Sometimes, after long drive, once the car is fully warmed up I see white smoke coming from under the car when coming to a stop or drive very slowly. Not a lot of white smoke, but still it is noticeable. Given that it is white smoke, it should not be engine oil.

Last week I had a long drive and again saw white smoke after returning home. I looked under the car quickly while the car was still hot and noticed that it is coming from the rear, from the area of catalytic converted. I am pretty sure oil is getting on hot cat and that's why it smokes.

The question is where is the oil coming from?

Today I raised the whole car high enough to do a thorough inspection of the underside. Surprisingly, both engine oil pan and tranny oil pan were fairly clean. (Especially given that I did not wipe them/clean them since last year.)

However, the area around the plate where rear differential is located was very oily. See pictures below.

Any ideas based on these pictures where the oil might be coming from? Rear differential uses tranny oil (or something similar)? But I assume it is not under pressure, so it should not blow out oil, right?

 

Automatic transmission fluid on hot exhaust piping makes white smoke. Spinning parts like a torque converter or yoke can sling the fluid onto the exhaust. I had that happen with the TH400 in a C3 Corvette once although mine went suddenly and catastrophically, like a stunt plane leaving a trail for the spectators to follow.

 

The differential uses gear oil.

Differential leaks are common; even small ones leave a big mess as the years go by.

The oil looks like it is migrating to the rear brakes (!)....which could get hot enough to create the smoke.

More likely is that the gear oil is getting slung onto the exhaust.

Don't forget to check the diff fluid level, by the way

Cheers
DD

 

The cat converters are well forward, not towards the rear

The converters get very hot and can easily burn fluids. In that area look for a trans fluid leak, as mentioned. Only a few drops are needed to make smoke

Cheers
DD

 

On the closer/second look, the area where the smoke was coming from is indeed forward of the rear differential. Closer to the middle. Though still much farther back from where transmission oil pan is located.

I just remembered that I actually took a picture of the exact spot last time it smoked. See below, with the smoke still visible. Maybe it indeed blows from the rear of the tranny? Are there any seals there where the oil can escape?

 

It appears that you have an "H" well rearward and oil is accumulating on it and then burning off.

The transmission does have a "rear extension seal" which merits looking at but, really, a combination of small leaks from various areas of the transmission is common. Eventually they all trickle downward and into the air stream...then get blown rearward

Cheers
DD

 

Thanks, Doug. For now, I will probably just keep my eye on it. Occasional white smoke does not bother me too much. It bothers me a little bit that other drivers who stop next to me at a traffic light on a hot day might think that my car is about to explode But that’s their problems, right?

I should probably first check rear differential and whether there is gear oil in it.

Do I access it by simply removing the black metal plate that covers the differential? It is not under load, correct?

 

For easiest access, yes, remove the tie plate. And, correct, it is not under load.

If you're double jointed and the stars align, or you have the car on a hoist, you might be able to skip removing the tie plate. But it's easier, IMO, to just remove it

Cheers
DD

 

There should be a fill plug on the differential rear cover. On my car it is accessible ( just) with a ratchet and a short extension. Doug is referring to the metal plate on the bottom of the suspension assembly, I have never needed to remove that plate to check the filler plug.

When refilling with oil, use an 80W90 oil for limited slip differentials, or if the oil isn't rated for limited slip use the friction modifier additive. GM makes one, so does Mopar I believe. The AC Delco number is 10-4003.

 

Thanks. This whole area around differential is so oily and grimy that it might take me a while to clean everything up before I can figure out where the fill plug is. Will take it step by step.

 

Above the fill plug is a breather, that also screws in. This breather is slightly sprung-loaded closed, and in some cases, including mine, it can lead to a pressure build up in the diff that forces oil out through the driveshaft seals. If you can get to it, unscrew it an modify it to remove the spring.
Having said that, the underneath of your car looks like oil is leaking from the engine/gearbox and is being thrown backwards by the airflow. Check that your crankcase breather system is functioning properly, as if it is not, the engine will leak - at least mine will!

 

In my extensive experience with leaking smoking Jags I'd have to say Trans fluid is more of a blue hue then white, raw coolamt is generally white and oil on hot exhaust is distinctive in smell and grey colour. Can you smell the smoke?

I had an over filled tranny that shot fluid out the breather hose that was quite an unpleasant smell. Maybe putting the car on ramps while someone red lines it in N while you look underneath will help isolate the problem.