2010 Ranger 4.0: Noticeable drop in fuel economy

[salt_monger]

Per the title, I have a 2010 Ford Ranger 'Sport' 4.0 Automatic Ext Cab. 220,000Kms (136k miles)

Note I'm a resident of Soviet Canuckistan, so I'll be mentioning both metric and imperial units, sorry, not sorry.

A couple of months ago the lower thermostat housing started leaking. I order a new OEM piece from Rockauto and installed it. Shortly after (maybe 2 weeks?) installing the thermostat housing I started getting a CEL code for voltage errors on one of my upstream O2 sensors. I ordered 2 new Bosch upstream O2 sensors from rockauto and installed them. I replaced both given the age and mileage of the originals that were in the truck (Ive owned since new). Since the above changes my gas mileage/fuel economy has gone to shit.

I used to get about 13.5L/100Km (~18MPG) with city driving and about 10.5-11L/100KM (about 22MPG) highway on longer trips. This has been fairly consistent since I bought the truck new in late 2010. Since the mechanical 'fixes' illustrated above I'm not getting more like 16L/100Km or about 14.5MPG. This blows.

Plugs/wires were done two years ago with OE Motorcraft parts. Regular oil changes and good maintenance. The drivetrain has 'not been fucked with' and engine has barely been touched since new except for basic maintenance. No weird wiring spliced in anywhere.

I've been over all engine bay connectors and fittings thinking I may have forgotten to plug something back in after the above changes. To replace the lower thermostat housing I did remove the intake completely which also included disconnecting the main computer harness in the engine bay. I currently have no CEL present or codes present when scanned (innova scan tool).

Engine runs and idles smooth.

Any ideas? Am I missing something obvious? Suggestions on what to examine to help diagnose?

 

[Brain75]

Is your Innova scan tool one of the $50 models or $300 models... how fancy can it get - can you see realtime stuff or just codes/emissions/reset?
If you can see realtime I would look to see if you are ever leaving warmup/limp - being in limp on other similar models we see 10-15mpg.

I would hazard that since you had good mileage, you put new O2 sensors in and now you have bad mileage that your new sensors are just duds... but that is a little surprising for bosch (bosch is one of the few exceptions to quality in today's crapshoot terrible aftermarket parts).

Voltage error on the emissions line could be bad sensor output or it could be way out of spec... I turn wrench on 2nd gen OBD1 primarily, so will defer to one of the more experienced modern engine guys here but I know one situation that should not happen that sometimes does - it sees more than 5v (valid range is 0-5v) from some foreign source (like 12v main power).

Do you remember what the codes were that made you get new O2 sensors?

 

[salt_monger]

Is your Innova scan tool one of the $50 models or $300 models...
If you can see realtime I would look to see if you are ever leaving warmup/limp - being in limp on other similar models we see 10-15mpg.

It was $250 CDN when i bought it, and I think it has some data logging capabilities. I'll check.

I would hazard that since you had good mileage, you put new O2 sensors in and now you have bad mileage that your new sensors are just duds... but that is a little surprising for bosch (bosch is one of the few exceptions to quality in today's crapshoot terrible aftermarket parts).

Been there, done that with cheap O2 sensors. Do not recommend. 0/10. Lol.

Do you remember what the codes were that made you get new O2 sensors?

Pretty sure it was a P0135. The typical running slightly rough symptom was there. After replacing the CEL went away and it runs/idles much smoother.

I'm going to check my plug wires again. I had disconnected them at the coilpack to remove the intake to get to the lower thermostat housing. One would think that if I didnt have one connected (or poorly connected) at the coilpack that the truck would run rough or seem really low on power...

 

[Brain75]

p0135 causes:

1. Faulty O2 Sensor: The O2 sensor itself may be defective, causing the heater circuit to malfunction.
2. Wiring Issues: Corrosion, damage, or poor connections in the wiring harness can disrupt the electrical signal to the O2 sensor heater.
3. Blown Fuse: A blown fuse in the O2 sensor heater circuit can prevent the sensor from receiving power.
4. ECU Malfunction: A malfunctioning engine control unit may not be sending the correct signals to the O2 sensor heater circuit.

most likely #1 in 90% of cases - the sensor itself, but as long as you are double checking for corroded, half connected plugs, also look for damage (at least where I am the plastic is so brittle it breaks if you touch it), and check the fuses.. Though fuse would be a continued problem, new sensor would not have fixed the CEL/codes..

 

[salt_monger]

p0135 causes:

1. Faulty O2 Sensor: The O2 sensor itself may be defective, causing the heater circuit to malfunction.
2. Wiring Issues: Corrosion, damage, or poor connections in the wiring harness can disrupt the electrical signal to the O2 sensor heater.
3. Blown Fuse: A blown fuse in the O2 sensor heater circuit can prevent the sensor from receiving power.
4. ECU Malfunction: A malfunctioning engine control unit may not be sending the correct signals to the O2 sensor heater circuit.

most likely #1 in 90% of cases - the sensor itself, but as long as you are double checking for corroded, half connected plugs, also look for damage (at least where I am the plastic is so brittle it breaks if you touch it), and check the fuses.. Though fuse would be a continued problem, new sensor would not have fixed the CEL/codes..

Re:
#1, sensor plugs, while a bitch to reconnect due to their position (passenger wide especially) were very clean and easy to take apart.
#2, Wiring: This is where I'm at a loss. I've been over what I think is everything I disconnected and ensure they were all clipped in properly.
#3, Blown fuse: I'll double check, but replacing the O2 sensors made the CEL go away
#4, ECU Issues. Again this is a a big ??? for me, as I *did* remove the ECU connectors underhood to get the main harness out of teh way to get the upper intake off to access the lower thermostat housing. See

for what I sorta did - my ECU had 3 connectors without a bolt holding them in.

My ECU connectors look like this

 

[Brain75]

To see if the ECU is the culprit - and again I am no expert on the post 2nd gen stuff - I would see what voltage you get at the o2 sensor with the sensor disconnected (if I remember the proceedure right).... this is where we need a more modern expert to verify/correct me or take lead.
Should be 5.0v max, if you are seeing 10/12v or something like that then either a stray wire is putting power there or the ECU is sending the wrong voltage out... i feel like the blind leading the blind here cause I was borrowing a haynes + chilton from the library and then returned em and ordered one of fleabay - have not got it yet, so I am working from memory on a book I don't have for a system I have never touched (the 2007 in my stable has had zero issues besides Key/PATS).

edit:correction - the heater circuit has 12v on it, the sensor should be returning something 0-5v... it is a 3 or 4 wire connector right, don't worry about the pin that supplies power to the heater.

from the pinout here I can see it would be pointless to test with the sensor unplugged it would all just be open circuit - so ignore my previous stupidity.

 

[salt_monger]

Just wanted to loop back to this post with a conclusion.

This ended up being a grounding issue.

In the engine bay pic above, there is a firewall mounted stud just right of the ECU plugs. This is where I reconnected my ground strap as I was sure thats where it had come from. In teh pic above it looks like the ground strap for the ECU harness DOES get bolted here. As I went thru the diagnosis I couldnt get away from the feeling that I was having some sort of grounding or continuity issue. I moved the ECU ground strap to the right stud/nut that holds the ECU in place and VOILA, my issues described above went away. I then removed and cleaned the firewall stud just right of the ECU housing and put the ground strap back there - my issues described above returned. So I moved the ground strap BACK to the ECU housing again, and when the the issues went away again. I decided to leave teh ground strap on teh ECU stud/nut instead of on teh firewall stud/nut and call it a day.