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[mt]

My P4 blew a head gasket about a year ago, they've both now been replaced with the heads skimmed and rebuilt and is running again, but it's smoking.

As the car's started from cold it's fine, then after about a minute at idle i see whisps of smoke coming out the exhaust which gets progressively thicker as the engine warms up, being very noticable when the engine is warm.
It was hard to tell due to wind and the background but the smoke didn't look entirely white, to me it looked kinda dirty/sooty but i can't rule out a blue tint though it wasn't obvious if it was. The exhaust fumes smell normal and not oily to me.
The engine runs very smoothly indeed, it started instantly from cold after not being started for a couple of weeks, didn't cough, splutter or hesitate at any point while it was running and was very responsive to the throttle, both when the throttle was pressed very gently and when the throttle was pressed to the floor as quickly as poss.

I didn't take it out but the mechanic said that he took it out a couple of time and when he got back the smoke had cleared but came on gradually again as it sat.

He thinks the most likely suspect is a cracked piston oil ring. My instinct says it's fuel system related, like an auto choke that stays on, but with the engine running so smoothly it can't be anything to do with fuel, so i am totally clueless.

Any ideas? As things stand i have no choice but to accept defeat, something i'd hate to do

[ZeNiTh-PbArM]

Hi,

watch fluid level in the coolant tank and oil level at the dipstick. If that smoke comes from oil or water you'll know for sure.
If the engine is burning a little oil, a cracked ring is not likely (consumption amounts to more than a little!) ; however, valve stem seals are a likely culprit. Did you have them replaced when the heads were rebuilt?
Fuel smoke is also possible, quite often due to a leaking injector, and does not necessarily affect smooth running. Hot restart takes a little more cranking than usual when this occurs.

regards,
zp

[mt]

[ZeNiTh-PbArM]

Hi,

i diagnosed a leaking injector last summer, i was driving on the highway at night, down a nice slope, an had my foot completely off the gas. All the way down the hill i kept hearing cracks and pops from the exhaust, which is common on carbed 33s, but on injected cars the computer stops injecting fuel above ~1300 rpm when the throttle is closed. I replaced injectors and the car felt quite different after that.
besides, never use injector cleaner fluid : they dissolve all the crud that has filled the cracks in the injector seat and they'll cause tight injectors to leak.

regards,
zp

[mt]

Good diagnosis
And cheers for the info on the injector cleaner, didn't know that!