Cummins Diesel Forum banner

air leak

3K views 17 replies 5 participants last post by  Sour-Diesel 
#1 · (Edited)
I installed a mechanical fuel pressure gauge last sunday running fuel to the gauge. ever since then i have not been able to build fuel pressure while throttling, my fuel pressure was constant while idling but dropped to about 13 psi at WOT but i never have had any symptoms of fuel pressure problems. i assumed it was the lift pump and my buddy has a couple spare ones so he gave me one to install that he swears up and down works. after installing his lift pump then reinstalling mine because i couldnt even prime the fuel system with his lift pump i can now prime my system up to 30 psi but as soon as i start up the truck my fuel pressure begins to drop and if i try giving it some throttle it smokes and loses power. i removed the lift pump and fuel heater bowl together because i planned on also deleting my fuel heater. i deleted my fuel heater and sealed up the pre filter bowl with permatex #2, sealed up the lift pump with permatex #2, and i replaced the line between the pre filter and the lift pump with 2 feet of regular fuel hose looped around. the only thing i can imagine is leaking is the 90 degree elbow off the lift pump that needs to face the pre filter bowl because i didnt want to over tighten the fitting and strip out the threads, im going to change my fuel filter and when im doing that im going to try giving that fitting another 360 degrees.
 
#3 ·
ok from what i can tell that trouble shoot guide will only help you eliminate a bad ofv. i just replaced my ofv with a tork tek one that works perfectly. my truck idled very low fuel pressure wise after installing the adjustable ofv but after hooking up the fuel pressure gauge i notice how low the pressure was at idle and adjusted it up above 20 psi at idle but the fuel pressure still dropped like a rock when throttling. thank you for trying to help but believe me all the generic troubleshooting guides arent helping me here. short of pressureizing the prefilter housing and lift pump and submerging it in water to look for air bubbles im really at a loss.
 
#5 ·
My Torktek came pre-set to 30 psi and works perfectly, you're either sucking massive air from somewhere or have a bad lift pump.

Another idea: I used to have a subaru that ran like crap and chased my tail for a long time until I found that the fuel pump was sucking air from a bad fuel pump.

I thought it was bad fuel, so I went to empty the tank by starting the car and letting it run with the return fuel line disconnected and extra hose (happened to be clear) running into a jerry can. You could see bubbles in the fuel as it was being pumped through the clear hose.

Maybe you could find a way to hook clear hose up between various fuel system parts to isolate where the leak is occurring.

This will likely make your fuel system lose prime. An easy way to purge air is to loosen the OFV and crank the engine until you or a helper starts seeing fuel burble out around the OFV. It should start right up with minimal hesitation after that.
 
#6 ·
ok from what i can tell that trouble shoot guide will only help you eliminate a bad ofv
It will also tell you if the lift pump is good.
Just because something is new don't assume it's good.

If you have an electric FP gauge double check it against a mechanical one before you go further.
 
#7 ·
i feel like you havent read my post at all and are just posting generic fixes. the first thing i said is i installed a mehcanical fuel pressure gauge with a tork tek snubber, and needle valve, running fuel to the gauge, no isolator. my OFV adjusted perfectly according to my fp gauge while idling. the initial fuel pressure, after getting the mechanical fuel pressure gauge hooked up and tork tek OFV installed, was a little low. i most notice the low fuel pressure while driving, my fuel pressure was dropping lower than 15 pis at WOT so i assumed it was a lift pump. now im thinking (or maybe just hoping) it was an air leak from the 18 year old hose that connects the lift pump to the prefilter housing. i replaced that hose with 2 feet of fuel line looped around, i deleted my fuel heater and sealed the prefilter back up with some permatex #2. but i still seem to have an air leak somewhere, i am going to try and troubleshoot it tomorrow.
 
#8 ·
poor run

i believe the lift pump could cause the condition , it would provide good pump at idle but once demand increases it cant keep up and the two lines one from the lift pump to the large filter canister and the one supply line from the corner of the valve cover to the preheater filter top can get rather rusty and probably need replaced if condition poor but that doesnt produce the condition your talking of but the lift pump sounds probable . take a look in on top of the tank from the driver side and you can see the condition of the sending unit , the top of the sending unit has about 3 inches visible of the lines where they change up and go to steel and enter the tank top . i am not thaqt knowledgable but trying to offer frequency as to the lift pump .
 
#10 ·
lift pump?

and i am told if your sucking air it wont run , even a small pinhole - it wont run but check those lines the two i mentioned
ive started my truck with loose fuel lines before when we were doing the head on my truck not long ago, i forgot to torque down fuel lines a couple times and the truck still fired up, it just runs like total sh!t, ive done the same thing the the banjo bolt not torqued on the fuel filter. i think your right tho its starting to look like a lift pump issue, ive been searching for an air leak all over and cant find one that i would have caused when i disconnected the prefilter and lift pump together about a week ago. i also tried clamping the return line like the troubleshoot guide says but that didnt improve my fuel pressure at all. i primed the system and started my truck a couple times tonight and i can actually keep the fuel pressure up by using the prime button on the lift pump, that really sounds like a lift pump issue to me any thoughts?
 
#11 ·
poor run

I believe you've got it , I can tell you if you disconnect the small rubber line on the lift pump from the fuel heater carefully and hook a hose to the side of the lift pump from a Gerry of fuel it should prime up and run great if pump is good .try that just be nice to the elbow hose so you can reuse
 
#12 ·
finally got a chance to switch out the lift pump on my truck it runs and drives now but still has the same fuel pressure problems as before. at around 2k rpms my fuel pressure is below 15 psi but it doesnt give me any problems
 
#14 ·
If you don't have any issues losing prime, your OFV is probably toast. You should have way more than 15 psi cruising, mine idles in drive at 25psi, then 30 at just over 1000rpm/25mph. Cruising at 1750rpm/65mph, It won't drop below 28 unless I'm really hammering it. There, I've never seen it below 24.

I am very happy with my TorkTek and would recommend them to anyone.
 
#13 ·
poor start

the ofv -overflow valve opens and returns fuel at 15 psi and above , they say to slightly pinch off the return line and it should climb . glad to hear its running and if your cold starts are good then your good to go . how were those two lines one from the top of lift pump to large filter and the one to the heated filter housing for surface rust ? if poor they can suck air and make a difficult or no start when holes are present - so be aware . hope this helped and great job
 
#15 ·
yea i was thinking it could be the OFV dumping too much pressure at WOT. i have no fuel pressure problems what so ever at idle but when i throttle it drops right away. so i guess i should get the clamp back on the return line and see if it helps my fuel pressure at WOT in order to test the OFV?
 
#17 ·
well i got a pair of vice grips and clamped down on the return line and now my fuel pressure is staying a lot higher, under throttle its not going below 20psi. so i sent tork tek a email requesting they send me another OFV hopefully they send me one asap.
 
#18 ·
so i talked to rob from tork tek and he says they test every single OFV that leaves the factory. at first he asked me a few questions like do you still have your fuel heater? i told him i deleted it 2 weeks ago, he asked what the fuel pressure was at idle, i told him i had it set to 30 psi. he asked if the fuel pressure dropped with no load just sitting in neutral while throttling and i said yes and told him ive got a clamp on the return line and it was helping my fuel pressure while under throttle. hes willing to bet im sucking in air somewhere and told me to run clear line from my fuel filter to my injection pump and look for bubbles.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top