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Engine Misfiring

ive got a 99 blazer with a 4.3 vortec in it and its missing on #5 cylinder ive put plugs and wires on it and its still missing any ideas?

Does it have a distributor? One coil? Three coils? Six coils? How many miles on it?

im geting fire at my plug so that means coil and distributor is good, thinking fuel issue

Fire doesn’t mean fire all of the time - so I wouldn’t rule it out.

Swap the #5 injector with another and see if the misfire moves.

You still didn’t say how many miles on it, but I either way I’d check the compression before doing too much.

good compression 185 psi and holds, so that rules out burnt valve and it dont have injectors is got a fuel injection spider and its got 135000 miles

I love it when all of the information comes out after the fact. Maybe when you post such a vague question that can have multiple causes you could have the courtesy to take the time to tell people everything that you have done and everything that you know.

If the vehicle has the spider assembly or CPFI system the problem might be with the poppet valve for #5 cylinder. These poppet valves rely on the fuel pressure to blow them open to introduce fuel into each cylinder. These poppet valves can become stuck from gum and varnish where the fuel pressure can’t blow it completely open. Since there’s a misfire only at #5 cylinder that poppet valve might be sticking.

What you might want to try is adding a can of SeaFoam Engine Tuneup to a half a tank of gas and then drive the vehicle. If #5 poppet valve is stuck from gum and varnish the SeaFoam might free it up.


yeah when i puld out the #5 tube it wuz full of carbon and gunk, i cleaned that out and gonna put it back together and try that, thanx for you help bud.

Now Tester has another name, I saw you get labled “dude” last year and now it is “bud” gotta love it. I just hate “Dude”

found a bad pcm causing #5 miss. In case anyone wanted to know

Nice to get the “Is that your final answer?” to the problem. Thanks.

How did you come to the bad PCM conclusion?

I am with you on that one Caddyman, this “miss” is probably going to come back. Very few shops would feel comfortable installing a PCM to cure a miss, not without crossing their fingers