1981 Cadillac Deville 8mpg

[istp]

Hi! First time posting.

I have a 1981 Cadillac Deville with the v864 6.0L engine.

I'm averaging 8 miles per gallon city, 10 highway (65mph).

It sounds and drives like there's a cylinder missing- no guts, and can barely make it up to 85 mph.

I have the service manual and have at different times gotten codes 19, 30 and 45.

I was wondering if there's anyone out there who had the same symptoms. The Caddy should be getting more than 200 miles to a tank of gas, right? Oh yeah, and the 8-6-4 is active and reading out on the sentinel.

-istp

[The Ape Man]

Best course is to totally ignore that this is an 8/6/4 car and treat it like any other GM fuel injected vehicle of that era. What is the condition of: Ignition parts? Vacuum leaks or mis-routed vacuum hoses? Fuel pressure/volume? Have you recently had a dead battery or beserk alternator? This could make the ECM flip out. Coolant temp sensor should be checked using proceedure in shop manual. MAP sensor vacuum hose should be checked for restriction. A compression test would be helpful to find engine mechanical failure. The catalytic convertor could be restricted at this point because of unburned gasses it has to deal with.

[istp]

Thanks for the tips!

Sounds like nothing a couple of thousand dollars can't fix. haha!

I'll get back to ya.

-istp

[Gwokable]

Hate to say it, but go through the emissions control system diagnostics, and engine diagnostics in the service manual. The only reason you'd be getting that kind of mileage is because the engine is being flooded by fuel that it isn't burning. It's horrible for the engine too; you can wash out the piston rings and badly contaminate the oil.

If you've got bad gasket leaks it's probably time for a new engine.

[turbocaddy]

before any of that, do a compression test to see where u are at...low compression will give u bad gas mileage also

[danomac]

I get between 400 and 500 miles to a tank of gas with mine on the highway with a moderate foot.

Like turbocaddy said, do a compression test first to make sure you don't have a dead cylinder. If the compression test comes out ok with all readings within 10% of each other, then check to see if one of the solenoids is stuck on.

A bad O2 sensor could really throw things off as well... has it ever been changed?

[istp]

I had the mechanic throw in a new oxygen sensor the other day. Filled up the tank, and will now drive around a while to see what's up.

Already it drives WAAAY nicer. Smoother acceleration and more power. Not sure if the mileage is any better. According to the sentinel it's slightly better, but I don't know how accurate that is.

I'll keep ya posted.

-istp

[istp]

Well, the verdict is in.

After filling up, driving around a bit. Some city, and 2 hours of some highway between 90 and 120 km/h (55-75 mph) the grand total is 14 mpg (US gallons).

There were a few pedal to the metal blasts in there too.

But for a 368 cu. in. motor, that sounds about right to me.

Whaddaya think?

-istp

[jayoldschool]

I think I will be putting in a new O2 when I get my 81 out of storage

[istp]

Here's another update.

I did a straight highway mileage experiment. 2 laner 90-120 km/h (55-75mph) the whole way.

17 miles/USgal

That's about as good as she's gonna get I think. I can live with that. And no black smoke.

-istp

[The Ape Man]

I very carefully tested several 81s out for mileage back in the day. Steady highway speed was 60 mph and no more back then. Checking with actual gasoline in the tank verses miles travelled. Filling it up myself. Not using the dash to figure things out. 21 MPG on a steady highway roll with airconditioning running. Summertime medium heat. Timing was advanced a few degrees over spec but otherwise no modifications at all. 90s RWD Fleetwoods with overdrive beat the 8/6/4 every time and the 93-6 weighs about 400 pounds more! Nothing like overdrive.