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Lb7 question

BigBurt

Active member
Joined
Aug 12, 2023
Member Number
6801
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39
Bought an lb7 2500hd from California because it has no rot. It’s got 224k on it and I have put 20,000 miles on it far and it’s been doing well. I heard a quiet pop before shutting it down yesterday but it ran well for 30 seconds more so thought nothing of it. No knocks or clunking. Went to start it today and it wouldn’t spin past a few seconds then stops dead with a metallic clunk. To make a long story short motor bars over off the crank until a certain point and locks, however if I bump the starter it goes past that point but I can’t spin it past with the bar? I pulled the glow plugs and there’s no fuel in any cylinders so I don’t think it hydro locked? I suspect the cp3 pump has been on its way out as I’ve heard the truck surging a lot lately. Can a cp3 lock up and cause the motor not to spin?
 
Not a chevy guy, but the CP3 can seize, like the CP4 and lock up. Could be a piston is coming apart. Probably have to separate it from the engine and try cranking it again. See if the hesitation is gone.

When the cp4 seizes in the fords it can spin the crank gear and mess everything up making it harder to diagnose and repair.

Funny though, I just diagnosed a lb7 with 215k miles. Would not start, popping back through the intake. Looks like it dropped an intake valve or busted a valve spring. I told him to pull the valve cover or do a compression check or both before he calls me again.

You don't have popping in the intake do you?
 
Tiha said:
Not a chevy guy, but the CP3 can seize, like the CP4 and lock up. Could be a piston is coming apart. Probably have to separate it from the engine and try cranking it again. See if the hesitation is gone.

When the cp4 seizes in the fords it can spin the crank gear and mess everything up making it harder to diagnose and repair.

Funny though, I just diagnosed a lb7 with 215k miles. Would not start, popping back through the intake. Looks like it dropped an intake valve or busted a valve spring. I told him to pull the valve cover or do a compression check or both before he calls me again.

You don't have popping in the intake do you?
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No popping in the intake. Going to compression check later this afternoon. Dropped valve also is on my radar for this as I don’t think I bent a rod etc. (just my feeling).

Going to pull that pump later see if it frees up. Going to work my way down. Pump doesn’t need to be timed right?
 
did the crank snap the pin that holds the balancer and timing gear? i have snapped them on my lb7 a few times before only running keyed cranks on builds....... bent a few and caught it before full snap. Ive built more than 20 lb7 motors and only seen valve issues on rebuilt heads, the OE stuff is fairly bulletproof. if you need parts let me know, i may have something in the shop. id check codes and see if it says anything. the lb7 is pretty basic so a lot can go wrong and the ecu not know it but id start there. is the motor stock?
 
carslut said:
did the crank snap the pin that holds the balancer and timing gear? i have snapped them on my lb7 a few times before only running keyed cranks on builds....... bent a few and caught it before full snap. Ive built more than 20 lb7 motors and only seen valve issues on rebuilt heads, the OE stuff is fairly bulletproof. if you need parts let me know, i may have something in the shop. id check codes and see if it says anything. the lb7 is pretty basic so a lot can go wrong and the ecu not know it but id start there. is the motor stock?
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Yes it’s stock, that was my draw to it asi knew no one had messed around inside it other than religious maintenance. I will check that out later thank you for that advice. When they bend/snap does it do so quietly or with a noise? Still trying to get all the damn California EGR bullshit off and out of the way. I’ll tell ya I do like how smooth the truck ran but all this shit being snaked all over its self sucks.
 
carslut said:
did the crank snap the pin that holds the balancer and timing gear? i have snapped them on my lb7 a few times before only running keyed cranks on builds....... bent a few and caught it before full snap. Ive built more than 20 lb7 motors and only seen valve issues on rebuilt heads, the OE stuff is fairly bulletproof. if you need parts let me know, i may have something in the shop. id check codes and see if it says anything. the lb7 is pretty basic so a lot can go wrong and the ecu not know it but id start there. is the motor stock?
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carslut said:
I’m not sure which pin you mean here?
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the pin holds the crank in line with the timing gear, hit youtube plenty of guys enjoy when it lets go and now the truck is out of time and the oil pump stops working. i killed the front pin a few times on my truck when it was totally stock. no idea if that is your issue but id def pull the balancer off and see what the front pin looks like, if its unhappy the 2nd pin is probably also not happy.
 
