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Rich fuel mixture help

15K views 26 replies 8 participants last post by  cory3122 
#1 ·
Ok I’m at a loss with this one and looking for some help.

So I had the Fj apart and installed a upgraded radiator, transmission cooler, replaced all the water hoses, and thermostat and housing. Put it all back together and pressure washed the underside and engine compartment throughly.

Now here is the problem. Fuel mileage dropped 4 to 5 mpg. I have been monitoring the system via odb2 for several days. No codes!! The A/f ratio seems to be very rich under modest acceleration and the exhausts temps seem to confirm this as well. TPS is within specs, MAF is within specs. O2 sensors seem to be in spec as well.

Anyone got any ideas?




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#3 ·
This same issue happened to me years ago on a ford F350. Performed normal maintenance, flushing the radiator and replacing the thermostat. It was a week of troubling shooting before I discovered that I installed a thermostat with the wrong temperature range. The engine would never reach optimum operating temp and the ECM kept the fuel mixture rich.
 
#6 · (Edited)
Possibly just water in the MAF sensor or MAF sensor connectors which is erroneously indicating to the ECU that there is more air entering the engine than actually is, and the ECU is commanding excess fuel.

Remove the MAF and clean it carefully, and de-mate the MAF connector and blow out both halves with clean, dry comoressed air.

Also, de-mate the connectors at all four of the exhaust gas sensors and blow out the connectors with compressed air.

But, much more info is needed to perform any kind of useful diagnostics.

1. What year model?
2. Original or aftermarket air cleaner system?
3. Have you verified that the engine air filter(s) are not water saturated?
4. How many miles on the odometer?
5. Have any or all of the exhaust gas sensors been replaced?

NEVER, EVER PRESSURE WASH THE ENGINE CMOARTMENT OR ANY PART OF THE WIRING HARNESS WITH A HIGH PRESSURE WASH SYSTEM

All the electrical connectors are “sealed”, but a 2,000 PSI pressure-washer will force water (or worse, detergent solution) past the seals, where it will stay and immediately cause false readings from sensors, or it will sit there for weeks or months, slowly causing corrosion
 
#7 ·
Possibly just water in the MAF sensor or MAF sensor connectors which iis indicating to the ECU that there is more air entering the engine than actually is, and the ECU is commanding excess fuel.

Remove the MAF and clean it carefully, and de-mate the MAF connector and blow out both halves with clean, dry comoressed air.

Also, de-mate the connectors at all four of the exhaust gas sensors and blow out the connectors with compressed air.

But, much more info is needed to perform any kind of useful diagnostics.

1. What year model?
2. Original or aftermarket air cleaner system?
3. Have you verified that the engine air filter(s) are not water saturated?
4. How many miles on the odometer?
5. Have any or all of the exhaust gas sensors been replaced?

NEVER, EVER PRESSURE WASH THE ENGINE CMOARTMENT OR ANY PART OF THE WIRING HARNESS WITH A HIGH PRESSURE WASH SYSTEM

All the electrical connectors are “sealed”, but a 2,000 PSI pressure-washer will force water (or worse, detergent solution) past the seals, where it will stay and immediately cause false readings from sensors, or it will sit there for weeks or months, slowly causing corrosion


MAF was the first thing I checked and cleaned, no moisture in the connector. I haven’t checked the o2 sensors connects but I will tomorrow.



1. 2012
2. TRD intake
3. Filter does not appear to be saturated and MAF reads within specs at idle and at 2,500 rpm per specs.
4 137,000 miles
5. I haven’t replaced any of the exhaust sensors but they appear to be reading voltages within factory specs.

That’s what’s getting me, sensors seem to be reading a rich condition, high exhaust temps seem to confirm this (along with the poor mpg) but it’s like the computer is not adjusting accordingly.

TPS sensor is within specs as well....



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#10 ·
Maybe check that the post-MAF intake coupling didn't get slightly bumped off (or cracked) from the pressure washer. Maybe some of the air going past the MAF isn't making it to the engine. The ECU is going to trust the MAF but, if not all of that air is getting there, it would run rich.
 
#11 ·
No, not the case.
If some intake air is bypassing the MAF, the MAF will report LESS air than is actually passing through the engine, and the ECU will supply only enough fuel to match the reported amount of air. Since more air than reported by the MAF is actually passing through, the resulting fuel mixture will be LEAN.
 
#16 ·
There is a vacuum-modulated fuel-pressure regulator in the fuel rail towards th rear of the intake manifold.

Check that your pressure washing didn’t disconnect or damage the vacuum line.

Also, if the diaphragm in the regulator developes a pinhole, it can dribble raw fuel into the intake manifold.
 
#17 ·
I checked that line today and pulled and cleaned the throttle body. It was in good shape. I will have to look up the test procedures for the pressure regulator.

O2 sensors should be here Tuesday so I can get them replaced as well.

Thanks for the help while I try to sort this out.


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#18 ·
cory3122,

I can feel your frustration. This may be a long shot but if I were in your shoes I would disconnect the the negative battery cable, wait 30 minutes, then reconnect it. Perhaps something in your system will re-set itself. Most likely won't be beneficial but why not try anyway.
 
#26 ·
Anyone got any ideas?
Could it be something unrelated to the work you did but coincidental or out of your control but happened at the same time?

For example could a brake caliper have started to drag at about the same time? That can be worth 4-5mpg right there. Do you have a wheel warmer than the others after driving?

Here, the weather fairly suddenly got a lot colder recently. My cold tire pressures on the FJ all dropped by 4 psi. I had to pump them all up a little. Slightly low tires use a bit more gas too. Lower ambient temperatures tend to make gas mileage worse for several reasons including more time driving with the engine not fully warm and using cabin heat during that time so more miles running rich. Short trips suffer most.

Did the area where you live just switch over to winter blend gasoline from summer gas? That costs mileage too.
 
#27 ·
Could it be something unrelated to the work you did but coincidental or out of your control but happened at the same time?



For example could a brake caliper have started to drag at about the same time? That can be worth 4-5mpg right there. Do you have a wheel warmer than the others after driving?



Here, the weather fairly suddenly got a lot colder recently. My cold tire pressures on the FJ all dropped by 4 psi. I had to pump them all up a little. Slightly low tires use a bit more gas too. Lower ambient temperatures tend to make gas mileage worse for several reasons including more time driving with the engine not fully warm and using cabin heat during that time so more miles running rich. Short trips suffer most.



Did the area where you live just switch over to winter blend gasoline from summer gas? That costs mileage too.


It could be unrelatedly I guess. I think it must be a leak somewhere. I have visually checked the fuel rails, injectors, etc. I still think I smell fuel at times towards the rear, inspected the fuel cap but it appears ok.

Guess my next step is to put some UV dye in the gas and look for a small leak.

Don’t appear to have any calipers dragging, about to do a front break upgrade and replace rear pads so I can further look into that possible cause.

Tire pressure is good.

I hope they haven’t changed the mix yet, heck it’s been in the 90’s all week.


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