Hi all, my 03 3.4 Monte has an afterfire issue over 3k rpm, (which you can read here: https://montecarloforum.com/forum/mo...over-3k-63277/ ) and has recently developed a constant misfire on #3. #3 plug looked real wet when I pulled it, I swapped it with #6 and swapped the coil pack and got no improvement. I used a screwdriver to listen to #2,4,6, and #3 injectors, they're all working but #3 sounds a little dull. I did head gaskets about 10k ago, and decided to replace the fuel injectors at the same time with a set from autozone. I'm going to check plug wire resistance tonight, even though those are new as well. I figure it's a good bet something is wrong with the injector, especially since I can smell fuel sometimes as I walk away from my car, but what is the likelihood that it just needs to be cleaned? I'd really hate to clean it, put it all back together, and then still have this issue. Thanks for your help yall.

 

Did you replace and lube the o-rings on the injectors when you put them in? Any sign of the injector leaking?

 

The o-rings were new, but I didn't lube them, I haven't spotted any leaks but I do remember that I had a leak from a rear injector during my post job checks. I should also mention that I have a code for "catalyst efficiency below threshold" but I'm not sure what that's about.

 

The plug in the socket is #6 which should be running normally. The plug in the fender is from #3, which is consistently misfiring.

The cel flashes when I'm driving, but will be steady at a light.

Edit: I should add that the car does stall on occasion, though this may be attributed to the IAC as I haven't cleaned that out at all.

 

I admit, I only replace injectors if they fail. I have seen plenty of cars run well past 200k miles on factory injectors.
If you did not lube the O-rings on the injector, it is potentially possible they did not seat properly. If is clear fuel is coming out of the injector, but you might not be getting proper combustion in that cylinder if the injector is not properly sealed. The other issue is that the injector is stuck open. I believe the 3400 has a main harness that all the injectors connect with (at least on the 3400 in the Grand Am does). If the Monte 3400 has the same, this is one BIG connector, I think near the alternator. This means you can use a voltmeter and ohm test all injectors from that harness without taking the upper intake apart. See if any of them (specifically #3) gives different results.

Also, double check the gap on the spark plug! if the gap is wrong can cause weird issues. And I don't care if they are "pregapped". I always check gap (and have found sometimes pregapped has issues).

 

I do know that the #3 injector is at least trying to open and close after listening to it with the screwdriver, seems like disassembly, cleaning, proper reinstall has a chance of fixing the problem, it'll just suck if it doesn't work. The ohm test at least sounds reasonably easy if I can figure which pins equate to which cylinders.

 

A wet plug can also be a sign the plug isn't firing. Under the coil pack is a plate that can cause problems. I would check that first. I also run Lucas injector cleaner and have never had a problem with them in any car I own. About every third tank I put a bottle in.

Here is the cheap way to do this. Buy one small bottle of cleaner if you don't have one. Then buy a gallon of cleaner, about $28. Then take a large Heinz ketchup bottle and after cleaning, fill it up. It has a squirt tip, so it makes filling the small bottle very easy.

 

Interesting, I'll keep that in mind, but the ICM is brand new and I tried swapping coils, and plugs, but I got no improvement so I think I've ruled out electrical issues except the plug wire.

 

I looked at a pic of a 3400 engine in a Monte. Looks like it has the connector I am thinking of by the alternator. For giggles, before taking anything apart, I would definitely see if you can ohm out the injectors from that big connector. If so and #3 has a much different value, then you can be certain that is the culprit. Another thought, with the car running, get some starting spray, mist it around that injector (I know, tricky to do with how that upper intake is). If that has any impact on engine performance, you then know you that odds are the injector is not sealed with the intake.

As GrayGhost1951 mentioned the ICM could be related, but having only one cylinder with an issue is a bit odd (as the coil fires two at the same time). But that spawns another thought, double check the firing order is right. My guess is that the firing order is fine (as I would expect more than one cylinder to have an issue), but it would not hurt to recheck and ensure it is not something that simple. Also, not sure if you tried swapping the plug wire out, but that could be worth while to ensure it is not a bad wire or connection with the wire (like the clip inside is snapping on or that is no weird wire damage).

Curious what the culprit is. Definitely keep us updated.

 

Yeah, good advice. Plug wires are cheap too, so wouldn't hurt to change them. Saw a thing on your tube with a guy pulling the plug wire and immediately finding something to ground it to, to see the strength of spark. He only missed once...lol. Good advice on the starting spray. Found many a vacuum leak that way. Again, being wet to me is a sure sign it is not firing, or the injector is overwhelming the plug and wetting it. Those are the two places to start.