Diesel loses power when woarm

_____ Hello All Mercedes Fans,
I own a 1981 W126 300SD car. The engine has been running well until recently. Now when it heats up (after about 30 to 40 minutes on the
road) the engine has less power, and idles poorly, stalling at times. Once i rev it up, it seems to fire on all five cylinders.
I have done the following: 1) replaced fuel pre and main cartridge filter about 6 months ago 2) pulled 5 injectors from known good engine and installed them. Problem persists.
Does this sound like an issue with the fuel supply from the tank, or could i be losing compression? I do have a compression tester. Engine has probably run for some 300,000 km. Valves were adjusted last summer. I drive about 10,000 km/year.
Hope to get some input from the experts out there.
Regards / John
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First thing I'd do is replace those fuel filters again, even though it was done 6 months ago. It's fast, easy,. cheap and I'd just rule that out for sure. A couple years ago I was having mysterious problems with my 80 300SD. It was fine at highway speeds but running very rough to the point of stalling when stopped. I had to keep my foot on the peddle at lights to keep it going. Now because it was fine at 60 MPH, I assumed it could not be a fuel supply problem. Luckily I decided to replace those two filters and as soon as I did the problem was gone.
I can't explain it other than perhaps the fuel pump doesn't have enough oomph to overcome the restriction at low RPM. I also had a problem years ago where it was doing the opposite, ie incapable of developing full power at highway speeds. That turned out to be clogging in the fuel tank at the strainer there due to infrequent use and sludge build-up.
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On 25/05/2012 19:43, snipped-for-privacy@nowhere.ca wrote:

The engine timing may be responsible. Look up the manual for the instructions. I think there is also some kind of ventilation to the tank and that could be a problem, and have you checked your tank for that brownish "seaweed." It grows in the tank because water can enter the fuel and feed the algae. You should be able to buy a small bottle of additive, such as Red Line 98 Plus. That should absorb the water in the tank and the algae won't bother the car. Apply regularly though. See www.redlineoil.com. If you live in a cold climate this problem may not occur.
Good luck!
PS. my car is 1984 240D, W123 240D.
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_____ Trader and Loony,
Thanks for your input.
Changed both fuel filters yesterday in the W126 300SD. Starts and runs OK and after 40 minutes of driving there is no sign of rough idle and stalling. However, it still lacks power. It occurred to me to check the banjo bolt connection that supplies the boost pressure signal to the ALDA (fuel adjestment device) on the injection pump. I essentially measure NO boost pressure. On my W123 300D turbodiesel i measure a few PSI boost pressure when revving the engine at no load. So, today i will pull the air filter housing and look into the air compressor side of the turbocharger. I can feel if the turbine shaft has axial and radial play. By blocking the line to the bypass valve actuator i should be able to confirm that the bypass valve is closed, and not stuck open. Removing and rebuilding the turbo does not look to difficult. I also have a known good spare turbo on an old engine in my workshop. Will let you know what i find.
Regards / John
PS: Where is Tiger these days?
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_____ I managed to hook up a boost gauge, and after cleaning the banjo bolt, checking the over boost cutout relay, and re-attaching the pressure sense lines to the ALDA device on the injection pump, all works pretty much as it did before. The maximum boost i get is around 9 psi to 9.5 psi. So, the most probable cause of my trouble was clogged fuel filters, followed by low boost pressure as another factor. I plan to install a boost gauge in the cockpit at a later time. The Garrett T3 turbo boost pressure can be adjusted, and there is lots of information about the procedure on the iNet. I may set it to 12 psi. The over boost cutout relay works fine from an electrical perspective. What i have not been able to test is the pressure switch on the back of the intake manifold. To test it one must remove it, and rig up a pneumatic air test connection to see if it indeed trips at 16 psi. When it trips a contact closed to ground, thereby activating the over boost relay, which in turn cuts out the extra fuel flow the ALDA device is asking for.
Regards / John
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