Anyone ever seen this style pop-off valve before?

azasadny · azasadny's Garage

My "new" engine has this particular style pop-off valve installed. Has anyone here ever seen one of these? I was able to get the engine started and when I was trying to set the timing had several LARGE backfires and then refused to start again. Could one of these backfire possibly have "blown up" the popoff valve? The car is in the shop (and has been for 2 weeks) but the mechanic hasn't started on it yet. Is there a way to tell if the pop-off valve is "good"? I haven't seen a valve like this one before. The airbox is metal and seems to be intact. I've got spark and am getting fuel but the engine won't start unless I spray starting fluid in the airbox. What do you think? Just a thought here...

Oh Haha · 07-22-2006, 07:53 PM

Bump 'cause I have no clue about this one.

Stophos · Stophos's Garage

I've never seen the pop off valve but I would look for a vacuum leak. I had the same symtoms and I found a crack in the intake boot where it dried out just above the clamp.
-Chris

smestas · smestas's Garage

Hey that's my pop off valves long lost cousin.

A previous thread on the metal valve:
Metal Pop-Off Valve

smestas · smestas's Garage

azasadny · azasadny's Garage

Thaks. I wish I knew how to tell if this pop-off valve was functioning correctly or not. I'll be contacting the mechanic tomorrow and if he still hasn't started working on the no-start condition, I'll have to get the car and bring it to someone who knows what they're doing. It's a hassle since I have to get a flatbed trailer from somewhere to haul the car around.

azasadny · azasadny's Garage

Simon,
Is your airbox made out of metal? Maybe these weird looking pop-off valves are used with the metal airboxes? I think mine is made out of brushed aluminum.

smestas · smestas's Garage

No its not metal only the valve itself is.

It looks and feels like metal with a crinckle finish but I think it is actually some sort of nylon/plastic or composit material.

azasadny · azasadny's Garage

Simon,
I think we have the same airboxes and popoff valves! Too bad I can't get my car to start!

Pkaaso · 07-23-2006, 07:34 PM

Yep, seen this with the aluminum air box. Anyone know what yr/model these were common on?

Paul

mb911 · mb911's Garage

art,

I had a 74 911s that came like that with an aluminum airbox. the funny thing was the aluminum airbox had a porsche part number on it with a porsche sticker. I know it was aluminum cause I cut it up for another project

mb911 · mb911's Garage

oh and the same blowoff valve

Pkaaso · 07-23-2006, 08:51 PM

I checked around and the consensis is: This air box was never made by Porsche, it is an aftermarket item. All pop off valves themselves are all aftermarket. We have one of these also. It looks like it was cast from an original.

Art, if the valve has failed it will create a vacuum leak and car will not start. Or, if the air box itself cracks will also cause a vacuum leak. Rare that these fail.

Paul

MotoSook · 07-23-2006, 09:19 PM

Quote:

The airbox is metal and seems to be intact. I've got spark and am getting fuel but the engine won't start unless I spray starting fluid in the airbox. What do you think?

Are you sure the airbox is metal? The pop-off valve is not all that uncommon, and it's a simple relief-pop valve with a spring behind it. If it is not seated correctly after it popped when you tried to start the car, it will leak air into the engine w/o the air going through the air meter. Without air going through the air meter the engine will not start or idle..well. It will be running lean.

Visually check that the valve is seated properly, prime the engine by lifti8ng the air plate while the ignition is on. Only do that for a fraction of a second, you will feel pressure working against your finger if you have system pressure. The injectors will also whine/scream a high pitch noise. That tells you you have system pressure and your injectors are opening.

If you don't feel system pressure immediately, but your pump and WUR are working, you will start to feel it after a second or two, and the injectors will scream.

Do you have correct static timing on the distributor?

azasadny · azasadny's Garage

Souk,
The car has been at the shop for 2 weeks and so I haven't been able to do anything. I'll be getting the car back as soon as I can get a car trailer/flatbed so I can pick up the car. The only way I can start it is to spray starting fluid into the airbox. I statically set the timing but I can't get the car running long enough to set the timing. When I was able to get the car running, it idled at 2000rpm and crept up to 4000rpm and would slow down...

mb911 · mb911's Garage

Its gotta be the pop off . I have a read a bunch of your threads on this and it sounds like it was a good running engine previous to it being shipped to you. Has the mechanic has had no luck?? If it idles so high that you suggested then its gotta be a vacum leak hence the pop off leaking ..

slotcar · 07-24-2006, 07:38 AM

Art what a bummer...I thought you were going to P20? Sounds like you might to have to take it to another shop...2 weeks is a long time to just sit there... (unless they told you they were backed up orginally). Best of luck...

Zeke · 07-24-2006, 07:58 AM

All symptoms add up to an air leak. It really should be reletively easy to fix, maybe not so easy to locate. I'd start with the pop off. I had a CIS for 15 years, but didn't work on it much. All I knew was if the air box blew and cracked, stick a fork in it. And, one day, it did.

Dan in Pasadena · Dan in Pasadena's Garage

I was going to post a picture but I realize I deleted it.

Last year I posted a thread on "Stupid Previous Owner Tricks". In it is an eternal classic - when my new to me '76 backfired after a months-long furlough for paint, I opened the airbox to find a (get this!) toilet flapper valve- WTF??!!! - epoxied in as a popoff valve! Apparently someone thought that would work as good, then upon realizing it would NOT work, they epoxied the hell out of it to shut it. So I can relate to odd looking things showing up in your airbox!

Does this metal one work?

MotoSook · 08:09 AM

Quote:

Originally posted by azasadny
Souk,
The car has been at the shop for 2 weeks and so I haven't been able to do anything. I'll be getting the car back as soon as I can get a car trailer/flatbed so I can pick up the car. The only way I can start it is to spray starting fluid into the airbox. I statically set the timing but I can't get the car running long enough to set the timing. When I was able to get the car running, it idled at 2000rpm and crept up to 4000rpm and would slow down...

You're not getting fuel (or enough). It'll fire on the cold start injector and then die out. Could be a lot of things, one being an airbox leak which is causing bad air metering...which in turn causes the fuel distributor to not provide adequate fuel to maintain combustion.

That CIS needs to be gone through and by someone who understands it.