The 4.3 and auto trans. in my van loses power and vacuum while driving on the hyway after an hour or so. I get vac. back by letting up on the throttle. When it loses power it wont stay at speed without downshifting to thrid gear on flat level road. I've replaced the distributor, cap,rotor, plugs, wires, fuel pump, fuel filter. Don't know what to look at next.
Catalytic converter?
Overheating coil?
Overheating engine?
Overheating computer?
I've had the converter backpressure tested and it is ok. To give you a little more info, when a timing light is connected it seems to have a miss, also the injectors seem to miss and clatter. This is all the time , the problem seems to fix it'self when I slow down or stop. It starts loosing power and vac. sooner than an hour after a stop.I don't know if a computer can cause these symptoms but that's what I'm leaning toward replacing.
Check the exhaust back pressure. Cats probably plugged up.
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