LB7 Duramax Return Flow Rate Testing

In this video I demonstrate how to test fuel injector return rates on an LB7 Duramax engine.

[@justdoit4492]

I did some injectors when I was an apprentice, no one else wanted to dig into it…but found some info online back in 2009. Was running rough but verified which injectors needed to be replaced… and a HPFP on another dirty max. Company was happy with me to say the least. Would have been 10k$ each to ship far away and get a dealer.

[@jhdiscgolfer]

Very informative Mike. I don't have a Duramax or a diesel but I am currently at work LITERALLY working a refinery unit making that beautiful ULSD. I'm very much the guy who would have just changed all the injectors, but it was very interesting watching the process of proving the bad ones and you were spot on with the suspect #1 and #5. Hope that Jeep is still running strong!

[@zmotorsports62]

Thanks for following along and watching another video. The Jeep is still running great. Mike

[@tommybogen3709]

Absolutely one of the most informative videos out there for LB7 return line flow testing. Perfect execution 👍

[@zmotorsports62]

Thank you and thanks for taking the time to watch. Mike

[@hotcheesesoup9776]

Thank you

[@brockbax]

Can you check cp3 return without special fittings or the kit?

[@zmotorsports62]

Yes. You just need some sort of graduated container to measure the volume and a couple pieces of tubing, preferably clear to connect to the barns one the CP3 pump.

[@billyzagrodny97]

Quick question if I may. I have an actually rail pressure of 1.50MPa desired 50. And when I check the return im only getting 15ML to the tank. Any idea what test I should do to check my issue? I'm new to diesels.

[@zmotorsports62]

Next I would use a bi-directional scan tool and force the rail pressure up. If the actual cannot follow the desired you would more than likely have either a stuck open FPRV or a bad high pressure pump. Good luck in your testing.

[@billyzagrodny97]

@@zmotorsports62 thank you for your response. I was thinking the FPRV, but then thought if it was stuck open I'd have more fuel being returned to tank.

[@zmotorsports62]

Yes, that would make sense but easy to check so I usually recommend checking that to confirm before jumping right to the pump. Good luck. Mike

[@nonenone1249]

@@zmotorsports62 is there a inexpensive that will it do a active test to demand max fuel pressure on the cp3 injection pump to test?for a 2005 gmc sierra 2500hd with a 6.6L duramax?if not do you know what bidirectional scanner will? thank you

[@royanguiano2610]

Yes I want to see more videos like this, can you show more in-depth the process of setting it up for both tests because I was trying to see where the hose fittings were connected to on each individual cylinder during the flow test an I’m assuming because the ficm relay were disconnected that the injectors wouldn’t spray to prevent the vehicle from running. There are just more things I need clarification on . I’m going to subscribe because I’m studying diesel an your right there aren’t any videos on flow testing, or testing the HVLP for these systems. But the flow test is a critical test for these an the Cummins so I’m wanting to learn everything that I can as I’m saving these videos to my library for my own studies. I really appreciate you taking the time to make this an please reply back or just post an give a shout out . It’s nice to know that the viewers are heard an recognized most people don’t do that .

[@zmotorsports62]

Thank you for the comments and for taking the time to watch. The return flow test lines are merely connected to the return port out of each injector after removing the return lines that daisy chain the injectors. Next time I have a Duramax in the shop for injector work I will show a more detailed shot of the hose connections. Mike

[@royanguiano2610]

@@zmotorsports62 no sir thank you, a gift is appreciated an knowledge is a precious gift. Your video was the most informative I’ve found all on KZbin. It’s the best one yet. really helps break things down for understanding. God bless you sir

[@zmotorsports62]

Thank you.

[@SeanCrimmins442]

Mike, When you inserted the test line into the return port on the head on the driver side near the firewall, did you also need to plug the banjo bolt line to isolate it?

[@zmotorsports62]

No need to plug the return line. With the flow going into the test vials there should be nothing in the hard return line that connects the injectors.

[@carlb1120]

Hi Mike That was most interesting. I had no idea of how to test injectors, but now thanks to you, I do. Please continue these types of videos as I find them most interesting and helpful. We enjoyed our trip out west into your world and we rented a jeep in Moab (You know I had to do it after watching all of your videos on your Jeep modifications). I asked the lady to recommend a trail for someone that didn't deal with heights very well and she told us about Shafer trail. I did OK for the first 15 miles and then we passed some very pronounced straight down drops that got my attention. We then headed toward a mountain that appeared to be about 1,000 feet straight up, but we continued. I kept thinking that we were going to be able to go around it until it was too late. We found our selves on this narrow dirt/gravel/rock trail that barely had enough room for one vehicle and no where to turn around. I later would discover that we were at the Shafer switchbacks. We kept going from one switchback to the next. I refused to look down and kept saying to myself, "Just drive the road, Carl, Just drive the road". Which I did as my Apple watch recorded my heart rate, which pegged the upper limit on it's dial for the entire time until we got to the top. My shirt was soaked and my wife said I was moaning a few times, but I told her that must have been her imagination, because I would have never have done that :)

[@zmotorsports62]

That’s an awesome story Carl. Shaffer Trail is not for those who dislike heights. Glad you did ok and hope it didn’t scare you off from Moab. Thanks for the kind words and for following along and watching Carl. Much appreciated. Mike

[@NOWELLSBLESSEDADVENTURES]

Hi mike all I can say high tech, high tech, high tech. Great video.

