Exhausting back pressure ... not the VX!!!

[tom4bren]

I was putting a new O2 sensor in Boy's Samurai. The threads in the manifold were goobered up & the new sensor wouldn't thread in. I filed off the threads on the new O2 sensor and used some high temp epoxy to bond it in place.

A week later the sensor blew out of the hole.

I borrowed a tap to clean the threads on the manifold so I'll be able to fix the problem correctly but it does beg a very important question:

Doesn't that seem like a LOT of back pressure on the exhaust? My past experience with high temp epoxy is that it generally sux but holy carp, that sensor shouldn't have blown out that quickly.

Do you think I'm looking at replacing a clogged catalytic converter? Thoughts??

EDIT:

Interesting read if you have the time.

http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

EDIT Again:

Looks like this guage reads from 0 to 8 psi. So I guess normal is somewhere in that range.

http://www.nationaltoolwarehouse.com/Exhaust-Back-Pressure-Gauge-P32592.aspx

[89Vette]

What temp is the epoxy good to? My guess is you underestimate the temp in the exhaust system. Unless you got something good to 1000-deg, it's probably not enough.

It also has to be fuel rated. Most epoxies aren't.

This company sells something that might last for awhile....
http://www.cotronics.com/vo/cotr/newprod.htm

Two years ago, I used 2300-deg 907GF (I think) to redo some custom sidepipe muffler packing in my vette sidepipes. AFAIK, it's still in place. (I used stainless screen to hold fiberglass in a sound chamber....with this 2300-deg stuff as the holding agent.)

OTOH, you could have an exhaust shop weld another bung in it's place....and be done with it.

[tom4bren]

Thanks vette.

I chased the threads with a tap I borrowed from our machine shop. I'm good to go without the epoxy now.

I don't know what temp the crap was rated for. It was a muffler bandage kit that I picked up at AutoZone. Rekin I got what I paid for.

The main reason for posting was to see if anyone had a clue as to how much back pressure was normal. During the fix, I cranked the engine with the O2 sensor out. I was able to hold the hole closed with my thumb (for a few seconds - it gets hot quick). I think we're good to go for now. We may ditch the cat & put on glass packs this summer anyway.

[Leon R]

EGT's, right out of the engine, are around 1500-1700*F, most front O2 sensors are close enough to see 1300-1500 deg. So your epoxy must be able withstand that. Mufflers rarely get above 500-600*F.

Then, there you pulses from individual cylinders, that just adds to the "pressure".

Ether find a Helicoil insert of proper size (can be very expensive), or just replace the manifold.

[tom4bren]

Thanks.

Job is complete. The tap repaired the existing threads so no helicoil was needed. The manifold will probably now outlive us both.

[blacksambo]

Nice job, and an interesting lesson for us all.