Hesitation with hard acceleration

[seth5051]

New here and first post for some help.

Ive searched through the forums and think that I know what the answers are going to be, but thought I would put a description of exactly what im experiencing just for others input.

'08 Expedition, 70K miles.

hesitation on hard acceleration from slow speeds (5 - 10 mph). 3/4 throttle gives some slight hesitation until the tranny shifts to the next gear, then is smoothes out. WOT, seems to confuse tranny, hesitation is worse, finally gets at a speed where it can shift and then smoothes out.

This doesnt happen at higher speeds, 40+. Starts easly and smoothly, idles smooth and if driven at 1/2 throttle or less, also if throttle is smoothly applied to WOT, never hesitates.

Has thrown p0304 once.


I've read about the difficulty in changing out plugs and im not comfortable doing that. COP's seem to be a common issue, and i may try that.
What about vacuum lines or checking for leaks? I've looked under the hood a little and its pretty clean and everything seems tight and in great condition.

Any ideas would be helpful, just trying to avoid the dealer starting troubleshooting things that I can and adding $$ to the bill.

 

[ecqm8000]

I like more info. You mention hesitation and later "smoothes out". Are you getting a rough engine until it downshifts then smoother when the engine revs? Or is there a pause in power (until the engine wakes up) then revs?

 

[seth5051]

From a stand still or rolling 5 mph start, if i apply 3/4 or wot engine starts to rev up, then hesitates, then continues revving, then hesitates.

Once it up-shifts, the engine revs up smoothly with no hesitation.

 

[ecqm8000]

HMMMMMMM....
Back in the carburetor days I would say you need a bigger shooter for the acceleration pump system.

Then and now, mashing the throttle opens an air valve (door, butterflies, etc). The exceptions are diesels and fly by wire TPS systems. The engine is at high vacuum just before the door is opened and the fuel metered by jets or injectors. With the big gulp of air is introduced, more fuel is needed to keep the engine (and driver) happy. Your rig sounds like it is not getting the proper amount of fuel from the injectors. If you have access to a Fuel injection pressure test kit, (pretty cheap at the auto parts stores) you can see your pressure at idle (I think it should be around 35-40 PSI) If the vacuum is removed from the fuel regulator, the fuel PSI is supposed to climb (about 10 pounds or so) That spike in pressure is supposed to get additional fuel through the injectors......

The fuel pump should be a constant PSI and if the engine makes good power in the upper RPM then I would suspect the pump and filter are not the problem. That test gauge can be worked with so that the gauge is held against the windshield during a test drive. I think the rail pressure should tell the story. If the PSI drops at "tip in" (your flat spot) that would help to get the problem figured out and the beast working again.

I just re-read your post and you mention it revs and then hesitates then revs. That does make me think of a fuel supply issue (fuel pump/filter). That gauge would show a drop below the operating range for fuel pressure and then should trip the MIL light. Let us know how we can help.......

When the engine runs lean (more air less fuel) it should trip the Check engine light for a lean condition.

 

[seth5051]

I think you are on the right track ecqm.

Brought it into the shop on Saturday morning. Take the tech for a ride to show him whats happening. And, NOTHING - it ran perfectly fine!! No hesitation at all. I didnt do anything at all to it.
The only thing I can think of is that i was adding Seafoam and the last time i added about double the amount - 6 oz instead of 3 oz to a full tank. MAYBE that cleared up the fuel injectors??

Its ran fine since Saturday AM.

On another note, I was going to go ahead and get the plugs and #4 COP changed out. The shop gave me a $390.00 estimate. Is that right?? Seems very high. $50 for the COP, and about $100 for plugs, leaves $240 for labor.

 

[cbiii]

I think you are on the right track ecqm.

Brought it into the shop on Saturday morning. Take the tech for a ride to show him whats happening. And, NOTHING - it ran perfectly fine!! No hesitation at all. I didnt do anything at all to it.
The only thing I can think of is that i was adding Seafoam and the last time i added about double the amount - 6 oz instead of 3 oz to a full tank. MAYBE that cleared up the fuel injectors??

Its ran fine since Saturday AM.

On another note, I was going to go ahead and get the plugs and #4 COP changed out. The shop gave me a $390.00 estimate. Is that right?? Seems very high. $50 for the COP, and about $100 for plugs, leaves $240 for labor.

Unless the later years are different, you can get a set of 10 coils from uneeksupply on ebay for ~115, spark plugs arent more than like $3 each. just do it yourself.

 

[tmajikman]

Yeah I think my plugs were 3.28 a plug, and they were Motorcraft plugs from Oreilly's, and I have heard that a set of 8 cops on ebay is right around $100 or so, its not too bad of a job with the proper extensions, swivels, ratchet and socket... Which even with all the tools you would purchase would be much cheaper than the stealership would charge for labor and mark up.

 

[Reconracer]

Swap the plugs, just take your time or have an independent shop do it. Dealer wanted $350.00. I did it for $50. Just sayin...