Slow loss of power and MPG, no CEL's or drivability issues

[joecrna]

This post is intended as a discussion and learning exercise regarding the gradual loss of power and worsening of fuel economy over time. I have already attempted some fixes but would like your opinions on what to try. Some of my maintenance actually helped, most did not. Currently the truck has about 210,000 miles.

We noticed a decrease in power while pulling our trailer the last time out. We only take it on a couple trips a year and this time it had been about a year. The truck is also used for moderate off-roading and as a nearly daily driver. US drivers now average over 15,000 miles per year, so I figure anything over this counts as daily. We are going on a long (8000 mile) trip towing a fairly heavy (5500 lb) trailer in a few months. So, in addition to trying to find my lost power/MPG, I need to do all of the usual maintenance.

How do I know the MPG and power have decreased? My wife and I had discused how the truck seemed to be slower, especially merging on to the freeway, over the months leading up to our last camping trip. We had also noticed a 12% decrease in MPG over the last year or so. Then on the trip, we left town over the same 4400 foot pass as usual. This time we were still able to maintain speed (65 mph) but the pedal was flat on the floor most of the time. Previously this had only been necessary on a few of the steeper sections climbing out of the LA basin. Throughout the trip over known roads it seemed that we were having a harder time maintaining speed or acclerating. The final capper was the hill to our temporary storage location. A very steep, narrow single lane dirt drive. To negotiate the steepest 50 feet or so near the top going a whopping 5mph the truck was flat on the floor and still loosing speed. I thought I might have to put it in low range. Again, this had never been this difficult.

As for the MPG, I record very detailed maintenance records including very drop of gas that goes in each of my vehicles. Comparing MPG over the last 10,000 miles to MPG over the same distance 100,000 miles ago, the truck was down 12%. It dropped from about 17.5 to about 15.5 mpg. Calculating mpg at each fill-up showed no precipitus drop at any time. Just a slow steady decline over that 100,000 miles or so. Strangly enough during the camping trip MPG was up from my usual 9.5 to 11.

As a final confirmation of a loss in power over this same time period, I compared performance now to some runs I recorded a few years ago. I used the same location (who knows about enviromental conditions but hey its always sunny and mild here in SoCal) and the same GPS based Gmeter equipement to record 0-60 and quarter mile times. Both were significantly worse.

What changes occurred over this time? None to the Explorer other than usual maintenance. The camper has gained 4 inches in hieght and 500 lbs in weight. However, the GCVW was actually lower on that last camping trip than usual.

So what gives? Why has my Explorers power dropped? Why is it getting worse gas mileage? Most importantly, how can I get them back? Remember, I have no drivabilty issues and no CEL's. The truck has had good maintenance over its 210k mile life, doesn't smoke and is reliable.

I'll let you in on the my attempts to fix this, the results and hopefully we can all learn some new ideas about trouble shooting along the way.

 

[drbenz]

The two things that come to mind are a mass air sensor drifting out of spec and a catalyst partially stopped up.

 

[joecrna]

OK, lookers but nobody wants to play yet. Any suggestions would be appreciated.

What I did first was fix the leaking rear axle seal. Shouldn't be related but it was leaking anyhow. Pulled the axles out, looked at the axle bearings (very good condition) and replaced both seals. While in the rear end I looked at the gears, bearings etc. All looked great except for a little wear in the carrier bearings. Actually very good for 200k miles and all I put the little truck through. Replaced the rear differential fluid with Ford synthetic 75-140 and LSD addative. Then topped it off with Royal Purple 75-140, ran out of the Ford stuff. I now have the famous 8.8 rear diff whine.

While I was doing this I changed the fuel filter. The truck seems to be down on power throughout the entire rpm range and not starving for full at the top rpm range. I did not expect this to fix my problem but according to my maintenance schedule it was time. So I pulled out the old one and put in a new Ford filter. I then drove it for about 1000 miles and repeated the performance tests.

To no ones surprise there was no change in mpg or power.

Any new ideas?

Edit: Sorry Drbenz, I missed your reply. If the cats are partially clogged, they are not effecting exhaust emissions. Just did a typicall California dynometer smog test. It had not changed from 2 years ago, within the margins of error for the test. I suppose I could flow test them but I have no idea what the proper flow would be to begin with. The MAF sensor is an idea though. Out of spec but still not enough to set a CEL. This has been a new concept for me while chasing this problem. What would you suggest I do, clean it. Is there a test to make sure it is in spec or do I just change it out and hope this is it?

 

[2000StreetRod]

ethanol fuel?

Does your current fuel contain ethanol? How does that compare to the fuel you were using 100,000 miles ago?

Does your engine coolant thermostat work properly? If the engine does not warm up to the correct temperature the air/fuel ratio will stay rich. Also, if the engine coolant temperature sensor is incorrect the PCM may think the engine is cold when it is warm resulting in a rich mixture.

Have you replaced your air filter? The MAF sensor and the O2 sensors should allow the PCM to adjust the air/fuel ratio for a clogged air filter but there is still resistance to flow resulting in less efficiency. An Amsoil drop in air filter is less restrictive while providing superior filtering.

