I bought my 1990 f150 (302) last week. The idle was kinda rough so I replaced the iac valve, also had done plugs/wires. I replaced my headers with some bassani headers and now my truck feels like it's lost power. I have a minor leak where the headers attach to the Y-pipe, but nothing major. The heads have been checked for leaks and are all tightened down to spec.

My idle is still rough, worse with the new headers. Every time I come off the highway, or when I've reached normal operating temp, the idle drops down and my truck dies.

Any ideas?

 

I heard of the o2 sensor doing things like that but other than that I wouldn't have a clue

 

I'm trying to track down the cause of a rough idle myself.

Really ... it can be any one of a hundred things.

Try going through the vacuum system and look for any leaks.

There's probably a set screw up on throttle body for min. idle speed. That could be out of adjustment.

Check and see what you're pulling for vacuum up on the tree and compare that to factory spec.

You could do a compression test on all 8 cylinders to get a general idea of the overall health of the motor.

Could be a problem with one of the sensors or actuators. Try downloading any trouble codes off the engine computer. Sometimes a trouble code will point you in the general direction of a problem.

One or more of the fuel injectors might be fouled. Pull your new spark plugs and see if one or more look different from the rest.

Try checking the fuel pressure up on the rail at idle to see if it's low.

Is there any chance that the new headers could be throwing the motor off ? If it idled well before, and then you got the idle problem afterward, there might be something about that installation that's causing a low idle / stall.

 

I've had the idle problem since I bought the truck (before the new headers). I checked the spark sequence and had 2 plugs mixed up :/ should've seen it the first time. Runs better now, idle is still off. Going to check for leaks, fuel pressure, etc by the end of the week. Will post findings. Thanks.

 

You're not supposed to touch the idle screw, it's factory set and will screw up the TPS. Someone may have touched it before though, so check the reading of the TPS. Also check for codes.

 

True.

The idle screw is held in position with red loctite. If it's still there it gives you a benchmark of where it's supposed to be.