Replaced the intake gasket which was leaking externally, now have P0300 multiple misfires and P0131 O2 sensor low. I'm thinking the two are not necessarily related to each other. I'm guessing we could have left the O2 sensor unplugged but am unable to find it. Before going after the leak, we had a P0301, Cyl1 misfire, which happened previously and was fixed with a new coil. Did look at the plug during gasket replacement, no water, no wear. So I guess my first question is where might I hope to find that O2 sensor plug-in, just to confirm it'* plugged in? Thanks

Those gaskets need to go on one way only. If it'* not installed correctly it can do this.

P0131 refers to the bank 1 sensor 1 O2 sensor. Follow the exhaust pipe from bank 1, the oxygen sensor will be the first one between the manifold and the catalytic converter.

Well! Come to find out that the guy who put the plenum down did not remember to tighten the damn thing down! We are now running considerably better, but still at rough idle (as opposed to NO idle) and a repeating P0100 MAF code. Once the fault sets, computer holds idle high, then clearing the code drops it back to a surging idle. Neither the O2 sensor code nor the misfire code has repeated. I suspect there'* still a vac leak somewhere, and we're looking for it. Thanks to you both for your input. We're confident that the gaskets are properly installed.

Be careful at this point, some junk might have gotten sucked in and might be causing your leak after tightening down.

Not out of the question for sure. We will recheck the mating surfaces for the plenum. Anything sucked beyond that has already done what it will do! Thank you!

Did that engine get warmed up? Even if it didn't I would replace the gasket again.Probably not sealing.

I'm afraid that'* the conclusion I'm coming to as well. Shouldn't take as long this time...

My son'* friend had a 5L Mustang that needed an intake gasket. They were wanting to go into the automotive business so I encouraged them to do the job themselves and save labor. It took them 5 hours to remove the intake. I believe it took them 4 hours to put it back together and it did not run well. Well, I questioned them and the first thing I asked was what brand gasket kit they bought. They put it back together and did NOT use a new gasket. I told them the problem was the gasket seals when it first gets hot and if that seal is broken a new gasket is the only way it will seal correctly again. They got a good kit and this time they did the entire job in three hours. Experience is a great teacher. There are better ways to learn but as far as turning wrenches the experience was good.