Another Power Loss Thread From a Total Noob!

[Rmagoon1990]

Hello Everybody!

I recently purchased a 1985 MB 300 CD locally here in Southern California. 178k miles, very good condition, no rust, and apparently well cared for. I have had no issues (aside from A/C issues), until this week when I was escaping the wildfires and drove for about 2-3 hours on the freeway. This was the first time I've maintained speeds over 65 mph for an extended period. I was noticing slight power loss midway through the trip going up steeper grades and getting passed by big rigs. Then later on I was getting extreme vibration for about a minute and had a blowout on my front left tire. I changed the tire, decided I better not pushed my luck, and turned around and went home. I didn't feel comfortable pushing it at higher speeds and drove all the way back at 40 mph max with my hazards on. Not a fun time...

When I got back on city streets I noticed the EXTREME loss in power. I was barely able to climb up the hill back to the house pedal to the floor, and the acceleration was far less responsive. I did a lot of reading over the past few days and have ordered two new fuel filters which will arrive on Tuesday, as well as an air filter as the one that was in it looked like it hasn't been changed since Y2K!

While I wait for those, I checked the ALDA this morning and it appears like it has been tampered with previously AND disconnected(?) See photos below. I did a quick test drive again and it is even less responsive than it was before, and is barely going above 3MPH on flat streets.

BUT! I noticed that when going in reverse it maintains a normal speed, yet going forward it is barely reaching above idle when fully accelerated. I will try and install the new air/fuel filters when they arrive on Tuesday, but wanted to check with everybody about the mysterious ALDA connections, and if the discrepancy between forward/reverse speeds might indicate a transmission vacuum problem.

I know it would be better to have started a thread after swapping out the filters, but am really curious about these two new bits to the puzzle that I've found this morning. Thank you so much for your feedback!!

-RJ

[SonnyMorrow]

I doubt the air filter is causing this unless its insanely filthy. I have a strong susupsion it is fuel related and you are on the right track in terms of ordering fuel filters. One thing you might have overlooked is the fuel tank strainer. I would certainly check that out before you replace your fuel filters. If your fuel tank strained is clogged then your fuel tank is probably in need of a cleaning. If you slap on new fuel filters up front you'll probably run into this issue again in the near future. There is also a possibility the fuel strainer is fine but it is best to start back there and work your way to the enginebay.

Here is a link: https://www.ifixit.com/Guide/Mercedes+W123+Fuel+Tank+Strainer+Maintenance/33401

[Diesel911]

If you went for an interview to work as a diesel mechanic the first question they often ask is what would be the cause of a sudden loss of power? The answer they are looking for is the Fuel Filters next would be the Air filters.

If your primary filter is the typical plastic one you can see into it to varying degrees. Look at it and tell use if you seen anything. If you see black gray greenish stuff in the filter you likely have something growing inside of the Fuel Tank that could be plugging up the Filters and or the Fuel Tank Strainer.

If the primary filter looks OK change the secondary Spin-on Filter and pump with the hand primer until you hear a squishing or buzzing sound indicating the air is out of the Fuel supply system. Then pump some more to be sure.

You can avoid having to use the hand primer as much if you fill the spin on filter with clean diesel fuel before installing it.

If you have the hand primer with the hard plastic knob be sure when you are done to bottom it out and turn it clockwise to tighten and seal it. If it leaks when you use it could be causing an air leak.

If you are still having the acceleration problems swap the positions of the fuel Hoses where they come off of the Fender well. This reverses the flow and bypasses the fuel tank screen. However you need to have at lest 1/2 tank of fuel as you will be drawing fuel from a higher position. After swapping the lines again use that Hand Primer to be sure the air is out of the fuel supply system.

Another source of loss of power is if you look at the where the throttle linkages come together on the fire wall. That bell crank has a plastic parts behind the plate that crumble away and rubber parts that delaminate. Also lately that part has gotten hard to find.



If after changing the filter.

[Diesel911]

The ALDA starts changing things after about 2000 rpms when the pressure from the turbo is high enough to do that. So it does not effect initial acceleration.

The Overpressure relay is a protection device that cuts the fuel down if the Trubo pressure gets unusually high. In normal driving it has no effect.

It is not uncommon for the at the very end of the intake manifold for the Banjo Bolt to be plugged with crud reducing or cutting off the pressure from the intake manifold to the ADLA. You can remove that and if needed clean it. Carful not to lose the crush washers.

[Rmagoon1990]

Hey thank you very much for your responses!

So I just went out to have another look at the primary fuel pump. I shined a light underneath it and everything looks pretty clean inside (consistent diesel color, no discoloration or particles). I've ordered both fuel filters for replacement on Tuesday just to be safe, so it will be switched out anyway.

I think I will have a look at cleaning out the fuel tank strainer tomorrow before the filters are replaced as @SonnyMorrow has advised. Starting at the tank does make the most sense, and even if it's not the problem, it's probably not a bad idea to give it some love either way.

I gave a cursory look at the throttle linkages this afternoon and everything "SEEMED" unscathed, but I haven't researched this extensively so I could very easily be overlooking something.

One quick question: I ordered supplies to do a diesel purge right before I change the filters. Does this seem like a reasonable step to include?

Thanks again for all your help!

[Alec300SD]

Order 2 sets of filters.
Change both filters.
Then do the diesel purge.
Change the newish filters out after the diesel purge.

Diesel purging with old filters in place will flush contaminants from the old, dirty filters into the IP...not a good idea.

[Rmagoon1990]

Good point and def makes sense. I'll order another set of filters for Tuesday. Thanks!

[Rmagoon1990]

Quick update. I went to get supplies for draining the tank today. Right before I left I moved the 300CD to a more level area and it was barely moving above idle with the pedal to the floor. It was worse than yesterday and I was getting no revving from the engine. Sure enough, I had another look under the hood and notice the throttle linkage components off of the firewall completely cracked in half, and barely moving the throttle forward (see attached pics).

It seems pretty clear that this is the issue (though I will certainly be switching out the filters this week as routine maintenance).

I admit that I have done very little research on replacing/maintaining the throttle components, and @Diesel911 I'm guessing this is the part that has become difficult to source?

If anybody could steer me in the direction of good resources/tutorials/part sources I would be extremely grateful for the help. Thanks again!

[tyl604]

Congrats; you may have found the culprit. Sometimes it is the simplest things that cause problems.

[SonnyMorrow]

Great, that is a simple fix if you can find the part and yes they are tricky to find. In the past I have repaired mine as best as I could with glue and some zip ties (ugly but a good fix until I found a good pivot mount for sale.) Part number is: 123 300 13 40

There was one for sale in the parts section the other day. Message greazzer I bet he has one: http://www.peachparts.com/shopforum/mercedes-benz-used-parts-sale-wanted/408041-sale-w123-parts.html

[Rmagoon1990]

I strapped some zip ties on the piece and it is running way better than it was before this mishap! Accelerates beautifully, shifts more smoothly, and goes up hills more zippily.

I will be ordering the replacement. I've seen just the rubber component for sale. Does anybody know if just this piece can be replaced, or should I search around for the entire firewall mount? I see it listed on Mercedes-Source for $62.95 but is also out of stock at the moment...

[Rmagoon1990]

Oh, thanks @SonnyMorrow I didn't see your comment. I will check it out!

[Diesel911]

Be careful what you buy as different years and models have different parts on them.