My '96 B2300 SE started running a little rough from time to time and losing power. It has an intermittent misfire. Finally an engine light came on and I got a cylinder 4 misfire code. Since then I have replaced the plugs and wires twice (in case the new ones were bad), replaced coil packs, injectors, catalytic converter and computer. It has compression. It still misses and I still get the light coming on with the same error code. My mechanic said he's out of ideas so I guess the dealer is my last hope...

I was thinking fuel pump is all that's left to try but why would I only get a cylinder 4 misifre code? Wouldn't all the cylinders misfire if the pump was bad?

 

Well, I'm stumped too. someone is bound to have an idea, but I dont.

 

vac leak would be my guess especially since it is on # 4 clyinder if I remember right the tee that supplies vac to the other accessories is in the #4 area on the intake.

 

I would have guessed vacuum leak or an intake manifold or head gasket leak from the back side of the engine.

As for replacing injectors, all you really needed to do to check those was to put a screwdriver up to the 3 known good ones and listen for a different sound in #4. The cat convertor would have given off a completely different code, not related to a misfire on just one cylinder anyway, and your coils and computer could have been tested without replacemnt necessarily. Apologies for telling you after the fact....but you didn't ask until after the fact.

Do a wet AND dry compression test if it hasn't been done. Maybe a bad valve will come up from those tests. As for a stealership helping you....you must be really expecting miracles!

I bet that if you had google searched somethng like "causes of cylinder misfire", you may have come up with some money-saving hints, too. Google is pretty great, IMHO. It's just too bad people ask for help AFTER they spend all that money....

 

The only other things I can think of that hasn't been mentioned, is the crankshaft reluctor ring, that generates the trigger signal for the CKP sensor, or a intermittent sticking valve, or collapsed lifter.

Maybe the CKP reluctor ring has some contamination on it, or a tooth has been damaged somehow, such that the sensors output is sometimes corrupted .

I had a "sometimes" problem with my then new 99 4.0L, that turned out to be a cracked #6 spark plug external insulator. Dealer found it with a wet down test. But this is probably a long shot in your case, unless your #4 plug/s are difficult to get access to & get misteated on installation.

If it's performance is ok when it isn't acting up, then maybe a sticking valve or collapsed lifter should be considered.

Had that happen to me once on my 78 Merc 3.3L, about 20 years ago .

A dose of Marvel Oil in the crank case & gas tank, some light driving & a oil & filter change, has taken care of that!!!! Yup I still have the old Merc & she starts & runs just fine!!!! LOL

So if you think it could be a intermittent sticking valve or misbehaving lifter, maybe a dose of Marvel Oil or SeaFoam, would be worth a try.

When my Merc's lifter acted up, I also had some mechanical noise in the valvetrain with the bad miss. So when yours happens, do you hear any mechanical engine noise from the valve area???? If so, suspect a lifter or sticking valve, If not suspect it's electrical based. IMHO

Just some more ideas for consideration!!!