Rover SD1 Forum Snippet #7 - Interpret the Colour of Exhaust Smoke

Click any word Here, below, to access Images of the Smoke Effect

However, don’t be fooled by plumes of similar white emissions occurring on very cold mornings because in such large engines as the RV8, the initial inefficient combustion process generates loads of H2O which evaporates visibly from those large tail pipes. Such emissions are of no consequence and disappear as soon as the engine warms up.

Plumes of thick white smoke are invariably due to combusting ATF caused by a damaged automatic transmission modulator valve allowing fluid to be sucked into the intake manifold via the transmission vacuum feed tube. Simultaneous transmission malfunction is expected!

Blue smoke (here) usually emanates from low temperature burning (or atomisation) of oil at start-up and carries no foul smell as such. Commonly, the cause is damaged or missing valve stem seals, mostly at the inlet valves, when oil from the rocker gear has drained down overnight and pooled in the area around the valve stem or valve guides.

Sure, oil will also pool around the exhaust valves and guides, but because the exhaust valve mostly operates at high positive pressure oil is not sucked down, unlike the inlet valves which operate mostly at negative pressure and oil is readily sucked down, more so, when the valve stems or their valve guides are damaged or worn. The seal, like a small umbrella, minimises the oil pooling adjacent to the valve stems and guides.

Other sources of blue oil smoke at lower temperatures can be via worn piston rings or maybe combined with a blocked crankcase breather system forcing oil upwards to be burned with a blue tinge at start-up. Blue smoke emissions usually disappear after the initial deposits have been driven off.

Grey smoke (here) has similar sources to the initial but transient occurrence of blue smoke, but producing a continuous emission from a hot engine together with an obvious foul smell of burning oil indicates more serious problems such as badly worn piston rings, worn pistons or bores along with continuous consumption of oil requiring frequent top-up.

Normal compression tests would probably confirm the source of such problems along with clearly visible carbon deposits on the spark plugs such as a formation of whiskers at the electrodes which then leads to constant misfiring due to variable spark intensity.

Different shades may indicate the above issues combining with simultaneous coolant leaks into the cylinders (lighter) or a simultaneously perverse fuel mixture problem (darker).

Black smoke (here) mostly results from over-rich air/fuel mixture burning so inefficiently that significant carbon particles are ejected into the exhaust system and beyond. Really bad mixture may result in vaporised but otherwise unburned fuel passing right through the system, with a strong smell of fuel in the exhaust emissions and the engine bay. Upon inspection, spark plugs will be sooty black or even wet with fuel and if the problem has been persistent, the inside of the tailpipe will be black with soot.

On carburettor cars the problem may simply be a matter of mixture adjustment or sticking needles or choke function but on injection cars there are several possibilities of malfunction within the injection system, such as the ECU, Coolant Temperature Sensor, Air Flow Meter and Throttle Potentiometer. Not so easy to diagnose without a program of logical tests - here;

Accompanying noxious black smoke will be the costly effect of poor fuel economy, misfiring, low power output and unwelcome cylinder wear due to oil being washed from the bores and rings.

Other common causes are a blocked air filter, faulty manual choke, leaking fuel injector. More seriously if the mixture is too bad, it contaminates the oil in the crankcase which will smell strongly of fuel. In such event, it would be prudent not to start the car.

Just when one thinks all causes of engine smoke had been discussed, up pop more. A light grey smoke may occur intermittently when accelerating hard or on a long downhill over-run.

Smoke on acceleration occurs because the big power banging in any combustion chamber forces gasses past worn piston rings into crankcase where increased pressure forces oil up past other piston rings into chambers that are not banging.

The function of the bottom set of piston rings is to wipe oil from the cylinder wall so that it does not pass into the combustion chamber. When smoke occurs only during acceleration, it is likely to be ring failure and a professional leak-down test using air pressure to test the rings may be needed.

Smoke on overrun is because very high induction vacuum sucks air and oil down from inside the rocker via worn valve guides Alternatively if piston rings are very badly worn then some oil might be sucked up from there also.

Smoke in engine bay or appearing randomly all around will likely be due to a rocker cover oil leak dripping down onto hot exhaust pipes from where it burns with smelly gusto.

Finally, back to the initial question which implicates worn valve stem oil seals: The follow-up question is: Can the seals be changed without removing the cylinder head? Answer: Yes!

O Disconnect the battery so the starter cannot function.

O Remove all spark plugs.

O Feed a long piece of soft rope down one plughole (don’t lose the free end of rope).

O Bring that piston to Top Dead Centre of the combustion stroke by hand.

O Bear down on the spring and collet with an appropriate lever. See here:

O Pop out the collets from both valves.

O Fit new seals.

O Re-assemble.

O Remove rope.

O Repeat 7 times.

So there you have it! All research from internet locations, cross-referenced with other sources to try and cover the whole range of exhaust smoke colour causes. However I am not an engine expert so if you have better information - Please contact me regarding Errors and Omissions.

And finally, for undiagnosed Pink smoke see here:

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Overloading Also Causes Exhaust Smoke!

Question: Why does my Rover SD1 emit blue smoke when the engine starts from cold which then disappears after a short while?

Well, most owners already know the usual answer to this question! Worn or missing valve stem oil seals, of course!

But not always! It opens up a host of possibilities and from there a variety of smoke colours might be considered. Let’s examine the possibilities. The basic colours of exhaust smoke are:

White smoke (here) generally indicates production of steam from coolant entering the combustion process via a blown head gasket, a cracked head, both usually resulting from prolonged overheating, and less commonly, a water leak into the intake manifold.

Such smoke usually has the sweet smell of antifreeze and, upon inspection, one may find an oily film of combustion residues in the coolant expansion tank and a creamy deposit in accessible parts of the engine, such as the rocker covers, oil filler cap or dipstick tube. This “mayonnaise” is simply the impurities carried around with the coolant deposited in these places after the water content has been driven off by engine heat.

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Ramon Alban


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