JoeH

Lol, I thought about that, but the shop foreman didn't think that would work as it's programmed for an automatic. I'm not opposed to trying it but I don't want to fry anything. The task at hand is to find/repair the "VCU Park Brake Switch" circuit. It's wire VJ1-10-0.8. I found the wire near the VECU, but I need to find where the VCU park switch is. So the dash instrument cluster has a parking brake lamp that functions, it works off the park brake sensor on the park brake knob. However, according to my wiring diagrams for the truck there should be a VECU park sensor. Any suggestions on where it'd be on an MR688?

Never ending. Dealer computer can't program idle speed because VECU isn't picking up that the parking brake is set, even though the dash indicator light comes on saying the parking brake is set. 🤣 Brought the truck home to dig into this myself....

What year is the truck? In general these are fantastic engines, if a bit small compared to modern engines. Weak point on these is the valve lifter tappets. They use a carbide face welded on. The carbide can crack, drop a piece off, then shave the lobe off the cam rather quickly. You'll hear a misfire and when you pop the valve covers one rocker will have excess lash from the missing lobe. Someone said on here recently that the E7 motor lifters are interchangeable?? But I am unfamiliar with this detail. Camshaft might be available through PAI. As long as you don't try and squeeze more HP out of the engine it should be very reliable. Mack engineered these engines to the perfect balance of durability and power. Deviating towards power will quickly sacrifice reliability.

The wealth of knowledge here seems to never end. Thank you @fjh! You pointed me in the right direction to sniff this document out!

Throwing this out into the abyss... Idle speed is adjustable by driver, assuming the feature isn't disabled. Punching the right cruise control sequence enters the "reprogram mode", and as you say you need a clutch pedal to set it once you've floated it where you want it. Or you need the Handy Dandy Mack software, which I sort of have but can't utilize because it doesn't recognize the DieselLaptops interface device I own.

Then how did it change in the first place??

Can I reset it with switches myself??

My 2003 MR688S AI350/Allison hd4560 suddenly just dropped it's low idle speed from 650 down to 500 or less. No codes, just had a ton of engine work done at the dealer, see my other "rollover" thread under Modern Trucks. Been running great til now. Firing up computer to check stats, and doing a visual inspection. Any recommendations?

Fan takes power from engine but also pushes you forward.... Like an airplane.Though it's probably inefficient. You should run 180°F thermostat, if the engine runs too cool unburnt fuel can gum up valves, exhaust, etc when idling.

Pistons are usually done in balanced sets. The C probably dictates what weight tolerance range the pistons are in.

Sounds awful trying to pull both heads together. Glad you have the service crane for it though.

Also, when you get those heads off, YOU CANNOT SET THEM DOWN RIGHT SIDE UP!! You will crush the injector nozzle tips!!

Go buy jam nuts they're flatter. Should be able to double nut a couple jam nuts.

Take those manifolds off first.... You're asking for trouble there...

https://www.thedieselstop.com/threads/any-mack-experts-i-need-help-advise.80033/ Reading this ATM.

You'll want to make sure your oil pressure is high at low engine speeds to handle the extra load on the crank bearings.

Maxidynes make 90%+ peak torque from roughly 1000-1200 RPMs up to the governor. I'd expect the fuel pump to be the biggest factor. For 5.9L 12v Cummins engines there's a fuel plate in the pump you can slide or replace to make substantial changes to the fueling. I've never tampered with fuel pumps other than to adjust idle and gov speeds, so this one is above my pay grade.

4 Years ago someone said you're right....

E6 designation started in the early 80's I believe. We've had trucks up to 1981 that still had ENDT676 engine designations. Our 1988 has an EM6-300L, so somewhere between 81 and 88 they changed it. Probably when the 4 valve heads came in.

Typo.

Which is pretty much the same thing as an E6.

Being a 1976 it should actually be an ENDT676 OR 675.

And my coolant runs cooler now, as the heads are running cooler from reduced exhaust temps.

How is your boost pressure? How hot does your pyrometer get when pulling hard? On hard pulls you should be hitting 30psi+, if you aren't hitting that and your pyro is pushing 1000-1200 degrees then I'd suspect cracked Charge Air Cooler. I found mine leaking a year or so ago. Pyro rarely hits 900 degrees now, and the truck will hit 34 psi in just the right scenario. 95 RD688, E7-350.

The E9 sounds amazing. I've only heard one once, and knew what it was right away. I'd love to own one, even if it's just a 350hp. If I were you I'd leave it alone. Shift your attitude away from trying to drive a "badass HP monster" and towards trying to drive an engine that will impress everyone worth impressing. Any trucker who isn't impressed by the sound of that engine isn't worth trying to impress. And you're not impressing anyone if you blow it up. Marine engines have a whole ocean to keep the heads cool. You have... A 3sqft x 3sqft? of air. Tank engines presumably have a lot more radiator surface area than you, too. So if you're going to aim for horsepower then you need to cover your weaknesses. There have been a lot of harsh opinions given, but recognize they're from people trying to steer you towards actually getting some use out of your truck.