I just purchased a 1984 300D Turbo yesterday. The car has 226k miles on it.
After I bought it I drove it around 50 miles to my father-in-laws house since we were so close. On my trip home I've noticed that it is now backfiring.

I drove it nearly 170 miles before it started. At first it was an intermitant pop, but eventually it would turn into a constant rumble. As I slow down the backfiring stops and it humms quietly at idle. After about 10 minutes back at speed (70 mph) it starts to rumble again. Oddly enough it appears that when the backfiring started the engine started running cooler. I didn't notice any loss of power while it was backfiring.

Perhaps 50 miles before this started the muffler bands snapped and I drug the muffler for maybe 2 miles before I got it tied back up.

Could the muffler falling have damaged it to the point that 3,500 RPM is too much pressure for it?

Is the exhaust pinched anywhere?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

I don't see any dings or crimps in the exhaust pipe.

does it sound like a wet fart?....i only ask because i have a mystery noise that sounds like that.

Diesels normally don't "backfire" because the fuel isn't really combustible in the tailpipe. If your compression was so low the fuel didn't burn in the cylinder, you would certainly notice it.

I suspect the exhaust is simply rusted away, and the the "backfire" is really just exhaust noise. Diesels are very quiet at idle, only get noisy under heavy load.

Internittant exposlons (backfire) would indicate to me a serious IP problem, so if you don't also get serious power loss, I'd not expect that.

If it started running cooler when the noise started, I suspect the muffler is collapsed inside, and the lower back pressure reduced waste heat load.

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!

Well I just had the exhaust inspected. Other than 2 places were I managed to grind almost through the pipe.. The exhaust checked out fine. I've driven the car several times since then and haven't heard the noise again. However, the trips have been more like 30 miles, not 200. The temp seems to be hovering around 85 degrees.

So I guess the question that begs to be answered is..

What other than the exhaust could have caused this?

A problem with the IP that only shows itself after 2 hours of driving at 65+MPH?

A sticking exhaust valve could do this....
or burned or a broken valve spring...

No I haven't checked the valve clearence. I'll admit that I am a little intimidated by the engine. I'm not sure why, probably because I've never touched a diesel engine before. I'll check the valves out.

They are intimidating.... good time to get a factory shop manual and read it ... first....

I didn't know diesels could backfire

I agree with psfred on this. Valve clearance increases as the engine comes up to temp. If it ran on all 5 when cold then there is clearance there. If it didn't loose power when "backfiring" then there wasn't a valve hanging up. The backfiring went on long enough that you would have noticed the loss of power. The problem IMO is an internally rusted muffler or a muffler baffle that rusted loose and is moving around causing the "backfire" rumbling noise to come and go. Expect it to return.