My '91 F-250 (5.0L) Automatic just started sounding like I had uncorked headers or something. After checking everything out, I noticed on the lower RIGHT side (passneger) exhaust manifold there is a threaded hole (approx. 1" or more diameter) that is missing something, right near the bottom. I called a parts store and they said it could be an oxygen sensor that might have fallen out. I looked around again and noticed there is not any type of connector that a sensor could even connect to. Also, down below the manifold on another pipe, there IS a sensor, so I'm assuming that's the oxygen sensor. I've got a Chilton's, but there are no blow-ups that show that particular area. HELP!! Any idea what's supposed to go in this threaded hole?

 

My truck has the 5.8, but if the hole is in the front it might be where the exhaust tube runs up to the EGR valve. I'm in the middle of an engine swap and noticed mine is loose.

 

My 90 F150 5.0 has a hole with a plug in the passenger side exhaust manifold near the bottom flange. Air is routed to the back of the heads and back between the two cats. My O2 sensor is on the Y pipe by the starter.

 

I've checked around the exhaust manifold again without finding anything such as a wire that would indicate it being a sensor that's missing. I think I'll try and find a plug for it but now I'm faced with what size it may be. Jimbeaux, sounds like you've got the same exact setup as me.