94 Dodge ram 4x4 2500 5.9 Gas

Hey guys. I was leaning toward an o2 sensor or a map sensor but I don't have a definite yet.

What I do know is at one point I was getting 12 mpgs. I went to fill up last night and I was at 5.5mpg. I'd say it dropped from 12 to 6 very rapidly. And it has been at 6 for a couple of months.

The truck runs fine, starts right up, roars down the highway, no stuttering or anything. No symptoms that I can see except my gas mileage **** itself. I've been chasing other problems so this hasn't been top priority but now it is

So the truck will start, and then after 10 minutes of running, the CEL will come on with codes 12 37 and 51. 12 being the battery... 37 I haven't narrowed down and don't see it in the Haynes manual. But more importantly 51 being the truck is running too lean. Once the truck runs lean for 11mins and 55 second, the CEL comes on and the PCM runs the truck as rich as it can. Hence the lack of mileage. Now as everyone else, I don't want to go throwing money around all ***** nilly. Is there anything I can do or read to narrow my problem down?

Thanks!

 

Have you check for oil in the back of your intake plenum. If you have oil present in the plenum you have a plenum leak which will cause a mirade of problems which do include leaning and bad gas mileage symptoms.

 

The from O2 sensor has a major impact on fuel ratio trim, but, if it fails the PCM will run the engine a little rich to be safe.

Something else is causing your lean issue.

 

I would replace the map sensor.

Dave

 

X2 on the O2 sensor, mine went while I was driving only 20 miles to get somethings at the store and went from 3/4 of a tank to 1/4 of a tank.
Did you check with an OBD I computer or the key method?

 

I was using the key method. I can have the guy at the store advance auto check the codes with the reader today if that will show something different. So I tested the o2 sensor per the haynes manual and it seemed to check out ok.

So I check the map sensor also per haynes and I was getting bad results back. So the manual says the voltage of the test wire will increase in voltage as RPM's raise up. So when you nudge the throttle, the voltage should steadily go up. It says as the throttle goes up, the vacuum goes down increasing the voltage. Mine the throttle goes up and the voltage does down. Which is backwards.

Does that sounds right? Figure I'll read the book again to make sure I read it right last night and replace the MAP sensor.

Also while I had the air filter off, I looked down into the Carb(read this on the plenum thread) And the entire inside is caked with burnt oil. So should i replace the plenum too? I'm not actually sure what the plenum is so I may be using that word wrong here.

 

Time to do the plenum...they sell kits for it on ebay or from hughes engines. Depending on your mileage, you'll want to look in to doing the timing chain while you're in there.

And it also sounds like your map sensor is bad. I'd try to find a factory replacement from one of the mopar parts websites or maybe ebay if you get lucky. For some reason, our trucks are really picky about their electronic parts.

 

I get 11mpg with mine. No matter what you do, your lucky to get 13 around here.

 

Well I swapped out the map sensor for a new one but the check engine light still comes on after a few minutes. But my abs and brake light turned off odly enough... Don't know the correlation there. Still getting bad mpg tho

 

reset the computer?