I'm pretty close to pulling my hair out on this one.

Started having occasional power loss issues a while back. Would have a period where the car would run great then the bottom would drop out on my power and I would be in pure limp mode.

I've addressed all of the basic issues and then some.

1. Fuel tank screen (wasn't in bad shape)
2. Both fuel filters.
3. Ran a purge.
4. Injectors cleaned, lapped and rebuilt with Monark nozzles
5. Return lines replaced
6. Air filter isn't clogged


To give an example of how the issue usually arises: Drove to the kids school the other day. I live in a rural area and its highway pretty much straight to the door. Cruised along at 60-65. Got to the school and idled in park for maybe 3 minutes. When I put the car back in gear NO POWER. Had to take back roads and could BARELY make it up any hill.

This scenario seems to be typical of how the problem usually arises. I was thinking about timing and or valve adjustment. I live in Michigan and the car has been starting fine on cold mornings so I doubt I have any terrible compression issues.

Wanted to get some input before I got into the pricier options like a lift pump, injection pump or timing chain.

The car generally idles very rough while in gear. Even when the power is good, if that is any help.

Any guidance would be greatly appreciated.

 

Where the Fuel Return Line comes off of your Fuel Injection Pump is a Fuel Pressure Relief/Overflow Valve. That Valve controls the Fuel Supply Pressure inside of the Fuel Injection Pump Housing.
The Spring inside gets weal over the years. If you have the type that you can take apart the Manual has that you can remove the Valve Spring inside and stretch it till the free length is 27mm.
Of if you can find one you can replace the Fuel Pressure Relief/Overflow Valve with a new one.
It would be nice to be able to check the Fuel Pressure before you start replacing stuff that but you need to fabricate a method to hook up the Gauge and even a liquid Filled Gauge Needle bounces a lot and you need some sort of restriction in the line to control the needle enough to get a good reading.

Then there is the 25+ Year old Fuel Supply/Lift Pump. Mercedes has a reasonably priced Valve kit for My Year and Model and they may also have one for yours. However, the one for mine lacks a tiny O-ring that I had to source from someplace else.

You could get a complete kit from a Fuel Injection Shop.

If your Hand Primer has a shiny Aluminum body and a hard Plastic **** on it is the old Style type. They are well known for being a source of Air leaks. You can replace that with a new style Bosch Hand Primer (the copies have quality control issues).
All you have to do with the New Type is to push down on it and release it and a Spring brings it back up.

If the Fuel tank Vent is restricted it can cause a low pressure in your Fuel Tank that combined with a weak Fuel Supply/Lift Pump can cause Fuel Delivery issues.
Don't do this with a full Fuel Tank but you can drive with out the Fuel Fill Cap on making it a temp Fuel Tank Vent. If you symptoms go away you have found the issue.

A more bizarre issue is that if you have a lot of Blow-by in your Engine to the extent that it cannot pass out of the normal Blow-By Vent it can actually push your Vacuum Shutoff towards the shutoff direction.

If your Fuel Inlet Hoses are hard or have a bunch of hairline cracks in them it they can cause air leaks. 5/16" ID Fuel Hose works fine as besides Gasoline it is rated for Diesel Fuel.

 

That is a ton of good information and should give me plenty of paths to chase.

Thanks