Have you tested the suction pressure at the inlet of the pump?

Should be around 3"hg. If its lower, you possibly have a suction leak in the fuel supply system. If you have much over 3" you possibly have a fuel restriction.

 

Hi Doug. No I haven't. How can I DIY the test?

 

I don't know of a diy way of doing it. They make a vacuum gauge that connects to the fitting with the plastic cap next to the alternator. The gauge with the special connector is around $150.

 

But if desired can get to 26k while actual is 21-22k wouldn't that mean it's sucking enough to get the right amount of fuel?

 

Having it not start without pumping the primer and being 5K short of the desired fuel pressure indicated to me that there is a fuel system problem. I suspect it may be in the supply system. The suction pressure test will either verify that is the problem or eliminate it as a problem.

 

10-4. Thanks for your input. I'll be a week or two before I have a chance to get the test done. I'll advise outcome. Do you see a lift pump in my future? May be too late? Would it work anyway?

 

I would do a lift pump anyway. Not so much from a performance stand point but for keeping your CP3 and your injectors happy. I will never figure out why GM decided to have the CP3 suck the fuel up from all the way back from the tank. Just more chances to have fuel related issues and inducing more air in the system. Putting a lift pump in made a very notable difference in the smoothness of the idle. Dean

 

I've had this issue as well, only in very cold temps though. Same issue?

 

Update-
Dean. Just had it at dealer. Tech II says 2" at idle. 4" at max RPM. He could only get 16k actual with 26k desired. My Stealth II while driving will give 21k actual. Looking like fuel pump? Lift pump would probably just prolong the inevitable?

 

Once the damage is done I would agree with you. When the CP3 has to pull fuel up to the pump it only shortens the life of the pump. This is where a good lift pump pays off big. It provides positive pressure to the pump and also removes air in the system. I can tell you working with 3000 psi hyd systems that air causes huge hammering effects on multiple components. Imagine how much worse it is when pressure at the nozzles can reach 27 to 30k psi! A CP3 pump or a few injectors will easily pay for a good lift pump install.

My truck is not some ramped up monster. A simple 70hp tune with EGR and DPF delete. I installed a FASS 95 gph system because that will easily handle up to 500hp. I own my trucks for the long haul. I know this set up will give me the best life for my CP3 and injectors. Dean

 

Would also like your input Doug.

 

I'm sure this is a different perspective than most, but these are my thoughts...

I would fix the problem you have before considering an electric fuel pump.

The gear driven pump that is on the back of the high pressure pump is completely capable of feeding the high pressure pump. An additional pump might help if you had a tune that required tons of fuel. That doesn't sound like your case.

I think the gear pump gets an unfair rap here. Big diesels have been using small gear pumps mounted on the engine to pull the fuel out of the tank forever. The system works perfectly as long as you don't have leaks or restrictions. I prefer this over a system that adds the complication of an added electric pump.

The Ford and Dodge diesels need electric pumps because the high pressure pumps on these trucks are not equipped with the gear driven lift pump on the back of the high pressure pump. They would not run at all without an electric fuel pump.

I hope you are able to find a solution easily....

Doug

 

Agreed. Question though. The FASS 95 was the one I was looking at also. It's installed by the fuel cooler. How much PSI is generated by this lift pump? I had to splice the new fuel lines by the transmission and then again a couple times at the firewall to get them installed. (idiot shipper bent the flex line in two to fit it in box). Is there enough pressure that could blow any splice with double clamps on each side? Marc

 

On the Duramax the pressure range is 8 to 10 psi. Hardly anything that will blow lines off. I also did the factory filter delete. I got into some bad fuel and clogged the FASS filters. The delete bowl has a plug installed in it. It is a 1/8th pipe thread. I found a cheap kit that uses a 5 psi switch to activate a led light. If my filters start to clog or I have issues with my lift pump while going down the road I will know about it now. Dean

 

The delivery line is still only 8-10 psi? I know the return line is that. I just thought the delivery would be higher. Marc

 

Thanks Doug. Words of wisdom. When newer, both my 2006 (until I met a moose) and this LMM towed 10k boats up hills on cruise control. The reman pump will be installed by the dealer next week. Like my mechanic said, It's the only thing left it could be. lol

For anyone reading these posts in the future, I have a feeling that when I started having lack of power issues around 500 Kms, (see Loosing power on hills) the pump was heading for a slow failure.

Marc

 

I know trying to diagnose these without having all the equipment is tough, but there are two test I would be sure to run before condemning the pump. There is a test that puts a clear hose between the filter housing and the pump. This would allow you to see if the pump is getting air from a suction leak. You also want to test the amount of return fuel to rule out leaks to the return. GM has specific tests for this.

 

I agree on replacing the CP3, if it's starting to go you might extend it's life by installing a lift pump, but you're risking a CP3 failure that will trash your injectors.
Replace the CP3 and then decide on a lift pump is what I would recommend.

 

Doug. Tech already did the return line test before I did injectors. Could only do passengers side because I was told by tech that the driver side lines were stuck and he didn't want to break any. Passenger side came back 4ml each. I was told that this was borderline for GM specs. That was part of what made me change the injectors. Didn't make a difference. Marc

 

Update- Didn't do clear hose test. Wrong decision.

Well $3200 for Reman CP3 fuel pump at dealership today and it didn't fix it.

Got the call that it was "finished" and "fixed". Even Left it outside overnight just to make sure it would start properly.

Went to pick it up today. Paid the bill. Started it to drive home then stall. Injector knock. No start.

