[updated:LAST EDITED ON 28-Sep-02 AT 07:38 PM (EST)]I have a 91 F250 with a 351W. The engine was recently rebuilt, and I've been having trouble getting a smooth idle ever since. Today's question is about ignition timing on this motor.

To time this motor, you pull out the SPOUT and set the base timing at 10 before, hot. I did this. With SPOUT back in the timing at idle is about 20 before. As you advance the throttle the timing jumps up, all at once, to about 30 before. It stays at 30 before at all engine speeds -- actually drops a bit at higher speeds.

Does this sound right? There is no "curve" it just jumps right up, and it does it just as the throttle comes off the idle stop. The engine speed increases when the timing jumps, but I don't know if that is a cause or an effect.

When the engine is stone cold, the idle timing is about 20 before, but jumps up as you just start to open the throttle to something off scale -- about +50! Again, no curve -- all at once.

I don't know what it did before the overhaul, but it idled better than now. I'm trying to diagnose why I can't get a good idle (hot or cold), and just trying things. Currently, I'm looking at the ignition.

 

Have you pulled any codes yet to see if the computer is happy? Also while it is running, pull the plug on the IAC motor. The engine should slowly die. If it keeps running, the throttle idle adjustment screw is too far in, or you have a vacuum leak. If the engine stops quickly when the plug is pulled, the throttle is too far closed.

 

Thanks -- I'll try these ideas. There are no codes, I've checked several times over the last couple of months.