My W124 Vacuum Problem

[shertex]

Just over a month ago, I started a thread looking for some basic guidance on diagnosing a vacuum problem: Basic Vacuum Diagnosis Strategy. Since then I've learned a few things and made some additional observations....so I thought it best to start fresh.

The symptoms on my 91 300D were and are very mild shift bumps and headrests that take a long time to retract. So I knew immediately that I had some sort of vacuum issue.

I measured a consistent 23" of vacuum from the pump at idle. As I THOUGHT (could have been my imagination) I might be experiencing some less-than-perfectly-responsive braking, I wondered about the brake booster. So, at the recommendation of my mechanic, I tried to get a vacuum reading off the brake booster line by trying to remove the line that goes to the shutoff valve. In the process, I broke the fitting.

So I took it to my mechanic to see what he could discover (and also to replaced the broken booster line). He concluded that the brake booster was fine and he wasn't able to find any leak. However, when he replaced the hose he noticed a bit of engine oil at the fitting between the pump and the booster line. As there should not be any oil in that line, and as the only two possible sources for an oil breach are the pump and the shutoff valve, he strongly suspects that I need to replace the pump...he thinks that it is likely that the pump diaphragm has been compromised and that that's the source of my vacuum issues.

The pump is original and of the original ticking-time-bomb design, but the car only has 113k miles and I'm very reluctant to shell out the money to replace the pump unless I'm 100% convinced I need to. Yes, I know some think I should do this preventively....

So, I got the car back from my mechanic and have made the following additional observations: (1) the part of the circuit that runs from the pump to the headrests and climate control is fine. I applied vacuum and was immediately able to make the headrests go down. (2) As I drive, vacuum builds up in the system so that the shift bumps go away and I'm able to retract the headrests.

The part of the vacuum system that goes to the passenger side of the engine is blocked off due to the wastegate conversion. So, it seems to me (though I admit that my knowledge of the vacuum system is very rudimentary) that I must have a leak or a failed component in the circuit that goes to the the vacuum modulator, etc... In particular, it seems to me that, if I get a solid 23" from the pump at idle, that should be adequate to make the headrests go down....but they don't. But, perhaps my assumption that a solid 23" of vacuum at idle means that the pump is producing adequate vacuum is incorrect.

I've reached the limits of my diagnostic abilities and am not sure what to do next. Given the above, is my mechanic correct that the pump is probably the problem? If not, what should I do/test next?

Thanks so much.

[funola]

Easy and cheap: install a clear inline fuel filter to look for signs of oil.

[shertex]

Originally Posted by funola

Easy and cheap: install a clear inline fuel filter to look for signs of oil.

You mean to confirm that there's oil entering into the vacuum system?

One thing that didn't make sense to me is that, since the pump produces vacuum and not pressure, how could oil get downstream from the pump?

[funola]

Originally Posted by shertex

You mean to confirm that there's oil entering into the vacuum system?

One thing that didn't make sense to me is that, since the pump produces vacuum and not pressure, how could oil get downstream from the pump?

Yes and that's a good question. The oil may be coming from other parts of the engine so you have to strategically place the filter to look for the source of the oil.

[JamesDean]

downstream?

All things flow towards the pump since its vacuum so it could be sucking oil from something plugged into it..I would think.

[shertex]

Originally Posted by JamesDean

downstream?

All things flow towards the pump since its vacuum so it could be sucking oil from something plugged into it..I would think.

To the best of my knowledge, the only other possibility is the shutoff valve. And if the shutoff valve is the problem, it's hard to see how that would account for the symptoms I'm experiencing.

BTW, it's not like there was much oil....just traces.

[JamesDean]

I need to go through my vacuum system. My headrests stopped working too.

[shertex]

When I disconnect the line going to the vacuum modulator and take a pump reading it goes immediately to 22-23".

When I disconnect the line going to headrests and climate control (i.e. with line to vac mod sill in system) I get a poor reading (around 8").

So it seems pretty clear to me that the pump is NOT the cause of the symptoms...that the problem is in that circuit I suspect.

Question: should the clear line with the blue stripe that runs from the VCV hold vacuum with engine off? It doesn't....so maybe I have a bad VCV?

[shertex]

As I'm getting a 5" output from the VCV with a 22" input, I'm fairly confident that my VCV is bad. Able to find part for $161....ordered.