96 freightliner fl70 5.9 mech cummins p series pump, truck starts and runs good except when the temp outside is say below 40F in the morning it will start right up and idle a min or two and start to miss unless you hang in the throttle, its like its making air in the injection pump, only does it under 40 degrees or so, once it warms up for half hour or more at high idle truck will run fine all day, if its 50-60 degrees in the morn, starts up and idles fine, any ideas ??
5.9 cummins cold weather miss
- Thread starter barkoguru
- Start date
You might want to check your intake heaters to see if they are working.I have the same engine & when I start it when it's cold the heaters come on & go off every minute or 2 until it wamrs up.I can tell because the voltage on the volt meter goes down when they kick in.It does miss alittle for the first couple of minutes.
No air heater on this eng. not set up for it either, I put heat lamps under the fuel tank last night and with the temp around 30 this morn did ok, im stumped .
I'm not that familiar with the types of fuel connections one would find on this truck, but I've seen problems with multiple brands with problems like you're describing with old, original fuel lines in cold weather. Older rubber and plastics will seal when hot but can leak air past fittings and clamps when the temps go down.
Try this: Get a fuel can, fill with clean diesel. Run a hose to the pump's intake port. Start the engine and run it until you're sure all the air is out. Leave it hooked up overnight. Start it again in the morning while everything is at cold ambient temperature. If it runs correctly with no missing or stalling, you've got a problem with air leaking into your fuel supply lines. If it runs rough and wants to stall, then the fuel supply system is fine and the pump is in question.
I'm not sure if the p-pump is susceptible to this or not, but older Bosch mechanical rotary pumps (similar to the VE? pumps on later 5.9s) had a seal under the pumping chamber that would shrink over time, working correctly when ambient temps were high but losing prime and being a pain to start if left overnight in a cold environment. In the case of the P-pump, it's possible that the pump shaft seal could be pulling air in. Not entirely sure, but similar things have happened to similarly designed pumps.
Try this: Get a fuel can, fill with clean diesel. Run a hose to the pump's intake port. Start the engine and run it until you're sure all the air is out. Leave it hooked up overnight. Start it again in the morning while everything is at cold ambient temperature. If it runs correctly with no missing or stalling, you've got a problem with air leaking into your fuel supply lines. If it runs rough and wants to stall, then the fuel supply system is fine and the pump is in question.
I'm not sure if the p-pump is susceptible to this or not, but older Bosch mechanical rotary pumps (similar to the VE? pumps on later 5.9s) had a seal under the pumping chamber that would shrink over time, working correctly when ambient temps were high but losing prime and being a pain to start if left overnight in a cold environment. In the case of the P-pump, it's possible that the pump shaft seal could be pulling air in. Not entirely sure, but similar things have happened to similarly designed pumps.
Slo-Ride
Veteran Expediter
Good to see a follow up post with what repair was done.. Did you find a blockage or kinked line? Or just luck and process of elimination?
I got lucky I guess, I figured since this hard plastic line went from the frame mounted filter over the the lift pump on the engine it was the most likely to be damaged by vibration and just the constant moving around from the torque of the engine, couldn't really see any cracks but it had to have been cracked or shrinking from the cold and leaking at the brass ferrules at the fitting.
This is exactly what I was talking about. This seepage means the seals have hardened with age and heat cycling. Fine in cold weather, too brittle to seal air out when it's cold.
Glad you found it, and I'm glad it was cheap!