What would cause two brand new emissions thingies to fail in a row? Each time, the little backdraft valve "fails" and lets hot exhaust gases back into the rubber hose, subsequently melting the f'n $750 hose. It's always the same one. It's the one nearest in the picture. The one in the back is fine. That thing was brand new, now it's discolored greyish white. This has happened twice now and I am trying to figure out what would cause this one way valve to fail. I have no problems with the other three. Kai Image Unavailable, Please Login
Ok I will post some theories I have..... Have you have replaced the back/white plastic diaphram valve at the other end of your $750 hose?? Do 348s have solid lifters? Maybe that exhaust valve is not seating 100% and causing extra heat in the exhaust??
Yeah, the diaphram was replaced the first time the valve failed because the hose melted it at the other end... It looks like each valve sits on its own rail supporting 2 cylinders. Wonder what those cylinders are? Kai
The metal valves are there to prevent heated exhaust gas from flowing back through the rubber hose, in the event of an increase of back-pressure in the exhaust system (such as a plugged catalytic converter). If only one of the metal valves (the "non return valve") is failing, that would seem to indicate that the problem is NOT due to a problem in the header, or the muffler, or the catalytic converter, since that should cause failure of both of the valves. You should not be able to feel pulses of exhaust coming OUT of the tops of those valves, the air is supposed to be drawn INTO the tops of the valves. Personally, I would pull all four of the spark plugs on that bank, and check for signs of overheating on the plugs. All I can think of is that you've got a compression problem or a mixture problem on one or both of the cylinders that feed that particular non-return valve, and the excessive heat has caused two failures.
I remeber when this happened to your car the first time, and would reccomend replacing both check valves. These fail all the time on a myriad of cars. Let me know if you you need a hand. I'm in SJ and have done door to door service on a number of F-cars. Mark
You might also check if the cats are plugged, creating backpressure. A Jag I worked on had the same failure. FWIW Jeff Pintler 89 348tb, 86tr
LOL, yeah, I posted about this a few months ago. Well, it happened again. The exact same check valve, serving cylinders 6/7. Brian replaced all valves after the last failure so it can't be "age". The same one failed again, I fixed it myself and now the exact location failed another check valve. So I think it's something more sinister that's causing the same check valve location failure 3x. The upsetting part is that every time the valve fails it lets the hot exhaust gases back up the rubber hose, melting it and the vacuum valve behind it (the check valves are cheap $15; the hose is $900 and the vacuum valve $83). I don't think it's the cats (new hyperflows) because that would kill both valves equally and not always the exact same one. Right now, I plugged both rails with screw on caps (wonder what that will break...) Kai
Save that valve!! It should lend some clues as to why it keeps failing. I see what look like white dust in the hose that's not normal and could be a sign of water moisture but it's hard to tell from the picture. Moisture can cause the valve to corrode and fail completely. If something is plugging up the exhaust manifold it could be picked up with a pressure test. We can always compare to my car as well. Capping them off won't hurt anything. Goth's idea of a bad exhaust valve is interesting too. Also there is a different (new) cap nut on that test port (why?) and if it's not seating right and allowing exhaust out it may be bringing more heat closer to the valve. It sounds like the valve fails entirely each time though. Anyways I'd be curious to look at it. PM sent. Mark
good deductions .......... if moisture could be causing this corrosion................. could it be ......... a bad head gasket... ??
Interesting ideas. The valve is <2 months old, so that would have been some fast rusting... I did keep it for "examination". Got your PM. Kai
