What can cause such bad mileage

I have another issue on the beast that I thought I would check on...our van gets terrible mileage..appears to be about 10MPG and not much better on the highway. I have added injector cleaner and keep the airfilter changed. It seems to run fine.

I wondered about the fuel filter...but Im thinking if it was an issue, the van would run bad if it starved for fuel..or is it an issue where it is starving and it compensates by adding more fuel? Man I miss the days of no computers and easy driveway mechanics.

It has 127K miles on it. I must say I have never had it tuned up though since we bought it with 65K on it...it always seemed to run good so the tune up hasnt happended. I think I need to pay someone to put plugs in her..cause I cant see the plugs on the back side of the engine...
Thanks

Well, even with the electronics on the new cars, they are still the same as older ones at some basic level. I assume you do not have a Check Engine Light? Is the mileage you're getting measured or on the display (it could be wrong)....

I'd look at it as what do you need for proper operation - compression, carburetion, and ignition.

On ignition, I suspect that worn or old plugs might cause a misfire that you should have noticed. If you did not feel an issue with engine response or acceleration, I'd bet that they are not the cause (but you should replace them...it's not too bad if you rotate the engine forward - check the archives for tips).

On compression, you could need more fuel to get the same power output if your rings/cylinder walls are worn and so you are not getting as good of compression. Again, if you have good full throttle acceleration and are not burning oil, you've probably have good compression.

Carburetion might be the culprit. On the fuel side, if your fuel filter was plugged, you should feel some loss of full-throttle acceleration. Even if the pump is having to work harder, the electric pump wouldn't pull that much juice to drop your MPG.

You might have a bad oxygen sensor. I've heard that they can deteriorate to the point of affecting mileage but not to the point of triggering a fault code. The sensor detects the amount of oxygen in the exhaust and adjusts the fueling appropriately to maintain the proper air/fuel ratio.

Clogged injectors could cause poor atomization of the fuel, but you said you've used injector cleaner, and I believe it would be noticeable in your performance.

One last (more expensive) issue could be plugging of the catalytic convertor. However, that would restrict airflow, and I would suspect you would feel it.

Don't know that my rambling has helped! Good luck...Glen

I'd replace the O2 sensor to begin with since they have a limited life span and see how that affects your mileage. Other culprits like clogged injectors or even a clogged fuel filter would affect performance and you'd likely notice it. A bad O2 sensor just causes the motor to run rich but you wouldn't really notice except for the lousy gas mileage. 10 MPG is awful in mixed driving. I routinely get about 25 MPG in combined city/highway driving.

If it hasn't had a general tuneup since 65k miles, I'd go ahead and start there. Plugs, wires, filters, PCV valve, etc. Should be less then $100 and an afternoon if you do it yourself. See how it runs then. Even if that doesn't solve the problem, it's a general maintenance thing and probably should be done anyways.

The O2 sensor usually gets replaced anytime between 80k-120k so that may be about time if a regular tuneup doesn't fix it. Unless you have had a problem, the catalytic converter still should be good, but there could be a remote chance that that could be it as well.

Thanks for all the ideas. I'm gonna do as much as I can myself and then prob pay someone to change the plugs on her...I guess the engine has to be moved or rotated forward to get to them..?? I think I am out on that deal if that's true.
I did get the front hub/bearing assembly replaced Sat without too much hassel and it fixed my front end noises for the most part.

Thanks again!
Todd

I guess the engine has to be moved or rotated forward to get to them..?? I think I am out on that deal if that's true.It sounds like a lot, but it was designed to be rolled forward pretty easy. I can have the engine rolled forward in about 5 minutes.

Remove the main intake hose from the air filter to the engine. Should be just 2 clips at the air cleaner and a screw clamp at the throttle body. There will be 3 sensors IIRC and a air tube that also will need to be removed but those are easy.

Then loosen but don't remove the 4 bolts that hold the "dog bones". These are the brackets that are between the engine and the top of front of the vehicle that prevents the engine from rotating forward and backwards. Once they are loosen, run a ratcheting strap through the metal loop on the right mount and around the hood latch bracket. Tighten up and the engine will start to pull forward. Remove the bolts and dog bones as they loosen.