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Old 08-06-2013, 01:56 PM
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Join Date: Aug 2013
Posts: 4
'93 300SD Performance Mystery

Steve on recommended this forum:

Well it's a mystery to me. I have a '87 300D with 302,000 miles and it runs like a top. I recently purchased a '92 300SD as its eventual replacement (180,000).

The 350 diesel has poor acceleration, especially on any kind of hill. So my local mechanic adjusted the ALDA. Lots of smoke and slightly better acceleration. Took it in for a smog test and it failed with a dyno of 55 hp . Mechanic backed off (leaned) the ALDA 1/2 turn. Took it back and it passed the smog test, but the interesting thing was it dyno'd 110 hp. Ran like great.

However, the over the next week performance went back to the old performance level, crummy. Checked the ALDA and the mechanic hadn't tightened the lock nut. So I set about attempting to adjust the ALDA myself. With a couple 'standard' tests, like running up an uphill on ramp and checking the speed, and 100 mile fixed course mileage test.

Now the mystery. At some adjustments I've gotten 64 miles an hour up the ramp (the '87 makes 62) and as high as 29 miles per gallon. A couple days later, touching nothing, she gets to only 54 miles an hour, and 23 miles per gallon.

I'm baffled. I've changed the air filter, and fuel filters, with injector cleaner additive a couple times this year.

One thing I have noticed one thing at speed the MPG (vacuum gauge) won't drop below 10-12 miles per gallon.

Any ideas? Thanks
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Old 08-06-2013, 05:06 PM
Skid Row Joe's Avatar
Join Date: Oct 2007
Posts: 6,804
Although I never owned a W140 300SD, I did test drive a used one in the late '90s in Memphis, Tenn. w/90K miles on it. I was surprised how doggy it was to accelerate. I drove the car for perhaps 5 miles entering and exiting the Interstate highway system during that test drive. I returned it to the dealer, losing any interest I had in the model because of my opinion that it was way underpowered.

You may be expecting more out of the car than it's capable of in the performance department. Other than acceleration, they're fine cars for the mostpart, sans the rod-bending engines some had.
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Old 08-06-2013, 05:26 PM
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Join Date: Aug 2013
Posts: 4
Thanks Skid? If it was just dogging all the time I can live with it. But as I found sometimes it outperformed my '87 in both power and mileage. What is making me crazy is that it comes and goes. Sometimes great sometimes a dog.
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Old 08-07-2013, 11:47 AM
is thinning the herd
Join Date: Mar 2010
Location: Columbus, Ohio
Posts: 3,319
Have you changed the strainer In the tank? Checked the injectors spray pattern?

Now I hate that everyone immediately jumps to this, but it would be worth compression testing your 3.5 to make sure its worth continuing. If you've got a bunch if dead cylinders no amount of tuning will fix that.

Like joe said W140s are big girls, and they are rated at as near as makes no difference the same power as the 87 300D. I had a 1991 350SDL and my 1987 300D was considerably faster. And my 350 was in great running condition and compression tested well when I bought it.
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Old 08-07-2013, 12:15 PM
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Join Date: Aug 2013
Posts: 4
Thanks. Its been suggested that I try testing the EGR with should be pretty simple. Failing that I'll have to try the compression test, even though I don't understand how low compression or dead cylinders could get better for a day or a hundred miles then get worst. I do appreciate your input though.
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Old 08-07-2013, 01:46 PM
Bio Brewer
Join Date: Apr 2010
Location: Napa, CA
Posts: 300
Not sure if that has a "CAT" on the exhaust but I gained a HUGE amount of "ooompphh" by that device mysteriously becoming hollowed out. My w124 was a complete dog when I got it but after that happening it's much more of a pleasure to drive.
'98 E300 Turbo "Juliette"
'87 300D Turbo "Roxanne"
'93 300D Turbo "Betty"
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Old 08-07-2013, 02:59 PM
compu_85's Avatar
Waiting for his Model 3..
Join Date: May 2008
Location: Springfield VA
Posts: 5,174
Remember that the turbo is controlled by the EDS computer on this car, just like it is on the 2.5 Turbo engines. The wastegate is normally open and the computer commands it to shut to build boost. If it detects any fault (like the EGR not working) it will go into limp mode and cut boost.

Stock the engine should make almost 150 HP and about 250lb/ft of torque off the line. The W140 is heavy, but it should still move decently with the 3.5.

1991 350SDL. 230,000 miles (new motor @ 150,000).

2013 Fiat 500E. 20,000 miles, 80 miles at a time.
2012 Passat TDI. 95,000 miles. More space, power, and fuel economy than the Benz
2004 Touareg V10 TDI. 150,000 miles. One of 450.
1999 Jetta TDI. 310,000 miles.
1992 Jetta ECOdiesel. 156,000 miles. 1 of 48. Sold.
1991 Jetta ECOdiesel. 430,000 miles. 1 of 700. Sold to VeeDubTDI, totaled in front of our house
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Old 08-07-2013, 04:06 PM
is thinning the herd
Join Date: Mar 2010
Location: Columbus, Ohio
Posts: 3,319
Totally forgot about the EDS system, compu is right that is definately worth investigating.

See if you can temporarily put a boost gauge on your car and monitor it while driving.
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Old 08-08-2013, 10:01 AM
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Join Date: Aug 2013
Posts: 4
Mystery solved. Turns out I've been running with intermittent turbo boost for
months. Went to check if the EGR would hold vacuum last night and got a sucking sound. The hose from the firewall to the EGR was rotted almost all the way through. It couldn't be seen as it was on the underside behind the EGR. Cut off the rot and reconnected, and its like a different car. Its good to have an effective turbo.

Thanks to Steve for his excellent explanation, and everyone who too the time to read my 'mystery'.
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