Random Misfire with 3.1
 

hey guys i havent been on here in a looong time, but im back and i need help. i have a random misfire going on that i cannot figure out... ive swapped the plugs out with the wires, no changes, swapped all three coil packs with 3 good spair ones, swapped out the mass air flow sensor, nothing, swapped out my ignition module, and it ran fine for 100 miles, then right back to the same problem, swapped out the upstream o2 sensor, nothing. swap out the map sensor, still nothing.. im really stumped on this one and kind of aggrivated lol. it will start up rough and idle then go to a rough idle, it runs really rich, accelerates really rough... any idea guys??

I've heard of people with random misfires having a clogged fuel filter, failing crank position sensor, failing fuel pressure regulator and worst case, failed fuel pump.

Any additional trouble codes?

naw last time i had it scanned it was p0400

Well the P0400 code is for your EGR valve which will increase your combustion temp and increase your emissions so it'll smell like it's running rich, which may have also clogged your fuel filter. If your EGR is plugged it can restrict your exhaust which can cause it to back up and studder.

I know the '94-'95 3100's use a 3 solenoid analog EGR (I believe in '96 they switched to a digital EGR, regardless, what I'm about to explain MIGHT still apply).

I know the '94-'95 3100's are known for having carbon deposits that "choke" an EGR tube.

If you look where the EGR sits (which regardless of EGR type, it's in the same spot). It sits on a platform that is cast aluminum that is part of the upper intake. Remove the EGR. The tube that moves gases between the EGR and the upper intake, CLEAN IT. Easiest way is get carb clean and then something you can force down that tube (I use an old piece of speedometer cable, it's stiff enough and flexibble enough to force through). Spray the inside with carb clean and work whatever your choice item/tool is into it until you are CERTAIN it went all the way through. If you CANNOT get it through, remove the throttle body (as the other end of that passage is right behind the throttle body). I've never seen an issue with the tube from the EGR to the exhaust manifold.
Also, clean the matting surfaces of the EGR and it's platform. I have done this without replacing the gasket IF the gasket does not appear all worn out.

When you first start the car it may start/idle funny for a little bit. It has to burn off all the carb cleaner that you sent down.

See if that helps resolve the EGR code, maybe even the random misfire.

yea im going to try that tom, i test the fuel pressure and when the fuel pump is primed, it sits at 40lbs, and i start it, it spikes higher, idk if thats because of the clogged passage, it was p0300 im srry. But im going to check the passage before i swap out the fuel regulator

I don't know the fuel pressure you should have, but without being started 40lbs sounds healthy to me and the fact the pressure goes UP when started also sounds good. You know you have pressure.
Remember all the injector basically just is opens and allows that pressure to squirt air into the cylinder a low or erratic pressure would tell me you have a pump or clogged filter issue.

Try the EGR passage cleaning. Hopefully that will help.

well i worked on her today *my monte*, i cleaned that tunnel and cleaned the throttle body, and swapped out my regulator. It fixed the problem, then the passage way got clogged again, so i went to work with the coat hanger, carb cleaner lol. Now she runs like a champ, i missed my system, and my cam lol. and with gas prices sitting a $3.17...... i smell a good cruise coming

That's awesome!! And not terribly expensive to fix those items!!
THanks for the update and happy cruising!

naw it was spare parts bro :cool: didnt spend a dime really lol already had the carb cleaner.:thumbsup::thumbsup::thumbsup: