Technical How will oil vapour effect the engine?

[AnthonyH]

I left an oil leak from the sump in one of my GP's for months, so there was oil vapour getting into the engine. Around the same time it started to miss-fire, gradually at first and now quite bad. So my question is how does oil vapour cause miss-fires?

I replaced the coils and plugs and there was no difference. So what are the other possibles? Injectors clogged, O2 sensor?

 

[portland_bill]

Your intial description is a little confusing. The sump is the pan at the bottom that contains the oil. A leak from there usually just drops to the grouond, unless it hits the exhaust, in which case it burns off. That will create a little smoke, and smell, but nothing else. So I'm puzzled how you think your engine has ingested oil from that. Perhaps a little more detail would help.

Firstly, diagnosis is needed, not a parts cannon.

You've replaced plugs and coils, but what about the plug leads? They are a weaker link than the coils, although coils do fail.

A misfire could be ignition, fuel, or a compression loss issue.
Ignition, you seem to have mostly covered, but if the leads are old, they should be considered a consumable item. Get NGK ones. You could disconnect one lead at a time, and see if the misfire changes, or not. If not, that cylinder is not doing much. The coils fire cyls 1 & 4 together, and 2 & 3 together. If a misfire occurs on one pair, swapping the coils will see if the misfire moves, or not. If it moves, a coil is suspect, or its connections. If the misfore stays, a lead or compression issue is suspect.
Injection issues would normally cause the computer to try to compensate, and if struggling will set the engine management light on and store a fault code. Reading the codes might give a hint, but a proper read of the fuel trims would be better. An oxy sensor fault will almost always throw a warning light. Fuel issues are rare.
Compression issues do occur occasionally. The head gaskets are stronger now than they used to be, btu do occasionally fail. A compression test would be a good idea.
If the engine has ingested a lot of oil, it can leave a lot of carbon on the backs of the inlet valves, and this can eventually cause them to not close or seal properly. Burning that amount of oil would normally cause the oxy sensors a severe headache before the valves suffered.
Likely a head gasket failure. If so, keep the new gasket in its sealed bag until ready to fit, it is impregnated with sealant, that will start to cure in contact with air, so needs to be fitted and bolted down. Make sure you get it the right way up, or you'll starve the cam of oil. New head bolts recommended.

 

[AnthonyH]

Your intial description is a little confusing. The sump is the pan at the bottom that contains the oil. A leak from there usually just drops to the grouond, unless it hits the exhaust, in which case it burns off. That will create a little smoke, and smell, but nothing else. So I'm puzzled how you think your engine has ingested oil from that. Perhaps a little more detail would help.

The sump was leaking from under the rear main seal, I left it so long that the underside of the car was covered in oil...

You've replaced plugs and coils, but what about the plug leads? They are a weaker link than the coils, although coils do fail.

Leads came with the coil pack so are also new.

Injection issues would normally cause the computer to try to compensate, and if struggling will set the engine management light on and store a fault code. Reading the codes might give a hint, but a proper read of the fuel trims would be better. An oxy sensor fault will almost always throw a warning light. Fuel issues are rare.
Compression issues do occur occasionally. The head gaskets are stronger now than they used to be, btu do occasionally fail. A compression test would be a good idea.

There aren't any dashboard warning lights.

If the engine has ingested a lot of oil, it can leave a lot of carbon on the backs of the inlet valves, and this can eventually cause them to not close or seal properly. Burning that amount of oil would normally cause the oxy sensors a severe headache before the valves suffered.

Inlet valves. This one looks most likely, there's no warning light from the sensors but the rev counter is noticeably wandering at idle.

Likely a head gasket failure. If so, keep the new gasket in its sealed bag until ready to fit, it is impregnated with sealant, that will start to cure in contact with air, so needs to be fitted and bolted down. Make sure you get it the right way up, or you'll starve the cam of oil. New head bolts recommended.

It doesn't seem as serious as this.

 

[GrandePunto PL]

Missing info - like engine. Which one is it?! And a year ("plate") of the car.
Valve stem seals can go bad and leak oil into cylinders and valves.
Bad oil rings and burning oil via PCV system (breather hoses).
What's the oil consumption?

Purpose of plugs swap is not just renewing but visual inspection (there are charts in the internet, how sparkplugs look and what's the potential problem).
Were old plugs black, oily etc.?

As for rough idle - multiple reasons. Compression, timing (mechanically and/or sensors), valves not sealing, bad valve lash/clearances (8V petrol engines), worn camshaft lobes (yes, they do wear, everything is nice and shiny, but it's bad, lobes lift way under 9,50 mm), fuel delivery (pump, injectors), exhaust system (leaks, worn lambda/oxygen sensors), PCV system (worn hoses or clogged), intake leak, EVAP system leak, other weird reasons (like failing accessories, A/C, alternator). Last thing is a ECU failure (it happens too, but it's rare).

 

[AnthonyH]

Missing info - like engine. Which one is it?! And a year ("plate") of the car.
Valve stem seals can go bad and leak oil into cylinders and valves.
Bad oil rings and burning oil via PCV system (breather hoses).
What's the oil consumption?

It's 1.4L 8 valves. 199 A4000 is the engine code. The plate is 2010. Oil consumption is now normal, i.e. close to zero.

Purpose of plugs swap is not just renewing but visual inspection (there are charts in the internet, how sparkplugs look and what's the potential problem).
Were old plugs black, oily etc.?

I inspected the plugs when replaced and on the chart it indicated oil vapour, they had a hard ash colour coating.