Misfire at high RPM?

[phiebert]

Over the past few months I have noticed what seems like a misfire at high RPMs (over 5000). I have used 94 octane gas since I got the car years ago and when it started to happen I thought maybe I just had a bad tank of gas. I refilled and it seemed to go away. Then it came back and I tried a lower octane fuel and it seemed to go away. Now it is back again with the lower octane fuel. It probably isn't a fuel issue at all.

The misfire is almost like a "warble" of the engine and very little power. It is inconsistent, it doesn't happen every single WOT. But it is happening more and more. Before I start down a long road to figure it out I thought I'd post and see where people suggest I start. I was going to replace the spark plugs first (I bought new high end plug wires last year so I hope that they are still OK). If that doesn't do it I'm guessing it is some ignition component. How likely is it that it is a fuel issue? Would it be possible that the fuel pump is gong and can't keep up at higher rpms?

Any advice is appreciated.

 

[LETHAL GTS]

You may know this already, but I'll mention it anyway. You sure you're not bumping it off the rev limiter?

 

[Schulmann]

Plug replacment is a good start.
Use the stock ones they are the best for your viper.
Check also your wires there might be an electrical leak.

Fuel quality should not be an issue, espectially in BC.
Even 91 oct is enough. Think ratherabout your fuel filter.

If you had a logging tool VEC2 or ODB logger you could see more.

An LM-1 tool could easily tell you if you are leaning out. If AFR is good then you have an ignition issue.

 

[Jack B]

Do you have a VEC2 because there have been some high speed miss issues associated with some older firmware versions? As far as fuel pressure, it is pretty easy to attach a temporary fuel pressure gauge to verify pump output. The Schrader valve on yours is behind the drivers side valve cover.

 

[GTSnake]

Coil Packs???

 

[phiebert]

I don't have a VEC2, just stock. I haven't had a chance to work on it yet but I will do the simple plug replacement first and see where it goes after that. This is probably a stupid question but what would one cylinder missing out of ten feel/sound like at high rpms? Would you even notice until more than one cylinder wasn't firing?

 

[vettekiller]

yes you could notice if 1 cylinder is missing. it will be slight but you can. I would do plugs and a fuel filter to start. probably worn plugs and not good enough spark or blowing out the spark.

 

[Gerald]

Plugs and Wires..... 1st thing to do without a doubt. Throwing ANY kind of code? When my 02 sensors were going, they wouldnt' throw a code, but car would break up intermitently in the upper RPM range... food for thought...

G

 

[big-n-italian]

i had that a similar problem before and found that i didnt have a plug wire securely fastened to the spark plug.

 

[LETHAL GTS]

If you haven't done plugs and wires in the last couple years, you should do them anyway. Chances are that would solve your problem.

 

[phiebert]

Not throwing any codes. I'll do the plugs and wires and then try the O2 sensors after that if the problem is still there. Thanks for the help!

 

[phiebert]

Thanks for the suggestions folks. I finally decided a few weeks back I would "fix" the problem. Before pulling plugs, wires, etc. I decided to reset the PCM and ICM (unplug it, leave it, plug it back it...lazy man's version of disconnecting the battery). Amazingly this has fixed the problem!

I'm guessing that because when I last changed the battery and did some racing right after with the Nitrous that the computer got confused because of the wierd fuel mixture. So perhaps the computer learned the A/F mixture or something else that is different when running with Nitrous at WOT. Then when running without it the car would misfire at WOT. Reseting it and running with normal octane (which is 94 up here) and no Nitrous for a while after reseting the CM's seems to have taught the computer the right thing. It even seems to pull a bit harder than it did before. I don't think it was running lean because I have a secondary fuel supply for it on Nitrous. I'm guessing that the computer changed the air/fuel mixture due to Nitrous.

Either way, I'm happy now and it didn't cost me a thing! Thanks for everyone's suggestions.

 

[Jack B]

Phiebert:

I am glad you solved your problem, but, there may be a different reason. At wot the PCM runs off of a fixed fuel map. There is no learning/relearning at wot. The only adjustments are for air temp, coolant temp and altitude. The O2 sensors are not controlling the a/f during wot. On the other hand, like any computer after many writes/rewrites (in closed loop) they slow down and or get confused, therefore, the reason for getting rid of the writeable memory.