Very light smoke at idle?

[bengal taiga]

This post seeks information regarding light wisps of smoke from the tailpipe- almost exclusively at idle. As explained in more detail, the smoke is not always visible but can be more pronounced as the engine heats up. A little background:

Per my recent posts, my pressure sending unit took ill and I managed to adapt other Djet parts to replace it, pending my finding a spare. I have since found my spare PSU and installed it. The engine seems slightly more tractable than with the other parts, but the fuel economy is not as good as with the replacement parts. (Down from 24 mpg to 21 mpg.) The engine was rebuilt about 6-7 years ago. Most of the fuel injection components are original, even the fuel injectors (although fitted them with new o-rings and feed tubes. I observed each injector to insure good even spray patterns.) With all of the idling, I noticed the temperature gauge started to creep near the red in traffic and this prompted me to replace the viscous fan clutch with one that engaged with less heat. The temperature gauge now sits at 3 O'clock.

Since all of the tinkering and considerable idling, I noticed that at idle, there are very light wisps of smoke emitted from the tailpipe. I have not noticed this before. It is hard to discern whether the smoke is whitish, bluish or dark gray. So far as I can tell, the smoke only appears at operating temperature or warmer. It is not noticeable upon acceleration, leading me to conclude it is probably not rings. I have tried heavy deceleration and do not notice any smoke either, leading me to exclude valve seals and/or guides. I have also rerouted the camcover vent tube to eliminate blowby as a source of the smoke so that it is in effect a downdraft tube. The receiving end of the intake manifold has been plugged. Blowby is very slight and hardly noticeable.

Pulling the plugs reveals five extremely clean plugs. One plug, out of number 4 cylinder, seems lightly darker than the others and seems shiny and almost wet. It is not fouled. Nor is it covered with oil burning deposits. It is NOT clean as one might find with a coolant leak due to a leaky head gasket.

To the best of my understanding, I am not loosing any brake fluid and the brake booster is dry. I am not loosing any coolant or engine oil. The coolant remains translucent and the oil is remarkably clean (no milkshake). Compression is 185 in all cylinders. I am currently using Mobile1 20-50W and valves have been adjusted to .011 Cold.

Given the above, I am scratching my head to deduce the cause of the smoke and remedy the situation. The one thing I haven't done is pull the fuel injectors to determine if one of more of them is either leaking or putting out an irregular amount of fuel. Since I did not detect any loss of brake fluid and notice that the master cylinder is dry, I see no reason to disconnect the brake booster vacuum hose.

Any constructive thoughts would be appreciated.

Thanks in advance.

 

[Malc]

Well it sounds like your motor is in good shape. The "classic" symptoms of BMW engines are usually:
Blue smoke on a trailing throttle - worn valve guides and or seals
White smoke - coolant and or brake fluid
black smoke - way too rich or burning oil

Now often a car will "smoke" due to the products of combustion, which includes water. A common sight on a Scottish winter morning is steaming exhausts! Unless you drive your car hard for a long time the exhaust system tends to fill up with water and with all the other fun chemicals makes acids... hence mild steel exhaust systems rotting out.
Sometimes petrol has water in it too which doesn't help.
I think that if everything else is ok water vapor is what your seeing. Remember there is a huge temperature gradient along the length of the exhaust pipe which doesn't help matters either
Take your car for a long hard thrash to get rid of the water build up in your exhaust and tip some pure surgical alcohol in your petrol tank when it's nearing empty. I remember you could get a product called Dry Gas in parts of the USA.

 

[bengal taiga]

Djet related?

Well it sounds like your motor is in good shape. The "classic" symptoms of BMW engines are usually: Blue smoke on a trailing throttle - worn valve guides and or seals White smoke - coolant and or brake fluid black smoke - way too rich or burning oil
Now often a car will "smoke" due to the products of combustion, which includes water.

Appreciate the prompt and thoughtful response. Unfortunately, the exhaust is not the usual unburned morning condensation, since I have noticed the smoke after some continuous (2 hours of driving and idling). In the morning or evening, I definitely see the steam which does burn off as the engine and exhaust becomes hotter. This is not the same thing of which I am complaining.

