So, I was doing some reading about "manual" turbo boost. I decided to get one, first I installed a boost gauge to check output. Before I install the manual boost controller, at idle I rev up the engine to see where the highest boost is. Around 10, installed the manual boost controller set at the lowest setting from the vendor.

rev the car at idle again, boost climbed to 15. Now as I drive it during my daily commute, I see 3psi at 40 mph and 5 at 50 mph. In addition during upshift at times it will climb around 18-20.

Where or at what boost should I see during normal driving condition? The shift point has change some, from first to second and third. The final drive takes a little longer, hence the higher boost indication.

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1984 300D, 1972 LT1 Corvette, 2014 Artic White 3LT/Z51 C7, 2013 650i Convertible BMW, 1994 Kawasaki Vulcan, 1992 Yamaha Seca II and 1996 Dodge Ram 2500.

This is what I got from the 1983 Mercedes Technical Data Passenger Cars.

Boost Pressure Driving in “L” Position at 4000/min bar gauge pressure 0.7-0.8

I interpret that to mean you put it in Low and drive the Car till you reach 4000 rpms. At that point you should have 10.15 psi - 11.60 psi
For Conversions Worldwide Wholesale Distribution of Valves, Fittings, Tubing & Flanges: World Wide Metric

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84 300D, 82 Volvo 244Gl Diesel

My recollection of posts in this forum is 12-13 psi max for cars similar to yours. There should be an overboost protection circuit cutting boost at about 15 psi. Did you remove the ALDA or disable the overboost protection system?

How do you "rev the car at idle?"

Sixto
87 300D

To the OP, where did you tap the boost gauge, pre or post overboost relay? I just installed an inline boost controller and my readings don't match up to the factory cut out value either.

Are you accelerating when you say you see 3psi at 40 and 5 at 50? You aren't going to see any pressure while cruising at those speeds. You only see boost build during acceleration until you hit hwy speed and you're around 2500 rpm.

I have my boost guage tapped after the protection relay, right before the ALDA and found out my max stock boost was 8 PSI. I installed the MBC and then got up to 8 PSI during moderate acceleration (4 PSI normally). I then adjusted it to reach 12 PSI and was successful until I continued the pull up a hill. Once my gauge reached 12 PSI, it snapped back to Zero and I lost power.

It only took a second or two and i regained power after letting off, it was definitely cutting back fuel, which would be the Overboost protection relay doing its job. However, I expected to see 15 PSI before the relay tripped.

I've thought about reasons for seeing low boost numbers while activating the relay and have come up with these scenarios:

1. the gauge has a leak and shows lower than actual pressure.
2. pressure sensor in the manifold is dirty and shows a different value to the relay (though this would usually result in the relay not functioning at all.)
3. the pressure signal from the relay to the ALDA is lower than actual manifold pressure.

Can you let us know if your gauge signal is coming straight from the stock manifold tap or tapped into line right before the ALDA?

At least in my case, I'm glad my relay was functioning because after all, I could have a faulty gauge, (my VDO 0-15 PSI gauge is made in India).

I thought I would update for anyone who may have this problem. I was getting fuel cutting out once I hit 12 PSI, but the OBP realy shouldn't be cutting fuel until 15 PSI. At this point my guage was tapped between the Overboost solenoid and the ALDA. My boost readings were inconsistent when read from the OBP circuit so I decided to use the tap on the manifold.

After tapping the manifold port, I found my boost would always reach 12 but the fuel would still cut out. Starting with the easiest thing first, I removed and cleaned the pressure sensor on the manifold, that didn't change a thing. Second, I removed the Overboost solenoid on the firewall and tried to blow through it. Turns out the valve was clogged and the ALDA wasn't getting the correct pressure signal. So I used some MAF spray to clean it out, black gunk shot out everywhere.

The result: Much better response pre-boost and pressure builds quicker.

If you haven't disabled your overboost protection circuit or removed your ALDA, its definately worth it to check on the solenoid and clean the pressure sensor. Take care.

Willson
85 300CD

Your OBP should activate about 16 psi, IIRC. I have run the old 300SD at 14.7 lbs maximum (wastegate controlled) for years and normal driving on the freeway at 60 mph will give 10-11 psi readings.

There really is no spec for boost at normal cruise, but 10 psi has been good for the past 100,000 miles or so...FWIW

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Did you just pass my 740 at 200 kmh in a 300SD?????

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



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership