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I have completed a full engine rebuild, down to the bare block, of my Tacoma. Replaced all the gaskets, cleaned and lapped all the valves tight, tuned the valve lashes to spec, cleaned everything, replaced the starter, torqued to spec etc. It was a 3 month effort working on it a couple of nights a week.

Last night was the turn key night. I must admit I was nervous, as it was the biggest automotive repair I had ever done. The engine started fine but then I notices something like smoke coming out of the catalytic converter and around both the exhaust manifolds. I thought it may be the excess engine assembly lube I used to put everything back together but soon I realized it wasn't smoke -- it was steam. When I went to the tail pipe, it was confirmed, the vapor coming out was definitely steam and there was some condensation inside the pipe as well.

I wonder if this is normal for a major rebuild that perhaps some moisture condensed inside the cylinders and it had to find its way out.

I ran it idle for about one hour, steaming had stopped, then drove it about 20 min and everything was fine.

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    In case you haven't seen this question: mechanics.stackexchange.com/q/11315/675 – Zaid Sep 15 '15 at 16:15
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    I suggest you monitor the situation. You may be seeing it because the engine was cold at startup. It could be that the coolant water is finding it's way into the cylinders and turning into water vapor. Keep tabs on the coolant level. If coolant level is dropping and there are no obvious leaks in the engine bay, coolant leaking into the cylinders might be the reason. – Zaid Sep 15 '15 at 16:20
  • it started right as soon as i fired up the engine so i doubt coolant had enough time to get into the cylinders. but if it had, wouldn't there have been a lot more damage instantly ? – amphibient Sep 15 '15 at 16:35
  • I agree with Zaid in that you should monitor the situation for a while. However I would also add that on a new engine rebuild there may be some air in the cooling system so a small drop in coolant level is not abnormal. However if the level keeps dropping after a few hundred miles, you may have an issue. – Nick G Sep 15 '15 at 23:15
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This is a non-issue. HC (hydrocarbons) and CO (carbon monoxide) in the exahust combine with oxygen to become Carbon Dioxide (CO2) and Water (H2O). When the exhaust is cold the water can condense enough to show as water vapor or even as dripping water from the tailpipe. The water is always in the exhaust, but when the engine and exhaust system is hot enough it 'boils off' the water vapor into steam, which you can't see with the naked eye.

This process is easy to see in action in the sky behind jet-powered airplanes; because the atmosphere is so cold where the plane is (~30,000ft), the water vapor in the exhaust condenses back into a 'trail' that we can usually see.

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What kind of climate are you in? It's quite common to see steam coming out of the exhaust of a cold car if you're in a damp climate - water condenses on the cold metal all the way through the system, then evaporates off as it heats up. It usually stops after a few minutes as the car gets up to temperature.

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  • mid atlantic. it was around 70 deg F last night, moderately humid. the engine had been disassembled for almost three months – amphibient Sep 15 '15 at 15:55
  • I don't know if that length of time would lead to a lot more condensation than just leaving for a few days - I'd assume there wouldn't be so much in the bores as you'd have a thin film of oil on them from the honing (I think?), but there may well be loads in the exhaust... – Nick C Sep 15 '15 at 15:58
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If it isn't condensed water inside the engine, which by the way accumulates if you drive your vehicle on short distances and not allow it to reach operating temperature, it's probably related with the coolant cycle and some gasket faulty cylinder gasket.

If it persist (I strongly hope not) and you want to be 100% sure, you could do:

  1. Pressure check on your coolant system for leak.
  2. C02 test on your coolant system for cylinder leak, i.e. faulty gasket.
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