: gas mileage

03-05-05, 09:45 AM
I wonder if anyone has any clue about what could be going on with my 89 brougham. I am only getting about 165-170 miles to a tank. this seems very odd to me. I know the epa regs on it is 17 mpg city and 24mpg on the highway. Im not expecting to get this same gas mileage 16 years later but if anyone has a clue to what could be going on I would greatly appreciate it. I bought it about 1 month ago so I dont know what the previous owner could have done to it. It only has 128,000 miles on it so if anyone has any suggestions...............also I dont know about the 89 broughams too much but do they require premium fuel. Thats all I put in my 91 coupe, but is it required with the broughams?

03-06-05, 06:16 PM
Hey, I feel your pain. I was getting around 14 or 15 MPG and now I'm getting 9. I ran 87/89 mixed for a week and a half or so and was doing well, though I did some highway driving. 87 octane just doesn't cut it, despite what the manual says. It pings quite a bit, even on minor hills. With the 89 it pinged only on steep hills and not so much...but for the extra few dollars it costs to fill up with 92 octane, it's worth it. So while 87 is okay, it's not that much more to go with the Premium.

But as for the poor mileage, I'm not sure what's going on. We're in the same boat, but why mine went from 15 MPG to 9...in about a week's time...well, I'm at a loss. The only real difference has been the Premium gas and not quite as much highway miles...

As a direct comparison, last week my trip odometer read about 205 or so after burning around 14 gallons. Today it was at 149 and took a little more than 16 gallons. Maybe they didn't really fill it last time, who knows? I'll search the forums some more to see if there are any suggestions.

03-06-05, 09:03 PM
same problem here except i have never gotten better than 10 mpg in the city

03-06-05, 10:46 PM
I've been getting a little over 15 mpg in my 88 Brougham. I use premium gas in it also. It does drop significantly in the city but on the whole it hasn't been too bad considering it weighs over 2 tons.

03-06-05, 11:03 PM
I get 20/21 mpg (combined). If everything mechanical is operating well on your car, fuel filter, air filter, transmission, carb etc etc then it could be driving habits.

Use "boat-fu". The objective of boat-fu is never to actually stop and when you're driving, drive slower. Time stop lights, if you reach a stop sign or are in traffic, accelerate gradually. You'll be surprised how driving habits helps your fuel economy. Fast take offs are death for our cars, as afforementioned they weigh over 2 tons!

03-06-05, 11:31 PM
Time stop lights, if you reach a stop sign or are in traffic, accelerate gradually. You'll be surprised how driving habits helps your fuel economy. Fast take offs are death for our cars, as afforementioned they weigh over 2 tons!

Bingo. Drive it like you're the 84-year old that bought it, it's a Cadillac. People expect it. Spock is right. The easier you start out from a stop, the better mileage you'll get, guaranteed. If you've got a lead foot, just think how many people more will see your Caddy if you roll by slow...


03-07-05, 12:10 AM
yeah, my 83 is rated at 17/24 but i get an overall of around 12.5, yeah, ive got a lead foot for sure,

03-07-05, 02:52 AM
man, its good to know that my bad mileage is a general 'lac thang, not juss my 'lac thang

03-07-05, 07:54 AM
i just got back from a trip to toledo and back. 300+ miles and i got 22 mpg. i am thinking that the canister purge solenoid is the culprit to the bad gas mileage in the city. i shall soon find out when i replace it.

03-07-05, 10:05 AM
I would look for the obvious stuff, air cleaner heat not working, make sure the tube from the exhaust manifold is connected to the air cleaner and proper vacuum lines are working, check to see that it is closed when it is cooler than 100F inside the aircleaner. This WILL contribute to the pining however. Olds 307's have a ECM progam that likes a lot of timing at part throttle, which makes them ping happy.

Most of the time when they get ping happy enough to not run on 87 octane it is the EGR Stack Tubes, or EGR Jet Extensions are they are sometimes called. Clean them out with a drill bit, or do as I did, remove them and install a plug in the intake with a hole drilled into it. This keeps the problem from happening for a much longer time. Cleaning them WILL improve performance and fuel economy. I was really surprised how much mine did. At 180K miles it was still getting low/mid 20's city/highway (granted, it was a Cutlass with 2.14's), but prior to that it was only getting 19 combined.

Make sure the EFE valve is not sticking closed, it will really hurt power and make it ping badly, as it overheats the intake manifold crossover. I removed mine, but the Olds 307 really likes this valve to help cold warmup.

Other thoughts, timing set wrong (set to factory, maybe up 2 degrees on 87 octane), bag plugs. (Bosch Platinums, not plus 2 or 4) do well for firing with a lower voltage, but the HE is a solid 45KV distributor, but if you have weak wires it may be contributing to it. Bad or wrong temp thermostat, 180 is as cold as you should run, it will NOT affect the computer going into closed loop, and anyone who tells you that is clueless and has never tested it, I have. I know more about the GM ECM in the Olds 307's (at least in Olds vehicles) than the average Joe. Take a look at mine and others' notes on http://www.442.com/oldsfaq/ofe307.htm and http://tech.oldsgmail.com/.