Took 406 out today for first real workout in years. Ran great at first, and then as the hours progressed it started running rough at progressively lower rpms (2600, 2400, 2000 etc). Figured I sediment was getting stirred up (was operating off-road) and clogging the filters so decided to change filters. Sediment bowl filter had some paint flakes, otherwise everything looked fine, but still changed sediment filter and two fuel filters with OEM issue (MB 411 477 00 15 sediment, MB 000 090 11 51 fuel filters). Ran worse than before. Chugged at low rpms (between 400 and 800) and then died. So......
Emptied the fuel tank and put in new diesel. Detached all the lines and blew them out (didn't mess lines into engine). Pulled suction pipe from tank and discovered paint flakes had been coming off it so I cleaned it. Still chugged. Changed OEM fuel filters for NAPA 3167. Still chugged at low rpms. Removed fuel filters, ran rough at first and them perfect. Reinstalled OEM fuel filters, ran like s*H*i*T again. Removed filters and now it is running fine.
Anyone have any insight of guidance? Obviously I need the filters, but why won't engine run with them?
The filter head with the 2 bowl’s comes in 2 types that look the same,
you have the 2 staged filter head ware fuel passes threw filter 1 (coarse) then threw filter 2 (fine) so it is filtered twice before exiting.
Then the parallel filter head, ware fuel is free to pass threw filter 1 OR 2 (both fine) before exiting so only passes threw one filter.
This is why the original filter 1 type (coarse) has now been omitted and is just replaced with the filter 2 type, so it can work in both staged & parallel filter heads with no loss of filtration at the fuel exit.
If you have traced a problem to the filters and you have staged filter head then its possible there is a blockage/restriction in the port between filter 1 & 2 in the filter head caused by running a course 1st filter in the past?
I'll take a closer look at the filter head. Blew out the connection between the two filters, or at least I assumed I did as air went in one side and out the other. That being said, I was not aware that there were two types of systems (and the filter assembly is what I have been mulling over in my head for the past few hours).
For what it's worth, the Mog is a 1979 406.121 (Case) w/only 300 hours on it and the filters I removed were NAPA 3167s.
You sure it's not something much simpler, like a leaky hose sucking air ? Next time it bogs, turn it off, loosen the bleeders on the filter housing and pump the primer. If it bubbles or does anything besides pump clear fuel, there's an air leak.
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What Mogasaurus said, air leak. When it gets worse over time it's almost always a clog or a leak. You've eliminated the clog, so check the hose connections. There was a point when mine had an air leak, as soon as it started to get rough, I'd hit the primer for 10-20 seconds and it would smooth right out. The hard plastic lines dry out and don't make a very good seal on the brass fittings after 30 some years.
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Seth
71 416Mog, 92 fj80 (worst fuel pig EVER), new 85 300tdt for dad, 2000 KLR650, 84 300td STOLEN!
Okay, will focus on air leak tomorrow. Just moved it to the garage, had it cranked up to 2600 running smoothly, and then after a bit it started to "gurgle" (for lack of a better term). Thinking filters, by increasing the flow restriction, amplify the effects of the leak?
so how dose removing the filters cause less vacuum to suck air in when the filters and everything past the lift pump is pressurised?
if it was an air leek between the tank & the lift pump then there would be less vacuum in that pipe if the filters are restricting the flow than if the filters were removed - unless its a very cracked pipe.
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