Can the engine be putting out codes if the Check Engine Light is not on? Assume the light works, engine runs fine, no sign of any trouble (sight or
audible), just decrease in mileage.

Is there such thing as a problem is not catastrophic, just an annoyance, so the engine puts out a code, but the check engine light doesn't come on?

Gas mileage has been going down, have done everything short of changing the 02 sensors. Just wanted to know if it was possible before I spend the $$?

 

Yes the engine can be throwing a code without the cel coming on. The check engine light will come on when an engine part that controls emissions isn't working properly or if a lean condition is detected. To check for codes, you could take it to an auto parts store that will check the codes for free. Then you could post the code here and someone will help you work through it.

 

Be nice if someone made a thread with the codes and explanations and posted it as a sticky............They are pretty much uniform for all models.

 

Thanks! I had a feeling that the computer could be throwing a code without the check engine light came on.

 

You & the guys are right, the computer stores codes in Continuous Memory (CM) & they remain there until the problem doesn't reoccur after 40 warm up cycles, after which the computer erases them, or after Fast KOEO codes have been read, or the code reader, or scan tools erase codes function is used, or a battery cable has been disconnected.

On code retrieval, CM codes follow KOEO (Key On Engine Off) codes in the data stream.

If you suspect "lazy" O2 sensors, monitor their PID output range & switching speed to the computer with a scantool, that'll read sensor PID's. If you have 100k miles on them, or have had the engine sipping coolant, or sniffing silicone spray, they may be suspect.

A number of, or combintion of things, can cause mpg to drop, so take the time to properly diagnose the problem, so you get good satisfying results for the bucks spent,

Don't begin throwing parts at the mpg problem on suspicions.

Take the time to properly test & make any suspect parts prove/disprove themselves, before replacing them.

Where are you on all past & present due scheduled maintenance items & what have you done so far????

Which year/model/ engine/tranny combo are we talking about & how many miles on it????

List any mods.

More thoughts for pondering.

 

Thanks paw paw, your knowledge is definitely deep.

2003 Ford Ranger Value Package, 4.0L, 4WD, 84000 miles.

I will get the codes read before I buy anything.

Will post results.

Thanx,

Kevin Kasson

 

P.S. No mods. Up-to-date on all maintenance (Ford dealer does some, I do some):
- new plugs 500 mi ago (ford dealer)
-fluids are all synthetic
-5 speed auto transmission

 

OK, good feedback.

With 84K miles on these O2 sensors, their switching range & speed probably belong on your suspect list.

So see if you can come by a scantool that'll read both banks PID's to the computer & post their switching voltage range & the speed at which their changing voltage.

Vacuum leaks, including the PCV valve & it's hoses & sealing grommets, EGR & Evap systems may belong on your suspect list, along with a clogged air filter, dirty MAF sensor & fuel injectors, or maybe fuel pressure probems.

Edit: If the engine isn't coming up to operating temp, suspect a stuck open thermostat, as that'll affect mpg & the mileage is about right for the thermostat to act up.

Good idea to have the computer scanned for any pending trouble codes, as they can provide good trouble shooting clues on where best to look next, so let them be your guide in the next trouble shooting step & post ALL of the code Numbers found.

More thoughts for pondering.

 

For 1995+ check out: Ford OBD-II Diagnostic Trouble Code (DTC) Descriptions

For EEC-IV codes (pre '95), see: Fault Code Definitions for EEC-IV System

 

Thanks for posting this,..........one of the most useful things here!

 

Quick question, 3 O2 sensors, right? 2 pre-CatConverter, 1 post-CatConverter that I could see underneath?

 

Yup, 3 O2 sensors.

 

Replaced the three sensors (w/Bosch) vs. Motorcraft ($300 vs. $150), I am an OEM man, but the auto store where I could get the Motorcraft sensors was closed (Memorial Day weekend.)

Also had to replace the driver's side door handle (dang steel rivets!)
Anyway, the passenger side O2 sensor was a real treat, the other
two were OK.

Paw Paw, did not shoot codes like I should have done, but did buy a
Actron OBD II Auto Scanner, model # CP9175. Did shoot codes after
the replacement and found none.

Driving right now through a full tank of gas and mileage does look better
at this point. Will update later on the gas mileage.

Oxygen sensors (2 of them, can't remember if one of them was post-Cat
or pre-Cat?) had white residue on them. I believe this is because the
truck may have been running lean?

I also read once that the synthetic oil I use has high Phosphorus content
and wonder if it could be that? If so, then my CatConv could be coated
with the stuff also. Maybe should change to Mobil 1? Anyway, probably
no big prob? Right?

 

Vacuum leaks, including the PCV valve & it's hoses & sealing grommets, EGR & Evap systems may belong on your suspect list, along with a clogged air filter, dirty MAF sensor & fuel injectors, or maybe fuel pressure probems.

Edit: If the engine isn't coming up to operating temp, suspect a stuck open thermostat, as that'll affect mpg & the mileage is about right for the thermostat to act up.

Paw Paw, I replaced the PCV valve several weeks ago (why are there coolant tubes
running around the PCV valve), to keep it cool or warm up faster? I clean the MAF
every six months with electrical component cleaning spray. I change the air filter
ever 12 months and use fule injector cleaner every 5k miles or so.

So that leaves Vacuum leaks, PCV valve hoses & sealing grommets, EGR & Evap systems, or maybe fuel pressure probems. Will let you know how the next tank of
gas ends up.

Although I think it was the O2 sensors because I had a case of the rotten eggs a
few days before the O2 sensor change.

Thanks,

Kevin Kasson

 

Bosch makes the stock sensors, so you shouldn't notice any difference. White residue is normal, unless it is a thick residue. Any residue can weaken the sensor, but that is why they should be replaced every 80,000 miles or so at the most. If your cat was having a problem you would have a P0420 code, which you don't so don't worry about it.

Anyone who says that the synthetic has high phosphorus is over generalizing. There are a lot of misconceptions about synthetic oil and misunderstandings about what a synthetic oil even is. There are only a small handful of companies that really make synthetic motor oil. However, I don't want to get in an oil discussion here, so I won't dig into it any deeper than to say that synthetic oil should have no more phosphorus than regular oil, if not significantly less. However, synthetic oil can hold more phosphorus in suspension before it becomes saturated. Some of the false synthetic oils might have elevated levels of phosphorus. I have not educated myself about the phosphorus content of various oils. I do know some oils also contain molybdenum, but that doesn't concern me because it acts as a metal lubricant and the concentrations would need to be very high to cause damage.

The white film is more likely something that came from the fuel. If it was slightly yellowish, it is more than likely sulphur. Sulphur is present in all fuels, and used to be in higher concentrations than it is today. New clean fuel standards limit the amount of sulphur that can be present. These standards came into effect back in 2005. Even so, the residue could have formed prior to that time, and there is still sulphur in the fuel today.

I wouldn't worry about it myself. Deposits from stuff that doesn't burn deposits on the sensors and reduces the effectiveness of the sensor element. This is exactly why sensors should be replaced on a long term schedule. 80,000 miles is a good number and I wouldn't wait much longer than that. Let us know if your gas mileage has improved. If not, let us know what your long term fuel trim is and we can try to us that to help isolate the issue.