Sudden gas mileage drop

[digitalmarker]

Hey folks,

I have a 1999 Land Cruiser with about 220k on it. Since I got the truck and lifted it + added 33" tires, I have been consistency getting around 240-245 miles/tank (at least that's what it reads on the odo).

About a month ago a coil went out and got replaced at a shop. A few weeks later the truck would start and then die immediately after starting. I cleaned the MAF and the throttle body, and the problem went away.

However, since then, gas mileage dropped to 200-210 miles/tank. I have had several fill ups since then, and it has consistently gotten 30-40 less miles/tank since then.

I thought cleaning the MAF sensor and the throttle body was supposed to improve gas mileage, not drop it.

Any ideas on what else I could check or how to troubleshoot?

Thanks in advance!

 

[FxFormat]

Oh wow 200 miles per tank is pretty bad, not entirely sure what’s causing the drop in MPG but I got a little better after I tuned up the truck. New spark plugs, fuel filter, air filter, oil I use 0w-20 oil and put 7 qts.

Check for brake drag, seeing if a stuck caliper or something causing you to increase rolling resistance.

 

[gungriffin]

Oh wow 200 miles per tank is pretty bad, not entirely sure what’s causing the drop in MPG but I got a little better after I tuned up the truck. New spark plugs, fuel filter, air filter, oil I use 0w-20 oil and put 7 qts.

Check for brake drag, seeing if a stuck caliper or something causing you to increase rolling resistance.

Plugs would definitely seem to be a good place to start. To the OP, when was your last tuneup?

 

[Biga27110]

tire pressure, i gained 5mpg

 

[gungriffin]

tire pressure, i gained 5mpg

Ha ha also not always idling my vehicle when I go off-roading with friends

 

[nissanh]

Did you get OEM coil packs? Aftermarket coil packs are no good for MPG.

 

[REZARF]

Check your O2 sensors as well.

Do you have any engine codes?

I would replace the suspected coil with a Denso Unit from Japan... they run about $50 or so on Amazon last time I bought a few (to replace one and keep a spare on hand for wheeling).

 

[digitalmarker]

The bad coil was replaced by a Denso unit.

Plugs were replaced about 15k ago.

No codes since the engine misfire one from the bad coil.

The idle is perfect and smooth.

The one other symptom is that the exhaust smell really smells like fuel. The dude at the shop I go to says “that’s just what a v8 smells like” but IMO it smells a lot more like fuel than my 2004 tundra with the same motor.

What are some of the things I could test for before I start throwing money at the problem?

 

[highonpottery]

Did you reset your ECU? It's free. I was always told to reset the ECU after doing stuff like cleaning the MAF or swapping o2 sensors, etc so that the ECU starts fresh again with the new parts.

 

[digitalmarker]

Thanks will try that tonight. I am 99% sure I did after I cleaned the MAF because I was installing a stereo and had the battery disconnected. Will do that again. Is there another procedure to make sure it resets?

 

[2001LC]

The bad coil was replaced by a Denso unit.

Plugs were replaced about 15k ago.

No codes since the engine misfire one from the bad coil.

The idle is perfect and smooth.

The one other symptom is that the exhaust smell really smells like fuel. The dude at the shop I go to says “that’s just what a v8 smells like” but IMO it smells a lot more like fuel than my 2004 tundra with the same motor.

What are some of the things I could test for before I start throwing money at the problem?

Did you happen to change or ad oil. Have been reports of high oil level dropping MPG.

Gasoline smell indicates rich mixture, some possible causes not yet mentioned:
Poorly functioning fuel injectors. Fuel system cleaner like BG 44k or Chevron techron may help.
Fuel pressure regulator. Reports they are sensitive to temp yielding varying MPG as they age.
Vacuum leak, intake gasket(s) or rubber line(s) going bad or not connected. Check them all from PCV to the two coming off the power steering pump.
Loose spark plugs.
Dirty air filter.

 

[digitalmarker]

@2001LC Thanks for the tips. I changed the oil a while before the MAF issue, so don't think it's the case. Air filter is nice and clean.

Entirely possible it's injectors, they have never been serviced to my knowledge. Going to pull those in the next few weeks and get them serviced.

Where is the fuel pressure regulator located? I tried searching for threads but the only thing I am finding are threads on replacing the fuel filter, which it sounds like is not a necessary measure.

Lines and vacuum look intact. I sprayed some carb cleaner onto the lines leading into the intake manifold, and the engine did not sputter at all, so doesn't seem like anything is broken there.

