Power loss mystery, not filters - PeachParts Mercedes-Benz Forum

Remy · #1 10-12-2014, 03:30 PM

I've been experience episodic power loss on my 1984 300TD (OM617), which I bought in January. It's my first car and I'm a new mechanic, so bear with me...

The problem:
First occurred when driving cross-country, going 70mpg on interstate in Iowa. Car slowly lost power (full throttle, slowing down gradually). I changed both primary and secondary fuel filters. Nothing happened for another 1200 miles until I was back in New Hampshire. Since then (the last 9 months), every couple of weeks I lose power... usually after about an hour of driving, or when going ~60mph on interstate, or going uphill, or on a low tank... but not always. I've also lost power right after starting the car up in the morning, or when slowing down coming off the interstate, or on a full tank... so no consistency. Usually I keep driving and start going downhill and pick up power again, or in worst cases the car will completely stall, but start up and run fine again after 'resting' for a few minutes. For a while I just drove it and let it rest since I couldn't figure out the problem.

Recently the problem got worse, and about a month ago I took out the filter screen at the bottom of the tank. It was gunked up and I replaced. Problem continued (no better), so I just checked the fuel screen again, expecting it to be clogged and planning to remove the fuel tank and clean it. The fuel screen is totally clean (see photo), so I'm not sure what to do next.

I double checked the fuel lines all the way from tank to fuel filters (I replaced sections of these back in January), and don't see any leaks or kinks.

My question:
Where should I start looking next? I was considering a diesel purge to get the injection pump and injectors cleaned, but it seems unlikely that this is the problem (I will do it eventually anyway). Do I have a faulty injection pump or lift pump? My air filter could be changed, but it was totally clean when this first started happening so I doubt that's the problem.

Other things I've done with this car:
new tires, exhaust pipe, coolant bypass hose, rear trailing arm replacement, valve adjustment, manual heater motor blower switch (CCU not working), bypassed auxiliary coolant pump... none of these should have anything do to with the problem. Car seems to run at pretty normal/consistent temp (80-90deg).

I'm hoping from the lengthy description above you all might have some insight (key feature: episodic, sporadic, gets better after 'rest'...usually, gradually getting worse), but I'm happy to tell you more.

Thanks so much!

See attached photo of screen tank filter I took out and checked today (totally clean - came out of the car like this), and the one I replaced about a month ago (dirty).

Mxfrank · #2 10-12-2014, 03:38 PM

On a turbo, the thing to check is the Alda feed. There's an article on diesel giant dot com that gives a pretty good tutorial (just plug in the URL, it would be edited out on this board if I tried to give you a complete link):

(dieselgiant dot com)/mercedesaldaboostsystemservi.htm

barry12345 · #3 10-12-2014, 10:36 PM

The fuel tank has a breather hose. If this obstructed many symptoms like you mention are possible. The fuel tank then develops a vacuum that the lift pump has a hard time dealing with. It usually takes some milage for the vacuum to really get deep enough.

As the car sits the vacuum gets released somewhat. A test is to drive with a loose tank cap but you may spill some fuel. If it was my car I would make sure that vent line is open.

Diesel911 · #4 10-12-2014, 11:01 PM

What barry12345 said. Also the Tank Screen was plugged the first time. To check for the a Fuel Tank Vent issue loosen the Fuel Fill Cap so that it will substitute as the Vent. If your Car runs better like that then you know you have a Fuel Tank Vent issue.

Why couldn't there also be crud in the Fuel Lines between the Fuel Supply/Lift Pump? You might try using some Startron additive in the Tank as it claims to have an Enzyme that breaks up stuff to the extent that it is small enough to pass through the Filters.

