I'm a first time poster on this site. Hopefully, I picked the correct forum.

My son's 90 Bronco w/ 302 V8, FI has a long running drivability problem that I've been unable to resolve. Vehicle mileage is about 140K.

The engine starts well, tends to idle high for just a few seconds, and then idle declines fairly quickly till the engine will stall if throttle is not applied. After warming for a minute or so, the engine will idle steady at about 700 rpm.

When engine is cold, acceleration from a standing stop is Ok, not head snapping but ok. After fully warmed, the engine will bog badly under moderate throttle and if pressed hard, will lightly backfire through the throttle body, perhaps a couple of times. Under light acceleration, the engine pulls steady. Under a heavy acceleration (flooring throttle), the engine pulls pretty good w/ no backfiring.

Timing was set closely to 10 deg. BTDC w/ SPOUT open. Engine has decent compressions, 145-160 psi, after 4 compression strokes on cold engine. It doesn't seem to burn any oil, but has a problem w/ pushing oil into the air filter housing through the PCV air inlet. Idle vacuum is OK, but system vacuum is lost under acceleration. This may be related to performance problem. PCV has been replaced and system checks OK.

I have replaced spark plugs and o2 sensor. I pulled the exhaust loose to check for blocked catalytic converter.

I blanked the EGR port w/ a no-hole gasket and ran the car, but same symptoms. EGR checks as sealing well when closed, and EGR behavior at high idle is correct.

I checked the MAP w/ an oscilloscope and determined the MAP is performing pretty close to spec. I calculated 166 hz w/ KeyOn-EngineOff and 111 hz at idle. These are a little high but my resolution of the MAP signal width is not that accurate from the scope. If it's off, its not by much. Spec I have says 159 hz at sealevel (Houston's pretty close) and 109 at idle. The signal was a nice square DC wave as it's supposed to be.

Someone told me the TPS could cause this problem. I checked voltages at idle stop (0.7V) and full throttle (4.5V) and they appear to be reasonable. Idle voltage is supposed to be 1V.

There are no codes set.

All I can think to do from here is to check fuel pressure and start replacing the ignition parts I haven't replaced; wires, rotor, distributor cap, coil. I really don't suspect ignition because the engine doesn't miss, it just bogs and backfires.

I would appreciate any leads or suggestions from those who may have encountered similar problems.

Regards

 

sounds like a small vacuum leak. May not pick it up on the gauge. I had a bad line going from the TAD or TAB solenoid to the one valve in the AIR line on the backside of the engine that was causing problems.