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Intermittent Vacuum Loss

2K views 13 replies 6 participants last post by  Golden Eagle 
#1 ·
Working through the bugs on a 1983 300CD that I bought earlier this week.

Last night I took it around the block just as a test run and it ran pretty well until about 3 miles into our journey when suddenly the brake pedal got rock hard. The brakes still worked but not very well and I had to stand on them to get a response. I suspected vacuum and sure enough when I got home and got parked it would not turn off with the key (it previously had turned off with the key without any issues). I shut it off with the stop lever under the hood and called it a night.

Today I fired it back up and the brake pedal was still too firm and it would not shut off with the key. While the car was running, I put a vacuum gauge on the two nipples attached to the main vacuum feed from the vacuum pump to the brake booster and sure enough I got a zero reading. I turned the car off and disconnected the main vacuum line from the vacuum pump and restarted the car. Suddenly there was full vacuum coming from the pump. I reassembled everything and now I have good brakes and the car turns off with the key as expected.

Thoughts?
 
#2 ·
Driving around the block last night and initially the brakes worked fine. After a couple miles, the brakes got rock hard again but this time the car sputtered and died. I jumped it and it started back up, but died again about 100 feet down the road. I jumped it a second time and this time after about 100 feet, it started to chirp/screech extremely loudly and then died. I gave up and dragged it the rest of the way home behind my truck.

I was immediately thinking alternator with the fact that it keeps draining the battery and figured the screeching was the bearing going out in the alternator. So i took it off and had it tested at the auto parts store and their machine claims it is still good. Spinning the pulley sounds a little gritty but not horrible.

Any ideas?

Also, there are two grooves in the alternator pulley but only one belt. Is this correct or should there be two belts?

Thanks.
 
#4 · (Edited)
After your initial post, I was going to suggest it might be the in line vacuum check valve at the pump. After your second post, it sounds like it may be an internal pump failure, as I've heard they make a lot of noise when they fail. The pump is mechanically driven by the engine, which is why I say this may be cause along with what you said about the car losing vacuum and then stalling.

What's mileage on the car? Is it the original engine?
 
#6 ·
I pulled the vacuum check valve at the vacuum pump and found it to be completely hollow inside. I assume there should be some kind of internals that constitute the 'valve' portion of the check valve.

Could this be causing the screeching/chirping noise I heard?

Is it worth looking for a replacement at the junkyard or should I just spend the $80 on a brand new one?

 
#7 ·
I don't remember It being hollow inside. I did replace mine along with a million other tings when I bought mine back in 2007 just for preventive maintenance. I know I bought one new and it wasn't over 30 dollars. 80 seems too high. All it's supposed to do is allow air to flow in one direction only, away from the pump. Don't buy it used, try peach parts, autohaus or Kent Bergsma. Sorry for the late response, frustrating when you're trying to fix a car thats kaput.
 
#8 ·
Well I've done tons of stuff to the car in the 6 weeks since I made this post and today I'm finally ready to get back to testing the vacuum pump.

I installed the new check valve and still have what I think is intermittent vacuum. I say I think it is intermittent because it was not working today but unless it has completely failed since my first post, it could work tomorrow.

Is it common for the vacuum pump to slowly fail over time or does it all go at once? Is there a procedure to definitely say that the pump is bad?

Thanks.
 
#9 ·
I think you're going about this all wrong....

Do you have a gauge on the vacuum line to the brake booster so you can see if you're getting vacuum?

If the line to the booster is cracked, or one of the hose barbs is broken off, or a hose connected to the barbs is broken or leaking, you'll lose vacuum as quickly as the pump can make it.

Understand that the small plastic vacuum lines on your car are extremely brittle and will break at the slightest touch unless they've been replaced. This being the case makes the possibility that one of them is broken or cracked and you're simply losing vacuum as fast as the pump can make it.

Without using a gauge to do some troubleshooting you're doing nothing more than swapping parts, a very inconsistent and expensive approach.

Dan
 
#10 ·
#11 ·
Reading the thread, it seems you maybe do for a new vacuum pump. Before I were to drive this anymore, I would pull the pump and inspect the bearing. All that noise your hearing, and the car dying. Leads me to think the pump has failed and you've lost the bearing. It may very well be tearing up the engine.
Thank you. I'm planning to pull the pump off today to inspect.
 
#12 ·
Pulled the vacuum pump off for inspection. First thing I noticed is that the small plastic piece and spring that broke out of my original external check valve were still rattling around in the short inlet tube where the valve screws in. The rest of the pump looked flawless--including the bearing, piston, and piston wall. I haven't reinstalled yet, but I'm hoping my solution is as simple as removing the plastic piece and spring. I'm thinking the plastic piece must've aligned just right in certain instances to block off the vacuum supply and cause intermittent vacuum loss.

Question: Do I need to prime the pump with oil before reinstalling? Seems like there should be at least a little lubrication for when the engine first starts until the engine can supply the pump piston with oil.
 
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