Have you checked the cylinder compression? As the valves and piston rings wear the compression will drop reducing power.

Have you replaced the pre-cat O2 sensors? Their sensitivity decreases with age. The PCM may not be adjusting the fuel trims correctly due to poor data from the O2 sensors.

 

[joecrna]

Dale, I knew you would have many useful insights. So I will roll this process foreward a bit more. Yes our fuel does contain Ethanol (up to 10%) but it is the same fuel and it comes from the same small collection of stations. Our formula in southern California also does not change with the season. That 100,000miles only took a little over 3 years to accumulate.

Your suggestion of the coolant sensor is very insightful as it plays a much larger role than many people realize. It appears to function normally, reading near ambient immediatly after start-up and rising to what seems to be normal operating temperature. I have not removed the sensor and tried to check its exact calibration. If the sensor was reading cold and sustaining the rich condition necessary for initial start-up, wouldn't the smog check catch this. High hydrocarbons, low Nitrates of O2 and CO.

Cylinder compression is consistant cylinder to cylinder but I have no idea what it should read as a baseline. If the compression in all cylinders was reduced enough from wear to reduce power to this extent and MPG by 12% I would expect to see many other problems. Oil consumption or no oil use/oil level rise due to blow-by, fouled spark plugs and the like. Doesn't that seem true to you?

Your next suggestion of pre-cat O2 sensors being inaccurate but not enough to trigger a CEL was a new thought for me. I had never considered that there must be a set of tolerances for them where they could be off but not out of spec enough to trigger a code. Naturally this must be possible or due to wear and contamination they would trigger a CEL almost as soon as the vehicle drove off the dealer lot. This tolerance range won't be much or it would effect emissions and run afoul of the EPA. If many sensors were out but not quite enough to trigger the CEL perhaps the cumulative effect could produce my issues. To that end, I have 4 new O2 sensors in boxes setting in my living room. I had decided to give this a shot before understanding that on the Explorer the post Cat sensors mearly monitor catalitic converter function. Oh well, I got a good price on them.

As for the air filter, it was time to change it. I changed the oil, oil filter and air filter. The air filter had 10k miles on it and appeared a little dirty but not bad.

Again, no surprise. 1000 miles with same mpg and a repeat of the performance tests with no change.

 

[joecrna]

It was time for the transfer case fluid and front differential fluid to be changed. That long towing trip is getting closer and I need everything to be ready. Front diff fluid was replaced with Royal Purple synthetic 75-90. The transfer case calls for Mercon ATF fluid and Mercon V says "not for use in transfer cases". What to do? Ford to the rescue with "transfer case" fluid. Probably just some relabelled Mercon with a significant price gouge. I don't know what would happen if I used Mercon V. It does have slightly different frictional properties than Mercon. Perhaps the electromagnetic clutch inside the transfer case would slip a little or maybe nothing. Who knows.

I drove the Explorer another 1000 miles and again to no ones surprise, no real change. Still within the margin of error for my testing techniques, road conditions and enviromental factors.

 

[2000StreetRod]

tires & ECT sensor

I notice in your signature that your vehicle has 31x10.5 tires. Did you have the same diameter tires 100,000 miles ago?

The most convenient and accurate way to check your engine coolant temperature (ECT) sensor is with a scanner. That way you know what the PCM is reading. If your cooling system is adequate and your thermostat is accurate then the ECT reading should only vary one or two degrees above/below the thermostat specified temperature when cruising.

 

[joecrna]

Same tires. Well, not really. They are the same size, manufacturer and model. When I converted to them about 150,000 miles ago they did make a big difference in performance, not so much in MPG. That was one of the reasons I have detailed performance data to compare my current perforamance to.

My previous statements with regards to coolant temperature were based on a scanner. What I don't know is what the setting is for our stock thermostat. It does seem to be a realistic temperature for a modern vehicle. I can't read the temp while cruising. My OBD interface is part of a large diagnostics system down at the shop and not readly portable. Think the receiving end of a telemetry system for an Indy car. I have avoided the computer side of modern vehicles and one of the guys handles this for me. I suppose I should just move into the modern era and get a scanner for the house. Then I can start my learning curve.

 

[2000StreetRod]

O2 sensor desensitivity

An O2 sensor's sensitivity does not decrease uniformly with age. A new sensor may output:
0.95 volts at 0.98 lambda
0.7 volts at 1.00 lambda
0.08 volts at 1.02 lambda

An old sensor may output:
0.83 volts at 0.98 lambda
0.65 volts at 1.00 lambda
0.09 volts at 1.02 lambda

The PCM adjusts the fuel trims to cycle at a set voltage above and below that corresponding to 1.00 lambda. With an old O2 sensor the fuel trims are shifted toward the rich end to achieve the expected voltage fluctuation. Also, for any age O2 sensor the voltage vs lambda curve is steeper for lean than for rich. That means the fuel trims are not symmetrical about lambda = 1.00. There is more deviation to the rich side to get the same voltage change as toward the lean side. Running rich will increase the CO and HC emissions. However, the catalytic converter reduces both and the measurements would probably be within smog inspection tolerances since the engine is not loaded.

