Help! Fuel mileage issue...

[96WillowGrnXLT]

Greetings, all! I've got a situation with my '96 XLT that has me baffled right now. A little background - it's got the 4.0 OHV engine with the automatic and 4x4 drivetrain. I have 59,900 miles on it and it has been getting pretty poor fuel mileage for a while, but it's been real bad on this last tank. I had been averaging about 13.8 mpg city and highway mix for the better part of the last year running 89 octane in it. However, the most recent tank, I switched to Amoco Gold 93 octane with a little Marvel Mystery Oil and my mileage has just been crappy! Right now, I'm at a half tank on it with just a little over 110 miles whereas before, I was running about 150-160 at the same level. Alarming to be sure, so I pulled out my scan tool and ran a live datastream to see what I was running for measurements...
First off, there are no DTCs now in the PCM. It used to have a P0420 - EGR insufficient flow, but I suspect that was from some carbon buildup that burned off as I've been driving it. (It really hadn't been driven too much until this last year) I found that my LTFTs are running really high. I'm running anywhere from +8 to +15 on both banks, yet the STFTs are reading in the -2 to +2 range. I found that the LTFTs are the highest when I have it at idle or if I let off the accelerator while driving, which would lead me to think it has a vacuum leak. I don't hear a leak and I sprayed the engine down with carb cleaner to be sure and did not note a change in engine speed. Also, the engine has a very slight stumble to it at idle under load or no load. It's not bad at all, but it doesn't run smooth like it should idle. It's had this for a while and I thought maybe plugs were a cause since they were never changed since new. I changed those out this last summer and it didn't make a difference in the idle. It currently has the factory plug wires and fuel filter in it and I plan on swapping those out this week now that the weather has warmed up, but I'm not terribly optimistic that either of those will solve the fuel situation nor the idle.
I've also noticed that the performance doesn't seem like it's quite there. It's never been a road rocket, but I would think it would have more performance than what it has now. It kinda feels like something's holding it back, or like it's kinda lagging. I wondered if maybe the MAF sensor is causing some issues because I've read about how the MAF on these can cause issues like this. The datastream showed a flow rate on the MAF of .7 lbs/min at idle and about 4.6 under a moderate load and this just seems low to me...
I'm planning on doing the maintenance I listed above and if that doesn't do anything, I'm thinking of running both a vacuum check on it and a fuel pressure check to see if maybe something is amiss in either of those. Right now I'm flat out at a loss as to what could be causing this and hope you guys can offer some insight as to what else to look for. This truck has been in our family since it was new and I can safely say that it hasn't really been maintained all that well since at least 2003 when I moved to Alaska for a few years. Now that I inherited it, I'm giving it the attention it deserves and want a happy, healthy truck again. Can you guys offer any help or clues as to what could be causing this?

 

[patrick112390]

well i can start off by saying run regular ass 87... running a higher octane isnt helping...unless you have a tune....

 

[1998Explodasport]

Not sure where you live but cold weather will also kill your gas milage.

 

[robertoa1a]

Whatever the case the fuel econ should be in the 18mpg to 21 range. Cold air is good for econ because it is more dense than fuel.

The problem lies, there and in when, the cold air is causing the PCM to think it is in cold start mode (like when carbs need a choke).

Back in the day (carbs) you used a choke to get adequate fuel/air mixture during starting. Now the computer decides that.

After the engine warms up the choke is no longer needed. Perhaps that isn't happening on yours due to poor OBD readings somewhere. But otherwise driving careful in slick road conditions isn't the same as 75deg driving smoothly down the highway.

Are you making a lot of short five minute trips?

 

[96WillowGrnXLT]

I planned on running the 93 octane for only this tank in the hopes that with the MMO it would help clean up whatever carbon might still be in the engine and maybe smooth out the idle a little. I'll try running Sinclair 87 in it next tank. I've always run 89 octane in everything just out of habit - Not sure exactly why...
It has been very cold here and I figured that's probably a good part as well. I'm in SE MN and it's been in the single digits below 0 F here for the better part of the last week. Only just today did the temperature climb above 10 F, though it's predicted to be in the mid 20s to lower 30s here for the next week at least. Even in optimal weather, though, I'm still running too rich and experiencing the same LTFTs, so something is amiss somewhere. I hear of this same setup getting mid to upper teens for mileage and I haven't seen that on this vehicle in at least 6 years. And that performance issue is another thing - it just feels like it's bogging down and not performing quite like it should. I don't remember it being this flat on acceleration before...
I should probably add that since 2002, this vehicle has primarily been driven in town on trips of less than 5 miles. For specifics, I can definitely say that in October of 2002, it had 48,200 miles on it and when I got it in March of 2009, it had just over 56,000 on it. I can practically guarantee that nearly all of those miles in between were short trips...

