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General Information => Troubleshooting => Topic started by: Rabbit79 on June 02, 2013, 08:47:14 am
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All of a sudden the old Rabbit is eating vacuum pump diaphragms. I've went through 3 in about the last 2 months. The first time I just changed the diaphragm... lasted a couple weeks.... 2nd time new diaphragm and check valves... that lasted about a week... 3rd time another diaphragm and it lasted about a week again. Is there any common problem that causes this? I've been getting the parts from NAPA, which I'm sure is all made in China stuff (but then what isn't these days), could it be I'm just getting sub-standard parts?
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get a vane vac pump problem solved.
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I agree that the rotary vac pumps are the best solution.
If you want to stay with the pain, make sure you pre-tension the diaphragm before tightening it down.
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How much wear and tear is on the other components of the mechanical side. That piston wobbling around in there like a loose marble? Bushings on other places OK. I have two other pumps that I have rebuilt in the past so if you need another like what you have give a yell. But honestly, IF mine goes out again I will be looking for a vane style as well. It may be spendy but then so are the number of d phrams you are going to go through in comparison.
Vane pump = problem solved if the pump it self isn't worn to the point of no or little pressure.
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I'll have to pull it out and check it over. I haven't really looked that hard at the pump yet, thinking maybe I just got a couple bad diaphragms, but after 3 I think the odds of coincidence have about run out. Luckily they have a 12 month warranty from NAPA so I've only had to pay for the first one. I didn't really notice anything obviously wrong with the mechanical side when I was washing it in the solvent tank, but as I said I didn't look that hard. The only reason I posted was to see if there was a common problem that causes this, but it looks like I'll have to dig into it. I've never really had much trouble with the pump, change a diaphragm every few years here and there, but all of a sudden 3 in a couple months is kinda weird. The engine is about a year and a half off a rebuild, and one theory I had was too much oil pressure, as there is a new oil pump in there (OP is about 70 at rpm, 30 at idle). However the old diaphragm lasted over a year off the rebuild so I don't know how much credence I can give to that theory. Plus I never really have looked to see how the oil flows around in there once it's in the pump. Just thinking out loud a bit on my oil pressure theory, I'm probably way off base on that. Anyway I'll take a look and post to tell what I find out.
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I don't know about that there theory of yours. I run 110 lbs pressure most of the first couple of miles till the oil warms up then it slowly works its way down to 70 PSI running. Some where between 30 and 45 at idle.
I have had the little O ring get buggered up and cause the oil to seep in there and weaken the whole diaphragm. Not sure what the flow pattern is on the oil. Up from the oil pump itself and out the hole in the top of the shaft isn't it? Then free fall back into the oil pan from the backside of the vacuum pump.
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Yeah I'm not all that confident in that theory myself..... pretty much just bouncing stuff off the wall on that one.
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there is a big plastic bushing on the piston that actuates the diaphragm.. that piston gets loose after a while, starts to wobble around, then it starts eating diaphragms..
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Thanks for the second to my reply #3. Has to be something loose in there.
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What is the life expectancy of a diaphragm vac pump when they were new? I kinda like how easy they are to rebuild. I took my out (diaphragm pump from the Caddy) to put a new diaphragm on. I wonder if it is possible to re diaphragm without pulling pump.
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Undoing one bolt and getting it up on a bench to snap those screws loose is worth it to get it out of the car. Really what is in the way? Only need to be aware of getting it lined up on the notch when you put it back together. You can spin the engine over to be sure before you fire it up to make sure it is down all the way. Highly recommended by me. Oil all over is just a mess.
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If you want to stay with the pain, make sure you pre-tension the diaphragm before tightening it down.
No mention of technique
didn't question your spelling of pretension, ha.
Just a joke, I don't know how to spell it,.. but did he read it?
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Rather than turn the gear and get the piston to pull the rubber into the body of the mechanism wouldn't you want to turn it so it was flat across the middle of the stroke? Get it to be at rest? That way it is equal tension and you aren't fighting getting the screws in there and cutting up the edge of the rubber.
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If it isn't pretentioned then every time it rotates it stretches the diaphram.
Compressing it is probably less straining.
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I can see how that would work. Pull it tight and clamp it in place and then it doesn't stretch any more than that. Whereas leaving it dead center it would pull it some and push it some but it is the pulling that makes it wear?
Not that I plan to do another soon I will be remembering this trick if failure of the last one comes prematurely.
Thanks for splaining.
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Sorry it has taken me so long to get back to this with some results. Right after I started checking into my vac pump problems I discovered I had a torn CV boot as well so the old girl has been sitting in the shop until I got un-lazy enough to fix everything. This time when I got the pump apart I discovered I didn't have a torn diaphragm but the nut that holds it onto the end of that little piston had stripped out and everything was loose. Perhaps I got a little over zealous with the ratchet when tightening it down. I checked the piston that actuates the diaphragm and it is a little loose in it's cylinder. Not horribly so but then it's not exactly tight either. Having never had one apart when it was new I have no idea how much play is acceptable. Just an estimate but I'd guess there's a millimeter or so of side play in it. To me it doesn't seem enough to be tearing up diaphragms, but sometimes it doesn't take much. For now I put a coating of grease on it to tighten it up a little and ran a die over the threads to clean them up and put a new nut on it. Band-aid measures I know, but we'll see what happens. If it keeps doing it I may bite the bullet and invest in a vane pump.
Just a note on what some guys were saying on pre-tensioning the diaphragm....... As I understand the way you were putting it you should run the piston to the end of it's stroke, whether all the way extended or all the way retracted wasn't specified (not sure that it would matter), so that the diaphragm is pulled or pushed to the extent of it's travel, and then tighten down the top cover. Is that correct? I've changed enough of these over the years that I'd gotten to the point where I just throw the new diaphragm in and had never really paid much attention to the tension on it. To be honest I've never had much trouble doing it that way. Up until recently I've never had much trouble with these diaphragms, maybe change one every 3 or 4 years, and I've had the old girl for about 17 years now. Anyway this time I followed the Bentley to the letter, and what they say to do is disconnect the piston from the driveshaft when you tighten down the top cover so there's no tension on it at all. I can't really see where it would make that much difference either way as there should be enough stretch in the diaphragm to handle the small amount of travel it goes through, but pre-tensioning seems to be the exact opposite of what the Bentley says, if I understand it right.
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yep the exact opposite.