Sometimes it’s hard to work out if a problem with an engine was there before you worked on it, especially if the car has been returned to you on the back of a tow truck. You can just about guarantee the customer will swear black and blue that the engine didn’t get hot – yet somehow the head gasket has blown.

Something we often see after a reconditioned cylinder head has been fitted is the engine blowing smoke on start up. Regrettably this is where the confusion can start. The big question is “where’s the smoke coming from?”  It wasn’t there before the old head was replaced. Naturally the first thing to blame is the replacement cylinder head. Maybe the valve stem oil seals are leaking.

It’s always a good thing to spend a little bit of time diagnosing the problem yourself before you start cursing the poor old head. There are a few simple tests you can follow that will help determine if it’s valve stem seals or the rings:

1. If the engine blows a constant jet of smoke after cold start but slowly gets less as the engine warms up, it’s more likely to be the rings.

2. If when holding the engine revs at about 2500rpm the smoke at first clears but then returns as a constant jet of smoke, it is also likely to be the rings.

3. Get the engine to operating temperature and leave idling for a couple of minutes. Then give the engine a rev, if it blows a puff of smoke and then clears, it will be valve stem oil seals.

Keep in mind, there is no 100% certain way of telling without rebuilding the engine but these tips should help isolate the likely cause and save some time and frustration.