So discouraged. I don't know what to think or who to trust any more. I'm $6K plus into this problem and still in the same position. They said they would look at it in the morning. I can't afford any more false diagnostic crap. Telling them not to bother. Starting to think that all he sees are $$ dollar signs when I walk in the door. I may be wrong but it's hard not to think it.

 

This must be frustrating as hell for you.....

Low rail pressure can only be a few things. Suction leak in the supply, restriction in the supply, fuel pressure regulator valve stuck, bad CP3 pump or leakage through the return either past the injectors or fuel pressure relief valve.

It sounds like you have eliminated most of them. The clear hose would identify air getting to the pump from a suction leak. Did they test the spill back on the relief valve? Are they able to control the rail pressure with the tech 2 to verify the fuel pressure regulator valve is working?

 

Brutal.

I have the valve shimmed. I can only assume he tested it with Tech II.

Yes he can and did control rail pressure. Could put it up to 26k desired and 21k actual at full throttle from Tech II. Around 5K at idle. He said that because there is no extreme fluctuation at idle and no fuel surge that it was very unlikely regulator is bad. It was reused on new fuel pump.

Fuel pressure Regulator valve works I assume. New CP3 pump. New Injectors. Fuel Pressure Relief Valve shimmed.

I guess that would leave Suction or restriction. Pressurized tank and no leak found. New lines up to firewall. Have not taken off hoses from filter to CP3 or from fuel cooler back to tank. Sending unit at top of tank was re sealed at 240k kilometers. New line in place then.

Anything left? lol

 

Another test that I have seen done is removing and capping the supply line at the tank. Then putting a mightyvac (hand vacuum pump) on the other end of the suction line where it attaches to the pump. This will pull a vacuum on the entire supply system including all of the filter hardware. Once a vacuum is present watch the vacuum gauge to see if the system holds a vacuum.

Being a mechanic for 35 plus years, I have been in this situation.... getting my ass totally kicked. At some point you have to go back through the basic troubleshooting again and make sure all of your bases are covered including verifying that the new parts you installed are working correctly.

I also feel like the dealership owes you some consideration after spending the money to put the pump on and not resolving the issue. When you are spending that kind of money to replace the pump, you damn well should have made absolutely sure that you have eliminated all other possibilities.

 

Update.
Mechanic tested the Fuel pressure relief valve. Got 75ml of fuel at 18k. Not supposed to be any.
My question Doug is, can the Fuel Pressure Relief Valve located on the rail by driver side firewall cause the no start and or injector knock? He said it's definitely the cause of my lack of power. The senior mechanic says no and the mechanic says possibly.

The dealer has offered to replace it. If it fixes it, I pay for the replacement and installation ($1000). If it doesn't , he removes it and does not charge me.
Marc

 

If it is leaking that much it is definitely the source of your low rail pressure. Its bad and it needs to be replaced. I am certain it has some affect on driveability. I would guess it will solve some of your issues and very possibly resolve them all. To get to the root of the problem you need to eliminate it as a cause.

I am not there with the truck so I am not sure what they are seeing and what tests they have run, but I am having a hard time understanding why they replaced the CP3 and would suspect the injectors were a problem before checking the return rate on the relief valve. That doesn't appear to be sound troubleshooting.

I am a a desk and a keyboard halfway across the globe, so I have a pretty distant view of what is going on. With that in mind, I would definitely replace the relief valve..... if you still have issues, I wouldn't be surprised if you have a suction leak somewhere on the supply system.

Good Luck!

 

Meyerperformance was having no start issues with the shimmed valve due to the spring being weakened by the shims.

http://www.duramaxforum.com/forum/06-07-lbz-lly-performance-parts-discussion/61345-ppe-race-valve-shim-kit-2.html

 

Hi Doug.
"I am having a hard time understanding why they replaced the CP3 and would suspect the injectors were a problem before checking the return rate on the relief valve."

The hind sight thing in full effect. It was my decision not to do the relief valve test. I didn't want to spend the extra money. (as little as it turns out to be). I had shimmed it and I thought I had a load of info to make the call. My bad and I've paid the price. Dearly.

Got truck back today and voila, starts like a champ. $1000 lighter though. New relief valve and you guessed it. Small pin hole in pressure line at fire wall were I had to splice the new lines I installed. I had charged the tank with a rag and a compressor and nothing. No leaks. He had did it the same way and nothing. This time he used an old cap and fitting and charged the tank to 7psi and left it. Sure enough. A wet line at the clamp. My sealer didn't make a complete seal. I have the piece.

His TECH II now shows 26k desired and 26k actual. My Stealth II with no load will get 22k desired and actual. Unlike before when it 5k off. The true test will be next week under load.

The only problem now is I still have the injector knock. I'll start a new thread.

Anyone reading this take note and spend a little to save a lot. Would my CP3 had needed changing, maybe. Not sure when though. My goal is one million kms. Would it have made it there? Probably. Will never know. I'll just have to listen to my wife nag for awhile and put up with it. lol

Can't thank you enough Doug. You're insight and troubleshooting is/was bang on. Even though you're " halfway across the globe". lol. It's only a 4 day run out to you from Nova Scotia. Funny thing is, I was just through Washington state in December. Drove right down I-5 from British Colombia. I could have save a ton of money. lol

Cheers
Marc

 

Awesome news!

I am glad you were able to use the information to help find the problem. The nice part about using these forum is you can get some great ideas on what direction to turn when you get stumped.
If you went down I-5 from BC you were only a few miles from my place. You should have stopped by for a beer!

Doug

 

Next time.
Marc