That kinda sounds like a broken crank to me but realistically it could be a lot of things and none of us are near it diagnose. The cp3 can lock up but it usually destroys the gear on it. If you pull the oil fill you can see it a little bit. An inspection camera is your friend.

If it didn't spin over easily with no glow plugs it's go a major mechanical problem. Weather it's a locked up injection pump, valve to piston interference, broken crank, or something else.

Pull the upper and lower valve covers and see what the valve train is doing. When I did headgaskets on my LB7 I just pulled the cab, made working on it way easier. If you have a lift this is the way.
 
philofab said:
That kinda sounds like a broken crank to me but realistically it could be a lot of things and none of us are near it diagnose. The cp3 can lock up but it usually destroys the gear on it. If you pull the oil fill you can see it a little bit. An inspection camera is your friend.

If it didn't spin over easily with no glow plugs it's go a major mechanical problem. Weather it's a locked up injection pump, valve to piston interference, broken crank, or something else.

Pull the upper and lower valve covers and see what the valve train is doing. When I did headgaskets on my LB7 I just pulled the cab, made working on it way easier. If you have a lift this is the way.
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I get about 330 degrees of rotation. I will probably do exactly that and do up pipes, down pipes etc
 
The last time I worked on a diesel that wouldn't turn 360* I found a rotated roller lifter, it turned 90* and would lock the engine... so I had like 300* of rotation.
Since you pulled the glowplugs that eliminates a few things.
Bottom line it has to come apart. I have a LB7 truck here that I may part out, the body is ate up from Michigan salt.
 
snapped crank on a stock motor that would be impressive. the stock firing order can def kill a duramax crank. i now run a non oe firing order cam to change the firing order after snapping a few cranks but that took fuel and lots of boost. def keep this thread going as you dig into the motor.
 
What did you find on compression test? It is the easiest place to start. If you are going to pull CP3, might as well pull valve covers. The CP3 is a pain to pull in them and you are basically to the same spot to pull the valve covers. Then you get to debate injectors 😬
 
carslut said:
snapped crank on a stock motor that would be impressive. the stock firing order can def kill a duramax crank. i now run a non oe firing order cam to change the firing order after snapping a few cranks but that took fuel and lots of boost. def keep this thread going as you dig into the motor.
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Perhaps the previous owner had a big tune in it.
 
OutlawRider said:
What did you find on compression test? It is the easiest place to start. If you are going to pull CP3, might as well pull valve covers. The CP3 is a pain to pull in them and you are basically to the same spot to pull the valve covers. Then you get to debate injectors 😬
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Well, life got really weird since I started this thread and unfortunately I haven't got that far yet, it’s on a rear back burner. I decided on downing the truck and pulling the motor out. When I get it into the garage (crew cab long bed so truck have to jockey some other projects) I’m going to take my time and start with compression test. I plan to drive this truck for as long as possible (I’m cheap lol) and with the body being so nice I have no issue doing a refresh on head gaskets, injectors etc in addition to whatever I find to be the cause.

Where are you guys hooking up to for lift points when pulling this generations motor out?
 
carslut said:
snapped crank on a stock motor that would be impressive. the stock firing order can def kill a duramax crank. i now run a non oe firing order cam to change the firing order after snapping a few cranks but that took fuel and lots of boost. def keep this thread going as you dig into the motor.
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as far as I know this truck never saw anything boost/fuel wise that would make me suspect a crank issue but I do have terrible luck
 
Sorry for multiple posts I suck at posting here from mobile. If I absolutely had to guess blindly right now I believe cylinder 5 is where my issue is
 
motor should have some hooks on it to lift it out. otherwise i use some of the holes on each head to hook up a leveler. nice thing on these trucks is you can take the cooling stack out and take the core support apart then slide the motor out the front fairly easy. by myself i can get a motor out in a long day. no lifting the body needed.
 
