My 1991 F250 7.3 IDI "Wait To Start" light goes off after only 4~5 seconds. The truck doesn't immediately start because I think the glow plugs aren't heated enough. I checked all 8 glow plugs with a test light and all indicate OK. If I leave the switch on for another 10 count, it starts right up.

The light on my 1991 F350 with the same engine stays on around 12 seconds and the engine always starts right up. Is the controller bad on my F250?

 

Did you ohm out each glow plug?
not sure of the resistance off hand but if you get one or two way outta sink with the others there's the bad ones.

 

I did the quick continuity check with a test light only. I believe I read somewhere that the resistance should be less than 6 ohms off the vehicle.

I bought this truck 7 years ago from the family of the original owner. It still has less than 90,000 miles on it because it sat for 12 years in a barn. Pretty sure these are the original glow plugs. Old age could be wearing them out from sitting.

 

I'd put a meter on them just to rule them out.
all the times the w.t.s. light went out that quick, it's been due to bad glow plugs

 

my guess is one or two bad glow plugs. i would just get 8 new ZD-9 plugs from rock auto for $9.33 each and then you will know it will be good for another 20 years.

 

Thanks, guys. You are probably right. My '91 F350 is also a low miles truck, but is still 26 years old. I replaced all glow plugs on it a few years ago because 3 of them actually checked bad with a test light.