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87 Chevy Caprice Wagon gets a whopping 7 miles to the gallon

Maintenance/Repairs
#1

Just bought this car because it looks really really cool. While i’m driving it I an actually see the gas gauge move down. It’s sad. I’m sad. It’s a good thing this car looks like a hearse so they can bury me in it when keeping gas in it kills me.

Took it in to get a tune up under the advice of the previous owner, the mechanic called me back after about 4 hours and told me that everything on it seemed fine, and charged me 40 bucks with no changes made to the car.

Car had a new engine put in it about 35,000 miles ago, stock tranny, carburetor, tires look fine. I’m way sad.

I’m also an idiot, is there anything i can do to get this mpg under control?

#2

7 seems very low, even for these cars. What motor do you have? Also does this have the 4-speed overdrive trans and are you using it?? Air pressure in the tires ok?? Brake dragging??

#3

When the key is turned on does the check engine light come on? And when the engine starts does that light go out? It may sound ridiculous but if the CEL bulb is burned out, broken or missing the engine will run full rich with no spark advance.

#4

The only thing you referred to was the gas gauge. Did you find out the actual mileage - using actual math? This would involve filling the tank and recording the mileage on the car or resetting the trip odometer to zero. Drive for a few tanks filling each time and recording the actual gallons to fill on the following fillups. Divide the miles driven by the gallons. If you’re only relying on a gas gauge then you don’t know anything yet.

I’d also not that if this is an '87 and “looks really cool” then its probably been messed with a lot. Find out if the wheels/tires on it are the OEM size. If larger wheels went on then your odometer (and speedometer) will be off.

#5

This vehicle should get around 15 MPG city and around 20 hwy. I had a similar vehicle with a 350 cid V8, and this is what I got, even driving with a heavy right foot.

#6

It’s got a 307, I’m averaging 100 miles on 14-15 gallons so right at between 6.6-7ish. I don’t see any lights come on when i start the car…

#7

It looks really cool because it’s just been kept in good shape. There really isnt any aftermarket stuff on here, no crazy rims or wheels.

#8

yeah it’s got the overdrive, and i use it. It does not seem to accellerate as fast as it should either. I’ll check on hte bulb stuff today.

#9

My late parents had one of these many years ago and used to get about 20 on the road with it. It was a good car, although not my cup of tea, and they finally sold it off cheap with 410k miles on it; with an untouched engine (350), transmission, and rear axle.

Without knowing what process the mechanic went through in checking things out, all I can tell you is what I would do.
Spark plugs out and a compression test to make sure the engine is not problematic. “New” engine could mean a worn out salvage unit that is new to the car.
Check tire pressure.
Check for transmission slippage.
Double check ignition timing to make sure that it’s not retarded too much.
Check for a clogged catalytic converter.

#10

Your car is among the last of the CCC (Computer Command Control) cars. If you have the 307 (actually and Oldsmobile engine), it has a big 4-barrel carburetor with an electric choke, electronic fuel control system, and an air injection emissions control system. Any one of these systems can cause mileage and performance to go downhill. You’re going to have to look long and hard to find someone still proficient in such antiquated technology, but once you do it shouldn’t be difficult to get this thing running right.

Note I didn’t say cheap. Those shops who still have tools and equipment to work on these computer controlled carbureted systems know the value of it and aren’t afraid to charge accordingly.

#11

If the carburetor that is still on there is essentially a Quadrajet then there’s also the possibility of leaking jet well plugs. This would basically mean a carb overhaul as it would have to be removed and gone into for any repair.

With the carburetor off and the float bowl filled with gasoline the bottom of the carb could be watched closely to see if gas starts dripping out the bottom which would mean leaking jet well plugs of course.
Normally this would cause a somewhat rough idle and difficult if not impossible to adjust carburetor idle mix screws.

