I recently purchased a used MPS from a reputable 914 parts dealer in my area. I decided to vacuum test it before installation. Started with 20in HG and timed the leakdown. These are the vacuum levels I saw as time passed:
1 min - 17
2 min - 14
3 min - 12
4 min - 10
5 min - 8
6 min - 7
7 min - 6
8 min - 5
9 min - 4
10 min - 3
11 min - 2
12 min - 1
13 min - .5
14 min - 0
Is this thing bad? I would think that if the diaphram was cracked, it would lose vacuum faster than this, but I really don't know. I hate to tear it apart to re-seal it and then ask for a refund if the diaphram is cracked.

Thanks,
Bob

It should not leak !!!

Leak down should be no more than 5 pounds in 15 minutes. I believe, someone correct me if I'm wrong please.

D-Jet Fundamentals

According to Banders, it should not go below 15" in 5 minutes.

Are the hoses to the unit tight? is the vacuum pump you are using tight?
I have never seen one with a small leak...it is ripped, or its not.
When they rip...they rip quite a bit.

I had alot of trouble getting a reliable seal using a mightvac hand pump. I could never trust that the hose or the pump were not leaking instead of the MPS.

I test them with my tongue...(fire away boys...but its true)
Sounds crude, but its simple and the tool is always with me...unless SLITS catches up with me and cuts my tongue out for shilling MIGHTY PUTTY instead of jb weld.
I digress.

Suck on the mps just enough to stick your tongue to the end of the unit...if it holds its weight...its good.

Rich

I've seen small leaks like this, and taking it apart and greasing the o-ring fixed 'em. I agree with the mighty vac comment.

The Cap'n

Send it back and get a good one if that's an option. If not then take it apart. I recently purchased a new one from fuel injection corp. It started to leak after 2 weeks. Second unit that leaked from them. I took it apart, cleaned the grease they use to seal the diaphram off and used a thin coating of grey permatex. Works like a charm.

The hose connection to the MPS from the vacuum pump is very tight. The vacuum pump is a Harbor Freight special (US General). However, I just applied 20 HG of pressure to my thumb and drank a beer (very scientific, I know)...and it still held at 20 - so I am comfortable with the testing equipment. My old MPS had a torn diaphram and would not hold anything.

Good for you for testing the pump/hose set-up Bob, thats good assurance. Always test the equipment before testing the MPS. But make a more air-tight connection than to your thumb if any leakage is noted. Clamp that hose down on something smooth and solid for an airtight connection if it leaks.

Using only the mouth to suck on the MPS and test it is no indication that it is a viable MPS unless there is abolutely no vacuum level obtainable from the MPS and it is a complete leaker. Try mouthing on the end of your Mighty Vac pump hose and see how much vacuum you can generate. Bet the needle does not move at all, a human mouth is not capable of producing the required pressures of an MPS and it is not sensitive enough to detect leakdown rate either.



Ideally the MPS will not leak at all and a new MPS should not leak - Agreed

They can develop leaks not only from a cracked or cracking diaphragm, but also at other locations in the vacuum chamber just from age - around the case halves (O-ring), the plastic electrical connection and at the interface of the diaphram to the case. The seals can dry out which usually manifests as a slow leak and a slow leak may not be a cracked diaphram.

Opening and resealing them can be a good repair, just be sure that the sealer used is not too thick so that it affects the calibration.

As far as a functioning MPS goes, a slow leak is often quite functional because of the forgiving way the MPS works. It responds to intake manifold vacuum levels that pulse and vary in intensity - not a constant pressure like a vacuum pump will apply. So it may slip a little between intake pulses but as long as the loss rate is not dramatic it will be serviceable. I have personally run MPS's with moderate vacuum leaks - much more severe than Bobs is showing - for many years without any issues.

I would not say your MPS is 'good' Bob, and it is probably out of factory spec for ideal - but those vacuum loss levels are pretty good actually.

Jeff got me curious. God I love the Internet...I would have thought about this for days.

Normal Atmospheric Air Pressure 1 atm = 101.3 kPa = 14.5 psi or 29.2 in HG.

A study of newborn infants showed that a baby produces 320 kPa while sucking a nipple...

So, if we use an infants capabilities, a human can suck 90 in HG


Rich

I still have the other one if you want to try it. Have you tried this one yet, or just tested it?

I installed it just to give it a try. The car would start right up and run for about 15 seconds...then just die. I disconnected the vacuum line to the MPS and plugged it and the car would stay running (not running well, but it kept running).

I have another MPS in transit. Let me talk to Ed about this one and see what happens with the other.

Thanks Evil!

Slight hijack here but, what is the difference between (or is there a difference) 1.7 and a 2.0 MPS??
Can you buy new diaphrams for these?

Tracks:

I do not think that new diaphrams are available. Even if they were, it would require re-calibration of the MPS. I don't know exactly what the differences are in the 1.7 and 2.0 MPS but according to the d-jet bible http://members.rennlist.com/pbanders/djetp...tm#troubleshoot - there is some compatability depending on the ECU. If you can't find a good used one, Parts Heaven rebuilds them for $165 + 50 core. I think I found a good used one though.

Bob

Not sure if this is a hijack or not?

Great thread. I too bought a used MPS and whole FI set up for $50... just for back up parts.

I too wondered as I was making the deal how to test it on the spot so I could assure myself that it was a workable part. So I sucked on it. And yes, it sucked slightly back on my tongue.... felt better about buying it.

So now... coming to this thread, have finally figured out, why my wife yelped so much when my son was breastfeeding.

Problem I have now is... was he holding 90HG for 10 minutes or was it intermitent?
My son is 19 now.....he probably has forgot. Think I'm gonna break down and buy the vac guage!

is this the tool, from harbor freight 39.99. I would like to test my mps too ??

jsayre:

This is the one I bought from HF: http://www.harborfreight.com/cpi/ctaf/disp...temnumber=92474

It works well and is about 1/2 the cost of the one you were looking at. Good to see some 914 guys in South Central PA!

Bob

thanks, i will check it out. harbour freight is right next door to me.

i just googled dillburg...

me and the kids just spent the day at lake pinchot all day saturday, the quaker race day use area. Right next to you.

Joe, your FI system is not a problem, yet. Dont mess with it!!!

I tested an MPS with a brake system vacuum pump. The tool for leakdown puts pressure on the MPS where you really want a vacuum. The pump worked real good. I tested the pump and the hose separately and it held about 20psi vacuum for more than 1 hour w/o any droop.

The MPS for my red car had leakdown numbers like shown above. I wasn't sure about the effects of such a leakdown until I tested the MPS in my 74 2L. It was much leakier and that engine ran and continues to run like a champ.