2007 Mercury Mountaineer 4.0L

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Oct 16, 2003
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So I inherited 2007 Mercury Mountaineer with 190K miles and a replaced 4.0L 6-cyl engine. Know nothing about its history, nor why the engine was swapped nor the miles on the "new" engine.
I get it the V8 4.6L is a lot more reliable, more power with about the same MPG. But I got to deal with this thing.

I keep reading that 4.0L had issues, wonder what to do, keep driving it until the wheels fall off or the next significant repair. It had more power than I expected, it's not a total slug.

Also have a 2008 Chevy Tahoe, with the 5.3L that I tuned to run only on premium Octane. A decent amount of power but it's a beater and has 262K miles on it so anything can fail anytime. No major repairs aside from fuel pump. Both bumpers need to be replaced, it feels like a moving parts car.

Also inherited a 1998 Subaru Legacy wagon (not Outback) with the "reliable" 2.2L engine that had the head gasket done twice and is on the second transmission. 200K miles and runs surprisingly well.

basically I now have 3 semi-junk cars.
 
THe 4.0l SOHC engine in the Mercury has an oil pumped tensioned on each bank. It is recommended that these two tensioners be replaced at 100K. Be aware that they are 2 different parts. Very similar they are but indeed different part numbers. One is longer by a bit.

The left one is easy to reach and the right one is a straightforward reach through the FR wheel well after removing the wheel.

What is recommended by mechanics who know is to turn the key to run/on and fully depress the gas pedal then crank the engine 5-8 seconds. Then, when I start the engine, there's no moment of clatter. The priming upon shutdown lasts a couple of hours but not overnight.

Earlier versions of the tensioners had a weak spring inside. The tensioners were redesigned/upgraded by 2004.

Another thing you might want to do is replace the 2 upstream O2 sensors. They're cheap and are the only sensors which wear out.

See if your MM has a plastic thermostat housing. They will split in time and there are metal replacements available. Mine is plastic but the previous owner had it replaced so I have some time.

What layout (2WD, 4WD, manual, auto) do you have? I replaced my trannie pan with a Dorman with a drain plug. The unit takes MerconV.
Band adjustment is easy when servicing the transmission filter.

edit: Just read Lubener's post. I'll gladly come down and take the MM off your hands.
 
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I have the worst version of this engine and it's fine. Pretty sure mine got updated parts earlier in its life.

I use Mobil 1 5W40 European Car Formula. Not saying that will do anything for it, but seemed like a good choice for 3 timing chains.
 
The 5R55 transmission is by far more of a problem spot than the SOHC Cologne engine. Replace the thermostat housing with a metal unit, change the transmission fluid, and drive it.

My ex's '01 Sport Trac had timing cassette rattle from 150k to well over 250k, never caused an issue. The trans gave up at 200k, and that was impressive.
 
Thanks for the info, it's pretty useful. Here it is.
Question. I want to remove the rear seats, all of them, the rear and the middle row. I did that with my Tahoe and it opened so much space. Made the car so much more useful. I never have anybody ride with me in the rear so the seats take up extra space and add weight.

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The transmission is the bigger issue in the 4.0 versions. Particularly that late in the engines run, they were decent enough. Previous history for me led me to the 4.6 and the 6r80 transmission on that Gen of explorer/mountaineer.
 
Your looking at the wrong part of the glass. Your glass is half full. You have 3 fully functional, paid for vehicles that all sound like they run great.

has 262K miles on it so anything can fail anytime
Anything can fail anytime, on any vehicle. Of the 3 vehicles in my footer, the newest one has had the most "failures" in the last 2 years. Those were pretty small, and covered under warranty, but the other two had only one - so there winning in my scorecard.

Enjoy the ride.
 
The transmission is the bigger issue in the 4.0 versions. Particularly that late in the engines run, they were decent enough. Previous history for me led me to the 4.6 and the 6r80 transmission on that Gen of explorer/mountaineer.
Yeah V8 when available is best in any Explorer, but somehow I have never replaced or even had any real trouble from the transmissions in my V6 Explorers. In fact I am a huge fan of that transmission family and hate the M5OD manual. 🤣
 
Yeah V8 when available is best in any Explorer, but somehow I have never replaced or even had any real trouble from the transmissions in my V6 Explorers. In fact I am a huge fan of that transmission family and hate the M5OD manual. 🤣
Three different 4.0 explorers in two different generations, one personal , two work. All of them had transmission problems, the work ones with under 125,000 miles and no towing... thus my comment... guess I was unlucky!
 
Three different 4.0 explorers in two different generations, one personal , two work. All of them had transmission problems, the work ones with under 125,000 miles and no towing... thus my comment... guess I was unlucky!
The craziest thing with my experience is the A4LDs have been the best. I don't care what anyone says, that is a good transmission. They shift great. I think early ones that didn't have enough cooling had the most issues.

My 2002 Ranger has 256k on its original 5R44E, it is getting clunky, but I really can't complain at this mileage. It has had fluid changes and one filter change at 150k that it didn't even need. It has been 60k since the last fluid change.

I do think the transmission in my 1997, a 5R55E, was rebuilt or replaced because a date and initials are marked on it in yellow paint marker. It shifts good accelerating, but does have a clunk when slowing down. Not going to do anything about it until it does something annoying, then I will drop it off at the transmission shop that can rebuild a light duty Ford transmission blindfolded.
 
The 5-speed auto transmissions are pretty weak on those.

We had a 2007 Explorer 4.0 in the family that experienced transmission failure at pretty low mileage, and was never towed with or used heavily.
Just found out the transmission was replaced in the Mercury. So this has to add some life to it. Although no idea if the replacement tranny was new or used.
 
You have three perfectly fine vehicles. When an engine or transmission goes junk it to thin out the heard. None of them are worth enough to bother selling. Now if you are retired (or work from home) and don't drive much and like to take long trips, that would be a different story since all three are really commuter cars.
 
So, have you taken the seats out yet?

Not yet, going to work on it when the weather is nice. It improves the practicality of the car so much.
With Tahoe, I kept thinking "what if" someone rides with me, yet I never had anyone actually ride with me so I did it.

It turns the SUV into kind of a mini-truck with a camper shell. And it opens up possibilities what you can put into it.

Plus, it should improve the MPG a bit, no longer carrying these heavy seats everywhere.
 
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You have three perfectly fine vehicles. When an engine or transmission goes junk it to thin out the heard. None of them are worth enough to bother selling. Now if you are retired (or work from home) and don't drive much and like to take long trips, that would be a different story since all three are really commuter cars.

That makes perfect sense. I doubt the Tahoe will get more than $1,000. The Mercury seems to sell for around $3,000, of that vintage. Which makes no sense either as the transmission repair will never be recouped.
 
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