b2600 getting stupidly shittty gas mileage

[farmer]

I just bought an 89 mazda b2600. I had an 87 b2200 for a few years, but I decided I wanted a 4X4. I searched online and they are supposed to get about 20 mpg on the highway. My old truck got around 23mpg, slightly above what google said it should get. I'm driving the same way, both are standards, the b26 is even geared a little higher, so I am running at lower revs most of the time.

The problem is I'm only getting about 12 mpg, which I just plain can't afford. That seems like it is well below what this truck should be getting. I changed the spark plugs, put injector cleaner in every other tank, tires are properly inflated. The aircare numbers for this truck are quite low (especially compared to me old truck, which barely passed), and it seems to be running fine as far as I can tell.

The engine is a little bit bigger, 2.6 L fuel injected vs a 2.2L carb (both 4 cyl). It shouldn't make THAT big of a difference though, should it?

Does anyone know anything i should check/ do fix this?

[Jethro]

I would get a code reader to see what the engine can tell you. You could run a compression test. New air cleaner might be needed. Just a few ideas

[counterfeitfake]

Maybe your O2 sensor is bad? If it's not working right the computer probably goes to a very rich setting. That would probably cause a check engine light, which you don't mention.

Do some looking around online, you should be able to find a spec for O2 sensors and how to test- it probably involves a propane torch and a volt meter.

Oh hey, I just found a page that could help: http://mr2.com/TEXT/O2_Sensor.html

[counterfeitfake]

Also, any auto parts store should let you borrow their code reader for free.

[stevesmith7]

Testing an O2 sensor and computer to see if the mixture control is working is really easy. Get a digital volt meter (not the old analog types) and hook it to the hot wire of the sensor. When things warm up you should see a voltage swing from near zero to near 1V. If it's pegged at either one you have an issue. It should bounce between the two extremes. Don't worry if zero isn't zero or one isn't one, it's the moving back and forth that shows it's working.

[volklpowdermaniac]

Fuel Filter?

[sweetdaddy]

Being the owner of many used cars and trucks over the years, I like the idea of changing the air filter. Its cheap,easy and the right thing to do as you don't know the history of the vehicle and how many miles since it's last been changed. It should be pretty obvious too, a filter clogged full of crap causing an intake restriction(and it wouldn't throw an engine code). Years ago I bought a used lease return 98 Maxima with 35k on it and the air filter had never been changed (factory replacement spec 36k), it got 20mpg, changed the obviously dirty air filter got 24 mpg.

Also, I like the idea of reading the codes. If a free to use modern OBD2 code reader from the local auto parts store won't work for you, you may be able to easily put a wire or paper clip across the engine computer diagnostic pin interface and get the check engine light to "flash" any stored codes. I had mid to late 80's fuel injected Toyotas where you could do this and the auto parts stores used to sell "shorter clips" for GM vehicles to read codes. Should be able to find out how to do it and what the codes mean online on a Mazda users group. O2 sensors, throttle position sensors, mass air flow and fuel inj coolant temp sensors could be bad and might throw a stored code.

Compression testing is good too and gives you an overall idea of engine condition.

Also, with a vehicle of this age I wouldn't rule out frozen brake calipers/cylinders or possible clogged catalyst(s) causing back pressure problems. Jack up each tire and see if there is resistance when you rotate the wheels (you obviously have to chock the other wheels and not have the park brake on when checking the rear wheels). If you've got a clogged cat, you probably don't have much power when you downshift to pass and engine the engine RPM's pick up. An exhaust shop can drill a small hole in the exhaust before the cat and do an inline pressure check to see if there's a restriction. These two items obviously wouldn't throw a code.

[farmer]

thanks everyone. I picked up an air filter today, along with a new cap, rotor and wires. hopefully that fixes it. The brakes are a likely culprit as well. When I first got it, the back brakes were grabbing. Haven't really noticed it since it has been driven a bit (it was sitting for a few months before I got it), but maybe they need to be pulled apart and looked at.

[Jethro]

I run a K&N on my 4runner. The filter oil contaminates the mass airflow sensor from time to time. When this happens, the vehicle goes into a rich tune. Kind of like the O2 sensor. A code read will diagnose either of these conditions