W123 240D hesitation/loss of power on highway Why?
 

I got my car back from the mechanic a few weeks ago and on Sunday I took it out for its first highway trip. Engine and transmission were well warmed-up from in-town driving before I hit the highway. After a few minutes at full speed (65-70 mph), I noticed a sort of intermittent "drag", like some force was trying to slow the car down. It would come and go in short spurts.

The highway ended soon. On the way home, I decided to take a different route, with more highway. The car drove perfectly normally on country roads (35-50 mph) for half an hour. Once on the highway, the hesitation/drag/whatever reappeared after a minute or two. It worsened to where the car would not keep up over 60 mph, but didn't continue getting worse. I put the car in neutral and revved the engine through various speeds...everything seemed fine...no unusual roughness or sounds.

We got into the city and the car drove normally in stop-and-go. Once out on the highway, it took a minute or two and the drag effect reappeared.

After getting home, I let the car idle while unloading it and telling my wife about the trip (10-15 minutes). I then checked the transmission fluid. It was a nice pink color and the level was between the marks. I had replaced it myself in the spring, and thought I might have overfilled it a bit but with the well warmed, then cooled, transmission it looks to be ok.

I am at a complete loss on what this could be. It really does not seem to be an engine issue, I'm thinking either transmission or some other drivetrain problem. Can a failing bearing act like this? The fact that it's taking a couple minutes to start makes me think something has to heat up for the problem to present, and in-town driving doesn't make that happen. Anything I can easily test for? My time is limited these days because of family stuff.

I can bring it back to the mechanic but would like to have some idea of what I'm dealing with. He commented that the car drives really well (has power) for a 240D automatic of its mileage. This is an original engine and transmission. Odometer died at 307,000 miles, my best guess is it's about 325,000 miles now.

Have you checked the fuel filters? To me it sounds like it is getting starved for fuel so there might be a partial clog. At highway speeds it needs more fuel consistently so the problem is likely to show up only there at first.

1. Fuel filters.
2. Tank strainer.
3. Tank vent.

When putting your filters on... use the clear 90 degree one up front.. so you can see if you are getting any little black specs growing in there... meaning cleaning of the other filters and a killing of algae will be needed to really solve the problem...
I took the tank out of my 123 to clean it... but would not the next time...as there are nice water blasting openings in it...

Fuel filter.

Thanks, guys! I would not have thought of that.

A restricted Air Filter can also cause the loss of power described though the Fuel Filters are the most common problem.

True.

Put a hill in front of a 240D auto and you will get the exact same thing....well technically any incline....throw in passengers...its even worse...

I never followed up on this. Replacing the fuel filters did the job. Once I got the engine running (it took more cranking than I expected, even after using the hand pump per instructions), the issue was gone. Instead of starting to hesitate at 60 mph, it gets up to 80 with no problem. Not bad for a $15 investment in new parts.

Thanks!

food for thought
 

this has happened to me more than once.

i like to keep a couple of filters, rags, wrench & scredriver sealed in a big ziploc bag tossed into the trunk as we are usually far from home when the power loss becomes apparent

I never leave home without them!

It`s usually a small problem with these engines that seems like a huge one many times.
Just the basic maintenance will keep them running a long time.

If you fill the big filter with first before screwing it on, you won`t have to crank on the starter so long.
I usually fill mine with ATF from a new bottle. That way I know it is coming out of a clean container vs
pouring say Diesel out of a container from the filling station.
This will lessen the possible issue of introducing dirt, water etc.. into your new filter.

Diesel Purge, or other diesel additives can be used as well to fill it.

Yeah, someone had advised me of that, and then when I actually went to do the job I forgot about it.
I would not have thought of that, I assumed siphoning a bit out of the tank (if you don't have a diesel can, which I don't) was going to be my only option. I'll definitely remember it if I have to replace filters again.

Side question: why do diesel cans not come in less than a 5-gallon size? You can find 1-gallon plastic containers, red, that say GASOLINE, easily. The equivalent in yellow that says DIESEL doesn't exist. The smallest one that does is 5 gallons.

No need for 1 gal diesel containers. Buy the cheapest Walmart ATF, keep a gallon in the trunk. It doesn't smell. Use it to prime filters if you have a roadside emergency filter change. Use it for emergency fuel if needed. The engine loves it.

Why did it take you two years to change your fuel filter?...;)