ZEEGEE66

1999 4.2L V6 manual Base
I recently started having what I can best describe as a hesitation while driving (feels like a misfire). It occurs under acceleration or while going uphill (even an overpass at 70 mph), but while at idle or off the throttle altogether, it runs fine. It is more noticeable as you move up in the gears, assuming that's due to the "heavier" load on the engine in those gears.

I just changed the fuel filter (it was DIRTY) and it seemed to improve but it's still very noticeable. I changed head gaskets back in December '13 (since I was unaware of the intake issues, I assumed it was head gaskets). Had the heads cleaned up, valve job, new IAC, plugs, wires, coil, - all the things you change when you want a fresh start on the top end. All parts were Motorcraft (thank you Rock Auto!).
I have run a compression test , fuel pressure test (didn't check before the fuel filter, but after the change it was 32 psi steady at idle and under load - still had the miss), cleaned and re-gapped plugs, and swapped plugs and wires chasing this and had absolutely no luck.

I think I'm going to run it a week and change the fuel filter again since it was so bad the first time, but I'm all ears until then.

I'm at the end of my rope here - throw me a line. Taking all the help I can get.

ZEEGEE66

Also, no codes whatsoever. Service engine soon light is on, but no codes.

jstockert

I know you recently changed the plugs but I bought a reman engine and changed plugs with NGK's. Had a slight misfire/hesitation under acceleration. Well the third time I pulled the plugs to check gap and condition I found a crack on the upper porcelain on one of my plugs. I installed the plugs while it was on the engine stand and I know I never dropped them. Changed the plug and misfire is gone. So I know the plugs are fairly new but pull them and check condition.

andyb8391

Have you considered cleaning the mass air flow sensor?? Or maybe intake leak?

ZEEGEE66

Thanks for the replies, I really appreciate your input!
When doing the compression test, I had all of the plugs out - I cleaned, and CLOSELY inspected each. I replaced the only one that looked suspect. Before leaving it in the final cylinder, I swapped the new plug in on each cylinder - one at a time - with no change in the "miss" while driving. It never got better or worse.

I have not cleaned the MAF, but will do so tonight. Can't believe that didn't cross my mind. I know it isn't an intake leak - That has already happened and I found out the hard way to check that first. No water in the oil and I sprayed carb cleaner around the intake areas and no change in idle or otherwise.

Keep them coming!

TLB4

Your code reader isn't reading right. The check engine light comes on for DTC's (Drive Train Codes) so if you say the light is on, there is a pending code. Work on getting it out of the truck.

I would still bank on a misfire. It might be subtle, but it's there. Your best bet is to find someone who can do live data logging and taking it for a rip and watching the spark plug graph, see if it misses at all.

ZEEGEE66

Cleaned the MAF - no change. It wasn't even dirty to begin with but I did it anyways hoping for change.

TLB4 - I don't have anyone with a scanner or other electronic tools of the trade so I think I'm shafted on that one.

On another note - since changing the fuel filter - there is a much more clear pattern emerging. Upon cold startup, it runs like crap - in every gear. As it gets to operating temp, it improves (total of 3-5 minutes) once at operating temp it is almost gone. First gear and low RPM in 2nd gear are really the only ones that are affected. Occasional stutter at speed while cruising.

ZEEGEE66

Any other ideas? All help is appreciated?

Carcrazygts2

Fixed for you. Could be a faulty EGR solenoid or EGR valve. SES light doesnot come on unless there are codes. Take it by Autozone or Advance and they will pull the codes for you.

ZEEGEE66

Have not driven the truck in a bit - drove it today and the problem is the same. Just found out that there is a correlation between not being able to read codes and the "cigar lighter" fuse. Apparently the OBD port is on the same circuit. I will replace the fuse and try to get codes pulled again.