Discoloured oil but no smoke? Could be oil block condensation

by Jim KerrBackyard Mechanic . Feb 03 2017

Question: I have a 2002 Mazda MPV with a 3.0-litre, six-cylinder engine. Recently, when I pulled the oil filler cap, the little bit of oil that was on the inside of the cap appeared to be very light in colour. It was approaching the colour of milk. However, the oil on the dipstick did not appear to be this colour.

I have not noticed any white smoke coming from the exhaust and the engine appears to be maintaining its normal operating temperature. My fear is that the colour of the oil in the oil filler cap means that I am going to need a new head gasket. What do you think the problem is? — Tony

Answer: A milky colour to the oil on the filler cap is caused by water mixing with the oil, but on your vehicle it is unlikely it is a head gasket problem. Coolant leaking from a head gasket usually causes a white cloud of smoke from the tailpipe when you first start the vehicle, and it would probably misfire or run rough for several seconds at start up. Instead, the moisture is probably coming from condensation inside the engine block.

As with a cold bottle when it’s taken out of the refrigerator, moisture forms on the inside of the engine block and mixes with the oil when the engine is first started. This moisture usually evaporates if the vehicle is driven long enough, and much of it is controlled by flowing air through the engine block with the positive crankcase ventilation (PCV) system. Short trips and cold weather produce more moisture to control.

I suspect that the PCV valve is not operating correctly on your engine and this is reducing the amount of ventilation. It is normal to have a little discolouration of the oil on the filler cap in winter, but yours sounds like there may be too much. Replacing the PCV valve should help and changing the oil to get rid of any extra moisture that may be trapped in it. Because the PCV valve is located below the intake manifold, I doubt if it has ever been changed on your vehicle.

Question: I have a 2008 Silverado 3500 HD pickup with more than 300,000 kilometres on it. It has been working fine but suddenly it decided not to start. There is no clicking, no cranking over of the engine and no gauges. The only thing I see working is the radio, and its screen says something about Theft Deterrent but it won’t play any radio stations. Do you have any idea what could be wrong? — Rick

Answer: This sounds like a major power or ground problem on your vehicle. There are two communication networks on your vehicle. The high-speed communications network connects the engine, transmission, OnStar and body computers together, while the low-speed network connects the other modules and computers to the body computer. The body computer acts as a translator between the two networks.

For your vehicle to start, there has to be communication between the engine and body computer. If there is not, then, in addition to not cranking, the radio will not play. As part of the radio theft deterrent system, the radio will play only if the serial numbers of the engine and body computer match the programmed number in the radio. Hence the fault message on the radio display.

The problem could be with the wires that carry the communications, but since it affects two different networks, it is unlikely that both are affected. The computers need power, ground as well, and since most features on the vehicle are not working, including the gauges, I suspect either a major fuse has blown that provides power to many systems, the power wire for the major fuse is not connecting or a common ground connection is bad.

If the major fuses are good, check for power at all other fuses. If there is no power at the smaller fuses, then the fuse box may be faulty. Moisture may have entered the fuse box and become trapped in the connectors on the bottom or inside. This corrodes connections and then the fuse box must be replaced.

If the fuses are powered, then check every ground connection to the engine, body and frame for clean and tight connections. One of these checks should locate your fault.