I have a 2004 Navigator, with the following problem. The engine runs smooth until you go to accelerate or go up a hill, then the engine seems to skip/jerk. It also does this with overdrive turned off. Additionally, under hard acceleration from a stop, there is some hesitation in acceleration.

So far I have:
-Changed the oil
-air injection cleaning done at Valvoline, including cleaning the throttle body.
-transmission flush
-tested all coil packs, all read fine
-replaced all 8 spark plugs

There is no check engine light on, it has 63,000 miles on it.

Any suggestions would be appreciated. My next plan of action is to replace the fuel injectors. Also looking for any suggestions on how difficult that task would be. Thank you!

 

The coils may be fine and working okay, but if the rubber boots are original then they could be "leaking" spark. When the insulation quality of the boots breaks down, the spark will jump to the side wall of the spark plug hole instead of traveling all the way down to the spark plug. A set of 8 boots is around $25.

 

I forgot to mention, engine runs perfect at idle and under normal accelleration. The boots all looked fine, very flexible/soft still. but for only 25 bucks it is worth a try.

 

That's par for the course. The more the throttle is opened the more air/fuel vapor in the cylinders. This has the effect of creating a more dense mixture within the cylinder. Because of the more dense condition it is harder for the spark plug to make a spark. As a result, the spark looks for an easier path to ground.

This is why you can have perfect spark at idle and mild acceleration, but get spark jump and a misfire while under load.

 

my favorite - check the injectors. Less than an hour's worth of work.

Pull them and look at the tips. If there are any that are cracked, gummy, split they need to be replaced. Chances are one is cracked, or at least may be really crapped up.

Get a can of carb & choke cleaner before hand. stuff works really well for cleaning injectors. Adn, IMHO, injector cleaner in the gas tank is useless.

To do the injectors, you only need a 7mm socket and some lightweight motor oil. (10w-30, 5w-30, etc.)

find the fuel pump cut-off switch - should be down near the passenger door footwell. Loosen the connector on it but not enough so it still works. Start the engine, let it run for a couple seconds, then pull that connector off. The engine will run rough, then die. Turn the key off and remove the battery positive cable. REINSTALL the fuel pump connector now so you don't forget about it! The fuel system is now de-pressurized and safe(r) to work on. It will still dribble a little bit when you pull the rail up, but nothing to worry about.

remove the two 7mm sockets from each rail and pull it straight up (it might take some force.)

Pull each injector straight up (might need a little wiggling) and check / clean the tips.

replace them in their holes & connect the electrical's

lube the o-rings with the oil (both ends)

push the fuel rail straight down & reinsert the 7mm sockets.

Shouldn't be more than 30 to 40 minutes per side. you DO NOT need to disconnect the fuel lines or anything else.

 

was able to solve the problem, there were two coils that were failing under load. both were replaced and everything is fine.

thanks for the help

 

So much for that COP testing procedure.

 

It is VERY common for stock coils to break down under a load, and not show a CEL. We build all of our coils, regardless of the application, not to lose output when placed under a load.

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.

By the way, for those of you who only need to replace a defective coil,
our SOS Coils are available separately.