What a rotten day, got a call from my wife that the truck 89 F250 302/aod would not start. I got there and found a bad battery(top popped up in a sealed battery), ran the rest of my work route. Got home and got another battery, went back put it in and found the alternator failed! PITA getting it off in a parking lot with only a 6" cresent and 5/8 wrench and stuck bolts. Anyway new batt/alt, now the rotten thing has a bad missfire under load and pinging. This has been a intermittant problem before and never could track it down as it missfires under load(3rd and 4th) at highway speeds. 6 months ago I tuned up with new plugs,wires, cap/roter, reseated all ECU connections and ect, still same..just lived wiith it and also no codes too!

But now after the alt/batt failure this missfire is all the time under moderate load, full throttle is good no miss at all. Problem occures with engine vacuum under 10" get worse around 8" vac then lessens under 5" vac. Bad MAP you guys think?

I am going to pull codes after this lighting storm levels the area(few trees took a hit in the back yard) I posting hopefully before I loose power!

 

Have you pulled the plugs to figure out which ones are doing the miss? I.e. the plugs do not have the golden brown look on them? Then there is the routing of plug wires as well can cause a crossfire condition that leads to miss fire. Another check that is a little more involved is doing a compression test on each cylinder that can tell you if you are losing compression in a cylinder.

 

Its a rebuilt engine less than 20K from the prevous owner said(I pulled bearing caps off to verify,all had 2005 dates on the bearing inserts).

A good possablity of routing of the plug wires may be the problem, I'll check them again and also pull the plugs to see.

Before it was a very intermittant miss, till today, I guess the ECU needs to relearn and it playing with timeing big time showing the miss.

 

Have you verified that the engine is timed correctly at the distributor, i.e. 10 degrees BTDC with the Spout removed? Another issue that may warrant investigation is to see if you have enough fuel pressure, and volume if you have a general miss condition rather than a specific dropped cylinder miss.

 

I had a misfire under load mostly through the higher gears and it turned out to be a bad fuel pump. It did it for a long time mildly then all of a sudden it got bad. Replaced the fuel pump and she ran like new.

 

Yep, its right on 10 deg Btdc. with spout removed, no codes again, Doubt its fuel pressure as WOT it runs with no problems. It is getting better now as the ECU is relearning from the disconnect of power. Going to dig up the wire diagram now to make sure the plug wires are not crossed.