LS7 running on 4 cyl
06-21-2017, 01:45 PM
#1
Instructor
Thread Starter
LS7 running on 4 cyl
Hey guys!
We really need your help. Long story short
A friend had his heads refreshed with new valves, springs, guides etc. The shop did a poor job and had to redo it because of leaking valve seats. Now hes put it all together, and we though all was great.
When he fired it up it struggles and only runs of 4 cylinders. Not random 4 cylinders, or only on one bank, but every second cylinder in the firing order. When you restart it, it runs another 4 cylinders.
We have checked everything, and no error codes except from random misfire with the Tech2. Fuel pressure is a bit low 48-50 psi at "idle". But that shouldn't cause 4 cylinders not to run?
Whats your take on it? We have tried another MAF and MAP. Where would you go next?
Thanks!
We really need your help. Long story short
A friend had his heads refreshed with new valves, springs, guides etc. The shop did a poor job and had to redo it because of leaking valve seats. Now hes put it all together, and we though all was great.
When he fired it up it struggles and only runs of 4 cylinders. Not random 4 cylinders, or only on one bank, but every second cylinder in the firing order. When you restart it, it runs another 4 cylinders.
We have checked everything, and no error codes except from random misfire with the Tech2. Fuel pressure is a bit low 48-50 psi at "idle". But that shouldn't cause 4 cylinders not to run?
Whats your take on it? We have tried another MAF and MAP. Where would you go next?
Thanks!
06-21-2017, 05:50 PM
#2
Melting Slicks
I would do a leak down or compression test, as it sounds like still leaking valves or the lifters plungers are bottoming out because of a poor valve job.
Another thought is that the springs maybe so weak in the closed position that the lifter preload could keep the valves off the seats, yet another point of a poor valve job, as I've seen this happen in the field.
Another thought is that the springs maybe so weak in the closed position that the lifter preload could keep the valves off the seats, yet another point of a poor valve job, as I've seen this happen in the field.
Last edited by extrapilot; 06-21-2017 at 05:54 PM.
06-22-2017, 11:38 AM
#4
Instructor
Thread Starter
I would do a leak down or compression test, as it sounds like still leaking valves or the lifters plungers are bottoming out because of a poor valve job.
Another thought is that the springs maybe so weak in the closed position that the lifter preload could keep the valves off the seats, yet another point of a poor valve job, as I've seen this happen in the field.
Another thought is that the springs maybe so weak in the closed position that the lifter preload could keep the valves off the seats, yet another point of a poor valve job, as I've seen this happen in the field.
Hes done both a compression and leak down test, both came out good. Leak down was about 5% on 6 cylinders, but about 12% on 2 other. Shouldn't be bad enough to cause this?
Have told him to check ground, will do again! My car has a ground connected to the back of the passenger side cylinderhead, but his does not. I believe his is an 08, did they remove it after 07?
Thanks, much appreciated!
Last edited by pierrel; 06-22-2017 at 02:27 PM.
06-22-2017, 12:08 PM
#5
Melting Slicks
Check the fuses in the box under the hood, there are two, one for odd side fuel injectors and another one for the even side, could as simple as that...
Last edited by extrapilot; 06-22-2017 at 12:41 PM.
06-22-2017, 07:47 PM
#6
Melting Slicks
Okey, thanks! We thought it might be a broken spring, but they all look good after having the heads apart twice.
Hes done both a compression and leak down test, both came out good. Leak down was about 5% on 6 cylinders, but about 12% on 2 other. Shouldn't be bad enough to cause this?
Have told him to check ground, will do again! My car has a ground connected to the back of the passenger side cylinderhead, but his does not. I believe his is an 08, did they remove it after 07?
Thanks, much appreciated!
Hes done both a compression and leak down test, both came out good. Leak down was about 5% on 6 cylinders, but about 12% on 2 other. Shouldn't be bad enough to cause this?
Have told him to check ground, will do again! My car has a ground connected to the back of the passenger side cylinderhead, but his does not. I believe his is an 08, did they remove it after 07?
Thanks, much appreciated!
09-23-2017, 07:17 AM
#8
Instructor
Thread Starter
Figured id just let you know what was the issue.
A stuck/broken lifter caused the car to run real bad and have a valve stuck open. Apparently he had not done any leak down nor compression test.
A stuck/broken lifter caused the car to run real bad and have a valve stuck open. Apparently he had not done any leak down nor compression test.
