The Exhaust FAQ
compiled & edited by Kristian #562
Please read the Disclaimer before attempting any work in this FAQ.

Colour of Smoke


Flash #412 & Marty #436
December ‘01


The colour of the smoke tells you what the source of a problem with your Engine could be.


Crude oil is comprised of ALL fractions cracked by the distillation process. If you burn highway or roof asphalt, does the smoke it gives of as it burns the same color as lamp oil? Both fractions are contained in crude. 

Assuming that you are using antifreeze in the radiator (and not straight water), the glycol in it has a distinctive sweet smell after it passes out through the exhaust pipe (same smell as when an engine overheats and "spews" coolant).


As others have said, the water/condensation makes "white" (scent-less) smoke, rich fuel mixture makes "black" smoke, and oil makes "blue" smoke. I had the head gasket on a Mitsubishi pickup fail, making the white, sweet smoke. Disassembly showed the head gasket had failed between the cooling jacket and a cylinder. Other modes of head gasket failure can also allow oil passages to leak into the cooling jacket (or vice-versa?) or the oil passages to leak into cylinder.


Exhaust's Heatshield


Bob#550, David#476


Occasionally the Foil Heatshield protecting the Side Cover from the Exhaust Peels off.

Other ways of getting rid of the Heat


David#476, Harl #380, XXX, Randy #478

December '01

My Shiny Exhaust (s) are becoming Discoloured

For the GS you should first read the Exhaust Modifications GS FAQ (General Questions) to see how the Two Canisters Work and what is inside them!

Discoloured Exhausts

Bryan #179, Flash #412

Typical Question: My 2001 F650GS exhaust cannister's look awful! They are badly discoloured. My dealer says that BMW will not warranty this defect in materials? The stainless steel apparently discolours when the bike is ridden hard. Has anyone else had a similar problem and has your BMW dealer been able to replace the discoloured cannister with new ones?

So don't worry about it.

For Cleaning your Exhausts, see What about cleaning the Exhaust.


One Discoloured Exhaust (GS)

For the GS you should first read the Exhaust Modifications GS FAQ (General Questions) to see how the Two Canisters Work and what is inside them!


Just became a member as I have purchased my first BMW; an 02 titanium blue GS (new to me with 5750km on the odometer). I am now waiting for the snow and salt to clear so I can take it out of the garage (believe me, snow in Southern Ontario in April is not the norm!). One reason this bike made it to the top of my short list is because of this forum. Very useful and often entertaining. My first message here has to do with the exhaust canisters: the left canister still has that new silvery look to it, while the right one is taking on an antique brassy glow. Is this normal? I also notice that while idling most exhaust fumes exit the right (hence the colouration?). Will the difference between the colour of the two canisters continue to grow? '02 F650GS, Ontario, Canada.

Rusty Exhausts

Very Few Problems with this. The Exhaust is Stainless.

I've got an '03 Dakar. Bought it at the end of September and have put about 9k miles on it. I expected the exhaust to blue and bronze some, but my pipes are rusting. All around the manifold is rusting, and the underside of the pipes and cans are rusting. I do not live near the ocean, and I have been pretty good about rinsing dirt and crud off the bike.

The mechanic at my dealer just told me that:

a) the pipes should NOT be rusting like that and they look like $#!t
b) BMW would not cover it under warranty since it is "ENVIRONMENTAL"
c) I need to call an 800 number to get a warranty rep to look at it.

So, I was wondering - how many of you have or have seen rusting 'stainless steel" pipes on their f650? I do understand that stainless CAN rust. But I thought that they'd last a little longer than 7 months (I've been living with it for about a month or so).

What about that little Cylinder in the Airbox – What’s that ?

It's the vacuum actuated valve for the Catalytic Converter Air Injection system. For more detail refer the Canisterectomy FAQ.

Typical Question


A while back when I installed my Laser muffler on my '97 F650, I removed the secondary air line hose that comes out of the airbox and connects to the exhaust header. I plugged the hole in the airbox with a piece of rubber hose and a bolt. Today I removed the air filter and took a peek inside the airbox to see where the hole leads, and there is a round metal cylinder with another small hose that routs through, and out of the airbox and into the RHS of the carbs. The other side of the cylinder comes out of the airbox to where I plugged it up (if you've seen it, you know what I'm talking about).


Here is my question: Can I remove that entire cylinder and plug up the hose going to the carb, or does that cylinder serve any other purpose? I could just leave it well enough alone, but the part of the cylinder that comes out of the airbox (the part I already plugged up) brushes up against the exhaust pipe, and I'm concerned that eventually it will wear a hole in the exhaust.”



What is that little Bolt on the the muffler ?

What is that Exhaust Extension Thingy I got with my Bike?


What about cleaning the Exhaust?
Haakon # 626, Norway


Some time ago some CG members asked how to clean the exhaust system. I then suggested to use a chemical used to clean up stainless steel after welding or heat-treatment. I just did a small test to show how well it works.


I have just finished "cleaning" part of the left hand exhaust.


As I had to do it in the shower, instead of outdoors (due to low temperatures outside), I just covered part of the "can" and pipe. The more you use of the chemical, the more poisonous gas is produced.

  1. I washed the exhaust with hot water and soft a brush. -picture: "Before1, 2 and 3".

  2. Wait until it is dry and apply the liquid/ gel. -picture: "Just applied".

  3. After 1 hour.- picture "1 Hour" - here you can see some lightening in the yellow/ blue/ brown.

  4. After 2.5 hour I washed the parts with hot water and soft brush and let it dry.

  5. End result, -picture "After1, 2, 3 and 4".

Here is a word document with what is IN that stuff. Pretty dangerous looking mix.

Why is my Exhaust Glowing?

Went down to fill up the coolant on my bike. It was dark and cold out. After I filled it up, I turned the bike on to idle. After two or three minutes I looked over and noticed that the large metal pipe that comes out of the top of the engine and heads to the exhaust was bright red at the top!! Is that normal? I rode around a little to see if it was something that needed wind on it to keep it from happening. But it was still there. IS something wrong with my bike? The overheating light does not come on. Just called my dealer. They said that it is totally normal...that in the dark every f650 header pipe will glow "cherry red" after a few minutes. Basically what the guy at the dealership said there should be NO warm up time with the f650 bikes. Get on and go. Seems strange to me, but when in Rome...


I assume you are referring to the header pipe. They often glow red, particularly in the dark. That's why they change from "chrome" looking stainless steel to yellow or blue looking stainless steel. Heat does that. Flash 412 (CO)

But sometimes, especially if you have a GS or CS:

What about Jet-Hot Ceramic Coatings?

What about Spark Arrestors for Forest Roads?

by Bill & DHP#711


Short answer. We don't know for sure. (Can anyone help?)

I have tried the coat-hanger-into-the-muffler test and the hanger butts up against a barrier at the forward end of the primary muffler (left) So it should pass a trail test performed by a Forest Service official. But does the bike have an USDA Forest Service Approved spark arrestor?


How bad is leaded fuel for the cat?


Q. I am going on a trip and in some of the countries I'll visit it could be possible that I will have to use leaded fuel in remote areas. I know that leaded fuel is bad for the cat and that it will block it, but would it be ok to fill up the bike a couple of times without destroying the cat? How much leaded gas will it take to destroy the cat? Since I hopefully won't have this problem I don't want to remove the cat. However if it is very bad for the cat to be exposed to leaded fuel and I notice that there is a problem finding unleaded gas in remote areas, I'll just let a local welder in that specific country remove the cat. Regards, Spakur #1117, Icelander in Malmö, Sweden, 1995 Classic Red F650 with 65.000+ KM