[@zmotorsports62]

Thank you.

[@arsgaragesc5774]

01 lb7 crank, no start & no codes related to fuel pressure just glow plug fault P0380 but ambient air temp is 90-100 this week so glow plug shouldn't be a factor . check fuel flow (on common return )while cranking from line you have capped in video with fuse out and get nothing. start to pump primer on filter housing and will get flow into mixing cup( couple oz come out). Scan live data show FRP at only 1-2mpa while cranking. Dont have tester to break down system( pump from injectors return flow). Any thoughts?

[@zmotorsports62]

Hard to say. I'd start by looking at fuel starvation to the pump. Could be losing prime or could be a collapsed fuel line which on these older trucks are becoming quite common.

[@steveale0711]

Great video. Any reason you can’t measure the cumulative return at the engine compartment quick disconnect vs back at the fuel cooler connection?

[@zmotorsports62]

No. Should be the same volume being returned, it's just at the cooler is easier and quicker access. Thanks. Mike

[@steveale0711]

@@zmotorsports62 thanks for the reply on an older thread. I was already checking my fuel delivery at the quick connect to verify my FASS was delivering. (It was not). Reprimed and it fired. Just did the return test and got 75ml on the return on a 15 second crank. New filter and separator on the way. What a relief.

[@norbertpozdol2959]

What is the max fuel in a single injector supposed to have flow to a container. Milliliters

[@zmotorsports62]

Maximum fuel return per injector is 4ml.

[@douglasgerrard5094]

Exactly what I needed to know.

[@zmotorsports62]

Happy to oblige. Thanks for watching. Mike

[@bannana6290]

lb7 does not have injector flow rates its the 2005 and up

[@zmotorsports62]

No, you cannot measure injector return flow rates with a scan tool monitoring any PID's. That is not what I'm showing in this video. In this video I am demonstrating how to measure actual return flow from each bank and then each individual injector, which is the only way to get this data.

[@JoseRamirez-em7zp]

Hi sr you said you don't want to see 60ml that means the Cp3 is bad thanks for the video

[@zmotorsports62]

Thanks for watching. Mike

[@GregoryJohnston77]

I know this is an older video, but i have a question. I thought i had bad injectors because when my truck warmed up, it white smoked. I decided to do a compression test, and i have zero compression in cylinder 1. My question is, could a leaking injector cup cause that? I know the only way to really know is to pull the head, but im trying to be optimistic here and hoping it's not a hole in the piston. I can handle needing valve train work or head gasket. i already bought new injector cups. I should have compression tested this truck before i bought all these other parts. 😢

[@zmotorsports62]

If an injector seal were to fail I think it would still have some compression, just low. For it to have zero compression I would think the injector would have to be completely out of the injector bore.

[@GregoryJohnston77]

@zmotorsports thanks for the speedy reply, so I'm probably screwed, it's either valve train or a hole in the piston. It just ran so good for it to be a hole i feel. 😥😥

[@zmotorsports62]

Hole in the piston wouldn't be my first thought. Although possible it's rare, I'd suspect valvetrain issue before a hole in the piston. Pulling the rocker cover should reveal the issue.

[@danielsandoval9012]

What is the vacume spec for the vacume pump on 02 chevy 6.6?

[@zmotorsports62]

Sorry, don’t know that spec off the top of my head. Thanks for watching. Mike

[@liftedlifestyle7765]

Hey I have an lbz that has crank no start and won’t hold a prime,do you recommend I buy a test kit and look at my injectors?

[@zmotorsports62]

You can, but if losing prime causing a no start I world look more at the filter head and/or adult lines from the tank. Sounds like more of an issue of sucking air vs. injectors.

