Hi everyone! I had a problem when I went to start my 06 Monte 3.9 Lt today. I put the key in, tried to start, it started to turn over, but not completely. I tried 3 times with the same problem, then let it sit for about a half hour. I thought maybe the starter went. I went back to try it again, and it started right up, but started to shake violently, and the rpms went up to a little over 1,000 and the CEL came on. When I hooked up my code reader, I got a P0305, which said it was a cylinder 5 misfire.
Any ideas on potential problems? I’ve been researching it on other cars since I don’t see much here, but everyone seems to be talking about a bad coil, or spark plug.
I was thinking when I tried to start it that the engine may have flooded, and maybe the plug is now fouled since I had starting problems. I’m going to try to check it tomorrow, but in the meantime, any other suggestions would be great as I keep looking for things to check.
Thanks in advance!

 

To my knowledge, you really can't "flood" a modern fuel injected car like people used to do on carbonated cars. Not saying it's impossible, but not likely.

Since modern cars can tell you which cylinder is mis-firing, it's a bit of a short cut in troubleshooting.
That being said, you need to examine what is related to a cylinder and being able to fire or cause a mis-fire. Assuming it's not internal to the engine (like a bad valve for example), it's common to look at:
- Plug Wire
- Plug
- Injector
- Ignition Control Module (ICM)
- Coil
- Other (wiring problem/internal problem)

If you are near/over 100K miles and still on factory plugs, this might be a good time for a plug and plug wire change out (they only last roughly 100K miles, 150K is pushing it but has been done).
My first step in checking this out, pull the plug wire. While the wire is off, check the coil and ensure there is not corrosion on the point the wire connects to on the coil. Moisture can sometimes get in there and cause surface rust that results in a bad connection.
If the wire and connection on the coil look good, remove the spark plug (with the engine cold). Inspect the plug. You may also want to check the gap on it and make sure it's still accurate (improperly gapped plugs or plugs worn to where the gap is no longer accurate can cause issues). Also, if the plug is really wet or very strong smell of gasoline, it's possible too much fuel is getting into the system.
You can clean the plug before re-installing (assuming you are not replacing it).

A little info about coils. Most my experience with GM coils are the 3100/3400/3800 (on those cars, a coil is next to the last thing I suspect).
The 3100/3400/3800 engines used one coil per two cylinders, the LS v8 engines have one coil per cylinder. These make it handy to trouble shoot by move the coil and see if the problem follows the coil.
I believe the 3.9/3.6/3.5 engines GM offered in '06-'07 (not sure which ones were offered in the Montes, I just know those three were around in GM v6 cars in '06-'07) have one coil for all six cylinders.
So, I would say on your car, a coil is not very likely. What you can do (and would take some research), at the time cylinder #5 fires, there should also be another cylinder firing at the same time. If you can find that out, you can swap those connections and see if the problem is still focused on cylinder #5 (ruling out, the coil and ICM).

If you put everything together and after checking everything and reseating the plug wire and nothing seems to add up, it could be a failed injector at that point (either hanging open too long dumping extra fuel in or staying closed and not allowing enough or any fuel in).

I hope this helps.

 

Thank you so much! That was INCREDIBLY helpful. Something I didn’t even think to mention until you pointed it out, I noticed that when the car was running with the CEL on and the hood was open, I definitely noticed a fuel odor by the engine, which is why I brought up the flooded issue. Supposedly the plugs were changed about 20,000 miles ago, but the thought did cross my mind that maybe they didn’t change all of them. On a different car I had, I was told when the alternator died and the car was running, the plugs became fouled, so I thought with the starting problem I had on this one, maybe somehow too much fuel went in there and ruined the plug in cylinder 5.
I should be able to look at it tomorrow and report back with those suggestions you said.
Again, thank you so much for the response!

 

Because of how fuel injection works, you should not smell fuel in or around the car It's pretty much a closed setup).
Fuel smells can be signs of a fuel line leak, an emissions control improperly venting, a plug or injector not properly seated. Just some examples.

 

Just got a chance to pull the cylinder 5 plug, and it looks like this and smells like fuel! I think it definitely needs to be replaced. I also saw it is a Bosch plug. I'm not sure if it is really a problem or not since I've seen many posts about keeping the ACdelcos.

 

Bosch Spark Plugs = BAD!
Stay AC Delco or NGK. You can get NGK platinums from RockAuto after shipping really cheap. I have them in a lot of 3100/3400 powered cars and they are great! I have had too many personal problems with Bosch plugs.

Since you found a Bosch plug, I would replace all of them.

 

Replacing them is now definitely on the to do list right away when the cylinder starts to work.
An update, I bought a spark tester today and tried it out. It looks like the ignition system is working correctly..I had the same results on a good cylinder as I did on the problem one, so I think I'm ready to rule out ignition problems. I also checked the wire resistance and new plug resistance and both look good. I also bought a compression tester which I think I will try out in a day or two..which I'm hoping will show no problems.
After that, on to the fuel injectors, which look like they're somewhat annoying to access on the 3.9.
Right now my main concern if the compression is good is actually a possible problem with the PCM causing the misfire and not directly the fuel system. On this specific car, I heard of the computer being reprogrammed a few times, which is why I'm now worried about that!
If I get up to this step, any ideas on if a noid light test might indicate a bad PCM? For example, if there's power going to an injector, and it looks good, but isn't spraying any fuel.
Thanks for all your help and the info!

 

I have trouble believing the problem is the PCM. it's very rare to be the PCM, I would keep looking elsewhere.

I'm not as familiar with the 3.9, but rather then jump to ripping into the injectors, try getting access to the injector connections (either at the injectors them selves or at a main wiring harness). Sometimes you can OHM test the injectors, compare the readings and find if you have an injector with a problem. In this case, you would OHM them out and hope that #5 have shows dramatically different, indicating that is probably your problem.

When pulling the rail, ensure you get new injector O-rings and replace them before re-installing them (only worry about the O-rings that are exposed from pulling the injectors). They age and once pulling, they may not re-seal properly. a thin coat of motor oil or dish soap on your finger rubbed on the O-rings makes a good lubricant when re-installing the injectors.

 

Another update: Checking the readings at the main wire harness was a great idea. The only problem, all of the injectors appear to be ok when I tested them. I set the meter to 200 OHMS, and I'm not sure which reading went with which injector, but the results were 13.9, 13.8, 13.7, 13.7, 13.7, 13.7. I then decided to test the readings the opposite way into the PCM...I'm not sure if it even means anything..I had to set the meter to 2000K OHMS, and almost each reading was right around 1500. Does that sound too high, or is it ok going the other way? All of them were almost the same, not just one. Also, is it possible the injector might be reading ok with the resistance test, but actually be clogged, or not letting fuel go through?

 

Update: I tried the compression kit out today...I'm not sure if I'm doing something wrong, but I decided to start with the problem cylinder 5...I got 0 compression. I tried again, same result. I then tried one of the front cylinders, and got a reading of 140. I'm taking a break, hoping that I did something wrong with the back one, maybe the connection wasn't tight enough, but it isn't looking good. I'll be checking it out again later or tomorrow