So here it is

I have a 1998 4x4 4.0 Ranger.

Started to see performance problems about six months ago. I just move to
San Diego and had to get it smog. It passed which suprised me for as bad as it was running.

Afterwards I had some time to do some maintenance on it. Changed the plugs, plug wires and air filter.

plugs 1 and 4 were broken in half and had been for some time. Needless to say the truck ran better afterwards. Still the gas mileage was really bad, 12 mpg. I notice the engine temp was not ever getting higher than 1/4 on the gage and it took forever for it to get there. Changed the thermostat. Temp now goes up 1/2 the way and does so in 10 mintues. also runs better with more power. Still it seems to be lacking pwr and after 120 miles I have already gone though 1/2 a tank.

Question: is there something to do next? there are no codes or engine lights. I had engine check light before the work but when I changed the plugs and wires I reset the ecu.

I was wondering if since the thermostat was stuck open the engine would have been running rich. Could this have cause problems for my o2 sensors, foul them up enough to cause mpg and performance problems but not enough to throw a code?

Also since the #1 and #4 were not firing for whatever amount of time would this have caused o2 sensors problems.

I have also run injector cleaner, gumout, for the last three tanks. How would you know if they was a clogged or partuially clogged injector? Best way to clean them?

Like I said it is getting better but still going through alot of gas and still a little weak when going up hills.

In closing it is a testament to the 4.0 and its ecu for the truck to pass a dyno smog with two broken plugs and a thermostat stuck open. That or the smog station really sucked.

Thanks for the help

 

You might try (1) replacing the fuel filter, and (2) cleaning the MAF sensor.

 

I forgot to post it but I had cleaned the maf already. Hadn't thought of the fuel filter. Truck has 50k on it. How does a clogged fuel filter affect mpg? I understand it affecting performance.

 

You would be better off running a complete diagnostic test on it to see where the engine needs attention.