dead miss on the highway under load

[joboss]

hey all,
I have an '03 with ~100,000mi on it that I purchased not long ago. When I bought it, I gave it a tune up and solid round of maintenance that included spark plugs, wires, air filter, coolant, oil, etc (didn't do the coil at the time). Recently when driving to work, when I go ~65mph uphill with A/C on I get a wicked shudder and flashing CEL that I assume is a dead miss. If I get out of the throttle and take it easy for a few seconds, it goes away and clears up. I replaced the coil yesterday with a Denso unit hoping that would fix the problem to no avail. The truck did it again this morning. It's strange that it only happens under heavy load of a hill climb at high speed and A/C on (I will try with A/C off today on the way home). I can't repeat it anywhere else. I've tried shifting to 5th at 35mph to simulate it and I can't get the same result. Has anyone else encountered this problem? I'm stumped since it has new plugs, wires, gapped correctly, coil, new air filter etc. I had heard a rumor that a clogged cat could cause this? I'm not really sure. maybe a fuel delivery problem? Any help is appreciated.

 

[Uncle Gump]

What brand of plugs do you put in?

I say if it is anything but Motorcraft or Autolites I would start there. But I've just never had any luck with any other brand in a Ford.

Being also your fuel demands are at the highest when it happens... I would check fuel pressure. I might also try a tank of fuel with some injector cleaner in it.

 

[joboss]

What brand of plugs do you put in?

I say if it is anything but Motorcraft or Autolites I would start there. But I've just never had any luck with any other brand in a Ford.

Being also you fuel demands are at the highest when it happens... I would check fuel pressure. I might also try a tank of fuel with some injector cleaner in it.

Yeah, I was thinking of trying to flush some cleaner through the system. I used Champion plugs. could be the issue but a problem in 5000mi? Seems unlikely but not unheard of.

I'm wondering if maybe there's a timing issue? maybe timing isn't being adjusted correctly for the engine load? would a bad CAM sensor cause it?

 

[Uncle Gump]

I personally would not change a single thing until it has the proper plugs in it.

Sure... there could be other things causing it... but experience tells me to start there.

 

[09fx4guy]

Don't overthink this. Start with the basics first.

UncleGump is right, it does sound like it could be a fuel pressure issue, I would start there. Maybe some injector cleaner would do the trick, maybe at some point you may need a new pump, or have a clogged pickup screen or junk in the tank.

It does seem odd that it only happens when going uphill and heavy on the throttle.

Changing plugs is not a bad idea either, but I would start with fuel. Either way, don't go firing the parts cannon with sensors and other stuff just yet.

 

[joboss]

Don't overthink this. Start with the basics first.

UncleGump is right, it does sound like it could be a fuel pressure issue, I would start there. Maybe some injector cleaner would do the trick, maybe at some point you may need a new pump, or have a clogged pickup screen or junk in the tank.

It does seem odd that it only happens when going uphill and heavy on the throttle.

Changing plugs is not a bad idea either, but I would start with fuel. Either way, don't go firing the parts cannon with sensors and other stuff just yet.

Ok, so I followed this thought process and went for cleaning the fuel system (kinda) since I have confidence in the electrical. I put some high mileage sea foam into the fuel and ran it yesterday. when I hit the hill this morning, I turned off the A/C and I down-shifted to 4th and floored it. The truck was happy and pulled no problem. I cycled the A/C and the engine didn't seem to care. I would guess that an injector or something was clogged? I think I'll replace the fuel filter to be safe at this point.


Thanks for the advice all

 

[09fx4guy]

Have you changed the fuel filter at all? That would certainly be the first thing to do.

I had an old buddy of mine buy a Mustang from an auction that was not running right, and one mechanic told him that it had jumped time and needed a lot of work, to the tune of over $1000. I told him to change the fuel filter first, turns out it was plugged almost solid, but just enough flow left to let it idle. After the new filter, it ran pretty good.

 

[Uncle Gump]

I would certainly do the hill tomorrow morning just like it was any other day you noticed the issue. A/C on and just cruising... That will give you apples to apples comparison.

 

[mikkelstuff]

My guess is the camshaft synchronizer. My '02 3.0L did the same. Ran fine until loaded down running 80 mph. I had changed cam synchronizers previously with a Dorman product. A Motorcraft cam synchronizer fixed the issue.