Engine/Transmission (1994 - 1998) fuel pump sucking air, out of ideas

[GAmes]

Truck was running fine but I have been getting tiny metal particles in the fuel filter the last 4 or 5 changes. Decided to replace the lift pump. At the same time I replaced the L shaped tube to the prefilter, the prefilter and the fuel filter. Fuel heater was removed long ago. I can pump it up and get the engine to start after a few revolutions (not quickly as normal) and the engine runs great. Shut it off and 5 minutes later it will not start without pumping it up again. I removed the steel line with the square cut o-ring that gave me trouble before and replaced it with a piece of new fuel hose connected to a barb fitting. I replaced the pre-filter o-ring. I reseated the L shaped hose. The only thing I haven't done is try to remove and plug the hole for the fuel temp sensor. For one thing I'm not sure I can do it without removing the whole assy and the other thing is I don't know if that will cause my problem. Any insight on the possiblity the temp sensor can allow air into the system? Anything I missed? OFV is less than a year old as is the return rubber hose, and I had no starting issues prior to the pump swap.

 

[kenny61]

I don't know if its a texas problem but up here with the snow and road salt it got my truck a few years back. The steel fuel and brake lines tend to rust where they run alongside the fuel tank. Also the top of the sending unit where the fittings go into it was getting bad as well I did a temp fix a few years ago one of these days I will replace it with the factory again...

 

[GAmes]

Thanks Kenny, but that is not a problem. Remember, I had no issues prior to the pump change. I replaced the whole fuel canister 16 months ago when I was troubleshooting the same problem. I had replaced the pump and was getting a lot of air. I put the original back in after replacing the canister, the OFV, removing the heater and replacing the rubber lines over the course of 8000 miles. That time it was cured with a little square cut o-ring on the fuel line that enters the prefilter. I just left the old pump on and have put about 84k more on it. This time the first thing I did was eliminate the square cut o-ring with no improvement.



I have eliminated the fuel temp sensor as the culprit. I globbed copius amounts of RTV over the plug and let it set up. I am now thinking the Chinese made Cummins pump is bad. I pumped fuel through it, had problems, reinstalled the old pump then stored it for 14 months. Tomorrow, rather than go fishing, I'm going to reinstall the old pump again and do nothing else. If the old one works I'll try to find a Carter, made in USA pump.



BTW. I still have the top part of the canister with the two fuel lines, the elec plug and the blocked off second draw straw in it. I've been saving it for someone who has had theirs rot off. I discarded the bottom portion because it was trashed, but the part I have is in good shape. Let me know if you are interested in it. I'll take a picture to verify it is what you need and you can have it for postage. It will fit in a priority box, I think, they are about $10 to mail.

 

[kenny61]

Much like our elected officials I did not read the whole post and offered a simple solution to a non existent problem :-laf I will now suggest that the tube rusted through at the same time you changed the pump and ignore my own ignorance



Thanks for the offer on the part but I will just swap out the whole unit the next time the sender goes. I should probably order one now before it gets discontinued.

 

[fest3er]

Inspect your steel supply line(s). If anything is well rusted, it's probably got a hole in it. Inspect the quick connect fitting; it might not be sealed any more, letting air in. You should smell some fuel up by the engine since you *do* have a fuel leak. ANd you should see evidence of fuel somewhere up there.

But, since you've been thorough thus far, perhaps the leak is not at all obvious. So stuff a rag in the filler neck with an air blower and pressurize the tank to 5-10 PSI (or install a schraeder valve in a spare fuel cap). That oughtta be enough to push fuel out whereever the leak is, since the leak is almost guaranteed to be before the lift pump.

 

[SCClockDr]

Some time ago I rebuilt the fuel pump on my 94. I was in the Maine woods and fuel was back draining with a 1/4 tank. I secured a lift pump designed for a chipper. The piston was quite a bit smaller. I gutted the valves from the new pump and installed them in the original. 2 were definitely bad.



