My truck has been pulling good since I bought it 2 years ago.
Currently pulling 30' travel trailer with no problems until now I have developed a misfire. It only happens at 2500rpm at 90kph under load. Any other speed at 2500rpm no misfire, and in a different gear at 90kph there is no misfire. 90kph and 2500rpm with no trailer on, no misfire. Any ideas

 

Have the MIL codes pulled or read or however one describes that process. When you have those in-hand suggesting a fix or remedy is infinitely easier.

 

I had a misfire in cylinder 1 about a month ago.... replaced the ignition coil on 1 and did all the plugs. Fixed.

 

How did you know the misfire was on cylinder 1?

 

Replacing the part(s) is easy. The hard part is figuring out WHICH part to replace and getting it right.

If there are no codes, use a scanner in mode $06 to look at the misfire counts (test $53). The computer is tracking the misfires to see if they exceed a calibrated threshold to set the MIL. You can read that data.

 

Popped down to local Ford today as it has started to misfire first thing in the morning, once it had warmed up misfire gone. They wanted $100 just to book it in to plug in the computer, they are also booking 1 week in advance as they are busy with winter tire changes.
Left it for now as the misfire is so intermittent that it might not happen on the day it went in.
Drove around all day everything fine again.
Back at home I decided to do some maintenance on the wheel hubs as they had been squeaking, started engine and big bang and misfire. Thought that an exhaust had blown off or similar.
Under the hood, found ignition coil floating around with broken mount. Thought this might be the problem all along. It wasn't until I went to replace the coil with a new one that I found the spark plug lying on top of the engine behind the fuel rail!!
Replaced the plug with new one and new coil. Misfire gone now on start up, but won't know now whether the misfire under load will be gone now until next spring when we'll be taking the trailer out again.
We'll see....
(Engine light never came on once even when driving on 7 cylinders)

 

Uh-oh!

A blown spark plug usually cannot simply be replaced... They usually damage the threads in the head on the way out.

That is usually normal on a OBD1 calibration as misfires are non-MIL faults.

 

Hoping it has just slowly loosened, hence the slight misfire that only happened at a specific rpm. We'll wait and see (fingers crossed)

 

The fact your COP mount was broken strongly suggests the plug was "blown out". Had it been simply loosened somehow you'd get a slight popping sound like an exhaust manifold leak and no damage to the COP.

Bad news is this does require a special process, good news is its not fatal to the engine, costs about $400 max at most independent shops.

 

Really hoping this isn't the case, can't afford $400 at the moment, and I suspect here in the Canadian Rockies that it'll cost more than that!!

 

sorry for such a late response...

My misfire got so bad it tripped the check engine light so I had the truck scanned for codes.... I pretty much knew what was going on, but the codes confirmed and told me which cylinder had the problem. In my case, bad coil on cylinder 1.

 

Thanks for the reply.
The check engine light never came on even when the plug was missing!
The new one seams to be fine at the moment. Day to day driving has been fine, the real test will be in the spring when the trailer comes back out and we hit the road again.

 

Good to hear it's fixed.

Yep towing a trailer will be a good test for sure.