2003 Mercury Marauder, DOHC 4.6 V8

Since maybe 2 weeks ago this has rapidly gone from occasional stutter to a severe annoyance. At part throttle, *especially* when going up hills or above ~40 MPH, the car has a rapid stumble/bucking/jerky motion going on. Feels almost like it hesitates, then catches with a jerk, then does it over again rapidly. If you lift off the throttle or punch it, the feeling goes away--seems to occur, roughly, between ~1200 and 2000 RPM. Worst at highway speeds but it doesn't really do it with the cruise control on, except if you try to accelerate using the cruise--and then it's less pronounced than it would be under normal throttle. Feels like it may be idling a little bit roughly, but that could just be in my head.

The strange thing is that there's no check engine light. Not a peep.

My previous car with an older version of this same engine would do this occasionally, but it wasn't this pronounced and it would only do it at highway speeds under very light throttle. Much more of a problem here. I never did figure out what it was on the Lincoln, the car got wrecked before I really tried to figure it out.

Ideas?

9/8/2010 5:04:09 PM

Just to get this out of the way...how old are the plugs/wires?

9/8/2010 5:11:32 PM

^ What he said + fuel filter.

9/8/2010 5:52:19 PM

my old blazer had the exact same symptoms. new plugs fixed it.

9/8/2010 9:19:24 PM

Wouldn't that throw a CEL? The plugs have been on the car at least since I got it (11/06) so they may well be original, I did consider that. No wires as it's coil on plug.

9/9/2010 2:12:02 AM

Lol cel isnt god. It helps but it doesnt catch everything. Change the plugs and see if it helps. If they are the same from 4 years ago its time.

9/9/2010 11:52:06 AM

You might have a stored code.

When one of the coils on my engine went bad (coil on plug) it exhibited the same symptoms.

No light. Worse gas mileage though. Platinum spark plugs looked factory new. After I pulled the stored code, I started getting CEL's. Not sure why it they never showed up in the beginning.

I replaced one and reset the ecu till I found the bad one. I also had that problem when the EGR ports were clogged. That might be different for you, as we have different engines.

9/9/2010 12:08:21 PM

change plugs first.. could be a coil breaking down if the plugs don't fix it.

Is yours still stock or do you have a tune on it?

9/13/2010 10:48:31 PM

i had the exact same problem with my crown vic (SOHC 4.6). It seemed like it only did it when the engine RPM and load was just right. It turns out the EGR port in the intake plenum was nearly clogged with carbon and junk. I cleaned it out, and also changed the plugs and DPFE sensor. No more problems after that

Your car is nice, I hope you sort this problem out

[Edited on September 14, 2010 at 2:52 PM. Reason : ..]

9/14/2010 2:49:51 PM

Quote :

"i had the exact same problem with my crown vic (SOHC 4.6). It seemed like it only did it when the engine RPM and load was just right. It turns out the EGR port in the intake plenum was nearly clogged with carbon and junk. I cleaned it out, and also changed the plugs and DPFE sensor. No more problems after that"

And those are typical Ford issues. I'd start there.

9/14/2010 3:13:05 PM

Changing plugs next weekend (couldn't do it this weekend, gf's family was in town for a visit) and will go from there. Will probably see if there is a stored code as well the next time I go past advance or autozone, just in case. Thanks for the suggestions folks, will report back.

9/14/2010 5:00:25 PM