1993 l 8000 engine vibration
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5th Wheeling
Joined: Sep 2020
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1993 l 8000 engine vibration
I have a1993 l 8000 crew cab with an 8.3 6cta Cummins and Allison md 3060. It vibrates from idle to 1000 rpm and is smooth up to redline in neutral. In gear it vibrates worse at idle but smooths out until it shifts gears, the higher gear it goes into the worse the vibration until it gets some rpm and then smooths out again. I have replaced the rear engine mount bushings with new ones and have checked the front mount and it has no cracks and is tight. I have also loosened each injector line and they all appeared even. The damper on the crankshaft has no cracks in the rubber. The transmission shifts at a low rpm which appears to load the engine and makes the vibration worse. This is all occurring without a trailer or load on the truck. Any ideas ?
Fleet Owner
Joined: Jan 2004
Posts: 26,007
Likes: 575
From: Brentwood,CA
There is your solution! Well it probably isn't obvious so let me elaborate. Its your fuel overflow valve on the injection pump. Your spring is weak and the valve is staying open and dumping fuel pressure. The Allison is eager to lock the TC and it will cause the engine to vibrate because you are essentially misfiring under load all due to low fuel delivery. Replace the overflow and road test! Problem should be solved.
Cargo Master
Joined: Dec 2015
Posts: 2,549
Likes: 331
From: Harned, Ky
I'll have to check the overflow valves on a couple, my 93 F-800 with 8.3 Cummins vibrates at idle but smooths out with a few rpm, no vibration between shifts but I'm usually into the fuel pretty good wick causes later shifts at higher rpm.
The 94 L8000 tanker at the FD vibrates bad at 600 rpm idle speed but is fairly smooth by 750 so I keep the throttle control turned out a little.
Mine has 181,000 miles, FD tanker has 28,000
The 94 L8000 tanker at the FD vibrates bad at 600 rpm idle speed but is fairly smooth by 750 so I keep the throttle control turned out a little.
Mine has 181,000 miles, FD tanker has 28,000
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5th Wheeling
Joined: Sep 2020
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My truck has 102,000 miles on it. It belonged to a horse ranch in Idaho and was kept in a barn off season, so it didn’t get driven a lot.
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Cargo Master
Joined: Dec 2015
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From: Harned, Ky
On P pumps the banjo fitting for the return line has the pressure relief made into the bolt
Google 5.9 Cummins overflow valve
Google 5.9 Cummins overflow valve
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Fleet Owner
Joined: Jan 2004
Posts: 26,007
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From: Brentwood,CA
No need, the overflow is replaced as one unit, it looks like a banjo bolt.
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5th Wheeling
Joined: Sep 2020
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I have noticed it takes several rounds of the starter for it to start up, which I thought was unusual as these mechanical engines usually fire off after a couple of rounds. This makes the pressure relief valve more suspicious. I will get one ordered and give it a try. Thanks so much for the help. I will post back how it works out.
Fleet Owner
Joined: Jan 2004
Posts: 26,007
Likes: 575
From: Brentwood,CA
That means all your residual fuel bleeds out of the pump and back to the tank. That is also a common them with a failed spring in the overflow. I remember when these trucks were still around in numbers and we would always be changing the p pump overflow valves as they never lasted.
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5th Wheeling
Joined: Sep 2020
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Well I finally got the pressure relief valve installed. It didn’t get rid of the vibration but it did change it. Before it would vibrate at idle in gear and not so much out of gear, now it vibrates in neutral and doesn’t in gear. But as soon as I accelerate it vibrates a lot until it hits 1000 rpm. It still vibrates after gear changes when under load. I’m still thinking it may be a fuel problem since the new pressure relief valve changed the vibration, but didn’t get rid of it. Any suggestions?
Freshman User
Joined: Jun 2011
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texas, not going be much help as my 3 - 8.3's are all manuals. perhaps injector or injectors? was wondering if that relief valve was the same number that 390gas listed above ? i believe mine drains back and our 1 cummins dealer here in interior Alaska does poorly. i will try to find someplace to order , but having correct part number would be great. Mine 1994 8.3 thanks dan