She's breaking up...

My jeep , 90 xj auto 4.0 motor is breaking up real bad when I hit 3 grand. It will still keep accelerating , but it sounds like crap. I'm thinking maybe coil but I am open for suggestions, it's to cold outside to guessing.
Thanx Digger

Do you still have the original plug wires? That would be my first guess. Spark plugs need more voltage to ignite the fuel/air charge under load, and cold weather doesn't help old wires. The stock wires aren't really "wires" at all '' they are fibers with carbon or graphite as the conductor, and as they age the stuff breaks down so that instead of a continuous electrical path you have a series of tiny gaps for the spark to jump. Never really thought about it, but I suppose in cold weather those gaps get bigger.

Loss of power at higher rpms can be attributed to a plugged CAT as well. Normally it doesn't feel as much like a miss, just a big loss of power.

old_man said:

Loss of power at higher rpms can be attributed to a plugged CAT as well. Normally it doesn't feel as much like a miss, just a big loss of power.

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It definatly sounds electrical. I have different plug wires on but I cant exactly call them new. Why would it happen right 3 grand all the time? Old Man that is what makes me think its not the cat. This is not just a loss of power.

digger4u said:

It definatly sounds electrical. I have different plug wires on but I cant exactly call them new. Why would it happen right 3 grand all the time? Old Man that is what makes me think its not the cat. This is not just a loss of power.

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It might also be a worn distributor. It might hit a harmonic and start wobbling at 3,000 RPM. But wires are cheap and easy to change, so if they are at all questionable it's a place to start.

It could also be fuel starvation -- pump, filter, or injectors. When a friend first installed his new stroker engine, he had a bad flat spot somewhere around 3,000 RPM. He fixed it temporarily with an adjustable MAP sensor to call for more fuel, but he ultimately went to larger injectors. Maybe your injectors are clogged a bit?

Thanks Eagle for the info, Several months ago when this problem started it wouldn't even go past 3,000 rpm. Since then I have added new 5.0 injectors, which solved another problem i was having and at least made it so I could get to and past 3 grand it just gets sick after 3. Ill look at my dist. sunday.

You haven't per chance pulled the distributor for anything have you? The indexing could be off on the distributor. .

If you free wrap the engine, does it cut out at 3krpms also? I also have seen CPS's that crapped out at higher rpms and it doesn't care if its under load or not. A bad plug wire or cap normally causes rougher running at an idle and smooths out as the rpm's go up.

Eagle said:

It might also be a worn distributor. It might hit a harmonic and start wobbling at 3,000 RPM. But wires are cheap and easy to change, so if they are at all questionable it's a place to start.

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While your putting on the new wires remove the cap and see if you can wiggle the rotor shaft. Mine was worn pretty bad at 142k.

JoBo