 

[1996BLKBauer]

I don't think this will help with any problem you have now, but you should add a small bottle of friction modifier to the rear diff, it should get rid of the whine.

 

[joecrna]

On Saturday I decide to continue with my servicing of the Explorer in preperation for the trip. I changed the transmission fluid (Mercon V), filter in the transmission pan, and the remote filter I had intalled. The intermittent slight hesitation in engaging drive went away. I am sure this will prove temporary.

It was also time to change the spark plugs and wires. No drivability issues, no misses, surging under load, no ticking from wire leakage, nothing. It was just time. This is my normal procedure for all my street driven vehicles. I used Motorcraft parts.

What the heck! A hugh seat of the pants improvement in performance. How could this be. The plugs showed some "dirt" but nothing abnormal. The gaps were still at spec. No plug to plug differences. The wires showed no signs of wear. The dielectric still seemed to fill the boots. No burns, kinks, tears or abrasions. Other than a few grease smudges they looked new.

Like I said, I do this with all my street cars and never before have I noticed even the slightest improvement. It just seems prudent to change them every so often and that trip is coming up. The trip is scheduled very tight and I only have one stop about the half way point were I can do any serious work on the truck. My uncle has a shop in Ohio were we used to work on heavy earth moving equipement. Otherwise I can't afford any breakdowns.

The percieved improvement in performance is truely amazing. At least a half second, maybe even a full second quicker. Power at speed on the freeway is most noticable but it is there across the entire RPM band.

This may be just my wishful thinking, as I see no reason why this would be true. I am currently accumulating the 1000 miles and will then retest with equipement to find the truth. Still to come are some ideas I got here in this post. The MAF sensor will get some attention and I still have those O2 sensors. Might as well change them out.

Any other suggestions would be appreciated.

 

[joecrna]

Friction modifier is in the rear diff as it is a limited slip.

Your O2 sensor info is spot on. I just didn't consider this when I started this little project. I am use to competition vehicles and everything gets changed between each event. I wonder about our street race and road course cars. Our smog test in California is done on a dynometer with the engine under load. It is a very small load though and the cats may still be able to clean the exhaust up enough to keep it within the tolerances required to pass. I am a bit less convinced that it would be within the tolerance of the testing equipement. Mine barely changed from 2 years ago. I will still change the O2 sensors and report back.

 

[drbenz]

One more thought before you give up on the cheap ideas, on my 97 it had a similar thing come up now that I think of it: low power and lousey fuel mileage, I (eventually) tried just disconnecting the battery for ten minutes and when the computer woke up it was like I had a new truck. Don't know how often that kind of thing will work though.

 

[Runnin'OnEmpty]

That's a good idea to clean the MAF sensor. The
heated sensor wires tend to build up a hard film over time
that gives a false airflow reading.

I use a spray can of O2 sensor-safe MAF cleaner to
clean mine. It's inexpensive and truly a tune-up in
a can.....

 

[drbenz]

Joe, the s.w.a.g. test for a restricted converter is to check manifold vac at idle and compare it to the reading at 2k or so. It should be as high or higher at the faster speed. The real test is a backpressure test usually done at the oxygen sensor hole.

 

[joecrna]

Still accumulating mileage to evaluate my surprise results from the plug and wire change. Have to disregard the first tank of gas after the switch as an outlier. I was so stunned by the perceived improvement that I must have run it a bit hard....12.5mpg. After 1000 miles, I'll compute mpg and run new performances numbers. Then compare those to my base-line from about 100,000 miles ago.

After that I will run the vaccum test and disconnect the battery to clean the MAF sensor. Then repeat mileage and performance calculations after another 1000 miles and then move on to the O2 sensors.

Just replaced the wideshield wipers and a head lamp. After all, this is part of routine maintenance in preperation for the trip.

 

[joecrna]

Logged about 1000 miles this week. Unfortunatly, I was running late during one of my longer commutes. This led to me driving a bit faster than usual. I also had the need to do a bit of light (muddy field) off road driving. My Northern commute only averaged 15.8 mpg over 350 miles. The return commutte averaged 17.9 mpg. This is even better than my baseline mileage but I tend to get better mileage going South. Needless to say, this was not a typical week.

I will accumulate about another 1000 miles this week and recalculate mileage. I have another vehicle to get out of the garage this week anyhow. Next weekend I should have time to do some more repairs/maintenance on the Explorer.

 

[Cranmaro99]

Thats amazing that all of this came with a plug and wire change out of all the other things!

How long were those plugs/wires on? Re-installed with stock motor craft parts?

 

[joecrna]

I change plugs and wires every 100k miles. They were indeed replaced with stock motor craft parts. This coming week we will find out if, and by how much, it really did help MPG and performance. It does seem promising. The really shocking part is that there was no indication that there was a problem with any of the wires or plugs. Even after removal they looked good.

 

[Lanky]

This may be a long shot, but could there be enough slack in the timing chain to cause spark detonation to be at the wrong time?