90 3.1L rough idle

So, I see a lot of posts to this effect, but few that show whether or not the problem is solved.
Symptom: Acts as though its missing on one or more cylinders. Engine shakes at idle, but is less noticeable at higher RPM.

I've tried: Replaced all plugs and wires, cleaned IAC, cleaned Mass air flow unit, sprayed carb cleaner all around engine looking for a vaccuum leak (listening to a change in the engine running). Drilled a hole in the exhaust pipe just before the catylitic converter, just in case it was plugged causing a lot of back pressure.

Im not sure what to do from here. I will keep trying and post back here if I find the problem.

If anyone has any advice it would be greatly appreciated.

get about 2 ft' of 3/16" vacuum hose and replace all the line connections. go to www.troublecodes.net (http://www.troublecodes.net) and see if you can get any flash codes to isolate your problem.

Thanks for you reply. I will get back to you when I have another chance to work on it.

The injectors on the 3.1 are trouble prone. That might be the reason for your rough running. The way to test the injecotors, with an injector balance test and coil test, is about impossible since the injectors are wired in series basically and access to the each injector is impossible with the plenum on. about all you can do is check the combined resistance at the injector harness. Have you tried running a big bottle of Techron fuel system cleaner through the tank?

Check all of your coils for STRONG blue spark....coils on these go bad and you have three. You can have one that is weak or bad on one side, good or weak on the other side. My son's old 91 had one that put out a weak yellow spark about 1/2", and a blue spark that was about 1-1/2". the others were all 3-4" blue. Cost you about 20 bucks each to replace if that is the problem.

All right. Thanks.
I have checked spark with a spare plug, but not to the chassis, especially not for an 1-1/2" spark! Will do.
Here is another clue. I put new plugs in and they turned black during the short running for testing. I cleaned them and they turned black again. So, this would seem to be insufficient spark or fuel. I will keep at it.

Well, I found that one of the cylinders has low compression, or should I say NO compression! So, I'm thinking one of the valves is stuck open, but this engine does not look fun to work on.
Anyone torn into one of these engines far enough to how much of a PITA it is?

You can change a broken spring by removing the valve cover. Either use compressed air to hold the valve closed or some rope pushed into the cylinder hole roll over by hand until the valves are held closed. Pull the negative terminal to prevent somebody trying to start it with the rope in the plug hole.
Search intake removal on this as they will have to be done if a valve is burnt.

Thanks for the information, rhandwor (http://www.automotiveforums.com/vbulletin/member.php?u=475028).
I will check into this. It would be great to get it running again.

Well, I found that the rocker arm was broken. Also a bent push rod. Not too bad. Looking for parts at the junk yard now.