Hello,

I have a 99 F350 V10 4X4 5spd with about 115K on it. About 18 months ago it started to have an intermitant miss. Most noticeable when pulling a load up a grade or if I lugged the engine. If I am pulling a load up a grade at 2600 RPM (or any speed above 2600 RPM's) and go from 1/2 throttle to full throttle, I don't get any more power the engine just seems to stumble and and run slightly rough. If I back off the throttle, to motor runs as normal.

It also has a periodic distinct miss that pops up ever now and then (every other day or so), but it's completely random. Only misses under load, and the motor always runs as smooth as butter at idle. One odd thing is if I dump a bottle of injector cleaner in the gas tank it seems to get better.

Replaced the fuel filter - no change.
Replaced the IAC (three times / last two were motorcraft) - no change.
Replaced PCV / Vaccum lines to PCV. - No change
Replaced all spark plugs (x2) - No change
Replaced all the coil packs. - no change
bought 5 more coil packs swapped them around - no change.
Rebuilt all the injectors - no change.
Pulled the cat off - no change.
Used an entire bottle of carb cleaner trying to find a vaccum leak, never got a change in idle speed.
Bought a fuel pressure gage, and don't see a drop in fuel pressure when this occurs.

I have had it to a mechanic three times (three different). The first time they told me the battery terminal / ground was bad. New battery and cleaned both grounds. Nothing. Second trip to the mechanic they told me an injector was unplugged (complete BS). Third trip they couldn't find a problem.

Anyone have any ideas? No codes / no CEL.

Don't know what to even try next...

Thanks,

Chris

 

I did not see in the list that you ever cleaned your MAF sensor (mass air flow ) which is behind the air filter . It requires a special MAF cleaner ( walmart sells it) . Search for the procedure on how-to, at this forum. MAF is one of the sensors regulates air/fuel ratio by telling PCM the amount of air passing thru , so ,it could be the problem giving the age of your truck.

While you are there , you may want to remove the air intake hose from throttle and clean the throttle butterfly area with a cleaner (MAF cleaner should do) . It may be sticky .

These are the elementary DIY to start with . If these do not resolve your issue , then you would need further diagnostic work , like fuel rail pressure readings (to understand the status of fuel pump ) , then may be hooking up a scanner that reads live data and try to find out what data is deviated from normal .

Good luck,

 

I concur, check the MAF - sounds like it might be dirty.

You don't see a drop in fuel pressure, but what IS the fuel pressure when it stumbles? At that much load, it should be 38-40 PSI.

 

Cleaned the MAF and no change. Pulled the MAF and ran on the default A/F ratios without the MAF plugged in. Immediately set off the CEL with a MAF code, and still had the stumble.

I have a scanner, sounds like I need to do some research on what live data I can get out of the scanner.

Thanks,

Chris

 

New boots with the new coil packs?