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1990 300D 2.5T Low power/no boost

5K views 0 replies 1 participant last post by  2.5Turbo 
#1 ·
I'm pretty stumped here. My 1990 300D 2.5 Turbo all of a sudden stopped producing any usable boost in December. I checked the usual things, like the overboost solenoid and the hoses and fittings to the ALDA, and everything was good. Turbo spins freely and there is no lateral play in the turbine shaft of any significance. Remember that this car is the one with the Electronic Diesel System (EDS) computer which controls the boost, idle speed, and EGR components. Since this car has a vacuum controlled wastegate (no vacuum = wastegate open), there is a vacuum transducer between the vac line and the turbo wastegate actuator, controlled by the EDS. Normally the EDS looks at the boost pressure off the manifold and adjusts the wastegate accordingly. When the EDS finds a fault somewhere in one of its sensors, it will cut all boost and the car will get about an 18 second 0-60 time (published spec is a skoshe over 12). Codes were cleared and it still was slow.

My mechanic looked at it a while back and didn't find much, but didn't test the transducer for the wastegate (Y31/3). He's doing that now. Here's a link to a copy of a service bulletin on the vacuum transducers: Vacuum Specs 1992 300D, Turbo Wastegate - ShopForum (go down to the last post).

Not sure what mine has as I've been away at school since the end of January.

If the transducer is good, I'm thinking about hooking straight vacuum to the wastegate actuator and lightly driving around to see if the turbo will produce boost if the wastegate is held closed, of course with the overboost solenoid and switch operating. If it does (and the ALDA enriches the fuel correctly), then we know the problem is upstream of the vacuum system. Hopefully it's not the EDS module itself...that thing is mega bucks.

Any other ideas on why this could be happening? It has been driving me crazy lately!
 
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