I have done a complete tune-up including new cops. It runs great at low speeds and under load, but when i'm driving in the 35 to 45 MPH range it is missing. It seems to be right when the truck should shift into high. This is driving me crazy. PLEASE ANY SUGGESTIONS ON WHAT COULD BE WRONG WOULD BE WELCOMED.

Dan

 

Did you fill the spark plug boots with dielectric grease?

 

Did you gap the plugs to spec? (If it's 3 valve = no adjustment / 2 valve = .052" - .056" which you can double check by looking under your hood at the engine spec sheet) The COP may not be seated correctly which dielectric grease is recommended also at the bottom of the boot, another common mistake is the COP wiring connector is not on all the way, to check that pull on the connector with a little force to see if it pulls off or not) Possibly may have a bad cop or if 3 valve have seen bad plugs straight out of the box, to find it without a datalogger you'll need to generate a code.

 

2 valve, plugs were gapped and i checked all the connections. It stopped or got better for a week or so after running it with premium fuel and with the overdrive off but started back up yesterday.

 

Would suspect you have a bad coil on plug. Would be easier to diag if you had a code. Possibly the COP boot has moisture causing a misfire also. Or a COP not properly seated.

 

Make sure your coil retaining bolts are snug.


Our new Ford COP’s, the Sultans of Spark(S.O.S.), have
been designed with both durability and performance in mind.
Please see them at:
http://www.performancedistributors.com/soscoils.htm
Quality was the main feature we stressed when we went to the
drawing board on these coils.
See your FTE special at:
https://www.ford-trucks.com/forums/793806-cop-fte-special.html#post6780133

 

sounds like a problem with the COPs i would check all of them over

 

Did you find it????? Just kidding haha. Just a thought why would it be "running rough at 35 to 45 mph" and not when at idle or something. Good description but I would put my money on a bad U-joint which tends to show itself at 40 mph via vibration.

 

Torque converter lock-up clutch chatter.

 

I have the 2000 F250 with the 5.4 and i am haveing the same problem. but mine gets so bad i have to turn the motor off then start it back up and it will do fine for awile.

 

Everytime at 35-45? What kind of vehicle, is this the 98 F-150??? Well if you count the shifts and after you feel the last shift 4th if it does it only after that then I would say you have a torque converter slipping on engagement. Not uncommon. Not cheap either.. A trans flush may cure but may not. Check that, I wouldn't dig into the engine much more. Ignition problems/ misfires would only be present at idle or load. Not at specific speed. 35-45mph is about where the torque converter will engage at light accell...

 

My problem is happens most times before shifting in to high (OD). I have notice it happening at other times now but still no check engine light....
New Coils/plugs/fuel filter have not solved the problem. i'm starting to wonder if it is in the fuel delivery (injectors / fuel pump)

 

I had a similar issue with my 5.4L. (Which I did find the solution to.)

It's a long thread, but you will have to read it all to follow my path to repair.

https://www.ford-trucks.com/forums/7...a-bad-cop.html

Stone