Random multiple miss fire P0300

I am a ASE certified and have a fairly good scanner. Im just wondering if there may be a common problem with this model vehicle. Since it is a used motor its hard to find a starting point. I was leaning towards a fuel injection problem but I have not checked for vaccume leaks so I will start there. That's very helpfu. Thanks for the pointer and if theres any way you can get me the cylinder identification so I can double check my wires that would be great. I assumed it was just standard General Motors. With 2 4 6 on the front and 1 3 5 on the back.

I have seen a P0300 caused by crossed wires but only if its 2 or more cylinders and the was they make cars now I doubt it would even start if that was the case. But I have been fooled before. This motor is such a simple designe there's not a lot to it and being that its a used motor that we know nothing about I want to be extra sure everything was installed perfect before I start pointing fingers at the used motor. This is exactly why I try to avoid installing used parts. But with todays economy you have to take whatever work you can get. I am going to put the smoke machine on it tomorrow and check for vaccume leakes. I will let you know.

Okay this is a good one. I just bought this 2005 equinox and the person had the wrong plugs in it, auto lite platinum so I bought ac delco plugs and new wires to change them. I broke the plug for cylinder #2 and we could not get it out so we pulled the head, and a 1700lbs torque snap on impact gun could not pull out the broken plug (with an easy out). We drilled the plug out and re tapped the hole for a healicoil (or how ever it is spelled). Then we put everything back together and new head bolts and gaskets, the works not cutting corners on anything. And cylinder #2 has only 30lbs of compression and is misfires on an idle but runs perfect once you start to drive it. The check engine light would flash for a bit then stay solid then flash again. After about 60 miles of driving continuously the light when out and would only come on flashing for about 2 secs every 10 miles or so. I am so F'ing aggravated and confused. Any help would be greatly appreciated! When I say "we" I mean me, my step dad who owns the garage and a friend with 20 years experience working on cars. I really do not want to pull that head again.

I did not do a wet test, what is a wet test? And I did not have the head checked because the vehicle ran fine before we tore it out and all the valves looked fine. I ordered a new head but I really hope I do not have to use it.