I have a new to me 99 QC 3500 Auto, and have been lurking here. I installed a Boost, Pyro, and Trans Temp gauge (Autometer) on the A pillar, and a Dipricol Fuel pressure (Mechanical with Isolator) on the steering column. Found a dead LP (Go figure it was the original @128,000) Installed a new LP from Cummins. Now after a 1500 Mile trip to NC and back, here is what I have found. When cold first starting i get 5-6 PSI, after about 10-15 MIles the pressure gradually increases to 12 driving, with my comp box on (Not pierced, I am a big wimp) pulling a 5% grade for 6 miles to the floor at 75+ MPH I had 9-10 PSI. What gives with the low PSI at start up? I have not seen anyone comment about this situation before. With 1/8th of a tank of fuel idleing warm i have 13 PSI. Is it just the cold Fuel? Should I re-bleed the FP line, I did when I installled it, but have since changed the Filter and LP. Thanks Murfman

 

This behaviour can easily come from a clogged fuel strainer sieve on the tank pickup. As the return fuel warms up the tank it gets less viscuous and will go through the small openings more willingly than cold. Can you pull down the fp when the fuel is cold more than when it's warm?

AlpineRAM

 

Welcome to the site. It is a good idea to fill in your siginature area of your profile with the info of your truck (year, tranny, and mods if any) so you won't have to type it in every time.
I would check the pressure at the pump (VP44) when then the gauge is showing low. It could just be the isolator is a little sluggish when it’s cold. And if you haven’t already, change out you banjo bolts to the high flow ones. That will help keep your pumps healthy by cutting back on the backpressure.
You can get them at Wildcat Diesel:
http://www.wildcatdiesel.com/
You can also get them at Genos:
http://216.167.41.9/CoastalData/show...ID=593&CATID=1

 

FWIW

When I first start up I see a fluctuation in my voltage due to the heater grids cycling. My volt meter drops and rises slowly, my lights dim somewhat then become brighter and my fuel pressure drops more than 1lb to maybe 2lbs (Pressure readings of 21+lbs to a little less than 20lbs).

I have read a theory that our lift pumps sometimes fail because the electrical connection becomes corroded and doesn't make good contact - additionally a proponent of this theory claims that the connectors are not up to the electrical draw of the pumps.

Billg makes a good point to double check your gauge first.

While trouble shooting you may want to take a minute and double check the electrical connections to the LP before dropping the tank.

Good luck,

George

 

Thanks for the replies, I am going to try re-bleeding the isolator, I have a feeling it has air in the line from the filter change. I plan on ordering a Banjo Eliminator kit, that should hold me over till the trans mods are done, and I can set out to try and break my "53" Murfman

 

I've got the PE pump on my truck and on cold mornings when the heater grids kick on, pressure drops to about 10 psi. After about 5 or 10 minutes, they shut off and the pressure goes back to EEE at idle and 16 at WOT. If I plug the truck in over night, I don't have that problem.