BigBurt said:
Well, life got really weird since I started this thread and unfortunately I haven't got that far yet, it’s on a rear back burner. I decided on downing the truck and pulling the motor out. When I get it into the garage (crew cab long bed so truck have to jockey some other projects) I’m going to take my time and start with compression test. I plan to drive this truck for as long as possible (I’m cheap lol) and with the body being so nice I have no issue doing a refresh on head gaskets, injectors etc in addition to whatever I find to be the cause.

Where are you guys hooking up to for lift points when pulling this generations motor out?
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I just pulled the cab on mine. Easier.
 
dug around to get some photos of things Ive seen in the bottom end of a few LB7 motors.

here is the pin that locates the harmonic balancer, these like to snap off. the oil pump and timing gear have the same kind of pin holding them. for whatever reason the factory decided not to use a woodruff key in these motors.

DSC01322-450x337[1].jpg


the OEM firing order likes to do this to cranks when pushed hard.


001-duramax-crankshaft-carnage[1].jpg


also seen many bent rods in lb7/lly motors but that takes fuel/boost to happen. not seen a stock motor kill a rod.

Ive had a few motors the timing gear has moved and the valves kissed the pistons.
 
Have it in the floor but not on stand yet or taken apart Nothing as far as compression and suction on cylinders 4-5-7. Trying to read up on things.

That crank pin design makes me scratch me head as to why someone decided it’s be a good idea. A key just seems superior?
 
BigBurt said:
Have it in the floor but not on stand yet or taken apart Nothing as far as compression and suction on cylinders 4-5-7. Trying to read up on things.

That crank pin design makes me scratch me head as to why someone decided it’s be a good idea. A key just seems superior?
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I think they did the pins to be cheaper. I have seen broken cranks on stock engines. A lot were shortly after a repair was done. I think the balancer wasn't torqued properly. I would have mine keyed if i was building one for sure.
 
So I’m sure there’s reasons why this isn’t a popular swap but…. Swapping a Cummins into this era Chevy and keeping the Allison, what’s out there to control the Allison with the Cummins?
 
BigBurt said:
So I’m sure there’s reasons why this isn’t a popular swap but…. Swapping a Cummins into this era Chevy and keeping the Allison, what’s out there to control the Allison with the Cummins?
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I think there are standalone Allison controllers.

I think the bigger issue is the Cummins is like 8" longer. I'm thinking that is going to involve a lot of work to get it to fit.
 
ive got a "destroked" standalone allison controller in one of my rigs. kinda pain to get setup but once it had the bugs worked out has been okay. ive been digging thru my bag of 2023 sema stuff for a place i grabbed a card from offering what looked to be abetter allson controller. im not a fan of the ATS setup, costly and mixed quality.
 
carslut said:
ive got a "destroked" standalone allison controller in one of my rigs. kinda pain to get setup but once it had the bugs worked out has been okay. ive been digging thru my bag of 2023 sema stuff for a place i grabbed a card from offering what looked to be abetter allson controller. im not a fan of the ATS setup, costly and mixed quality.
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What was the issue?I had a pcs controller in a Fummins and the throttle position sensor was my biggest problem. Couldn’t really calibrate it right. Ultimately I’ll put this lb7 back in but looking at the empty engine bay got me thinking about putting the more simple Cummins in.
 
Went a bought a new(used) motor and stuck it in with the computer to match that motor. Fired right up after priming and resetting the security setup. Now there’s a delay when you bump the key to fire that wasnt there before? Turn key and it takes a second before starter fires, am I crazy or do some of these trucks do that for some sort of “self check” reason or like on a Peterbilt or Mack? Motor/computer is out of an 02 federal emissions.

Started to tear down the old motor to keep what I need but progress is slow. So far cp3 pump was good and not the cause of my lock up.pins were in decent shape and not bent. Gears on front of motor are all decent and no metal came out with the oil and pan.Plan on keeping cp3, injectors, FCIM, random lines etc. for spares. If anyone is in need of any random little stuff just cover shipping.
 
BigBurt said:
P1781 and u2106 on new motor now yayyy
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Looks like modules aren't talking to each other. I would start by reseating all of the main harness connectors. Also how are your batteries? Could be a low voltage issue
 
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