I can hear a low rumbling under the hood and took a look. I think the idler pully for the one-does-all fan belt is causing the problem. It is bouncing a lot. Rumbling is worse with the increased load of the A/C compressor.

Did a quick search - know this has been discussed before - but can't find it.

So, I'm now looking for tips on belt removal and idler arm pully replacement.

Records show it was replaced about 50K miles ago. Apparently not a long life thing.

It could be another device, but this pully would be the first suspect. It is running straight, not sideways, so maybe not critical at this point.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

To remove the belt:

- remove the 13mm head nut in front of the plastic hinge that holds the upper end of the spring

- put a tire iron in the big hole on top of the plastic hinge and pivot the hinge so you can pull back on the 13mm head bolt that goes through the head and hinge

- swing the hinge the other way and pull the spring off the hinge

- pull the belt off the idler and check the idler for smoothness

- check also the tensioner on which the idler pivots for smoothness

The idler shouldn't wobble on the tensioner, the tensioner shouldn't wobble on the timing chain cover, and the tensioner shouldn't click when swung through it's range. You already mentioned that you checked that the idler is parallel to the other pulleys.

If you have to get to the tensioner, the bolt that goes through the tensioner pivot takes an 8 or 10mm hex bit. The bolt goes into an oil passage so don't be surprised to see oil. If you're lucky you can replace the tensioner without removing the fan or other pulleys. I think you can replace the idler by popping the cap and undoing the bolt.

Sixto
95 S420
87 300SDL

I had to replace the idler pully this past spring but have no records of when it was last replaced. My guess is much longer than 50k ago. i was amazed that when my pully let go the front berings were released and the whole outside assembly was leaning to the passenger side. but it held the belt for another 10 miles. changing it out was a snap. the clearence was tight but i took a 6mm allen head socket to remove the center bolt.
if you have a bouncing problem the tensioning spring or the pnumatic shock my have weakend. remove tension from the spring side, slide the belt clear and check to see if your pnumatic shock has gone wimpy on you. as i understand it the springs also have a lifespan too.

little tip. the stock lug nut tire iron fits quite nicely into the hole on the top side of the upper spring tensioner mount. use the tire iron to release tension while you remove the upper long bolt that comes out by injector #1. and again use the tire iron to pull the spring back up when you are done.

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sold
1987 300 SDL 318K
126.125 (OM603.961) (722.321 trans) (head#14)
Born on date 11/07/1986
2.5" Flowmaster Exhaust
flex-a-lite dual electric cooling fan conversion

now have a 1985 toyota diesel pickup

Well, I just had a chance to investigate this - and thanks for the input. Certainly better than the manual.

I knew the belt was getting old because the inside ribs show cracking, and I've had a replacement on-hand for some time now. So its good to learn how to do this. Actually pretty easy relative to how it looks complicated at first glance.

I can't really find anything in bad shape. The pully runs smoothly, there is only the slightest looseness to it along the axis of the shaft its mounted on. The pivot operates smoothly and with no looseness. The shock absorber is a lot more tight on compression than extension, so I may go ahead and replace it. The spring looks to be possibly original equipment, so that will get replaced too.

All the other device pullies are smooth and easy to turn, except of course the crank.

So, I think I'll go with a new shock, spring and belt and see how it goes. Those are the easiest things to replace too.

Thanks,
Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Just to be clear, pulleys should turn smoothly and NOT make noise. New pulleys feel smooth and heavy. Pulleys on their way out feel smooth, light and make a whirring noise. What's a light whir when spun by hand is a growl when spun by a loaded belt.

Sixto
95 S420
87 300SDL

Do yourself a favor and take the fan off -- piece of coat hanger wire, an 8mm allen, and it's off. Takes less time to R&R the fan than to fish the damn..d belt around the bottom of the engine.

I suspect your belt is the problem -- when they are cracked like that, the pulley is riding on the bottom of the grooves in places and it's grabbing.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I have a '91 350SD and have had that pulley replaced. I keep wearing out engine shocks though. When mine idles, the grumble that I hear is a vibrating shock. When the shock is replaced it is as quiet as can be.

I have had 2 shocks in 2 years. Any ideas?