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Idles rough when warmed up.

909 views 6 replies 4 participants last post by  wrenchmonkey 
#1 · (Edited)
I have a 1997 3500 4x4 Dually that starts great and initially idles OK. After the truck has been run for awhile it does not want to idle very well. At a stop I am forced to shift out of drive and into nuetral to avoid it dying. Most of the time shifting to nuetral will keep in running, occasionally if it is really hot out I may need to up the RPMs while in nuetral to avoid it from dying. Temps are showing within normal range. Not sure what is causing this. Runs fine when accelerating. I tow a 28' trailer with no issues, until I pull to a stop and then I am back to shifting to nuetral to keep it running. Looking for some help with this.:flag1:
 
#2 · (Edited)
check your fuel temperature, I have the same problem when it gets hot. I noticed in the middle of idaho pulling my 28' trailer that my truck was running like s**t, i was at a rest area had my wife hold the throttle to keep the truck running, dumped some water on the ip and it started to run better. I'm in the middle of finding a good setup to make a fuel cooler with... btw, what part of wa are you at?
 
#3 ·
Pretty sure I saw somewhere that when the only way to keep the truck running right is to dump water on the injection pump its either time to find a new pump or have it rebuilt. Your problem isn't not having a fuel cooler. That is only putting a band aid on the problem.
 
#4 ·
my truck did the same thing,
-i had to clean my pre filter
-then had a diesel shop run some kind of lubricant cleaner into the p pump cus it was sticking when warm,
-then i bumped up my idle cus it was a bit low, and after all that the truck has been running grate again
 
#5 ·
when you put a lazer temp on the fuel tank and it says 170 degrees thats too damn hot, add the hotter fuel from the injector nozzles and show me a 7100 pump that will still run like normal... or for that matter any engine.

most newer engines have fuel temp sensor that will derate the engine when the fuel gets too hot. why? the fuel gets too hot it does 2 things... 1 it loses viscosity and therefore is harder on the ip and injectors and for 2 it loses its energy. I've seen a few big trucks with fuel coolers for that exact reason.

I have a quote verbatim out of the cummins service bulletin on fuels

CUMMINS SERVICE BULLETIN 3379001:

Temperature causes changes in engine power because it affects both viscosity and density. An increase in fuel temperature will cause a decrease in viscosity, which will reduce power due to internal leakage in the fuel system, as described above. The maximum recommended fuel pump inlet temperature for Cummins® engines is 70°C [158°F].

An increase in fuel temperature will also cause a decrease in fuel density (increase in API gravity), which will reduce power due to lower energy content of the fuel. On Cummins® engines using the PT™, Quantum®, or HPI® fuel systems, the power loss due to increasing temperature is less than that on engines using the in-line, distributor, or CELECT™ systems (less than 1 percent per 5.5°C [10°F]), due to the inherent viscosity compensating characteristics of these systems.
 
#6 ·
Never heard of anyone needing a fuel cooler on one of these trucks. I live in Florida where the norm in the summer is 95-100 degrees and I live in the city where it is stop and go traffic and have never had a problem like this with hot fuel.
 
#7 ·
do you pull heavy loads much? also your engine has to work the hardest at freeway speeds pulling long grades which raises your egts as well as all your other temperatures. pulling a 15k trailer in 120 degrees up a 6% grade for 5 or 6 miles is hard on a lot. in town you are giving the engine idle time and not putting your engine at 100% load for any longer than it takes to get to speed if that.

some bigger cat engines come with fuel coolers from the factory, and I've seen a few freightliners, peterbilts and kenworths with fuel coolers
 
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