01 LeSabre misfire

Been working on my son's 01 LeSabre. It has a misfire now and then. I checked the code and its showing P0304, which the book says is a misfire in cylinder 2. I changed plugs and wires, noticed that one of the coil packs had a rusty looking connection, so I changed that as well to no effect. I swapped out the rest of the coil packs with ones off another car that I knew were good just to test it out and it made no difference. I also unhooked the MAF and ran it for a bit, but that also had no effect.

The miss is mostly felt during hard acceleration through all the gears and also when the motor is lugging when going up a steep hill in 4th gear. If I shift down to third when going uphill, it will smooth out and feel ok. I did notice a fuel smell around the car, but I can't find anyplace that it seems to be leaking.

Any help in diagnosing this would be greatly appreciated. :thumbsup:

Regardless of what "the book says" P0304 is a misfire on cylinder #4 (center of back bank), not #2. Have you changed ignition wires?

Ok, number 4 then. Yep, changed the wires when I did the plugs.

How about fuel injector on that cylinder? Have you used a noid light to monitor the activity of that injector? How about the injector o-rings, are they leaking fuel? Are you loosing any coolant? Have you pulled the #4 replacement plug to check for excessive fuel/oil/coolant contamination? We will have to assume, that the compression on cylinder #4 is within specs.

I haven't checked the injector yet, but I thought that might be my next step. It does not lose any coolant or use oil. The spark plugs I pulled out looked quite good, but I probably could pull the new number 4 plug and look it over. The car sat without being driven much for about six months, so I'm wondering if an o-ring may have dried out in that time.

You mentioned the smell of fuel around the car, is the smell more pronounced under the hood?

Under the hood is where I first noticed it when I was changing the plugs on the back side of the motor.

From your description, it certainly sound like a seconday ignition misfire....you have changed plugs and wires(hopefully you used OEM plugs and checked the gaps....should be 0.060).....

While you changed coils, it doesn't sound like you checked spark output....need to do that one wire at a time with a spark tester attached.....this will test coil output....if bad output from a coil, swap coil with another.....if the spark output is the same for the same cylinder, bad ICM...if the misfire moved to another cylinder when you moved the coil, the coil is bad....

If spark is ok, and compression is ok, then we may have an injector problem.....if a noid light shows a pulse at #4 injector, it could be the injector....unfortunately, about the only thing you can do is an ohm check....if it is out of limits, then you need an injector....however if you have the same reading as other injectors, it proves nothing....it could still be bad....since no DIY'er has the equiptment to test the coil in the injector, or flow rate....about the only way to know for sure, would be to remove the rail and swap an injector.....if the misfire moves, you know it was the injector....

Under the hood is where I first noticed it when I was changing the plugs on the back side of the motor.

I noticed that wiggling the fuel line connection, the quick release snaps, when I work around the back top of the motor will release a few drops of gas on my 98. I assume there's an o-ring that is set and a little movement causes it to lose the seal because of the pressure residual from the pump still in the line. You might try wiggling those lines and watching. I have tried turning the key to ON and letting the pump pressure up and I get no leak--only if I move the line out of its resting place.

Did you put in the ACDelco plugs recommended by the books?


Since you moved the coils, did you rerun the code check to see if the misfire P number changed with that coil movement?

The slight miss at load at relatively low rpms in 4th such as a slight upgrade is exactly what I have had on my cars with wires aging. I can't duplicate the same miss as a lower road speed but same rpm in 3rd, even on a greater slope giving a little more load, because the higher ratio handles the load with less strain on the engine apparently. It would happen only in 4th.

Related to that, is it variable with how hot the engine is? E.g., does it happen only only warmer days after longer run time on a slope but doesn't happen when the outside temp is cooler and the engine hasn't been running as long? I am thinking you might have a seep of coolant around a gasket that is affecting only certain cylinders and only when cooling system is under high pressure (15 pounds).

My thinking that a small coolant seep might not show on short drives in loss of coolant in the reservoir tank. The mechanic who did my UIM and LIM gaskets showed me the throttle body gasket and how they were all too thin originally and end up giving a seep. You have not said anything about UIM and LIM gaskets having been replaced on this car. It should have had a recall on it where GM put stronger nuts and washers to press harder on the throttle body gasket and put in the ground pecan shell/water pump coolant pills (Barr's in Gold package) to fill seeps and block them, hopefully. If you really want to take a look, you could remove throttle body, clean it, and replace gasket with a new one.

Total Miles on the car hasn't been given in any post, but you might want to do UIM and LIM gaskets.