2002 Ford Expedition 4x4. Engine Size : 5.4 V8.
Question: I have a check engine light with codes P0171 and P0174 dealer said to replace intake manifold gaskets. I replaced intake manifold gaskets. Now the truck idles fine but when you put a load or run the truck hard it runs like crap,

it's missfiring and can hardly make it down the road.

I've try to replace the mass air flow sensor (MAF), fuel injectors, checked all ignition coils (at idle but they are firing). I don't know what else to do. Could the timing have jumped? Not sure how that would have happened but I don't know what else it could be. But won't the truck idle bad if the timing jumped?

Answer: These two check engine light codes ARE common for intake manifold gaskets leaking vacuum. This causes a lean condition on both banks- thus the codes. A misfire under load is usually caused by a problem in the ignition system. Most often a spark plug wire or spark plug breaking down. If this problem was not there before doing the intake gaskets, you may not have gotten the coil connectors mixed up and on the wrong cylinders, or a coil is breaking down under load.

I would scan the engine codes again to see if you have the lean codes came back or misfire codes like P0300 or P0301 P0302. The 1 or 2 means that cylinder #1 or #2 is misfiring so you can concentrate on the one the code points to.

Try spraying water while doing a brake torque to see if you can narrow it down to which cylinder(s) is misfire. That means start the truck, put it in drive with your foot on the brake firmly and then give it some gas. This will put a heavy load on the ignition system. Now if it is running bad have someone remove spark plug wires one at a time. If engine runs worse with each one, that means that cylinder was firing just fine so that one is not the problem. If one of them causes no change then that one was not firing to begin with so there is where the problem is.

Sometimes just disturbing these coils is enough to cause them to start to screw up. They are quite common to fail on this Ford engine. But first make sure the connectors / coils are on the correct cylinders. If that is OK, you may have gotten some debre down in the cylinders and is fouling out a plug or two. This would not make sense to be a timing chain issue since you were not doing anything with that.

It is also possible that you got the wrong gaskets. If they are incorrect you might have several intake ports that is blocked with the gasket. That ill cause low air flow into a cylinder and the engine to not run right.

Also check fuel pressure. Low pressure caused by a weak pump or plugged filter will cause the engine to run lean. This would cause the running bad on acceleration and could also have been the root cause of the lean codes P0171 and P0174.