The Fuel Pressure Relief/Overflow Valve on the Fuel Injection Pump can also cause the issue.
Long Fuel Pressure Relief Valve/Overflow Valve Thread
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum

This does not meet your exact symptoms but if your Valve Clearances are too tight when the Valves get hot the Clearances close up and it effects the power driving and also idle if it is bad enough.

ah-kay · #5 10-13-2014, 01:20 AM

What car? It could be the lift pump. I had the same problem with one of my 300SDL, an OM603 engine. I did almost the same things you did, filters, fuel tank strainer, check for leaks, by-pass fuel thermostat heater and all to no avail.

The problem manifested itself when

1) Fuel tank is low. When I top it up and it gives enough head to the lift pump then the problem goes away.
2) Going uphill and press on the pedal. It will slow down and lose power. If I go up hill steady and no sudden pedal movement then it will be OK.

I replaced the lift pump and all is well. The problem is the mechanical fuel pump is not delivering enough fuel to the engine at high rev.

Good luck.

Remy · #6 10-13-2014, 03:30 PM

Thanks everyone for the initial responses.

ALDA - I actually cleaned out the banjo bolt the first time this happened, when I pulled over on the interstate back in January (didn't really know what I was doing, but heard it might help). I will recheck this and plastic tubing as well. A guy who said he knew what he was doing actually did "turn up" my ALDA (backed out that screw?) when I first got the car... I keep hearing "don't mess with it unless you know what you're doing" so I get that I should be cautious. How do I know if the ALDA or switchover valve are broken? Could my move (from NM 8,000ft to NH 5,000ft) and associated change in ambient pressure have anything to do with it?

Fuel lines - It's entirely possible there is crud built up in my fuel lines (especially somewhere in the half I didn't replace). I may replace them completely soon. I was going to run a diesel purge product (Lubro Molly comes recommended?) before refilling the tank this week, so it will be interesting to see if I get the regular slow idle and any of the power loss issue just running it for 10minutes off a bucket of that.

Vent hose/valve - This seems like a likely culprit, although I've never heard the 'woosh' sound when opening the tank cap to fill up. I will try driving this week with the tank cap loose and see what happens (very curious to see if any difference). I need to get under my car to figure out where it is - I'll report back on this one.

Lift pump - How hard/involved is a rebuild? Pros/cons of a rebuild vs replacing the pump? Sounds like there aren't any kits but I found this from whunter: http://www.peachparts.com/shopforum/mercedes-parts-reference-library/183448-om617-912-lift-pump-repair-kit-w123-190-300td-w123-130-300d.html

Ah-kay your problems sounds similar (especially #2) to mine... when it's doing the power loss thing I can sometimes keep momentum and cruise uphill with no loss... but if I accelerate hard or back off it has trouble and won't make it. Is a lift pump rebuild a good precautionary step anyway? mine is '84 with 240k. If I remove the secondary fuel filter (or primary, forget which is which - the white one that looks like an oil filter) and the lift pump is having trouble, would I expect the filter to not be entirely full? I've tried manually pumping the lift pump when the car is idling after having trouble with power, and I didn't notice any difference... does this make the lift pump a less likely culprit?

Thanks again everyone!

ah-kay · #7 10-13-2014, 03:53 PM

if your remove the spin on filter and it is not full, then you have leak somewhere. It may not be the lift pump. The symptom is fuel starvation to me. Air leak, blockage, or weak lift pump can all be plausible. Basically I did elimination to find the culprit.

cooljjay · #8 10-13-2014, 05:26 PM

Don't forget to look at the throttle pivot bushing on the firewall....this can come apart and cause acceleration issues...

Remy · #9 11-02-2014, 08:28 PM

Hey folks,

Quick update on this story - so I eliminated the fuel tank vent issue, I'm pretty sure that's not my problem (had some power loss, drove the car with cap undone and it made no difference).

