Hey Everyone, I have a problem, I just rebuilt my '90 5.0 because it burnt the #8 piston, and cracked the head above it between the valves. It stems from what I am nearly sure is an ignition problem. The truck has had a mild misfire of shudder at any consistent or crusing speed since I bought it (about 4k mi ago). It seems to be that the computer is advancing the timing to far, or erraticly. I have fully rebuilt the motor (.030, new pistons, reman heads off of a '95 mustang w/46k mi and same casting# e7te-pa). That all went really well, the engine runs great, aside from the ignition thing, as this wasn't my first 302 rebuild. But the same problem is still there, just like before. I have replaced : cap&rotor, plugs, wires, double check timing, before the burnt piston. Since the rebuild, I've replaced the Ignition Module, inspected the TPS ( about .988V at idle, and full smooth transition to a bit over 4V at WOT). I tried another ignition coil, and a lot of head scatching. Also, I've noticed that when you hold the gas pedal at a consistent above idle speed, like 1500 RPM, it will step up RPM on it's own about three time to around 1625RPM and then drop to 1450 RPM and sputter, and repeat. This cycles about every 10 seconds that you hold the pedal. Seems like the ECM is searching for proper timing advance. And it idles normal and WOT on the road seems fine too, it is most noticeable at a consistent speed. Sorry for the long post but I'm trying to provide as much info as I can. Thanks all for the help or even moral support, I'm pulling my hair out and I sure as hell don't want to damage the new motor!!

 

Have you already replaced your dist. stator?

 

Make sure your plug wires aren't running side by side for any
length. I have heard the 5.0L has a problem with cross firing
if the plug wires are running close to each other.....

I'm going on memory, but I believe the worst ones are cylinders
7&8, maybe somebody can help me out on the numbers???

 

7 and 8 are the most likely to crossifre. There is an old TSB that talks about this but i dont remember the number.

 

Ya, I was aware of the 7&8 cylinder crossfire, so when I put the new wires on, before the motor burned up, I routed them as far away from each other as possible to try to correct the problem. No luck. And the distributor stator? Is that the "hall effect" sensor under the cap that reads the position? If so I checked with an ohm meter when I put in the new ignition module and it didn't seem to out of line.

 

yea, thats the one, the hall effect. As far as checking, i think you would have to look at the square wave form on a scope or just replace it. i have used a blinking light on a logic probe but that wont detect an erattic signal, just the on off switching.

 

thanks again Dro Man, well I have a scope, I could scope it but I'll have to investigate what I'm looking for as far as wave patterns. I still haven't ruled out the ECM, remember I asked about ECM part number reference? This is the problem I'm trying to fix.