Heavy load irregular random misfiring

[jimmy57]

I have a couple of vehicles that have irregular misfiring when towing heavy loads. 2006 F350/2wd/auto/4.10 and a 2006 f450/auto/2wd/4.30.

The 350 is towing a long horse trailer and the trailer with horses is easily 15K pounds. The 450 hauls a heavier horse trailer and a 36 foot deck trailer with hay or tractor and goes 20K lbs.

I have ridden and know that porpoising is not the issue although the 450 with loaded flat trailer does have a lot of porpoising.

Both trucks will exhibit this if driven up a grade where I can hear it in exhaust note simultaneously with the felt jerk from misfire.

Both have had had a lot of work like head gaskets, coolers, one had a FICM, and both have had some injectors but not full sets. No coolant usage on either. 350 has 124K miles and 450 has 75K miles. Both complain of this occurring for a long time. Both have no long cranking problem and show to have had supply tubes and dummy plugs when head gaskets were done. One I know has had STC fitting repair.

I have measured fuel pressure and it is 54 psi on one and 61 on the other when this occurs.

Scoped ICP is smooth. Cylinder misfire (speed deviation) graphics via Autoenginuity has transient small irregularities that are not cylinder specific and are not out of line with what I see on 6.0's w/o this problem.

EGR is clean on the one with functioning EGR and it makes no difference if the EGR is defeated and test driven.

I wetted the engine on one of them and drove it to see if the injector wiring may have been arcing with no improvement.

Has anyone ever chased a misfiring 6.0 problem like this?

 

HELP!

[Bruce Amacker]

You won't likely have injector wiring arcing when wet as they only operate at 48v. It's going to be really tough to diag these without an IDS so my first suggestion is to find one so you can take advantage of the balance/misfire graph. The only backyard test I could suggest is to load them up with a lubricity enhancer like Stanadyne Performance Formula and see if it helps which would indicate an injector problem (sticking barrels and plungers that would cause intermittent misfire).

 

Injectors are on the top of my hit list for the complaint you describe. AE should show you the MPWR, make sure it stays above 47ish volts. Are the buzz tests consistent both cold and hot? Has the oil been changed recently on both?

 

AE is not the tool for this and I can't say that Mode 6 is accurate, but it "might" show you something.

 

Here's a screen capture from a 6.0 showing countable misfires on cyl #4:

 

 

Good Luck!

 

[jimmy57]

Thanks.

I have measured MPWR and it stays at 49 or 50 under load (one of those strong FICM's) on one of the trucks and 48 on the other.

Neither has any cold running issues and start and run smooth at the coldest temps we have had in Texas (24F) lately. Oil changes have made no difference. Both have short cranking time.

One truck has been using Rotella synthetic on 5K changes for a while.

Injectors was my first suspicion given no other direct traceable issues I didn't want to do that and not have fix. I was fishing here in case someone else had chased this or if it was something that was being seen by others with the age on these units now.

I really didn't think the injector wiring was at any great risk with the 48 V signals but the list of things that has been eliminated makes me leave no stone unturned. One of the the suspicions I dreamed up is fuel vapor pockets but diesel is really resistant to something like that as far as I know. The reason I'd entertain anything like that is that both of these trucks do this with heavy loads and do it most repeatably when you have been out of throttle and get back in it again. The engine temp would be high and the fuel would have been sitting in cylinder heads for that period and could get heated. The pressure waves from injector filling cycles then could give opportunity for vapor pockets but I have not seen that as an issue with Diesel. Has ULSD changed that? I had a THL case I worked once where a gas motor had an irregular misfire under high load hot and it was a faulty fuel rail pressure damper. That car felt like these trucks.

I've pulled a full panel of misfire counters without catching anything but I haven't done it the last couple of times I've had either one of these trucks in.

Buzz tests are always consistent hot or cold.

The lubricity additive sounds like a great idea. I will try that on one of these and let you know.