Clean the overboost protection valve; don't bypass it, if it's "slow". If your wastegate hose cracks or falls off (it could happen ) the T3 turbocharger can put out 25-30 psi (with concommitant fuel enrichment). Without the overboost protection valve limiting it to 1.1 bar, you can have a 300SD with a really low 0-60 time, BUT a really short melted piston time (est. ~ 5 minutes at load ).

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

So I can just spray some brake cleaner (non-chlorinated) through it to clean it or what? I was under the impression that when they got slow there was nothing you could do but buy a new one or bypass it.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

OK guys, some points here:

1- 15 seconds 0-60 is about right for a W126 with an OM617.95x engine. You might sqeak that up to 14 but that's about it. That's pretty close to factory spec, IIRC.

2- More boost does not always mean more power. More boost with no extra fuel means superheated intake air, which could possibly REDUCE power. Spec for this engine is 0.75-0.85 bar (about 10-12psi). Don't believe me? Test your boost pressure now (probably 9-12psi). Pinch the rubber wastegate hose shut and repeat the measurements. My 1984 300D ran IDENTICAL times (0-60) with 10psi (wastegate working), as it did with 16psi (wastegate disabled - hose pinched). You need to turn up the fuel delivery, which is an INTERNAL adjustment (NOT the ALDA), and requires pulling the oil filter housing off to access the rear governor cover. Big job.

3- I also don't recommend bypassing the overboost protection circuit if it can be helped, but I must admit it is bypassed on my '84 (which I have confirmed won't get anywhere near damaging boost levels anyway).

4- A handy "boost gauge" is the MityVac Silverline 4050 kit (~$60 from thetoolwarehouse.net), it's also a vacuum & pressure pump - this is a MUST HAVE tool for any Mercedes owner. Install a "T" at the ALDA, or in place of the overboost solenoid temporarily. Then run a length of hose from the T, out the hood seam, through the driver's window. Check max boost in third gear at 4000 rpm (a steep hill helps).

5- If you ALDA is maxed out and you believe it still need more fuel (no smoke and/or still slow off idle), add shims
underneath. This is a band-aid fix, the real solution is the internal adjustment.

6- Last point. The G-Tech Pro really is a piece of junk. I own one (it's going on eBay shortly). I replaced it recently with a Race Tech AP-22 unit, which is conservatively about a billion times better - way too many cool features to list here. It costs a little more but is worth every damn penny, IMO.

Here's some sample data output from the AP-22:
http://www.meimann.com/docs/mercedes/87_300D_blue_perf.txt

Here's the link to AC-22 & AP-22 meter info:
http://www.cb-racing.com/rt-index.htm

Here's a link on adjusting the OM603 internal full-load setting, the 617 pump is similar:
http://www.meimann.com/docs/mercedes/Bosch_full_load.pdf


Good luck,

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