: 81 fleetwood brougham

10-29-12, 01:57 AM
Just wanted to know the good and the bad about the 8-6-4 motor. How much mpg do you get too.

10-30-12, 08:41 PM
the 8-6-4 or 6.0 or 368 motor is a dependable low power motor.. The best gas mileage I ever got on the highway was 19mpg...But let me say that was with the 8-6-4 disconnected, thats running on all 8 all the time... around town 15 at best... but that was years ago...

10-30-12, 10:48 PM
A friend said he saw 21 mpg on the road, with 8-6-4 functioning. Bruce Roe

10-31-12, 02:43 AM
Oh ok thanks guys. I just bought my 81 and can't wait to get on the road with it. I think I'm just goin to keep the 8-6-4 functional

The Ape Man
11-05-12, 07:25 PM
Bruce has the correct info.

There isn't much of a downside. The 6 cylinder mode vibrates but is only a transition between 8 and 4.

All the original complaints with these cars EXCEPT the 6 cylinder vibration were DEFI related and later accepted as normal.

11-10-12, 04:42 AM
How do I know if the 864 is turned off cuzz I keep checking it on the electronc gage that tells u how many cylinders are running and its always on 8.

11-10-12, 11:34 AM
The orange wire from the transmission is likely cut.

The Ape Man
11-11-12, 10:07 AM
There were also PROMs available to disable the 8/6/4.

11-13-12, 01:20 PM
There were also PROMs available to disable the 8/6/4.
Hi, The Ape Man
What do you mean, could you tell us more?

11-13-12, 05:56 PM
He means that the dealership disable the cylinder deactivation feature (when customers complained that 6 cyl mode was vibrating) two ways. One (like my car), they cut the orange wire that sends the high gear signal to the computer that the trans is in 3rd; or, by actually swapping in a different computer that does away with the deactivation. Check under the car first.

The Ape Man
11-14-12, 09:01 PM
Not the whole computer just the PROM chip. Somewhere I've seen a list of GM PROMS and some were listed as 8/6/4 disabling.

11-15-12, 03:18 AM
I'm gona have to checc on that...both the wire and the PROMS.

11-16-12, 06:30 PM
If the wire is cut, run the wire thru a toggle switch. Mount the switch at the bottom of the dash. Switch off until you reach freeway speed then flip the switch to engage 4 cyl. mode. The only thing problematic is the plastic valve covers tended to crack if the owner or mechanic was not careful replacing the gaskets.