misfire in upper rpm range

[cyrusdrum]

At highway speeds in 4th or 5th gear usually right about at 4k rpm or 80 mph it starts to misfire. It loses power then comes back in making the car shake/jolt. The tach needle also bounces from 4k to almost around 6k. I checked pocket logger and it is reading normal rpm, no changes in timing, and no knock. From searching around I think it may be the CAS. But it only does this in upper rpm range. It almost seem like when it goes to open loop mode, but I don't know why that would cause this. Any help would be greatly appreciated, Thanks.

Tim

 

[VegasTSI]

check the vacum lines going from your intake manifold to your fpr , one of my lines actually fell off and I was getting backfires at 5,000+ rpm

 

[Jingx284]

how are your plugs? Are they gapped properly? Are you wires in good condition? Have you done a boost leak test?

These are the first things that should be done. Start from the easiest.

 

[JayRolla]

I had the same problem and it turned out to be just some bad spark plug wires

 

[cyrusdrum]

Spark plugs were just replaced to try and fix the problem so those are new, wires are fairly new, maybe 4 months old (bosch wires). I just went out and checked all the hoses going from the intake manifold to everywhere. I have not done a boost leak test cause I don't have what I need with me, I guess I can do that when I go home for the summer. Seems like a boost leak would make it misfire but the bouncy tach thing I don't under stand. The wires are held right next to each other, I wonder if that might be it. I will try getting a wire seperator. Any other ideas, Thanks again

Tim

 

[JayRolla]

Ive been told by a few people to stay away from bosch wires but if there that new probably not the problem. Yea so I would say maybe a boost leak.

 

[ez1286]

I think you were right with the cas, because the tach gets it's info drom the cas so it could be sending out a faulty signal.

 

[cyrusdrum]

Any ways to test this? I guess I could just visualy check it.