Hey everyone, I recently bought a super clean s4 vert and its developed a misfire I cant seem to solve. I drove it around a couple times before storage and it ran great, hasn't run in over 10 years and with a little atf bath, new fuel filter, and some spark plugs she fired right up and has been running strong until now. The car developed this miss while I was warming it up as I do occasionally, I've tried coils and it has brand new plugs but still no difference. I did however find if I pull the engine fuse and "deflood" it, the car runs much better for a few minutes until the miss comes back. the only place from here I was thinking would be to pull the manifolds and check the injectors? I will also say the car fires nearly first crank every time, never takes more then 2 seconds of cranking before it fires and "idles" haha. any help is appreciated! thanks everyone!

 

I would suspect injectors, or a vac leak.

If you're going to pull the injectors to clean them, you'll have to take apart the upper intake to get to the primaries. If you're going that far, might as well rebuild the intake with new gasket, vac lines, etc.

 

I would also suspect a vacuum leak. These cars have so much vacuum hose for the emissions system and it tends to crack, especially on the ends where it's tough to see it while assembled.

Also, the LIM > Engine gasket can sometimes start to leak. If you replace it, instead of getting another multi-layered steel gasket like the factory used, get one of those blue ones Atkins sells.

You can test using the propane method, but you have to be careful as it is flammable. For that reason alone I highly recommend the smoke method, if you have a smoke machine or know a mechanic with one.

 

Sorry for the late reply guys, ill be sure to follow the advice given. I figured if i was going through the trouble of pulling the manifolds id redo the vacuum lines and gaskets too. No one has dove into the forum saying "REBUILD" or "sounds like low compression" and that's basically what i needed to hear lol. Thanks again guys I appreciate your help!!!

 

My 89 vert has a rise and drop in the idle while cold until its warm then its fine. I learned to live with it and just keep the throttle slightly higher while warming up. Weirdly, it only happens when in neutral and not in gear.

It does not bother me much since its only when cold.

 

Alright another quick update if anyone is still following, ive redone the intake manifold and injectors. no difference . BUT for whatever reason i was messing with the coils again (just hoping something would change probably) and the car stopped missing, i unplugged the leading wires and cranked it over. went back and plugged the leading wires back in and it still didn't start. went back over and moved some stuff around making sure everything was seated and the car fired right up, no misfire but I lost my tach! wtf is going on!!!

 

The tach is driven from the trailing coils, so I would check that connection one more time. I think there are two connectors, make sure they're both in firmly.

Progress is progress though

 

OK I have finally fixed it!!!! turns out the problem was with the grounds (of course) I followed Aaron cakes ground wright up and it fixed it right up. Cleaned the ecu ground (without taking the manifold off, don't let anyone tell you it cant be done) and it still had a slight misfire, but it cleared up a lot of it. I then cleaned the main battery ground on the strut tower and added a 4 gauge wire between the main and a bolt on the AC bracket. The car runs like a million bucks now. 80s wiring can shove it lmao

Thanks again guys for helping along the way!!