If the internal valves will not hold prime any leak & air through your return will allow the lift pump to lose prime. If loss of prime only occurs below 1/4 full then the air is coming from the end of the return in the tank. If this is happening with a full tank then there is most likely an air leak in your return as well. In both cases the internal valves are allowing the back flow.



BTW the chipper pump was ~$75. 00 as I recall. (Several years ago. )



To simplify:



If you are getting air when running then the issue has to be between the tank & lift pump, if you are getting the air after shutdown then the issue is the pump & the return line if it happens at any tank level.

 

[GAmes]

I have no rust what so ever. Once the engine starts it runs perfect. WOT is 30 pounds of boost and 1300 degrees, the fuel plate is just forward of where the stock plate was. Fuel tank is full. I don't have a fuel leak, I'm losing prime. I guess the title is misleading, sorry. I'm leaning toward the internal valves being bad. I'm going to disassemble a pump to see what they look like.

 

[GAmes]

I disassembled the two pumps I have. The original and it's replacement. Contrary to Joe G's diagram they only have two check valves, not three. Now that I know how they work I'm thinking the outlet check valve on the new pump is not closing. If that is the case I have parts:-laf

 

[JSTC]

I've found in more than 1 case, the o-ring, inside the quick disconnect, on top of the fuel sending unit to the tank goes bad. You can either replace it with an o-ring that holds up to diesel fuel, or replace the hard plastic line with a rubber line. Hope this helps

John

 

[SCClockDr]

I disassembled the two pumps I have. The original and it's replacement. Contrary to Joe G's diagram they only have two check valves, not three. Now that I know how they work I'm thinking the outlet check valve on the new pump is not closing. If that is the case I have parts:-laf

Wow, glad you have a plan. Hope you have enough good valves to "git'er done"

 

[GAmes]

Since I was saving the original pump just in case a rebuild kit ever surfaced I used it for the rebuild. It was replaced because the spring had got a bit weak and it was ticking, plus the boot had deteriorated. I pulled the spring out of the Chinese pump, it was about 1/8th inch longer than the old one. I also used the boot. After installation all was good. No leaks and the engine would restart after shutting it down for a few minutes. I thought I had the problem solved. WRONG!! I secured the supply line so it wouldn't be flopping around and the engine wouldn't start, but at least that gave the real problem away. I'm pretty sure the steel braided flex line has a hole in it somewhere and when it is bent just right the hole opens. So I have it covered with duct tape and subsequent starts have been successful. I need to run it really low on fuel before I cut the flex portion off since it is below the current fuel level. If I'm right or wrong I'll post in a few days. Thanks for all the suggestions. Meantime I'm off to my favorite lake.



Thought of something else. I can truthfully say that with 715k I'm still running the original lift pump:-laf

 

[GAmes]

Back to square one:{ I cut out the steel braided hose and replaced it with a new fuel line. No improvement. What am I missing? I started out by replacing the lift pump, fuel filter, prefilter screen and the L shaped hose. Prior to that I had no starting problems, just metal in the filter canister. To trouble shoot I've put in a known good OFV and cut out the steel flex line and removed the inlet line to the prefilter. I know the pump is good. I pulled my boat 350 miles round trip.



Has anyone had any success with spraying the lines with soapy water or WD40 or something to find a suction leak? I'll even replace my 16 month old fuel return rubber line if someone can explain how that causes starting problems.



I'm getting really frustrated here.

 

[SCClockDr]

GAmes Hi!



You are up against hydraulics and gravity.



If your return line can suck air and your pump will some allow reverse flow. Your fuel system will drain.



No different than a garden hose.



You could block off the end of your return line with a bolt etc. and pressurize it with fuel. If it leaks you should find it after a bit of time.



Remember the return straw in the tank module could be the source as well. Any pin hole would do it.



Also a static vacuum is more difficult to maintain then static pressure.

