Just put on a new Bosal Catalytic converter on my '86 740 and a few days later I started hearing a loud exhaust leak from under by the Cat. I looked underneath
and there was a hole in the upper end of the cat where a plug had been. I presume this is for an oxygen sensor on certain models. That dang plug was loose from the factory and had rattled out in a couple of days. Now I'm faced trying to find the right sized plug or taking the Cat back to AZone and bitching and having them order me another one. So, a word of warning, if you have a plug on your new Cat, put some Locktite on it and crank it in!!!!!!!!!!

'86 740

To seal the replacement plus, you can use a product called Muffler Mend, or similar exhaust pipe dope.

I was lucky enough to find the missing plug from my Starla cat.

I lost this plug on my replacement cat on our 88 740. Found a Champion spark plug (at Autozone) that fit the hole perfectly, 15K miles ago. It's for a tractor or something- plug thread was about 3/4 inch diameter. It is a standard Champion part...and it cost $1.50.
--
Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K

Had the same thing happen to me twice on bosal systems - one 240 and one 740!! Plug worked itself out. First time it took a few days to figure out why it was quiet then suddenly loud. I took off the header looking for leaks and finally heard the leak much lower. They must leave the plugs loose to accomodate o2 sensors.

I ended up buying a plug for $5 at Midas muffler shop of all places - they keep them on the shelf! I used muffler acoustical sealing stuff to make sure it stayed in place.

Bob Weber
49 Volvos since 1979
Hamilton, Ohio 45013
VCOA member

There are cars that use the port you describe for air input from the 'smog' pump.
--
There are no stupid questions only those that are not asked.

that plug should be a M18x1.5 thread, same as the larger size spark plugs and practically all oxygen sensors (small ones are M14). My 740 has the oxygen sensor in that spot. I doubt that they would even have a plug to fit that hole. You don't happen to have an old oxygen sensor lying around, do you?


Cliff Scott
89 DOHC740

Loctite is not designed for, and will not work at exhaust temperatures!
Colin.

Save yourself the labor and headache. If it were me, I'd just go back to Auto Zone, tell them what happened, and demand that they give you a new plug. (even if they have to scavenge it from another cat. (they can get a new one to replace the one they give you)

There's no reason you should have to do the leg work to find a plug that fits.

Just my 2-cents. (Arguably worth that!)

Jeff Pierce
--
'92 Mercedes 190E (my daily driver), '93 945 Turbo (a kickass family car), '53 Willys-Overland Pickup (my snow-plow truck/conversation piece)

a spark plug from certain ford v8s will fit in that hole, ask me how i know :)

because u fitted an after burned...lmao at the image of a honda up your chuff when ignited....one roasted honda...lol

Fire Fire Fire lol.

Thanks for the warning. I'd go a step further than locktite and have it welded at a couple of spots.
--
Erwin in Memphis, '88 745t 183,000 miles, '95 855t For Sale