I had CEL on with po171 and po174. I cleaned MAF sensor and checked thoughly for vaccum leaks and found none. CEL went of and but my mpg is still around 14.
1999 4.0 4x4

 

I did a little searching on the site, and found some things that maybe I should check into replacing. DPFE sensor, O2 sensors, and Idle air control valve, I know my gas mileage goes down in the winter but not to 13 or 14 mpg. any other suggestions

 

To be sure about O2 sensors do an emission test. Lean ore rich mixture - replace both O2 sensors.....

 

I like Pablo-UA's, idea to check the O2 sensors. If you have access to a scantool that'll read PID's, you could monitor their switching speed to see if they're lazy.

It'll also be able to monitor other things like the computers engine coolant temp sensor, IAT sensor, fuel pressure, fuel trim ect, so you can get an idea about how other fuel consumption affecting things are acting.

Seeing as how you had lean codes for both cylinder banks & think you fixed them with a MAF cleaning, makes me want to ask if you've modified your air intake system????
Also, did you reset the computer, so it could begin learning new fuel trim, after cleaning the MAF sensor????

Usually but not always, a lean code on a 4.0L has to do wth the intake manafold gaskets leaking. I know you said you've carefully checked for vacuum leaks, but did you check carefully around the upper & lower intake manafold gaskets????

I suppose you could also have a sticking open PCV valve & that too could cause a uncalculated for upset, in the metered air flow to the engine, so check the PCV valve & it's hoses.

Don't know what kind of gas you've been using, but if it hasn't been a good quality "top tier" brand, with extra detergents in it, I suppose the fuel injectors spray pattern could be off some, so maybe consider adding 20 oz of Chevron Techron Concentrate Plus at the pump, then tanking up with Chevron, or Texaco which already have Techron in them. With the additional added Techron you'd have about a 5x stronger dose, so it should begin to clean things up & maybe do it in one tank.

Then run most of the treated tank out & see how things go. If the injectors are dirty enough to cause the mpg to drop, the Techron should be ble to clean them up enough for you to measure a gain in mpg.

Did this drop in mpg come about slowly over time, or suddenly after some event????

 

Check fuel pressure too...

 

It may take a couple days for your leaking intake gaskets to cause the check engine light to come back on.

 

Yeah, made of plastic intake with 6 o-rings..... but exaust leak cause MPH drop too.

 

You can test the O2 sensors and clean them with a torch. do a search for testing o2 sensor. get a multimeter and a torch, some silver antisieze and the socket. How many miles do you have on the truck?

 

I have 121,000 miles on the truck

 

The CEL came back on when temperatures dipped below 40 degrees F.

 

Temp related fuel consumption, check thermostat, engine needs to be 180deg to run under the computer or it runs in limp mode.

 

OK interesting clue, pull the codes again & post ALL the code numbers.

If it's the lean codes again, rig a propane "sniffer" & with the temp below 40 deg & engine at cold idle, check closely around the upper & lower manafold gasket areas, for a vacuum leak. If it gets a whiff of propane, the idle speed should increase.

Then check the manafold fasteners in that area, if loose, try re-torquing the fasteners to spec & see if you get favorable results. If not, suspect the gaskets & replace them.

Did you ever check out the O2 sensors switching speed?????

 

I rehooked it to the scan tool yesterday let it warm up and all O2 Sensors had readings I cannot exactly remember what they read. I also had one code PO171. Which I originally had PO171 and PO174 before MAF cleaning.

I have tried looking for an upper intake leak but never with the temp below 40 degrees I can try that assuming we will have another day under 40 degrees. If i reset the computer the CEL will not come back on until next winter when the temps dip down lower again.

I had my ABS light come on a couple weeks ago but instead of speedodometer not working my tach seems to jump back and forth???? I changed the VSS on the rear main, and the ABS light is off for now but tach is still jumping any clue on this?

 

The O2 sensors numbers should have been switching rapidly.

If they were slow/sluggish, not rapidly changing, then suspect the lazy sensor. Seeing as how you have over 100K, they belong on your suspect list, until they prove themselves.

If the O2 sensors check out ok, then the lean codes returning with cooler temps, makes me want to suspect a intake manafold leak. So maybe the propane sniffer test would be worth doing. It isn't difficult to rig or to do.

Not sure about the ABS lght, look in the "Tech Info" thread atop this forums thread listing page for a link on how to pull the ABS codes & let us know what you find, as that could offer up a good clue on where best to begin your ABS system troubleshoot.

 

Where can i find what my O2 sensors should be reading?