Vacuum system question
 

I just bought a 1993 300D 2.5 with 188K miles, and have been having power issues. Mainly, the car doesn't seem to have power, either from no boost, or shifting too early.

After reading some threads and such, the first thing I wanted to check were the vacuum lines to make sure there weren't any leaks. Here are two pictures that show something that I think is odd (click the images to enlarge):

http://iamstuffed.com/blog/wp-conten...04-300x225.jpg http://iamstuffed.com/blog/wp-conten...05-300x225.jpg

My first question is, what is the purpose of sticking a wooden dowel (the blue stick) into the vacuum rubber junction thingy? Would that be the cause of the power loss?

Everything else in the car seems to work.

I don't know how much power this car should have to begin with, but it doesn't have any power on the highway. I can floor it and it doesn't downshift either.

The bigger problem isn't the blue dowel but the branch of that splitter that goes down to the vacuum pump is disconnected!

Sixto
87 300D

That model has the dreaded, at least I think it's dreaded, vacuum actuated wastegate. Conventional turbos use a pressure signal to open the wastegate to control maximum boost. This engine uses a vacuum signal controlled by the ECU to close the wastegate allowing boost pressure to build. Not only do you have to have a vacuum source available to close the wastegate, the engine has to have all its stars aligned for the ECU to concede boost.

I'll have to check a vacuum diagram but I believe the line plugged with the dowel supplies devices on the exhaust side of the engine including EGR, wastegate and a throttle flap of some sort. Of the two lines going towards the driver seat, one supplies the AC system, the other supplies the transmission vacuum control system.

The transmission vacuum system shouldn't affect downshift ability. There is a bowden cable that communicates throttle position (and possibly an acceleration derivative) to the transmission. There's also an electric kickdown switch at the far end of the accelerator pedal arc. You should feel a click as you press the pedal into the carpet. Peel back the mat and carpet to find the switch.

Diagnosing this situation will take time. I'd spend as much time researching gsxr's pressure wastegate conversion.

Sixto
87 300D

Thanks for the replies. I disconnected that tube just to take the picture. It isn't normally disconnected.

Yes, I noticed in the first picture.

Also to check, there is a hose from the intake manifold plenum to a squarish device atop the injection pump. This squarish device is called the ALDA (German acronym for alitude and/or pressure compensator). The ALDA resists the IP's natural tendency to provide fuel for a max boost condition. The ALDA's influence is reduced as pressure in the intake manifold increases. The ALDA has to get a good boost signal from the manifold or the IP will think there is no additional air and not provide additional fuel. Make sure the line from the intake manifold to the ALDA is clear. There might be devices along the line. Those devices should allow air to flow freely as well. Don't forget to check the nipple on the manifold. It takes a 12mm wrench and it tightened to a very light torque. I've had no problems cleaning the line with brake parts cleaner.

On that subject, any tar you find in that line coats the inside of the intake manifold. You can thank EGR for that. At some point you will be advised to disable EGR because it's hurting as much as it's helping. The vacuum controlled wastegate complicates disabling EGR since EGR function is one of the ECU checks before allowing boost.

Sixto
87 300D