 

[GAmes]

If your return line can suck air and your pump will some allow reverse flow. Your fuel system will drain.



Remember the return straw in the tank module could be the source as well. Any pin hole would do it.

I think I eliminated the pump by installing a different one. I am not ready to buy another one when I have three that pump fuel in my possession.



I'm having trouble believing the fuel return system can cause a problem when I have done nothing to it. The rubber hose and the module are only 16 months old AND I was having no problem prior to disturbing the inlet fuel system and pump.



I guess I need a better explanation of why the return hose could cause hard starting. The unneeded fuel has to go back to the tank and it is not under pressure.



I don't really believe it will fix anything, but I'm going to stretch the spring on the OFV. I thought I had a spare fuel cap, but I can't find it.



Need some more theories!!!

 

[hsmith]

Man, you have a lot more experience on these fuel systems than I do. We both know a very small leak in the system can cause a loss of prime. Don't think you have an OFV problem. I have a new one I can send you for trial but don't think it will help. I 'm thinking you need to apply low pressure to the system and check with a soapy solution. If I have anything you need, let me know soon. Leaving in a few days 'till next year.

 

[kenny61]

I don't know the physics behind why the return line can cause hard starting However having had to replace it for that very reason I can put my hand on a bible and swear that it does without worry of lightning bolts. My guess would be the leak allows the the line to bleed down probably though the overflow valve because it is designed to only hold pressure one way. If I was in your boat I would break the system down into sections. disconnect lines at tank and fuel pump put pressure on it and see if it leaks. Disconnect lines between tank and fuel return see if it leaks. crimp off fuel line at return from p pump. pump priming pump see if you have any leaks between fuel pump and p pump return then if you have to drop tank and check sending unit and associated pickups and returns. To check the entire return line you can plug the return at the tank and and pump up the primer pump until you cant push it in anymore you could also do it with the truck running but the pump will build enough pressure until something pops. I just did that on my buddys case when the supply line cracked and it ran dry and I was trying to get it to prime... diesel everywhere

 

[GAmes]

Thanks for the offer Smitty. I don't reallu think the OFV is the problem either. Happy Holidays!



Kenny I think you have the best plan. I'm going to hook my boat up in the morning, go fish a tournament over the weekend (manually pumping it up every time I need to start it) and have at it again next week.



If anyone can offer any other fix I'm all ears.

 

[fest3er]

... I guess I need a better explanation of why the return hose could cause hard starting. The unneeded fuel has to go back to the tank and it is not under pressure. ...

If the entire return line is secure, it will hold fuel in its entire length, just as a straw will hold your sweetened iced tea in it when you block the top with a finger.



If there is a hole in the return line, the weight of the fuel between the hole and the tank will pull air into the line. Because air is lighter than fuel, it will travel as far up as it can, displacing fuel as it goes. The displaced fuel usually flows to the tank. Over time, the fuel level can, and usually does, drop below the injection pump. Thus it can take a few grinds of the starter (and thus a few strokes of the fuel pump to fill the p-pump) to fire the engine.



Fuel can come out of the leak hole for a reason very similar to why water pipes can rattle when you shut a tap off very quickly. You might think air is flowing in through the leak. But it's really bubbling in, and the fuel coming down the line is sort-of bouncing. That bouncing can 'hammer' the fuel near the leak, pushing a little out the hole.



The supply line (if stock) is bigger than the return line. It is possible (normal, really) for the lift pump to supply more fuel than the engine needs; the OFV then opens returning the excess fuel to the tank. This return fuel can be under significant pressure. Try poking a tiny hole in a straw, then try blowing a mouthfull of iced tea back into the glass through this straw. You should observe tea coming out of the little hole.

 

[kenny61]

T

If anyone can offer any other fix I'm all ears.

Find out what's wrong and I'll tell ya how to fix it :-laf

 

[hsmith]

I think when Gary figures out the problem he will have a fix for it. Happy fishing man.