questions on 8.1 motor in 2004 Chevy truck

[Kegracing]

All,
I posted in the support section, but we don't get much traffic down there.

Looking at a 2004 Chevy dually with the 8.1 gas motor and Allison trans.

What is considered high mileage on these motors? (assuming good condition, good service, not abused, filters oil etc all taken care of correctly)

Known problems with these motors?

thanks
Ken

[Alan Roehrich]

I have a 2003, I love it dearly. Wouldn't trade my 8.1 for any diesel of any type or brand.

Some of them have slight piston slap when cold. Don't rev it up cold. It won't hurt a thing.

Some of them use a little oil, maybe 1 quart in 3000-3500 miles. The vast majority of them are actually sucking some oil from the lifter valley under the intake manifold due to the design of the PCV system. It is not getting oil past the rings, or past the valve guides and seals. It's simply picking oil up through the PCV system built into the intake that has the pick up in the wrong location, and a serious lack of baffling.

The PCV system design will cause deposits to build up on the plugs, usually at 25-30K miles they'll look a little rough. Change them, the deposits can cause enough resistance to create a misfire, and could cause enough load to kill a plug wire or coil pack.

I use 20W-50 Royal Purple in mine, and because it is a 2 wheel drive, I use an AC PF 932 two quart filter. My truck will be down maybe 1/2 quart or a little more between oil changes.

At 96K miles I replaced my plug wires with Taylor wires to resolve a slight misfire under acceleration. And I went back to the GM AC Delco double platinum plugs.

It's a tough big block that makes really good torque, with proper maintenance it should go 200K+, and possible 300K+ before needing anything serious.

If you live in an area where they don't require emissions testing, or at least don't look at the exhaust, at least get rid of the catalytic converters, better still, put a nice exhaust system on it, and get the ECM tuned. Power will increase considerably, but so will fuel economy. You can also put a GM 180 degree thermostat in it, big blocks don't like to run hot.

[Kegracing]

Thanks Alan.
Appreciate the excellent feedback.

Ken

[Rusty Davenport]

One of my best friends has several of these trucks that roll everyday....he is in a REPO business and they have cars hung on them everyday ......they must be hardcore tough original motors and at least three of them has turned 250,000 with store brand CARQUEST oil changes and filters......amazing trucks no joke

[Mark Howes]

Running the 2004 8.1 Allison combo in a motor home. Lots of power. Hauls motor home and trailer up and down the hills as fast as you want to go. Only 40.000 miles so far so sounds like lots of life left

[Alan Roehrich]

I'm behind on my projects, but I'm actually developing a kit to convert the 8.1 to adjustable shaft style rocker arms, under the stock valve covers. If you're keeping the stock camshaft, you can use 1.8:1 rocker arms for a little boost in power. If you're changing cams, you'll probably want 1.7:1.

The intake manifold can be modified to solve the PCV problem, and also for about a 30HP gain, by cutting it open to remove a baffle or two and doing some porting. The stock PCV system inside the intake manifold is closed off, and the PCV system is run externally at the oil filler tube (you won't want to add oil with the engine running).

My personal truck will be getting a set of Hedman headers and 3" true dual exhaust.

[Chipper Chapman]

I had one with 280,000 Miles. It had been used hard its whole life. Gave it a tune up and it still ran like a champ and worked great. Sold it to a friend and they work it everyday still. I'd love to have another

[Dwight Southerland]

What's the fuel mileage pulling enclosed trailer?

[Alan Roehrich]

Dwight, I got 8-9 towing a very high profile 28' enclosed camper that had probably 2,000 pounds of extra weight in the trailer and close to 1,000 pounds extra in the bed, and three people in the truck, much of that trip was into the wind, and I was running 70 MPH, uphill and down. Truck pulled it and handled it with ease. This was before I upgraded to 19.5" wheels and tires, and better brakes.

There is more power and more fuel efficiency to be had from the truck. I have a friend with a chassis dyno and various EFI tuning software. I've been pressed for time, so this is moving slow, but I'm going to do some stuff and document each change as it happens. Since I'm keeping this truck, I'm going to go fairly far to see what can be done with it. I think over all there is 100HP and 100 ft/lbs to be had, and I think that if you do not use the power to show off how fast you are, the changes will result in better fuel efficiency, especially when towing. I know what some other guys are seeing with some work in the right places.

I think the single biggest problem is that the power band of the engine is too narrow, it runs out of power and useful RPM for the gearing it has. The one complaint I have is that it downshifts too early, and then runs out of useful RPM before it will upshift, so it has a hard time getting to that next shift to drop the RPM back down. So my goal is to increase the power available so it downshifts less, and spends less time in the lower gear. Using too much throttle and turning too many RPM is where the fuel goes.

I wish this truck was a 2006, as the 2006 has a 6 speed Allison. It's not a conversion that can be done easily and affordably, I doubt I will ever do it. I got a sweet deal on a one owner low mileage truck, so I settled for a 2003. A 2006 is a better truck to start with, but they 8.1L trucks get harder to find every day, especially the newer trucks, as the Duramax got more popular, and GM was intentionally killing the big block. I think it might be possible to make a 3.73:1 gear work well with the 6 speed, but I'm not sure it is a good idea with the 5 speed, my truck is a 5 speed with a 4.10:1 gear. I swapped to 19.5" Vision aluminum wheels and Hankook tires, which made a big difference everywhere, I think the RPM drop was worth at least 1 MPG.

A new truck like mine would be $70K, if you could buy it. I don't mind spending $2K or $3K on this one. It's still under 100K miles, and as Bridges says "that truck will go well over 200K miles, and when the time comes, we'll just make it better for the next 300K miles".

[Bob Don]

I tow with a 2002 Silverado 2500hd 2wd crew cab, long bed, tow package with Allison trans and 8.1 Vortec. I've got just over 100k miles on it (I bought it with 82k miles 4 years ago) I get about 10 mpg towing a 24' enclosed with a 3705# car plus tools, spares and a scooter. Other than downshifting for hills, you'd barely know you're towing anything. I get the piston slap when i start up cold like Alan said but it doesn't seem to have any ill effects. The engine uses about a half a quart of oil every 3500 miles but by then it's time for an oil change anyways. The only issue I had was a leaky trans cooler in front of the radiator.