1988 Volvo 740 Fuel Consumption Problem (12 miles/gal)

aem7 · Post by **aem7** » 03 Aug 2006, 20:14

1988 Volvo 740 non-Turbo 242,000 miles.

I just bought this car and don't know the repair history on it. The car is doing about 12 miles per gallon and about 180 miles on one tank of gas in a mixed urban/highway driving environment. I checked the tyre pressure and they were reading 35-40 psi.

I googled it and found the manual at VLVworld.com.
http://www.vlvworld.com/indexframe.html ... engine.htm
I smelled the Fuel Pressure Regulator vacuum line as described, and that seemed normal (i.e. no fuel smell). I wasn't equipped to perform any of the other tests.

Took it to a mechanic and he said he's gonna replace the fuel filter (which was apparently rusted out and old) and that might fix the problem. I did ask him to run some of the tests suggested on that webpage -- he seemed confused and asked me why I thought those things needed checking.

Can anyone suggest anything else I should be checking? Is 12 miles/gal normal for Volvo gas mileage? The EPA claims 20 miles/gal should be achievable. I have heard that other old Volvos also develop this problem. What is the most likely culprit?

Thanks in advance!

cosmiclobster · Post by **cosmiclobster** » 03 Aug 2006, 21:21

I have a '86 740 WITH turbo and I get still 17 MPG when I drive with a lead foot!
I think you must be leaking gas somewhere for it to be that low.
The engine would have to be running extremely rich to be consuming that much gas and you would know that by poor performance and by the exhaust pipe spitting out wet black fluids.
Park your car and put newspapers under the engine, under the fuel pump (under the drivers seat area) and under the gas tank.

Take the covers off and inspect the tank itself. There are 2 big nuts like the top of a large thermous that hold the intank pump and the other holds the fuel level sensor unit. If one of those have worked themselves off then you will be spilling gas out as you drive.

Good luck!

wojeepster · Post by **wojeepster** » 04 Aug 2006, 18:28

If the fuel pressure regulator is sticking you could have this problem. An indication would be black powdery plugs when you pull them. If the coolant temperature sensor for the computer (there are two!) has failed it could also cause this problem by telling the computer it is much colder than it is and so the computer richens it up. Also check the function of the oxygen sensor It should vary from .1 to .9volts If it is stuck on one value this could be your cause. You can temporarily fix it by taking it out and burning the contaminants off with a propane torch (a trick used by used car lots) it is only temporary and if this is the cause you should replace. Hope this helps.

Kushballz · Post by **Kushballz** » 08 Aug 2006, 17:06

This happened to me, but only logical to happen during wintertime... My Thermostat for the engine cooling was bad... Failsafe style thermostat, so the engine temp never warmed up during winter, so I was getting about 8 mpg due to engine being on "choke" mode, when really the thermostat was bad.

aem7 · Post by **aem7** » 09 Aug 2006, 22:02

Well, its me again. I haven't figured it out yet. I had a mechanic replace the fuel filter (he thought it needed replacing) so I had it done, and now gas mileage appears better. But frankly I haven't driven enough to get a real handle on it... so I'll drive around a little more and see. I pumped gas for the first time tonight since the repair and it started smelling like gas in the car... I read somewheres that is also an easy repair, so I will probably inspect the fuel tank and try some of the repair tips given here once I have determined that new gas mileage.

Thanks for all your tips I am sure these will come in handy. Going to check things in sequence.

cosmiclobster · Post by **cosmiclobster** » 09 Aug 2006, 22:19

Kushballz wrote:This happened to me, but only logical to happen during wintertime... My Thermostat for the engine cooling was bad... Failsafe style thermostat, so the engine temp never warmed up during winter, so I was getting about 8 mpg due to engine being on "choke" mode, when really the thermostat was bad.

This is a very good call! If the computer thinks that the engine is cold, it will produce a very rich mixture, as if you were driving with the choke on, and your mileage wil go down the toilet. If this is the case, you will see black smoke and residue coming out of the exhaust. A good test is to hold a paper towel up to the exhaust. If it blackens very quickly, that is your problem.

Good Luck!

petershen1984 · Post by **petershen1984** » 12 Aug 2006, 23:27

My fuel economy figures are on average 18 mpg, with about 14 at worst and 23 at most effecient. The latter figure was attained on my recent long distance trip. I traversed 320 kilometers of highway consecutively in 4 hours at an average speed of 85 km/h.

My findings from calculated figures are that in city traffic conditions, the car is only 60% effecient.

aem7 · Post by **aem7** » 14 Aug 2006, 17:20

i just drove to nyc and back, and discovered that my fuel gauge was screwed up. on I-95 i was getting about 23-25 mpg, and somewhat less when driving to suburban places (something like 18-20). so i guess whatever the fuel problem was, it seemed to have gone away. phew!

thanks for all the help and the technical advice.