While I had the tank empty to look at the tank screen (a couple weeks ago) I did a diesel purge - I took off the incoming and return fuel lines at the lift pump and ran liqui-moly diesel purge through from a bucket. The car ran great for 15min while I was doing this, and I didn't notice a lot of gunk or obvious dirty diesel coming back from the return. I reconnected the fuel lines, put 4 gallons in the car and tried to get it to the gas station. It died once, I puttered home with almost no power, and I went and got 5 more gallons. The next day I got the car started up pretty easily and was on my way to work, and we lost power about 1 mile into the drive. Car died on side of the road and I couldn't get it started again. I used AAA and towed to a mechanic who has worked on my car and knows the old MBs well. He checked pressure on the lift pump and said it was fine. He had some trouble starting the car but when he got it running he said it ran great and he didn't know what was wrong. He said fuel lines looked good, no leaks, pretty new (I replaced most of them recently)... He said, based on my story of power loss, that it was possible there was something "broken in the lift pump that was working intermittently" and that he could take it apart and check. I decided not to spend the money "to check" and drove the car home.

I've been driving the car for a week with GREAT power, no problems yet and HE DIDN'T DO ANYTHING and all I did was a diesel purge - before the car broke down and got towed.

I'm driving 2000 miles to CO in a couple weeks and figured I should take the lift pump off and check it out before I get on the road. I have the oil drained and lift pump repair kit arrived, but I am totally stuck at just getting the lift pump off the injection pump. Does anyone have a diagram or know of a post that describes this for the OM617? I can't find anything. I just need to know what bolts to take off in what order, and ideally what wrenches people use... any leads very much appreciated.

Thanks!

scottmcphee · #10 11-02-2014, 11:40 PM

You might be sucking air into a fuel supply line. Accumulating air until the amount is enough to disrupt normal IP pressurization. Put a section of clear hose leading into the IP and see if you are getting bubbles. Some small ones are ok. Big bubbles and youve got troubles. Put a section of clear hose on the IP return line too. Bubbles flying out of the IP return loop pretty much confirms youve got air leak into fuel.

Secondly, dont be afraid of the ALDA. Remove it from the IP by undoing the nut holding it on, and let it sit there rattling around on top with hoses connected. The only thing an ALDA can do is restrict rack movement... i.e. Limit the range of fuel delivery in the IP. I.e. Reduce power. A leaking ALDA can choke your engine.

ROLLGUY · #11 11-03-2014, 10:54 AM

Quote:

Originally Posted by scottmcphee

Remove it from the IP by undoing the nut holding it on, and let it sit there rattling around on top with hoses connected.

My first thought was: what kind of advice is that? Is there more? Just let the ALDA sit there and rattle around forever? Seems like bad advice to me....Rich

vstech · #12 11-03-2014, 12:22 PM

Quote:

Originally Posted by ROLLGUY

My first thought was: what kind of advice is that? Is there more? Just let the ALDA sit there and rattle around forever? Seems like bad advice to me....Rich

I think he was mentioning this method, to see if it made a difference.

Diesel911 · #13 11-03-2014, 07:20 PM

Concerning the Fuel Supply/Lift Pump on the MW Fuel Injection Pumps the Fuel Supply/Lift Pump has 3 nuts that hold the Lift Pump to the Fuel Injection Pump. The Bottom Rear Nut is harder to get at then the rest.

When you back off the Nuts try to turn one Nut some and then another Nut all the way around incase the Cam inside is pushing on the Spring.

If you get the Nuts off it pulls straight out off of the Studs.

Before you re-insall the Lift Pump get a Mirror and Flash Light and look into the Hole the Pump goes into and rotate the Engine until you see the lowest part of the Cam Lobe inside of that hole. That way you can insert the Lift Pump with little or no Spring tension on it.

If there is any Spring Tension at all on the Lift Pump tighten a Nut just a liitle and then move on and reapeat that on all of the Nuts trying to tighten the Lift Pump down slowly and evenly so you don't crack the Flange off.

Diesel911 · #14 11-03-2014, 07:26 PM

paradox1559 · #15 02-08-2022, 08:00 PM

Old thread, I know. But I just had pretty much the exact same issue and am trying to narrow it down.

Did you ever figure this out?