Hello all,

I have a 1985 300SD with 208K miles. This past weekend I decided to inspect the turbocharger boost output, so I measure it directly after the switchover valve. On a full load run uphill I was getting 9 psi of boost. I believe that ~11psi is the factory setting. So I moved the measurement to just before the switchover valve and the same results. Next, I adjusted the wastegate 2 full turns clockwise or 2 mm. There are 1.1mm of threads from the stem tip to the adjustment nut. Re-tested boost and 9psi again? The boost is smooth, but I would have figured that i should have gained 2 psi based on some reading of how to adjust the wastegate.

Did I not adjust the wastegate enough? Or do I not have enough fuel output from the injection pump?

I was thinking of temporarly pluging the wastegate reference hose to determine if the injection pump was providing the required fueling. Any thoughts?

Additionally, I did check the fuel pressure just before the injection pump (after all filters) and it is low according to the FSM spec.:

Idle = 8 psi (should be 11psi)
3000/min = ~11 or 12 psi (should be 19 psi)

Just replaced the fuel filters too. Additionally, I pinch the return line for a second and the pump was able to build 20 psi.

I am wondering if my relief pressure valve is bad.

I am wondering if the low fuel pressure might be a cause of the low boost situation, would this significantly affect output at the injectors?

Within the past 8k miles i have performed the following repairs:
Check injection pump timing (via A-B light method)
Adjusted the valves
Adjusted the throttle linkage (huge performance difference)

I was wondering if anyone has any input. I think I am going to try plugging the wastegate hose first. Then maybe look into the fuel pressure relief valve.

Any help is appreciated.

Thanks!

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1985 300SD 200K

So long as you don't let the engine overboost, it is ok to disconnect the wastegate for a short test run-just be sure to watch the gauge like a hawk.

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Hmm. fuel pressure sounds like you need to at a minimum, stretch the spring on the lift pump. possibly replace it.
if that makes no change, next would be to adjust or remove the ALDA

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Thanks for the input. I think I'll try plugging the wastegate to determine whether I have enough fuel.

Unfortunately, my SD has the unserviceable style relief valve! I think I might have read on the forums that my relief valve is interchangeable with the serviceable valve, but can't remember where I found it. Does anyone know if this is true?

Testing was performed with and without shimming the ALDA up ~1.0mm. Would removing it and testing it change the fuel output? Does anyone know if there is a certain boost pressure where the ALDA would act like it is removed?

Another thought, are boost leaks common on these cars?

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1985 300SD 200K

Plugged the wastegate line able was able to get 12 and 13 psi. So must be I need to adjust the wastegate further.

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1985 300SD 200K

Sounds low with a plugged wastegate, you should see around 20 psi (depending on turbo/engine condition). The wastegate springs also get weak at this age.

• Stretch xx to 27 mm and recheck fuel pressure reading.
• Bypass ALDA & Overboost Valve and measure boost directly from turbo w/ wastegate dissconnected
• Warm the car up, take it somewhere where you can drive it in 4th gear at 2500+ RPM.
• While in 4th gear and 2500 RPM, go full throttle and slightly apply the brake
• Watch the boost level, repeat with another gauge to insure your getting an accurate reading.

This reading will be the MAX boost the engine will create in its current configuration (compression, fuel, injectors, timing, etc...). Now you can connect the wastegate hose, repeat and try to adjust the boost level.

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Sorry for any confusion, I forgot to clarify the conditions of the plugged wastegate testing. Testing was performed uphill, 3rd gear at somewhere around half to 3/4 throttle position. I was feathering the throttle to prevent overboosting and just wanted to verify that I had enough fuel to provide the factory 11 or 12psi. Thank you for your suggestions and if I get time this weak I will try your suggestions. Is it possible the wastegate spring is so weak that it 1.0mm of adjustment does not correspond to 1psi of boost anymore?

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1985 300SD 200K

It might be-the wastegate spring has been seeing 800°-900° for 27 years

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

I do not check boost at idle or under no load. In general, a stock turbo won't spin fast enough to build boost at idle. I check at 60-65 mph cruise (where I make a solid 10 psi) and WOT in high gear, where my W/G blows off at 12-13 psi

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1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



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