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2008 MP8 engine



Working on 2008 Mack with MP8 overhead cam engine. Ive got problem where the gear on air compressor keeps getting chewed up. Removed tranny from truck and removed timing gear cover from rear on this engine. Found the intermediate idler (double idler) gearset rubbing into aluminum timing cover. Wanted to know if anyone has come across this problem before.


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Yes, TONS of Mack and Volvos have or will have this problem. The double idler gear retaining nut backs off and causes the problem. Its a huge job. We have 2 MP7s in our shop right now getting the repairs. We always have one truck in the shop with this problem.

Normal needed repairs

Replace all gear train, replace oil pump, check/replace rod and main bearings, remove oil cooler to clean metal debris, blow out all oil passages.

Volvo came out with a Service Bulletin that calls for tech to drill through the flywheel housing and double idler gear/nut and then drive a pin through it and install a freeze plug in the hole. The pin keeps the nut from backing off. But of coarse they ran out of the parts for the last few months but recently started sending them out again.

I steer clear of these jobs, just involves to much time. Good luck

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I really appreciate the advice. I had a customer bring me this truck all taken a part with the tranny removed and engine hanging. They have a maintenance shop and got in over their head with this one. Now I am the lucky one. Just curious when you receive these jobs do you remove the engine or do the job in the frame and remove transmission?

I had 2 local Mack dealers told me they never heard of this problem. I am having hard time getting parts.

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Every dealer is different but we've been seeing this for atleast a year now. We've been doing them inframe but the 2 we have now we pulled the engines. We are still trying to figure out the best way to complete the repairs. So far one guy says he likes it out of the truck the other one says he likes still in the truck. Trail and error on this one. One thing we do know is if it has an Allison its better to pull the engine. We have engine stands, so that helps alot with it pulled. You can do it inframe or out, just pick which one you feel better with and go with it.

How bad was the failure? Is there alot of metal in the pan? The oil cooler needs replaced too.

We dont have any problems getting parts. What problems are you having?

Its been taking us about 60-80 hours for each engine. Volvo pays somewhere around 35 hours, warranty time. Warranty time doesnt mean anything.

After you complete the repairs you will know more then your Mack dealer.

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We have had only a few go down at our shop however our other shop north of us does them all the time. I wouldnt wish one on my worst enemy. If I wasnt a dealer I'd pass on that one. We have some tech's here that like to do them, but not me. 5 year/ 500,000 mile warranty on that failure

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Sorry for the delay in writing back. I'm assuming this could have been a lot worse. The teeth on the air compressor gear are almost completely gone. The intermediate gear loosened up enough that it wore a circle groove in the aluminum cover. I am getting all parts in tomorrow incl. gears, bearings, compressor, oil pump, cooler, and gaskets. So it looks like Monday I will be starting assembly. There was a ton of metal in the pan. I haven't done too muck work to these engines, so I dont mind this job. I def. dont want to be doing one of these a week, thats for sure.

Have you been pinning the gear nut after repairs are done?

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Gear pinning is service bulletin so its done by vin # only. We get an exact number of pins with a list of vins. So no, we dont pin after the repair. I dont know how they make the decision what trucks get pinned, we just see the list. I would think all trucks should get pinned.

We've been staking the shit out of the nut and threads on all double idler gears while completing repairs.

I replaced a flywheel housing once and tack welded the nut while the cover was off. Maybe it was a good idea, maybe it wasn't. Taking a punch and staking the nut and threads is probably enough.

It would be interesting to see what others do or dont do to the new double idler gear.

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Finished this MP8 up today. Runs good. The new double idler gear came with the nut staked. I staked the nut even more. Truck runs great, customer happy. Thanks for all the help.

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I have a 2008 granite with 500 hp mp8 it has a water leak at the bottom of the radiator where the fittings go into the tank and they are telling me I have to buy a new radiator to fix the problem !!

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Its been a while, since you guys were discussing the issue with this double idler gear. I am wondering, if there is a chance to get any specs for the torque, thats required for the nut, thats coming loose. If the nut came loose, the driver shut down the engine right the way, no damage is visible, no Metal is in the pan, gears are fine, is there a chance to tighten the nut again and pin it, or tack weld it?

That would save a lot of money. The problem is, there is no dealer who knows how much torque is on that nut, since it is only replaced as an assembly.

Does anybody know how much torque is on that nut?

Thank you!

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