Just did a head gasket on my 4.0, also put reconditioned head on. new rod and main bearings and complete go through checking everything as I went.
got it back together and starts right up reaching in through the window. will sit and idle for hours. Problem is, when running down the road, I have very little power, seems to lose power as it goes, and has no torque either.
I have checked everything I can think of and no luck.. like I said, it starts and idles like a champ, but just no ***** at all.. any ideas>? thanks..

 

What is a 4.0? You mean 4.9?

 

Well, couple of thoughts.

Did you check compression before you did the head? I assume so, but I was once told engines are like a chain, if there is a weak link it sometimes shows up when the other links are made stronger. Or to put it another way, if the valves were letting some compression by the rings it may have run ok, but now that the valves are sealed the rings may not be up to it. Would most likely get some blow by if this is the case, and another compression check would tell you for sure.

Could be timing. That could make it idle and start fine, but not have much power.

Plug wires. They might be routed differently enough now that they would cross-talk under a load and be ok at idle.

Fuel pressure. Sometimes when you disturb a system it loosens dirt/debris, or what have you and redeposits it where it can cause a problem - possibly restricting injector flow making the engine lean out.

Some things to think about anyway.

 

thanks, but been down those roads already, the only thing that I have come up with so far is that I may have matched the dished pistons with the larger head chambers, thereby creating to much combustion chamber space.. I know that it has the dished pistons, and I was told by local machine shop that Ford produced two different sized heads for the 4.0.. He thinks I may have big head and dished pistons.. SO... thanks for the info though..

 

Yes, excuse me, that is a 4.9 liter inline Six... sorry for the mistake.. I seem to do that every time... just old age I guess...

 

Well, check your compression, then. That should tell you whether you did or not without you having to take it apart.

Also, check to see what timing does as you climb the rev range.

 

you may have your primary fuel filter plugging up on it . I had this happen on a 1987 i owned.
If you have one it is on the frame rail drivers side. It is black and has a cover on the bottom that you can use a filter wrench on to take off .
I found out the hard way on a trip accross montana after stopping at 3 ford garages the last one knew exactly what and where it was.