'87 300D (603) Low Boost Pressure (9 psi max)
 

My car runs great but the turbo seems to be MIA these days. There is a noticeable lack of boost from 45-65 mph. I installed a boost gage between the switchover valve and the intake manifold boost line. In 3rd gear under load @ 4000 rpm, the max boost that the gage is reading is 9 psi. From a standstill, the boost will go to 5 psi quickly but afterwards it's a dog until the rpms build up. I'm thinking the wastegate is stuck open or out of adjustment. How is the wastegate adjusted on this particular model? Is the procedure similar to that in the w123 turbo's? Any replies would be greatly appreciated.

603 boost
 

Do a search on this topic and you will find an evening of reading. There are two different turbos found on the 603 engine-KKK and Garrett. The KKK is adjustable, the Garrett needs a restrictor placed in the hose to decrease action of the wastegate.

I think you should pull off the cross-over pipe between the turbo output and the intake manifold. You can inspect the interior of the pipe and the area around the EGR valve to see if it is restricted due to a buildup of soot/oil sludge.

While I don't think its necessarily your problem, it is an inexpensive thing to check first. I've seen pictures of them so clogged that it would probably affect boost.

Ken300D

Best reply to a Turbo problem...
 

In the Mbenz 124 posts, there is a turbo problem reply. I will repost it here as best as I can....
His post: The turbo is probably fine. the problem could be a occluded signal line from the intake to the ALDA, that allows fuel enrichment when boost is present. To pronounce the Turbo as not working you need to connect a pressure gauge and prove that there is low or zero pressure, and then make sure the wastegate is OK. Garrett turbos have a rubber hose to the wastgate, but if that breaks or fails it will not limit power- it will prevent the wastegate from opening and provide too much boost. this will trigger the electrical protection circuit and it will kill the signal to the ALDA.
The ALDA is on top of the injector pump, under the intake manifold. The ALDA requires no maintenance. However the plastic tubing that feeds it does, especially the electrical switchover valve, and the fitting at the intake manifold, both of which can get clogged with soot. Disabling the EGR eliminated the soot problems.
The 124 was found to be the rubber hose to the wastegate that was cracked and broken. The overboost switchover valve was clogged with soot and was also replaced. Problem solved.
I hope this will be of some help to you!

Thanks guys for your replies. EGR is non existant on my car ever since I took ownership of it ;) I flushed all boost related lines and both switchover valves with carb cleaner and still no luck. Then I decided to adjust the wastegate. I took removed the 10mm nut and then thightened the adjustment screw with a 3mm allen key as much as possible and took it for a spin. 3rd gear, 4000rpm, under load, I'm now getting 12 psi boost which is much better than the 9 psi that the gage was reading prior to the adjustment. It's still doesn't accelerate the same as when I bought the car but its better than what its been lately. I guess I'll have to replace the wastegate someday.