 

[96WillowGrnXLT]

Whatever the case the fuel econ should be in the 18mpg to 21 range. Cold air is good for econ because it is more dense than fuel.

The problem lies, there and in when, the cold air is causing the PCM to think it is in cold start mode (like when carbs need a choke).

Back in the day (carbs) you used a choke to get adequate fuel/air mixture during starting. Now the computer decides that.

After the engine warms up the choke is no longer needed. Perhaps that isn't happening on yours due to poor OBD readings somewhere. But otherwise driving careful in slick road conditions isn't the same as 75deg driving smoothly down the highway.

Are you making a lot of short five minute trips?

Most of the driving I do with it is not what I call long trips, but definitely not in-town hops. Last month, I took it to Eau Claire, WI and Owatonna, MN and I averaged 16.6 mpg. That was pure highway miles. Otherwise, I normally average 13.5 to 13.8 on a combination of city and highway as it runs now.
I definitely think the recent cold weather we've had affected my fuel economy. I don't, however, think it would affect it as much as this. I figure I'm running probably around 10-11 mpg right now, and this tank has been mostly highway driving. I wondered if maybe an ECT sensor may be causing an issue. The datastream is reading ECT to be between 185 and 190F, so I wouldn't think so. But I suppose anything is possible...

 

[Fuzion]

im having the same issues, I have been getting around 200-210 miles to the tank. That full to the top. I do have code that reads p0420, im thinking about changing both the sensor and the cat, maybe that will solve the mystery questions.......what do you guys think?

 

[budwich]

perhaps if you had an egr problem that never had "attention" to it, then your "firing" has probably been a "bit on the rich side".... in terms of maybe a bit of "pre ignition".... the end result might be a slightly plugged cat that is resulting in poor flow thru and loss of power / umph. But I would do more "regular maintenance" (sensor cleans, checks) before going towards the "back". Winter time gas mixtures aren't the best for trouble shooting loss of mileage cause they certainly aren't designed for that (improved mileage).

 

[Millertime1987]

Perhaps its your distributor? When you replace the spark plug wires make sure that isn't an issue. Otherwise a fuel filter would be the next thing to replace. As far as "wintermix" fuel being a cause, I have never had it. Maybe a mile or 2 difference but I chaulk that up to its colder so it takes longer for the truck to warm up so its using more fuel.

 

[jremington59]

My last 97 with a 4.0 got about twenty in the summer and about 14 in the winter when it was really cold.

 

[patrick112390]

Whatever the case the fuel econ should be in the 18mpg to 21 range. Cold air is good for econ because it is more dense than fuel.

actually quite the opposite. more dense air=more fuel to avoid lean scenario. hotter air=less fuel, put it this way {air=*} {fuel=+} (dense air, more molecules)*****=+++++ (less dense) **=++ requires less fuel for perfect ratio if you want to get technical, its the stoichiometric ratio 14.7/1 parts air 1 part fuel. your fuel economy will suffer in winter but because the air is dense, the computer adds more fuel = more power!!!

 

[96WillowGrnXLT]

Actually, when it was summer, that's when I pulled the average of about 13.5-13.8 with it and that seemed to be the norm except for this tank and the tank that I had when I went to WI and Owatonna. On that tank, the temperature was down in the 20s, so my mileage of 16.6 could have been higher I suppose if the weather was warmer. But I don't think it would have been much more - maybe one more mpg...
I started to wonder if maybe the cat is plugged like budwich said. It wouldn't surprise me I guess - I'd be willing to bet that at least half of the mileage on the odometer has been in-town short trip driving, and that's gotta be a good chunk of unburned fuel reaching the cat and the cat not getting hot enough to burn it off. But I'm not experiencing any smells and the cat isn't getting any hotter than it normally would, so I don't think that's the case. I guess the best bet would be to run a vacuum check or better yet, a backpressure check on the exhaust.
One quick note - I pulled the air filter box apart and shot some MAF cleaner into the filament wires on the MAF, but I haven't gotten the chance to run it yet since I did that. The filaments didn't look too dirty, but I figure it was worth a shot anyhoo...
I may just let it ride for now until spring comes again in a few months and I can get a better idea of what the engine is doing on warmer intake air and optimal fuel. But I know for sure that even when the weather was nice and warm last summer, fuel mileage still wasn't near what it ideally should be getting. I've just recently begin to notice the lack of performance issue...

 

[2000StreetRod]

Airflow & ECT

Congratulations on owning a low mileage Explorer!

Do you have 4 wheel drive and engage it frequently? Running synthetic axle lubrication improves fuel economy - especially in winter.

Your scanner will greatly assist you in improving your fuel economy. Your MAF airflow of 0.7 lbs per minute at idle is comparable to my SOHC. My flow at 46 mph is about 4.6 lbs per minute but I have a modified intake system.

Does the ECT increase rapidly after cold start? The faster your engine warms the better your fuel economy. Also, what temperature thermostat is installed? After warmup my ECT fluctuates between 192 and 198 degrees. A 195 degree thermostat should help your fuel economy.

Does your PCM enter closed loop within a few minutes after cold start and stay there except during moderate or greater acceleration? Anytime the PCM is in open loop the fuel economy suffers.

What is the voltage swing on your pre-catalytic converter O2 sensors?

Your LTFTs should be 0%. The PCM increases the LTFT to keep the STFT within range. I agree that you probably have a vacuum leak that is causing a lean condition on both banks. If you're confident it's not a hose then it's probably your intake manifold gaskets. Leaking manifold gaskets worsen in cold weather since they shrink. It's a more frequent occurring problem with the SOHC than the OHV.

 

[96WillowGrnXLT]

Congratulations on owning a low mileage Explorer!

Do you have 4 wheel drive and engage it frequently? Running synthetic axle lubrication improves fuel economy - especially in winter.

Your scanner will greatly assist you in improving your fuel economy. Your MAF airflow of 0.7 lbs per minute at idle is comparable to my SOHC. My flow at 46 mph is about 4.6 lbs per minute but I have a modified intake system.

Does the ECT increase rapidly after cold start? The faster your engine warms the better your fuel economy. Also, what temperature thermostat is installed? After warmup my ECT fluctuates between 192 and 198 degrees. A 195 degree thermostat should help your fuel economy.

Does your PCM enter closed loop within a few minutes after cold start and stay there except during moderate or greater acceleration? Anytime the PCM is in open loop the fuel economy suffers.

What is the voltage swing on your pre-catalytic converter O2 sensors?

Your LTFTs should be 0%. The PCM increases the LTFT to keep the STFT within range. I agree that you probably have a vacuum leak that is causing a lean condition on both banks. If you're confident it's not a hose then it's probably your intake manifold gaskets. Leaking manifold gaskets worsen in cold weather since they shrink. It's a more frequent occurring problem with the SOHC than the OHV.

Thanks on the congrats! My mom owned it since new in March of 1996 and I got it when she passed away last March, though I had already pretty much been the owner of it by then. She rarely went anywhere and when she did, it was always in town here. I found it pretty sad that it had only 8000 miles put on it in nearly 7 years...

4x4 is hardly ever engaged. Even when it is, it's usually only for the slick intersections in winter around here when I need traction, then it's switched off when I have firm traction. But I plan on swapping to Amsoil fluids in the trans, transfer and both axles. Just gotta free up the funds a bit first...

I replaced the t-stat in it in 08 with what I thought was a 195, but perhaps it wasn't. The factory one got stuck open at about 55k and it was barely building any temp. Maybe I'll replace it again and confirm I'm getting a 195 stat this time. It seems like the one I have now may be a 185. As for building temp, that seems to be in line with what it should be, so no problems there either...

PCM is in closed loop once it's warmed up and doesn't go open as far as I've seen. Everything seems like it's working as it should be, but those LTFTs are the only thing that's thrown a red flag. Well, that and the fuel mileage I've been getting. I checked the O2 voltage as well and the pre-cat is reading normally on both banks with normal fluctuation, with the post-cat reading correctly varying from .4 to .6 v.

I was also thinking perhaps I have an intake gasket that is leaking. If it is, it must be pretty small as I sprayed down the intake area and all the vacuum lines pretty liberally with carb cleaner and it didn't change how the engine ran. But maybe I'll try running some propane in the intake area and see what happens. It sure feels like it may have a small vacuum leak when the engine idles, though...

Going back to the plugged cat idea - If this were the case, wouldn't the engine be running a bit hotter as well? I had a Nissan Altima one time that had a slightly plugged cat and the engine ran a good 10 degrees warmer than spec and the fan ran constantly. Also, the Altima had the same issue with LTFTs and STFTs. I don't experience any of the former on the Explorer, but the lack of performance and high LTFTs lend some credit to the idea.

I started to rethink on the MAF as I'm not experiencing and sort of hesitation in acceleration or anything that would indicate a dirty MAF in that sense. I still haven't driven it after I shot that cleaner on it and I'll get the chance to this morning. But I really don't think that will do anything...

Thanks for all the help so far, guys! It's great to have a forum like this to bounce ideas around on and gain from your experiences.

