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tom1361

Truck cannot maintain speed without lurching.

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Hello All, I have a 1971 R795ST with the ENDT 865 V8 engine. I'm having a problem with the truck lurching wildy when I'm not totally off the fuel or complete on the fuel. I can't simply maintain say 25 or 35 mph without the truck lurching. If I try backing off the fuel then easing back into it the truck will start lurching again. It dose the same thing if I floor it then back off and try to maintain  speed. The linkage and pedal are in good shape, all fittings are not loose fitting nor are they overly tight. I was told at a truck show 2 weeks ago that it sounds like an issue with the governor? Any help would be appreciated.  Thank You, Tom VB.

jake and the mack 1.jpg

Edited by tom1361
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Very nice looking truck. Bobtailing them dogs are very responsive; not sayin you don’t have problems 

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I'm no Mack expert but it does sound very much like a governor issue. In case you need it worked on, DaveHummell on this forum was very impressed with Diesel Power Service in Williamsport PA.

Edited by h67st
Wanted to add more info
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I assume you're not new to trucks, but just in case you are: PLANT YOUR HEEL! If your heel isn't planted on the floor then it's impossible for YOU to maintain a steady throttle position. 

But like I said, I assume you're not new to trucks, so I would guess a governor issue as well. Any clue how many miles on the truck? I'm sure someone on here is savvy with the 865 governor and can tell you what's likely going on with your governor.  Not sure if there were multiple fuel pump options on that motor, post a picture of your fuel pump just for expediting the solution.

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H67ST , Thank you. Williamsport is just an hour ride for me across rt.80. I'll give them a call. JoeH, Thank you for your reply. I'll get a picture of what the pump looks like. Although I believe the only pump used on that engine was the  AMBAC APE mini V8 design. I've been driving mostly Macks for 32 years now. I've never had a truck do this in the past this bad. If you're not completely on the fuel or totally off the fuel it bucks like a bronco.

REMAN-MACK-Fuel-Injection-PUMP-AMBAC-APE-V8.jpg

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Just thought I'd cover that base; every time I've had someone ride along to train they leave their heel up in the air. Doesn't work with an air ride seat constantly moving over every bump!

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It maybe a governor issue buy more than likely it is not. I drove a few trucks that were really touchy just bobtailing around. Get a trailer behind them with a little load and they smooth right out. Some engines just like a load on to run right at any speed.

The old v-pumps V8's do act like that more than a 6 cylinder. The only engine that is worse is my old supercharged 320 cummins that has a slightly turned up PT pump and I only run around bobtail. Got to slip the clutch when it starts bucking or lunging at part throttle to get it to settle down and keep in the throttle a little after that. Keeping the RPM's up seems to help the lunging as it is bumping on the high idle governor limit. Planting your boot heel helps steady your foot to.

Another thought, I don't think your year engine has a puff limiter but that would certainly help dampen the throttle response and lunging.

Edited by AZB755V8
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Sounds like typical 325 to me. Every one we have does that bobtail. Hook a trailer on and it's fine. I put a cable throttle on the one to cure it.

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