Oil in Coolant Plus lean and misfire code
 

I was getting my 1996 Chevy Camaro ready for CA SMOG and I had a exhaust header leak due to a crack in in a primary tube. I removed it to have a friend weld it for me and re-installed it. Prior to this the engine ran smooth and no codes (just thought it wouldn't pass with an exhaust leak.

After this was fixed the exhaust leak was gone but it skipped and sputtered at idle and I got a P0155 code for the bank 2 O2 sensor. I found that it was just a bad connection and that code went away but it still ran rough at idle. I took it out for a drive and got a P0304 misfire code. Figure I messed up the #4 cylinder spark plug and changed it. This time I got a P0300 general misfire code. So I figured the O2 sensor on the passenger header got damaged during the install and changed it. Now it still runs rough and got a P0174 code. So I switched back to the old O2 sensor and get the same P0174 code. Everything got checked and no vacuum leaks etc.

I went and looked and found oil in the coolant (but no coolant in the oil). It was a brown milkshake (definitely no corrosion as I have had that before and know what it looks like). Several years back I had the same oil in the coolant but no coolant in the oil and it turned out to be an intake seal.

First question: What else could cause that? - head gasket, cracked head or block is what I can think of - but the exhaust doesn't seem to be burning water (which if there is no coolant in the oil I guess that's possible).

Second question is that since I had an exhaust leak with no codes and ran ok, now that the exhaust leak is fixed, could that exhaust leak masked the misfire/lean codes or make the the mixture OK but now that its fixed what ever problem there was now causes this issue?

Last question, can a blown head gasket, or intake gasket or cracked head or block cause a lean code and misfire?

BTW - this has 330,490 miles on the engine.

Thanks ~ Erik

Re: Oil in Coolant Plus lean and misfire code
 

How close was the primary crack to the head? Closer the leak, the greater the likelihood of a burned valve.

Re: Oil in Coolant Plus lean and misfire code
 

It was just above where the primary is welded into the collector but the primaries are short tubes, the crack was maybe a foot away from the head.... didn't think of a burnt valve. Given this do think think that's still a possibility?

Re: Oil in Coolant Plus lean and misfire code
 

After parting out the car, I finally got down to the heads and found that the cylinder number 4 valve (don't remember if it is exhaust or intake) had a crack in it, probably causing the lean code. Figures that it was the only cylinder that I could not get a compression gauge on with the headers installed. (all the other cylinders were good.

Funny that the code only showed up after I welded a crack on the number 6 header primary.