I first noticed the smoke when I was looking at the cv boots, with the engine idling. A tissue over the tailpipe does not get wet, nor does it get oily, nor does it get particularly sooty. But I can definitely see the smoke. It is not billowing and seems irregular. If it is a valve guide or seal, I would expect to seem blue'r smoke and not necessarily at idle but on deceleration. Having owned 02's I am well versed in guide and seal replacement. For some reason, I have never suffered that malady with E3's and E9's presumably because I have run them cooler. I wondered if an oil ring could have crapped out, leaving the compression rings to do the oil rings' work. Yet, I haven't seen any billowing blue or even whiteish on acceleration, suggesting that not to be the case.

Because of my recent spate of fuel injection related issues, I can't help but wonder if one or two cylinders is running richer than the rest, and the smoke is more mixture related. As you likely know, disconnecting the temperature sensor on the manifold will cause the system to default to a limp-home rich mode. In the case of this engine, I have tried this and even though the engine speed may change, the wispy smoke remains. If I had not seen the smoke, I would not be concerned as all vitals seem good. Unfortunately, now that I noticed it, as much as I would like to, I cannot ignore it. As noted before, other than the "wisps," the engine runs very well (although I would prefer enjoying better fuel mileage).

 

[Malc]

Have you checked that the engine breather pipes are all clean and clear? It could be possible that your burning oil vapour,especially if your using a "light" engine oil.
The engine maybe running rich in one or two cylinders, have you had the injectors checked? They do wear out eventually.
Sorry I can't be much more help
Malc

 

[bengal taiga]

Chasing the tailpipe wraith

Have you checked that the engine breather pipes are all clean and clear? It could be possible that your burning oil vapour,especially if your using a "light" engine oil. The engine maybe running rich in one or two cylinders, have you had the injectors checked? They do wear out eventually. Sorry I can't be much more help Malc

Breather pipe/s? If you are referring to the vent at the top of the valve cover, I have redirected the hose so that it is not connected (at all) to the manifold, thus eliminating this as a possible cause. I am not familiar with any other vents on this M30.

I now suspect the fuel injectors along with possibly other injection components. For grins, I pulled two adjoining injectors and did a rough measurement of dispersed fuel volume. The volume was different suggesting the two cylinders are receiving differing fuel amounts. Problem is the volume seems to change each time I repeated the test. This might explain why the wisps are not always present and only sometimes one or more cylinders run very rich. On the other hand, I have observed wisps and disconnected each injector, one by one, and the smoke remained - suggesting one or more injectors leaks or produces fuel without being energized. This makes little sense because even with the slight wisps, the idle is regular (does not hunt) and the fuel economy seems acceptable or not terribly thirsty.

Toying with the throttle switch (by moving it and disconnecting it) seems to completely eliminate the wisps. Running the engine through normal operation, the wisps return. Per the several FI manuals I have consulted, the switch functions correctly and I can obtain the same erratic results - with an old spare switch.

I have also noticed that disconnecting the air temperature sensor in the manifold will typically result in a richened mixture. This is not always the case here. I have tested the sensor with an ohmeter under different temperatures and it appears to function as designed. Nevertheless, I will be switching it out with another sensor I have on hand.

One other thing. the cold start valve was also exchanged with a new one. The new one, when energized, does not leak. By happenstance, I found the car starts just as quickly - without the cold start valve connected. The valve has been left disconnected - thus eliminating it as a possible source of the wisps.

Again, the smoke is not billowing and sometimes takes a discerning eye to notice, so that most people might not notice it at all. It seems dark bluish gray indicating more of a over-rich condition with a dash of oil. Consequently, some of the few things I have not tried yet include: disconnecting the power brake booster, serious bolt-on injector cleaner, changing to straight 40 weight mineral oil, STP (or similar viscosity index improver) or even different petrol brand.Idea (Its times like these I wish I still had that Bosch FI tester that was used to only collect dust! Who would have known it might have been useful 15 years later!Rolling Eyes)

Thanks again for trying!

 

[bengal taiga]

Re: Chasing the tailpipe wraith

As noted: "Toying with the throttle switch (by moving it and disconnecting it) seems to completely eliminate the wisps. Running the engine through normal operation, the wisps return. Per the several FI manuals I have consulted, the switch functions correctly and I can obtain the same erratic results - with an old spare switch."

I think it safe to conclude the smoke was a product of the switches being dirty or improperly adjusted. I opened both switches and discovered they showed evidence of exposure to the elements. After cleaning and resetting the first switch, the smoke has disappeared or at least it has not reappeared after several days of driving.