MPS leak down, What now Geoff Options

[lmcchesney]

I rebuilt my MPS and used sparing gasket sealer. The copper diaphgram is pristine. However, when tested after a 24 hr rest, I still get a slow leak down, (-15mmHg/10min). What do I do next?
L. McC

[Demick]

That is a very small leak. As long as it doesn't lose more than about 4 or 5 mmHg per minute, it should work fine. Remember, when the car is running, you have a very large capacity vacuum source which is constantly changing. A very small leak will not affect it.

Demick

[Bleyseng]

Fix it! Even a small leak gives you crappy throttle response IMHO. If I stuck a pristine MPS in for a test drive and then a leaker you would see what I mean.

My secrets are to seal the inner face of the diaphram witha light grey sealant. Then assemble the MPS halves witha light coat on the inner and outer surfaces of the o ring. Let the damn thing dry for atleast 24hrs before testing with a vacuum pump. It should not leak down!
Then adjust it with a A/F meter or a Wavetek meter.
I can give you my spec for a o43 mps if you need them.

Geoff

[anthony]

What's the brand of the sealant?

[KenH]

Well you might try this...

Pressurize the MPS, instead of vacuum, and put it in a bucket of water. I found one that was leaking from the Electical connector this way.

Note - not a lot of pressure, just a few pounds should work.

Ken

[pbanders]

I strongly recommend against pressurizing an MPS to check for leaks. The problem is that the full-load diaphragm is unsupported when you pressurize it - under vacuum, it is supported by a thick stop plate. When unsupported, it's very easy for it to rip out around the adjustment flange.

BTW, this is the same reason that many people have reported that their MPS's were "blown" after they spun the car at the track. Why? Because when you spin, and you're going backwards with the clutch out and your foot off the throttle, you pressurize the intake manifold, which produces positive pressure in the MPS. Pop! goes the full-load diaphragm.

[Bleyseng]

(IMG:style_emoticons/default/agree.gif) good point.....BTDT (IMG:style_emoticons/default/sad.gif)

I use Permatex Motoseal 1 Ultimate Gasket maker Grey. Use a very light coat to replace the diaphram gasket and coat the O ring sealing surfaces.

This post has been edited by Bleyseng: Jul 26 2004, 11:33 AM

[KenH]

That, why I said a FEW pounds of pressure. If the leak does not stop when rebuilt, it probably would not hurt to see if it is leaking from some odd place. Nothing to loose at that point.


Ken

[Bleyseng]

The diaphram can leak around the press in stop or around the o rings in the inner and outer screws also.

[lmcchesney]

Update.
I did follow Ken's suggestion and submerged the MPS, connected hose and blew gently into the MPS. No air bubbles. I increased pressure until I could blow no harder, still no bubbles. Re-connected the hose and sucked, crimped and attached vacuum gauge, still slow leak. I added hose clamps to the MPS, still slow (14mmHg over 5-7min) leak. I do not understand loss of vacuum without bubbles. Suggestions?
L. McC

[SpecialK]

Are you sure it's not your gauge or hose that leaks?

[Bleyseng]

Thats what I would test next.....

[lmcchesney]

That is what I have suspected as well. However, it is my only vacuum gauge. Is it reasonable to think if there is no bubbles with pressure that there is no leak with vacuum?
L. McC

[Bleyseng]

Leaking down to 1 lb in 5-7 min is still not acceptable. You gotta figure it out as its either the MPS or the vacuum pump.

Geoff

[BMartin914]

Having MPS difficulties...

Thought I had one good one out of 3. Thought. Held a tester on it tonight and it will only hold 20 Hg for about a minute. Starts leaking down pretty quick. Down to about 5 in 20 minutes.

Can the slow (relatively compared to other units) leak be repaired with a sealant or do I need to a. find a good diaphragm or b. go rebuilt by Bret?

[dralf]

I too think I have a bad MPS (IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif)
I bought a Mityvac from my FLAPS for about $20
and had hoped that the 5 inches of vacuum loss in 5 minutes was due
to the tester rather than the MPS. I then put a rubber stopper in the end of the tester line and pulled a 20 inch vacuum. Hell the thing still would be reaging 20 inches if I did not finally stop the test. Seems that the MAPS is starting to go. I hope this weekend to change out mine for one that is thought to be better and see what happens. It is a 1.7 and I have a 2.0, so far everyone I have talked to about that says for testing purpose that should be close enough. My engine is running so rich now that I see dollar signs dripping out of the tail pipe. Paid $2.69 for a gallon of premium.

One last stupid question, could someone once again tell me why the aftermarke MPS are no good, and I see that AA sells used and rebuit ones, are they bad and only their top shelf Porche Original one for $1600 something any good ? Heck I put something like that in the car it better be made of Gold!

(IMG:http://www.914world.com/bbs2/html/emoticons/ohmy.gif)

[BMartin914]

dralf,

The aftermarket (rebuilt) MPS units - the ones by Bret anyway - use a stainless steel diaphragm with fewer pleats than the original. It does not flex exactly the same and is therefore [much] less desireable than the original units [diaphragms].

[bd1308]

ya ready man......

we're going to get this thing running real good!

[Bleyseng]

The two that I have seen from AA were repaired via soldering a new copper diaphram to the center stop. They ran but with very poor throttle response.

I have a box of junk diaphrams I could take pics of.... the box of horrors!

[BMartin914]

QUOTE

ya ready man...... we're going to get this thing running real good!

Ya got a good MPS for me Britt?

QUOTE

The two that I have seen from AA were repaired via soldering a new copper diaphram to the center stop. They ran but with very poor throttle response. I have a box of junk diaphrams I could take pics of.... the box of horrors!

So what's the recourse besides a rebuilt (remanufactured) MPS?