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You definitely have a hydraulic problem.
How many miles on your LPOP and HPOP?
You getting a lot of smoke?
Did you reoring your injectors? Was that your first experience installing injectors?
 

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You seem ok without that trailer, just cruising, as you said.
Seems to me that it can’t keep up with a higher need, hens the duty cycle maxing out at 65% with a resulting effect of only about 50% in pressure that you are seeing.
You said that your replaced your lpop. What did your front cover look like? Did you put a new oring with screen in the base of the hpop tank?
Is your oil level in the crankcase ok?
Are you positive that the bolt for the hpop bolt was properly torqued?
Positive the key on the crank was put in place when you mounted the new lpop?
You positive all injectors seated before you torqued the hold down bolts?
You did not have these hydro numbers before your rebuild?
What all did you do in this engine rebuild?
Did you install a new oil pick up tube to front cover oring?
After bad numbers, have you looked into the hpop tank to see if the oil looks aerated?
 

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I had a thought about something that maybe causing your issue. You rebuilt that thing. Perhaps when you siliconed the pan, you went a wee bit heavy with the silicone bead.
I found this when I opened a customer’s engine. That bead was an excess push out bead on the inside. It detached. It was the in the oil. Eventually, it got sucked up onto the pickup tube screen.
Try to remember laying your bead and how heavy you went. Then get under your truck and inspect the pan surfaces, all the way around and take note of how much push out you have.
You get a bead up onto the pickup screen like that, It will be ok until you put it to it and your pump won’t be able to put it to those injectors and that’s exactly what you have going on. We just have to figure out why and what happening will definitely do it.
Check to see how much pushouf you have, all around.




Automotive lighting Automotive tire Motor vehicle Vehicle brake Headlamp
 

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That needle gets awful twitchy when holding at 2300. Twitchy needles are generally an indication of aeration. Aeration will cause an uneven idle BUT aeration might not be present at idle. To test for this, you get get your oil hot, bring up to 3400RPM while standing still for 5 minutes. Watch your gauge. You should be at around 1200 RPM. Needle should be steady not twitchy. It should also not rise or fall in pressure.
Now, When you are at 2300 range when it’s twitchy and you let off, it spikes up over 3k a second before coming to around 500.
This can be a sign that the HPOP gear is slipping on the shaft when resistance is high. When engine RPMS are climbing, the rotational force is in one direction and is facing resistance. When you let off the rotational force unwinds/lets off. If that gear is starting to slip against resistance in one direction but the resistance decreases as you let off it can grab. Once it grabs, there is going to be a spike in pressure. Then the cycle begins again.
I am not positive that is what you have going on but if another shop called me in on a problem comeback or somebody dropped that truck off here after saying to me, Hey, I had this thing apart and it’s not right”, I would check to make sure whoever did the work, had the gear properly seated on the shaft by rotating the crank to watch for any wobble at all with the gear and he the torqued the washer and bolt to 95 ft. pounds.
Then, with regards to your tach not working, Ide pull the gauge cluster and look for the brick of weed that the old hippie left in there, that knocked something loose. Ide be bummed out if I found that it was just the result of a mouse.
 
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