300D still gasping for power on long hill

My 1981 300D is still 'gagging' for fuel after a while on a long (~1 mile) hill. I've replaced the tank strainer (which was dirty but not clogged), the prefilter, and have a newer FM100 canister for mail filtration. I'm tending to believe it must be sucking in some air during heavy fuel use. Frankly, I've lost track of the variables I've introduced: this problem may or may not relate to the FM100 installation and/or the new (original style) primer pump installation. Earlier, the Benz would pull nicely up this same hill. Some have suggested the lift pump itself, but my gut instinct is to suspect something simple first. Any ideas? The car runs wonderfully at other times. Just the prolonged hill problem.

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Diesel Engine Specific Aid

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Questions and Answers

Chassis Model Number

W123

Fuel delivery

From my experience this sounds like a fuel delivery problem (ie, restricted fuel flow or low fuel pressure under heavy demand). How have you installed and routed the FM 100 filter? What type of fuel are you using? Are you still using the factory fuel filter as well? Let me know and I will try to help. Kent

300D gasping on long hill

I routed the input of the FM100 to the output at the primer/lift pump. I routed the output of the FM100 to the injection pump. I abandoned the factory main filter. I am currently using straight petro diesel. I had drained and not reused the tank contents when I did the tank strainer (how can you do this without your special socket??) I had run 4-5 tanks of B99 during the summer and after the FM100 installation. My hunches: the FM100 connections at the lift pump and the injection pump are leaking. I cut off the factory hose flush with the connectors at these points, then installed and clamped your supplied hose over the remaining snippet of factory hose. I did this because the FM100 hose ID just looked too big for the lift and injection pump fittings alone. OK, maybe not the best idea. Also, I left short lengths of hose clamped off with bolts in the ends at the old main filter. Earlier, I had replaced the factory main filter prior to FM100 time, and I wondered about the hollow bolt seal - I see that it is a common replacement part. I did not replace it. If this all conveys the image that my 300D is held together with bailing wire, I can only say that apart from the oddities mentioned above, all is stock and seemingly in good shape.

Things to check

Did the power loss problem happen immediately after you installed the FM100? Have you double checked the routing direction through the fitler head (see arrows by the fitting inlets and outlets)?

Some other things to consider/try:

Have you tried changing the FM 100 fitler element? Not all B99 is created equal. Depending on source and where stored it could have had a high content of moisture in the fuel and that could plug the element is short order (since it is a very efficient water separator.

If you have tried everything then I recommend you return to the basics. Go back to the way the engine was plumbed and filtered originally. Take the FM100 out of the equation and see how the engine runs. If your power returns running on straight diesel with the factory filter, then reintroduce one change at a time and retest power output. This will help you narrow down the problem.

Good luck, Kent

300D gasping on long hill

Kent, Reconnecting the main filter and taking the FM100 out of the loop did the trick. Wonderful, accelerating power up the culprit hill now. The FM100 had definitely been connected correctly per the in and out arrows, and I had swapped in a new canister sometime shortly after the problem first appeared. Interesting that the hand primer worked much better with the FM100 out of the loop. Since several tanks of biodiesel in this period were from the same source, I have to wonder. Another idea: when I did the final tightening of everything for the FM100 bracket installation, the result yielded a tilted assembly. Not a lot. I asked about this, and Kaia replied that up to 30 degrees was OK (if I remember correctly.) This was more like 15-20 degrees. Could it be that sensitive? I know you were very clear about vertical alignment in your, as always, great instructions. Anyway, I guess I'll scope this out by the time warmer weather returns. I'll also make some enquiries about my biodiesel sources. Thanks for everything, Doug Knutsen