Archived Bucking while coasting, Miss fireing

[EDankievitch]

[Ok, I've got a 05 ram with the G56 6 speed. The engine is stock. Miles are 126,500. I've developed a bucking when I coast does not matter if I use the cruise or not. The engine will miss fire in nutual if I rev the engine. When it first started to show up it would only miss above 2500 rpm now it will miss above 1400 with no load. I've checked the FCA, replaced it. Checked the MAP sensor, replaced it as it was giving a fluctuating reading. There are NO codes and the truck will run ok when working/ towing. Also the MPG over head gauage is reading around 45 Mpg. I did a driving test and monitored the Fuel rail pressure under full load it was 45,550Kpa at Idle it was 65,000Kpa when coasting it was bouncing fom 34,000 Kpa down to 900Kpa. Any thoughts??? it's getting wrose and I want fix before it does not drive... ... /B]

 

[Missouri Mule]

Seems your rail pressure sensor is wacked out or your loosing your cp3 or ecm since you already changed out the fca.

What is kpa equivalent to psi. I know aprox in the 25,000 psi range the pressure relief valve opens up.

But the pressure sensor would send data to the ecm so maybe the ecm is sending out wrong data to overhead mileage gauge and fca.

Sorry don't know how to check out cp3,ecm or pressure relief valve.

Doesn't seem to be fca problem not the right symptoms. Your injector are overdue to be changed out but the symptoms don't fit bad injectors either.



With out codes it is hard to pin point the problem and playing hit and miss is to expensive.

 

[EDankievitch]

Thanks for your thoughts. I agree, it's hard with out a code. I know the injectors are getting old but the truck runs strong and smooth at idle. It's just when the load is removed that it starts this miss/ bucking. Geno's offer's the fuel rail with the blow off valve and pressure sensor for a decent price I might give that a try but if I do I think I should replace the jumpers also. I've had one crack a short while back (about 60,000 miles ago) Volvo ( Diesel school Volvo Penta)had said not to re-use the jumpers.

 

[JJPage]

As long as you have provided decent filtration to the injectors, I disagree that they are overdue for replacement, there are numerous CR's on the road with 200k+ miles on original injectors. I'm not sure where this myth, where at 100k miles the injectors need replaced, started, but it's not correct.



How's your lift pump? I would ask about your fuel filter, but if it was plugging, I believe your truck would be behaving just the opposite, problems under load. Sounds like it could be electronics also... . are your battery terminals clean and tight? Batteries in good condition?



You could also change out the rail pressure sensor, seems there are two different cummins part #s, depending on the type of alternator you have? #3949988 was the original, and #3974092 is a newer version. There is a very recent thread on here discussing these sensors, I believe it stated if you have a Delco alternator, use #3974092, but I am not positive.



When you figure it out, be sure and follow up this post with your findings, it's always helpful to the rest of us when posters do. Good Luck!

 

[JJPage]

I was mistaken about the sensor thread being on this site. But here it is.

Rail Press. Sensor Part #? - Dodge Diesel - Diesel Truck Resource Forums

 

[EDankievitch]

Thanks one more bit of info the truck ran awesome when it was cold this morning then as it warmed up the trouble started warmer it got the more pronounced. Thanks again everyone for your input.