My 2001 dodge cummins has been testing me some lately I had an air dog 100 fuel pump on it and never could get pressure to settle in pump finally gave up so the guys at air dog replaced it  no charge( which I think is awesome customer service by the way) any way put it on and ran great  for about 150 miles then I noticed my pressure would jump up and down not staying consistent and truck did what I will call loped  rpm would go up and down  and pressure gauge jumps accordingly.  Then i couldn't get my heater to blow out of vents  just defrost  so figured vacuum hose not hooked up  also 4x4 sometime engages and sometimes not the 4x4 light does not light up.  so messed with line and heater started to work   next morning no heater again  so that night got it going again but it is hit and  miss  noe i see oil leaking on grond  and noticed oil leak at vacuum pump and power steering  so am thinking of replacing seals there  just wondering if all of this could be related or just a bunch of small things all at once  any ideas

Sometimes the oil leak is coming from the power steering pump resevior cap and can't be seen running down the side so looks like an oil leak. If your lucky like I was with the same area leaking it turned out to be old fluid that behaved poorly by expanding and running out over the top under the fill cap. More problems can happen with expansion using synthetic P/S  fluid so I used organic fluid and flushed the system and the problem went away

 

My fuel pressure also was erratic, jumping up/ down swinging back and forth after the 1/2'' line kit upgrade. I had a combination of problems.

 Mainly because I did not drop the tank and was not cleaned of debri and with 190,000 miles dirt was reducing flow at the tip of the new draw straw. This also caused the new lift pump to suck in air at one or two of my incorrect fuel fittings on the suction side of the L/P causing the pressure bounce. Air also was accumulating at the top of my prefilter head or bracket making it more difficult for the L/P to draw consistent fuel.

 

I could tell I had a dirt clog because after I removed a hose and blew 10 lbs of air back in the tank the fuel pressure would go up to the proper level, but not last long.

 

Make sure you don't have a problem with the tank vent too. Try running with the fuel cap off to see if any improvement.

Model of airdog might play a part.  Mine is model DF 100.  My pressure didn't "bounce" but pressure would go up and down 5-10 pounds maybe driving a mile.  Found out the adjustment screw was loose.  I reset pressure and no more problems with pressure.

Edited January 20, 20187 yr by 015point9

We have the vacuum pump rebuild kit in stock in the store.

 

As for the heat only out the vents that because of a loose vacuum line or broke vacuum line. 

 

Edited January 20, 20187 yr by Mopar1973Man

The fuel pressure bounce may be a measurement issue.  Where is your gauge mounted?  What gauge do you have?  Do you have a needle valve &/or snubber to damp the pulses caused by the fuel pump?

 

Alternatively, consider what you need for sufficient flow.  The pump has to put out enough fuel flow to fill the lines and keep them at a given pressure.  Note that larger lines allow more flow but will have lower pressure than smaller lines.  Smaller lines will naturally have more pressure but the quantity of fluid will be less.  Think of a fire hose vs garden hose.

 

 

Mine was definitly air... When I would rev it up right after the fuel pressure would bounce higher than the average pressure. It was weird.