My '03 Coachmen RV is on an F450 chassis powered by a Triton V-10. The chassis battery has been bad for a few months but since I use the rig infrequently during the summer and can start the engine from the house battery, I have put off the replacement. However, last week, it began shuddering rather severely when under load (accelerating or pulling a grade). No check-engine-light, but I did pull a code: "P0300", multiple misfire! I have read a few dozen posts here about plugs, coils, etc. and a couple mentioned the possibility of a bad battery screwing up the sensor voltages enough to cause a misfire. Is that a real possibility?

The rig has less than 40k on it and has had faithful maintenance. I replaced the battery yesterday and would like to gather some info from you-alls' experience before I attempt to pull the cowl and facing the daunting task of replacing spark plugs or CELS.

Any encouragement from you will ease my mind. Thanks
VCB

 

I changed my 2003 spark plugs @80K miles few months ago , the gap was a bit larger , factory recommends @ 100K miles, some changes earlier . So, it is quite likely your plugs are still OK , based on my spark plug rubber boots at 80K, yours should be fine too at 40K. Assuming that your COPs (coils) are original Motocraft , if so , they should be OK @ 40K mile too (they should last 100K+).

Did you wash your RV's engine compartment ? did it get wet for some reason ? if so, it may cause that CEL error .

Here is a link for possible causes for PO300 ;
OBD-II Trouble Code: P0300 Random/Multiple Cylinder Misfire Detected

I would start by checking vacuum hoses see if any unmetered air leak causing this .1- PCV hose (90-degree elbow at throttle @ passenger side, 2- Clean MAF sensor @air filter housing with an MAF spray cleaner (do not touch it), check MAF rubber grommet at the bottom of air filter housing to see if it is tight ),3- Check all size of rubber hoses for cracks and tight fit . That is what I would start with .

Good luck,

 

If you changed the battery, did it make it better?

A "random" misfire usually isn't plugs, COPs or boots, or water in the holes, or anything spark related. It would be a misfire on a single or multiple cylinders, but not "random".

I'd start looking elsewhere. Misfire under load usually means fuel pressure is too low.

Hook up a fuel pressure gauge and check the pressure under load.

 

Thanks for your reply. Yes, I have replaced the battery. I have only tested it sitting still with the brake on and applying load. It seemed to be alright. I'll take it out on the road later for a better test. I'll post more after.

VCB

 

Don't rule out battery. I met a guy a few years ago that had a low mileage F150. Had similar problems very intermittently. Was at dealers many times. He finally got tired of it and gave it to his son, who thought he could figure it out. The son hadn't had it very long when the battery suddenly died. He had no more performance problems after he put in a new battery. The thought was that the bat had a loose plate the would sometimes shift and short out a cell internally, thus causing the computers to freak out.

 

Good news! Test run yesterday and running fine. No codes. No misfire. The battery was so weak it was affecting the voltages at sensors. That's 3 Super Start batteries in 6 years. Kirkland this time. Thanks for all the advice.

 

If you don't have a battery maintainer hooked up to keep the battery charged I would consider adding one. Lead acid batteries like to be kept charged. Some maintainers (like the Schumacher) come with a pigtail you can install on the battery so you can disconnect the maintainer quickly without screwing with the battery.