I have been having fuel mileage issues with my 1990 pickup for nearly a year, just can't seem to find the problem. I've checked for vacuum leaks and tested all the EFI components I could. Normally, the truck gets about 28 mpg, it's barely getting 20mpg now. The engine starts and runs fine. If I wasn't checking the mileage, you could not tell there was a problem. The thing that baffles me the most is that the fuel efficiency of the engine is down by more than 25%, but no check engine light or trouble codes. The tailpipe O2 sensor(s) can't detect such a major fuel mixture problem? I've replaced the O2 sensor and cleaned and checked all the EFI sensors I could. I guess I'm at the stage where I need to start replacing EFI parts, but which one(s)? Since there is no CEL and the engine is running fine, I thought there might be some kind of fuel leak. Unless the injectors are leaking, I was not able to find a fuel leak and the cold start injector is working properly.

Trying to figure out which EFI component to replace first, I recall cleaning the throttle body because the butterfly was sticky from dirt/carbon buildup. I'm wondering if some of the crud may have made it into the TPS and messed it up. The TPS tests within spec, but I wonder if it's not working correctly. I keep looking at the coolant timing switch or the coolant temp sending unit, thinking that a faulty sensor is making the engine think it's cold and running too rich. After that, I don't have any solid ideas about what's wrong. I plan to clean the EGR valve and probably take off the upper plenum and clean it up, other than that, I am open for any advice on how to get to the bottom of this.

 

What's the mileage on your truck. A clogged cat will destroy fuel mileage

 

About 160K. The truck has been well maintained. How do you tell if the catalytic converter is clogged?

 

Youd really have to remove it. Throw a test pipe in there and give it a run. You should notice a considerable difference if your cat is clogged. You may be able to look thru it if its ok.

 

I took the throttle body off, removed the Throttle Position Sensor and cleaned everything up. I'd cleaned the throttle body before the best I could without removing it from the plenum but it didn't make any difference in the mileage. Cleaning the throttle body without removing the TPS made me think I had filled the sensor with crud, so I took everything apart to have a look. The throttle body was not excessively dirty and there were no blocked vacuum ports. The throttle position sensor moved freely with no signs of dirt fouling or other damage. Cleaned the throttle body, reinstalled the throttle position sensor, calibrated the sensor and put it back on the truck. After the first tank of gas, I was happy to see I was getting about 25mpg. Not the 28mpg I should be getting, but better than the 19-20mpg I was getting before this repair.

I don't think cleaning the throttle body was the reason for the better mileage, but a slight adjustment to the throttle position sensor started me back to normal fuel mileage. I will probably try another adjustment to the TPS until I get closer to 28mpg. Since the throttle position sensor is now the main suspect for the poor gas mileage, I may even spring for a new one.

 

Is this a 2 wheel or 4 wheel drive? If it a 2 wheel drive you should be able to get 28 mpg. I have seen where some say they get 28 mpg with a 4 wheel drive and I have never gotten that out of a 4 wheel drive truck. On a 4 wheel drive I do good to get 21 mpg on the open road with a 22re manual transmission.

 

It's a 2WD and has gotten a solid 28mpg since I drove it off the lot in 1990.