Rail Pressure low only at temps below 40 degrees

[JohnnieBishop]

Truck is a 2003. The rail pressure is about 6000 psi at idle and when running about 70 mph will only gets to about 8000 psi unless I am running about 80-85 then it is at about 15000 psi all while temps are below 40 degrees and the truck runs like crap. If the temp is above 40 I have no problem with rail pressure and truck runs fine. Have replaced numerous fuel related parts to figure out what it is. Have replaced injectors with new bodies and nozzles, new injection pump, new crossover tubes when I replaced injectors, as well as the fuel rail which came with a new overflow valve as well as a new sensor, check valve at back of head and replaced all fuel lines coming from tank to fass pump and from pump to injection pump. All battery connections are clean and tight as well as the ground wires. Also have replaced both cam and crank sensors with cummins sensors. Also have marked and checked to make sure pump and gear is not slipping on one another and it is not. All parts came from local cummins dealer except for injectors and injection pump.

 

[ranger 393]

Any codes?

 

[MarkEkberg]

I am similar to you. Under 30 degrees its a crap shoot if it will fire off or not when cold. Above that temp it fires and runs great. My rail pressure is low for initial fire even with strong crank. Hoping you figure out as I have given up until problem gets worse.

 

[AH64ID]

Are you running a tuner?

 

[MarkEkberg]

Don't want to hickack original thread...... But I think we may have the same problem.

Yes I have a smarty. Does not matter if it is in or out. Same pattern of hard start when cold and low RP exists.

 

[Prairie Dog]

Also not wanting to hijack but does the exhaust stink at idle when the rail pressure is low?

 

[JohnnieBishop]

I am running a smarty as well but it doesn't matter if it is on the truck or not. Just replaced the air inlet temp/MAF sensor, coolant temp sensor, and MAP sensor as well. Gonna try it later this week and let you know. Also don't have any codes either. Exhaust doesn't smell any different than usual and no hard start issue exists with mine cranks fine.

 

[JohnnieBishop]

Finally got cold enough outside this morning and drove the truck same problem no rail pressure. Done an experiment and heated the ecm with a heat gun and no change on rail pressure. Next I heated the ambient air temp sensor and finally got it to about 130 degrees. Drove the truck ran fine until it dropped below 45. Went to local parts store and purchased an ambient temp sensor and no change in rail pressure.

 

[AH64ID]

Sounds like a programming issue, not a sensor issue.

 

[JohnnieBishop]

Trucks ecm was reprogrammed about 2 years ago at the dealership. The reason for reprogram was when this issue started and wanted to start with the basics of getting it checked and reprogrammed.

 

[xDiesel03]

It's sort of a programming issue. This is a normal thing for 2003 trucks and I think maybe 2004 trucks. The only way to fix it is to switch to HP Tuners so you can control more of the ECM tables. I haven't messed without UDC PRO but I know you can't fix it with regular UDC. If you could data log you would see that the ECM is calling for lower rail pressure and higher timing values than what you are commanding in the base tables when it's below a certain temperature. With HP Tuners you can turn these modifiers off so your ECM only pulls from the base tables. Lavon at Firepunk tuned mine and I couldn't be happier. Gained ~75hp over UDC, truck doesn't defuel until 5000rpm, no more weird rail pressure in cold weather, and the truck runs better than ever.

 

[AH64ID]

It can be fixed with UDC Pro.

Anyone can PM me if they want more UDC Pro info.