Weird misfire

Okay, I need some advice from the big boys.

I have a weird misfire on my 348. First the car was misfiring at about 3,000rpm, then fine until 6,000 rpm. Turned out that I had a bad spark plug wire, and found a pinched fuel line. After I changed the plug wire, and got the fuel line straightend out, the car seemed to be running fine. It runs great at idle, but as soon as the engine speed gets to 5,000rpm it starts to misfire. I get no check engine lights at all. I have disconnected the battery for 5 minutes, to reset the ECU's, then reconnected it. Turned the car on and let it idle for 15-20 minutes without touching anything at all. I have disconnected the cat ecu's, and reset the motronic ecu's again. I have disconnected the crank possition sensors, made sure that they are clean, and then reconnected them. Check to make sure that the cam possition sensor at the back of the intake cam on bank 1-4 was connected properly. Made sure that the connections on the injectors were connected properly. Check the connections on the coils. Even made sure that the connectors that are at the back of the intake plenum, or either side of the oil filters were hooked up properly. The sparkplugs only have maybe 1,000 miles on them at best. The O2 sensors are also pretty new.

Now I'm at a loss? So do any of you know of a way I can find out what is causing the misfire? Or, anything that I have missed? I'm really scratching my head here cause, I have gone over everything I can think of. Again the car runs perfect through the rpm range until it hits 5,000rpm or so.

Thanks in advance.

Pretty complicated engine & engine management systems to guess at based on what you've posted.

For starters the engine has dual everything and any failure can cause poor running of one sort or another and honestly there are many components in the system that could cause a high speed miss..

I almost always start with an exhaust gas test to define which bank is running bad/worse. That test can lead to any number of other tests, ie: compression, fuel pressure, intake air leak, manifold vacum, exhaust backpressure, scope test. All of these tests require special tools and the order in which they're performed can vary depending on results of a previous test.


I'll think about this, but without some data about which side of the engine or what type of miss, I'm not sure I can help.

Regards, JRV

As for the type of miss. I think it is related to spark. The reason I say this is because one of the times I had the car sitting going through the different rpm's. idle, 2k, 3k, 4k, then around 5k rpm it would start to misfire. I then went from idle right to 4k, and it sort of started to hesitate, then as it got closer to 5k it would start to miss and then back fire. You know the loud pops. So that is why I think it is electrical. It really seems that it isn't getting the spark it needs.

One mechanic that I talked to said it could be that I have a coil pack going bad on me. I don't have any of the fancy scopes, just a guy that does my own work. I really don't wanna have to take it to a shop cause they will rob me blind. I'm hoping that there is some way of figuring this out at home. Also I even double checked the ground connections at the rear of the car, on the drivers side. Those were okay too.

Personally I doubt the coil pak theory..although in defense of the guy suggesting it, coil paks are on the dart board.

I would think perhaps throttle crossover valve malfunction, MAF meter, fuel pressure problem, throttle syncronization problem, dirty/bad injectors, vacun leak, coolant temp sensor problems, not neccesarily in that order.....

The list above is why it's neccesary to build a data base on running condition...rich would mean further tests in one direction, lean would mean tests in a different direction for example.

Without a wide variety of special tools to define & isolate, I really don't know how complicated problems can be solved simply, short of replacing two of everything until the problem goes away.

Regards, JRV

Yeah I was kind of affraid of that. That is why don't wanna take it to a shop. I don't want to play that game. Because if they replace a part and that doesn't solve the problem, I'm gonna be stuck buying a part I don't need. However I may just take it in and pay to have them diagnose the problem. It sucks that I don't have the equipment to do this myself, cause if I did I would. The cars aren't hard to work on, especially with the right equipment. I have done my own engine major, but this little problem has me puzzled.

Not all mechanics are stupid idiots just out to rob you Ernie......

You might be surprised how smart & educated some mechanics are!


the reason why car diagnoisis costs money at a shop is because all the years of training and all the equipment and the building it's all housed in to diagnois problems correctly costs money.

LOL, well back in the old days the barber would do just that.

I know all too well how smart some mechanics are. I do understand that they have many years of experiance and training. Yes they deserve to be paid to do their job.

However I have spent close to $10,000 having a mechanic work on my car, before I started to do my own. Now that I have done my own work, I now know what it actually takes to fix something. With this new knowledge I looked at the old bills, it just didn't add up. For example I was charged two hours labor for something, that I later found out only took about 45 minutes to one hour to do. Anyway I don't wanna fight. I respect your knowledge and know how.

I understand what you are saying. I just hope you understand what I'm saying. All I'm trying to do is get this problem figured out as cost effectively as possible. That's all. If I offended you that wasn't my intention.

Thanks for the help anyway.

Do a fuel volume check, The pressure maybe there but the volume of fuel is not. This is a common over sight when looking for problems such as this.

mmmm, let's see.. my wife's recent hip replacement cost over $15000, she was in the OR for 45 minutes, so your logic would be that I should do my own surgery because the system has been abused and mismanaged?
As a professional mechanic for over 40 years, having invested my time and money in tools,equipment, training, I still lurk on these forums and try to offer reasonable insight from personal experience just as many others do. I enjoy learning from their experiences, they help me understand my craft and stick to it despite the stigma many seem to wish to attach to us.

As to your problem, as JRV states, difficult to pinpoint, but I would start by establishing a baseline by reading off everything from charging system, exhaust gas and fuel pressure. then go to specifics of every sensor reading. If you can provide any of those figures, I am sure that the wealth of talent on this board will help you get out there running strong.

Regards, Brian

Thanks for the advice guys. I'll keep you posted on what I find.