I've exhaustively searched the forum and can not come to a conclusion... thus, a NEW thread is born!

I have an 83 300sd. I'm trying to diagnose if my vacuum pump needs attention or not.

Lot's of info below... my plain and clear question is in BOLD RED at the bottom of this post!

Symptoms are:

*Brake pedal is usually very hard after startup. Takes a bit for it to get enough vacuum to function properly. While driving, the hardness of the pedal is intermittent. Sometimes the pedal is easy to push and the brakes work awesome! Other times the pedal is VERY hard and it is difficult just to keep the car stopped at a red light.

*Trany is shifting at the right times but has big flaring. This problem is also intermittent. Sometimes flare is just right, other times the engine revs between the long soft shifts.

*Climate control center valve is not working properly. The Center Vent valve defaults to the defrost position when no vac is applied(as it should). When I select center vents on, it splits about half defrost vent, half center vent. I don't know if this is because of a faulty center valve, a vacuum leak or just not enough vacuum to operate the valve.

As you can see I have a few problems that could be caused by any number of things. And one problem might be causing the rest.

PLEASE HELP... I can not find any vacuum leaks under the hood... I have already checked and replaced a few connectors and vac hoses to bring back up to “like new” shape. I need to start somewhere with the rest of the diagnosis process and I decided to start with the vacuum pump.

Is it possible for the vac pump to be working but not creating enough vacuum for the systems to function properly? I took the main vac hose off the top of the pump and applied my thumb over the check valve while the engine was running. It did create a vacuum on my thumb but nothing impressive.

Does anyone know how much vacuum the pump itself is supposed to be making??? Should it suck my thumb to it tightly when I test the vacuum? How can I messure it? Lot's of threads here about how many inchs Hg should be making it to the trany vac system. What amount of "Hg should the Vacuum Pump itself be creating?

Once I know if the problem is my vac pump or not, I'll start digging into the rest of the more complicated stuff (ie. the climate control BARIED in the dash... uhg ).

I appreciate GREATLY any thoughts and help you can offer!

__________________
Cousin Dave
83' 300SD 250k+ and going strong!

Take the best that exists, make it better!
If it doesn't exist, create it!
Accept nothing nearly right or good enough!

A properly working pump will create a minumum of 20" vacuum. Some do better than 20".

You need a Mityvac or a suitable vacuum gauge for testing.

A Mityvac is a basic necessity for working on these vehicles.......almost as important as a 13mm socket.

I had recently many of the same symptoms you report -- intermittent hard brake pedal, hard shifts, no engine shut-off, etc.. Because it was intermittent, I had a hard time getting a vacuum tester in there in time to "catch it in the act", but the problem finally progressed enough to where I measured just 5" vacuum at the pump. The spec I think is 22" for my '84 300D. I ended up putting a new pump in ($$$$$), and everything is back to normal. I kind of wish I had saved some money by rebuilding the pump I had. But it was a busy week, and I just couldn't find the time.

Another dead giveaway that your vacuum pump needs rebuilding is black oil in the vacuum lines. The source is the engine oil where the pump interfaces with the engine block. If the pump allows engine oil into the vacuum system, you will need to replace all the hose up to the brake booster and even the brake booster inself if it has oil in it ($$$$).

Another thing, once you rebuild the pump and you have good vacuum, you can expect the next weakest thing in the vacuum circuit to pop. 24 year old car = needs all the mission critical rubber/plastic replaced.

Thanks for the quick responses!

After reading your posts, I am suspecting my Vacuum Pump to be weak. I will get a vacuum gage and test it for proper Hg. Thanks Brian!

Thanks MDF. I was wondering if the Vacuum Pump was one of those things that either work, or they don't. In your case the pump was working but only partially... this further increases my suspicion of my VP. If my test show a bad pump, I'm going to get a rebuild kit for sure. MUCH less $$$$ than new!

Thanks Lifer... I'll check for oil in the vac lines when I disassemble. I'll cross my fingers on that. Sounds like oil can cause saver damage. Hopefully everything else will stay together once I get proper Hg again.

Thanks Folks! I'll post here when I get this figured out.

__________________
Cousin Dave
83' 300SD 250k+ and going strong!

Take the best that exists, make it better!
If it doesn't exist, create it!
Accept nothing nearly right or good enough!