Jbrew

You have to be sure because the rear end can and will shake both once the fluid breaks down. It'll shake the truck off the road it can be so bad. You should be able to differentiate between the rear end and misfire but, there's been plenty of occurrences the rear end has fooled people.

bigmikek

If slip the clutch above 1500 rpm to get going does it still shutter?

Carcrazygts2

If its a V8 it coule be coil packs. Is there a check engine light/

DevonF88

Truck doesn't shutter when I start off revving higher than 1500. No CEL, haven't checked for codes yet.

Jbrew

Lets sort of start over, not totally but some of your description and questions are sort of confusing on page one. Well to me anyway lol.

Idling in park you should be at 725 -735 Rpms @ idle when the vehicle is running perfect. Ford Service has between 700-750rpms as being normal. Anyway, that's how you can tell if you have vacuum problems.

Also when pulling the regulator line when checking fuel pressure, IF there is no change, that regulator is shot.

Since you replaced coil packs and such, well, non of that matters so keep that in mind. BUT, if the exact problem is repeated to a T, then yes, common sense tells you that they weren't the problem. But the exact same problem has to reoccur right. Process of elimination.

Sounds to me like you have a Low Grade Misfire. This can be caused by low compresion in those cylinders showing up in the KAM (KeepAliveMemory).

On the other foot, you may have a pressure regulator problem if it was checked out properly, - which will stress the pump, burn it up fast, specially if the tank is a 1/4 full. Fuel cools the pump and that really only comes into the picture if the pump is working overtime. You'll burn it right up w/bad reg.

Does the engine miss at all stationary and in neutral ? Or just under low loads in drive ?

DevonF88

Idles perfect when in neutral, can't tell the revs because of lack of a tachometer. Only misses when driving at low rpm and getting on the gas. Problem occurred very suddenly while driving on the highway up a hill in 5th gear. So the relief valve is in the fuel rail? Easy fix or will I need a new rail?