 

[2000StreetRod]

Posssible leak source

Sorry for your loss! Now that I'm retired I'll probably only put 3000 miles a year on my Sport.

The following thread discusses a possible source of a vacuum leak. I think it's worth checking.
4.0L Upper EGR Tube O-Ring Is Constantly Leaking

Also, I believe your model has a tube connected to the underside of the throttle body that sometimes leaks. See '96 Explorer Vacuum Leak

With the low mileage you have it seems unlikely the cats would be plugged. One thing I haven't tried yet but plan to is to disconnect one EGR hose to the DPFE and briefly connect a low pressure gauge (my vacuum gauge will read up to 8 psi) to read the back pressure on bank 2. It should be less than 4 psi.

Mercon V is semi-synthetic so I didn't bother switching to full synthetic ATF. Full synthetic in the engine is definitely going to help performance and fuel economy.

Have you checked your tire pressure? Cold weather results in lower tire pressure.

Changing the fuel filter is an excellent idea! See Why change fuel filter?. I never put anything in my tank besides fuel and Techron fuel injector cleaner. Other cleaners can release particles small enough to pass thru the filter and collect on the valves or clog the fuel injectors. You may want to burn the tank of fuel with the Marvel Mystery Oil before replacing your filter. Add the fuel injector cleaner (two bottles) after replacing your filter to clean things up.

Then I suggest you perform the Fuel pressure test procedure. Your model has a return fuel pressure system with a fuel pressure regulator mounted on the fuel rail. You could have a weak fuel pump with all the infrequent driving over the past years.

 

[96WillowGrnXLT]

Well, a little more info to go on now. I got to drive it this morning and as I suspected, that shot of cleaner on the MAF did absolutely nothing. I'm still pulling about +11 at idle on both LTFTs. However, I noted a couple of things today that I think helped narrow down what is going on...

First off, right on the cold start it did something weird. It started up like it was already warm. Right after I started it, the idle dropped down to 800 for about 10 seconds, then it was like it said, "Oh yeah, I'm supposed to be at fast idle!". Shot up to where it should be on a cold start and stayed that way until it got warmed up, like it should be. It didn't run bad or anything, but that was interesting. It's never done that before. Though it is a bit warmer out this morning, I don't think it's enough to make it go right to idle after startup!

Also, I think the idea of an intake gasket leak is picking up steam. I drove it a good while today and popped the hood on it when I got back to spray things down again with carb cleaner. Well, the carb cleaner didn't do anything just like last time. But I heard what sounds like a muffled vacuum leak and it sounded like it was coming from the upper intake area. I sprayed more carb cleaner all around the upper intake but it didn't affect the engine speed at all. Part of me was wondering if what I'm hearing is just the air moving through the upper intake, being as that intake is just plastic. I tried to spray through the intake runners down to the lower manifold gasket underneath, but it didn't change anything. The sound I hear doesn't sound like it's near the throttle body, so I don't think it's a hose leaking. I may try it again on a cold start this afternoon and see if I can find it better...

 

[ThePhoenix]

the intake gasket cleared up my stumble...but didnt improve my mileage at all

i need a scanner

 

[2000StreetRod]

IAC valve

I looked back thru this thread and saw no mention of cleaning or replacing the IAC valve. Were you monitoring the ECT, rpm and commanded idle speed (can't remember exact PID) when your cold start went to low rpm. It sounds like either the IAC valve did not respond to the commanded idle speed or the PCM thought the ECT was above 130 degrees and commanded a low idle speed.

I suggest that you clean the IAC valve and then monitor your next cold start to see if the IAC valve is following the PCM commands.

 

[96WillowGrnXLT]

Oh yeah, forgot to mention anything about that. I cleaned the IAC back in '08 for some reason or another that I can't remember anymore. But I do remember cleaning it...and the horrific whistle I had for about three hours after that whenever it idled. Guess I didn't blow enough air through it to dry it out!

 

[2000StreetRod]

IAC valve

Does your IAC valve look like the one in the photo below?

On my SOHC the IAC valve has a hose that goes to the main intake tube. That way it uses filtered and metered (measured by the MAF sensor) air. If yours is like the one in the photo then the IAC valve is using unmetered air and there is a small filter (under the cap) to clean the air. There is also a thin gasket between the IAC valve base and the intake manifold. Some people forget to install the gasket when cleaning the IAC valve. The gasket is often a source of intake leak. If the small filter is clogged the IAC valve can't function properly. I suggest that you examine your IAC valve thoroughly and check its function using your scanner. The IAC valve has been the source of other members' poor fuel economy.

Does your engine idle smoothly when warmed up?