[@liftedlifestyle7765]

@@zmotorsports62 thank you for the quick reply and great video!👍

[@hotcheesesoup9776]

I am assuming pulling the ficm fuse, and cranking the truck over commands full rail psi? 23k or so? To check bleed off. I have only seen bi directional special function to command full rail pressure to do this test. Very informative. Would like to see more

[@hotcheesesoup9776]

My rail pressure drops after 22k down to 17-18 Desired stays 22-23 actual 17-18 and injector knock and lots of smoke. All new sending unit, lift pump and fuel lines from tank to cp3. PPE relief plug. Ice picked and di electrical greased every pigtail on injectors. Checked ficm and di electrical greased those connectors , and clean MAF

[@zmotorsports62]

I'm not sure if you're asking a question or just stating your specs. Low RP is usually either pressure regulator, injectors returning too much fuel or injector pump sucking air and not getting a good supply of fuel.

[@hotcheesesoup9776]

@@zmotorsports62 just stating issues and what I’ve tried. Lift pump pushing 8psi fuel pressure to the injection pump. Thinking about trying injector cleaner as one or more of my injectors is sticking open at throttle and blowing smoke and knocking

[@nonenone1249]

@@hotcheesesoup9776 what scanner are you using to test the cp3? thanks

[@ajsumner3267]

Nice video

[@justfishing6067]

Never knew you had this channel! Mike has rescued my projects a few times now... lol Started with an old aluminum boat, later my s10 frame cracked around the control arm bolts. Now I'm in need of Mikes rescue service once more. Mike how do i get ahold of you these days? 🙃

[@zmotorsports62]

I remember those projects well. I’m quite active on both garagejournal.com and irv2.com under the username zmotorsports. You can send me a private message through one of those forums. Mike

[@justfishing6067]

@@zmotorsports62 I'm under the same user name on the garage journal. Looks like i need 10 posts before i can message you. 🤔 Feel free to message me, it might take me a while... lol

[@justfishing6067]

@@zmotorsports62 kzbin.info/www/bejne/h53Ze418qtl0hKM details!

[@zmotorsports62]

Responded via your PM. I’d be happy to help. Mike

[@mitchburhite2507]

Can any of those cause it to start then shut off? I have an 01 and every time I bleed the fuel and pump it up it’ll run for about 30 seconds then die. Only way I can get it to restart is if I crack the bleeder and repeat process. I have done orings on the primer replaced the two rubber hoses and replaced the rubber hose on the 2nd ficm bolt. I’m at a loss. I know it’s not the lines under the truck because I bypassed them by running 2 rubber lines from the passenger side fuel lines to a 5 gallon jug of fuel.

[@zmotorsports62]

I think your problem is still on the suction side. An injector or CP3 pump with high rates usually won’t cause a restart condition. They’ll usually cause an extended crank symptom.

[@mitchburhite2507]

Okay thank you for the reply. How would I go about finding where the lines are sucking air at then? Pressurize the tank?

[@zmotorsports62]

Pressurizing the tank and look for leaks is one way. Don’t use more than about 3-5 PSI though. Another way is attach a clear line to the filter head and look for bubbles. That’s air being introduced into the fuel. Then work back until you locate the point where air is getting in. Most often it’s the filter base but you mentioned you already rebuilt that. Next check the quick disconnects at the driver’s side rear of head and at the tank. Mike

[@mitchburhite2507]

Ok can If the fittings at the head are bad can I just run fuel hose from there to where it needs to be? Thanks again mike

[@zmotorsports62]

Yes. You can eliminate the quick disconnects and run fuel line.

[@SaDMax33]

Very informative

[@zmotorsports62]

Thank you and thanks for watching. Mike

[@bryanbeane7328]

We’re do you get that test kit thanks!

[@zmotorsports62]

I'm not sure where to get them these days. Mine is an older Kent Moore J-45873 kit that I bought years ago. This is the same kit that the dealerships use or used. I just Googled it and I see they're still available. I would stay away from the cheap import kits as they don't seal very well and will leak all over and give your erroneous readings. Mike

[@bryanbeane7328]

@@zmotorsports62 thanks I deal with a lot of Duramax trucks been using the balance rates but would like this kit for a more accurate reading. Spent alot of time on the ford 6.0 still work on alot of them but I’m starting to see more and more Duramax trucks and been doing well diagnosing issues with them would love to have that kit!

[@zmotorsports62]

It is a well made tool and definitely better quality than the cheap import kits. It would be a hard pill to swallow for a one time use but for a career mechanic it's a good investment for proper diagnosing of the Duramax return rate.

[@Dieselpwr]

Seems like a convoluted test overly complex

[@zmotorsports62]

The test is what it is. There's no other way to perform a flow test on an LB7 other than to measure the actual flow.

[@Dieselpwr]

@@zmotorsports62 just pull the return line

[@zmotorsports62]

The return line will show cumulative not specific to a component. I mention that in the video.

[@Dieselpwr]

@@zmotorsports62 yea I’ll just ohm them out by unplugging them

[@zmotorsports62]

Ohming them will test the coil but not determine the return flow.