BUT

I did follow @highonpottery 's advice and reset the ECU (unplugged and plugged back in 10 mins later). The results are promising - just did a long range trip to the mountains, and got 275 miles out of the tank (compared to 200-205 I got on the past few fill ups). This is actually the highest mileage I've ever gotten out of a single tank before. Granted it was a mostly freeway trip, but about half of it was on steep hills, and the rest of the time I was driving 75-80 mph, so not exactly conserving gas. But either way, hopeful that this was the answer.

Going to drive it around town for usual commuting over the next week and will report back on what kind of numbers I am seeing. Either way going to get the injectors serviced by FIS in the next few weeks.

Thanks all!

 

[abuck99]

@2001LC

I did follow @highonpottery 's advice and reset the ECU (unplugged and plugged back in 10 mins later). The results are promising - just did a long range trip to the mountains, and got 275 miles out of the tank (compared to 200-205 I got on the past few fill ups). This is actually the highest mileage I've ever gotten out of a single tank before. Granted it was a mostly freeway trip, but about half of it was on steep hills, and the rest of the time I was driving 75-80 mph, so not exactly conserving gas. But either way, hopeful that this was the answer.

Going to drive it around town for usual commuting over the next week and will report back on what kind of numbers I am seeing. Either way going to get the injectors serviced by FIS in the next few weeks.

Thanks all!

Did you unplug some module or disconnect battery? I had the understanding disconnecting the battery for 15 min would effetively reset the ecu.?

 

[Spike555]

Did you unplug some module or disconnect battery? I had the understanding disconnecting the battery for 15 min would effetively reset the ecu.?

This is not true. Otherwise anyone with a check engine light could unhook the battery, reset the ecu and sell it with a fault code.
You must use a OBD II scanner to reset the ecu.

My bet is on something was left unplugged when the shop did the coil.
A vacuum line, or a electrical connection and the ECU is in default mode, default mode is a "good enough" engine mapping program sp that you can still drive with something unplugged or out of whack.

 

[hifu]

Had a bad o2 sensor when I purchased mertle, got about 10mpg and when I changed out both I’ve gotten 15-16 the last 6 years

 

[abuck99]

This is not true. Otherwise anyone with a check engine light could unhook the battery, reset the ecu and sell it with a fault code.
You must use a OBD II scanner to reset the ecu.

My bet is on something was left unplugged when the shop did the coil.
A vacuum line, or a electrical connection and the ECU is in default mode, default mode is a "good enough" engine mapping program sp that you can still drive with something unplugged or out of whack.

Curious what function do you select in the OBDII scanner to reset a ECU if there are no codes present?

Maybe the word “reset” is misleading. My understanding was disconnecting the battery for 15 min after cleaning MAF sensor for example clears the fueling parameters and provides the ECU the opportunity to relearn fueling to driving behavior.

 

[Spike555]

Curious what function do you select in the OBDII scanner to reset a ECU if there are no codes present?

Maybe the word “reset” is misleading. My understanding was disconnecting the battery for 15 min after cleaning MAF sensor for example clears the fueling parameters and provides the ECU the opportunity to relearn fueling to driving behavior.

I suppose that would depend on the car, some need to relearn fuel trims after battery disconnect. And I could see how in some cases that might do a blanket reset of the ecu.
But I think those cases are extremely rare.

There should not be any need to reset anything after doing anything to the truck, cleaning MAF, replacing plugs, etc.
the reason for cleaning the MAF is to get the dirt off of the heating element so it gets a better reading of incoming air, it will see that right away.

 

[digitalmarker]

My understanding was disconnecting the battery for 15 min after cleaning MAF sensor for example clears the fueling parameters and provides the ECU the opportunity to relearn fueling to driving behavior.

That is my understanding too, and that's why I did it. This ECU is 20+ year old technology, so a reboot after changing the air intake parameters was likely necessary.

This is not true. Otherwise anyone with a check engine light could unhook the battery, reset the ecu and sell it with a fault code.
You must use a OBD II scanner to reset the ecu.

There was no way to reset it with the cheapo OBDII reader I have unless there is a code. Then there is an option to "clear codes". Otherwise it just says "no codes found".

Also, if the vehicle is throwing a CEL, wouldn't it just come back on since the faulty part would still be faulty? That has been my experience.

 

[digitalmarker]

Just curious, what are you guys getting on a full tank? I have 33" tires on 18" Tundra wheels, and Slee sliders. No other armor or weight.

Before this whole thing started, I was consistently getting 240-245 on approx 21 gallons. So 11.5 on average, which seems terrible.

 

[abuck99]

Im averaging about 12mpg on 33” tires (lifted, armored, bumpered, and drawered) but most of my driving lately has been short trips city driving ( 15 miles or less each trip).