#12

It might be a great deal cheaper to retrofit a carburetor and distributor from a 1979 GM engine rather than attempt to de-bug the car’s CCC system. It can be a nightmare. And the car should get 20 +/- mpg as others have mentioned. Unless the car is a Canadian emission model the carburetor has a mixture control solenoid and when the key is turned on the engine light should come on and the carburetor solenoid should cycle, making a fast clicking noise for 25 seconds and then stop. If those things aren’t occurring the engine is washing itself in raw gas. Pull the dipstick and smell and pull a spark plug and notice the black soot. The catalytic converter is by now a total melt down and, in fact, it has likely been removed already.

#13

20 mpg on an expressway only trip is something I’d expect. But if your driving is pretty much general in town stop and go driving, these cars did get lousy mpg in such instances. Perhaps not as low as 7 mpg, but this is a big heavy car and not at all aerodynamic, so you can’t expect much mpg in city/stop and go driving.

I had an '85 Mercury Marquis wagon (mid sized wagon, same as Ford LTD) with a V6 and the best it did was just over 20 mpg on highway trips. Around town it was about 14 on a good day. Your Caprice wagon is full size and much bigger and heavier. You also have a bigger V8 motor. I’d think if all is running properly you are looking at about 10 mpg city driving, and 17-18 highway. These cars did have lock up torque converters but otherwise were pretty primative transmissions. Big heavy transmissions with a lot of sloop and inefficient in lower gears until the torque converter locks up. If you torque converter isn’t locking up that would cost you about 2 to 4 mpg.

#14

I had an 88 Caprice with the 305 V8 engine and in normal city driving I got 17 mpg and on a long highway trip I got 20+ mpg. This car had the 4 speed automatic. These are the kind of figures you should aim for. IF the car has a trailer towing rear end with a 3.23 ratio or higher, you may get less.

Some reasons for poor mileage could be a stuck float in the carburetor, stuck choke valve, dirty air filter, gas leaks from the tank or fuel lines worn out spark plugs, faulty wiring, etc.

#15

One thing you might want to do is to check the odometer to see if it is registering the actual number of miles traveled. If you are traveling more miles than indicated, check the tire size. Sometime in the last 25 years, someone may have put on oversized tires.

#16

The check engine lights do not come on at all.

#17

The mechanic i took it to said the plugs and filter looked brand new, i just spoke to the previous owner, and he said that at one time the carb was running too lean, he took it into a shop and they re-built it, then the check engine light came on, he took it back to them and the check engine ligh stopped coming on but he car started using up alot more gas. So i’m thinking we MAY have found the culprit. I may not have to be buried in this thing afterall.

#18

I am more familiar with those carburetors than most mechanics these days and the entire fuel and spark control system is more difficult to diagnose and repair than current electronic fuel injection systems. Rebuilding the carburetors requires a special set of tools and quite a bit of hit and miss experience to properly set up the mixture control solenoid for the proper “dwell” and as earlier stated the engine light is integrated into the logic sequence of the computer. If the key is turned on and the computer sends triggers the light but there is no light to carry the current the computer will not operate. When the computer does not operate the mixture control solenoid remains fixed in the full rich position and also, the timing remains fixed at the base setting so the engine is sluggish while burning excessive fuel. If pushed, the exhaust manifolds will likely get red hot but the car will likely never exceed 60 mph. To confirm that you have that fuel system this picture http://www.autozone.com/autozone/parts/Autoline-Carburetor/1987-Chevrolet-Caprice/_/N-iietiZ8vd1l?itemIdentifier=56748_0_0_1277%2C135607 shows the electronic Q-jet. in that carburetor the blue button on the top is the connector for the mixture control solenoid. Does your carburetor have such a connector? Regardless of color.

#19

@FakeDad–I wonder if the shop that rebuilt the carburetor for the previous owner may have bored out the carburetor jets. That would eliminate the lean mixture problem at the expense of fuel mileage. It is also possible that they set the float level too high.

#20

it is way way hard to get it to go past 60…