that you may think are leaking. If it engine revs up then you have found the leak. Just be careful not to start a fire.

 

I don't recommend starting fluid, HIGHLY flammable..... could be just a touch TOO exciting. Carb cleaner, or brake cleaner works as well, and isn't quite so exciting to use. Squirt some directly down the throttle body, to see what it is going to do to engine idle, then hit spots that have the potential for leaks. (basically, any gasket surface exposed to intake vacuum. not to mention vacuum lines....)

 

Ive always been told to use water, as it'll make it bog instead of rev, and you're cleaning the carbon out.

 

That sounds like a good idea. Thank you. Spray bottles are a dime a dozen, and water is, for the most part, free. (yeah, don't use the garden hose for this test..... )