Emission Control System Clogged

[DQuintana]

My 1991 Q45 Check Emission System light came on yesterday.

I took my car to a Nissan dealership to have it checked out and they said that the EGR system is "clogged".

If the clog is before a certain point, the charge to clear it is $800. If the clog is after a certain point, the charge to clear it is $1,500, due to the location and time required to get to the clog.

What happens if I do not have this work done?

Does anyone have any comments?

Thanks in advance!


Modified by DQuintana at 5:16 PM 10/8/2008

[Infinitiguy19]

Yea do it yourself.

Read this: zer...tures

Trust me your EGR is clogged.

Or it might be the carbon canister but thats a rare ting to break.

[Unnatural1]

If it was one of the EGR codes. There is no way for it to be a canister problem as they are unrelated systems.

However, there is a VERY GOOD chance that the EGR passage is clogged. This is an extremely common problem with older cars especially with high miles.

If you are good with your hands and tools you can do this. This no small job. But, is learning worth $1500 to you?

[Q451990]

You must have a "California Q" since the federal emissions Qs didn't have the EGR temp. sensor in 1991.

It is technically possible to do this without removing the plenum... although I have never done it that way. If you follow the instructions in the previous thread, you should be able to remove the throttle body and the EGR valve as outlined in the thread ppastos linked. Have new gaskets for both on hand and also remove the EGR temp. sensor. Ignore the yellow box - the red arrow is pointing to the EGR temp. sensor.



Here's a pic to show you where the EGR feeds the plenum just behind the throttle body... note that there is a 90 degree bend there - and that's where the vast majority of your carbon will be.



At this point you have access to both ends of the EGR port on the plenum. The carbon in there will be so hard that you'll think it's supposed to be there, but just keep chipping away at it. I use an old dental pick... the heavy duty kind, and an old toothbrush that happens to be very straight, and a lot of brake parts cleaner. I also found that a drill bit with a diameter just a little smaller than the port is a good tool for breaking that stuff up.

Anyhow, as with everything there's a right way and a wrong way to do this... and what I'm describing is not ideal - but it's better than nothing. Ideally, you'll pull the plenum, clean everything to as new condtion, and replace the hoses, knock sensors (if cracked), it's harness, etc. - but you'll probably have to pull that stuff soon enough as your fuel injectors start failing from ethanol "enhanced" fuel unless you are lucky enough to live somewhere that doesn't have it.

Good luck!

Heath

[Q451990]

And to answer your question about "what happens if I don't fix this" - the EGR valve introduces exhaust gases into the fuel mixture to slow combustion. Slowing combustion helps prevent spark knock (much like high octane fuel) - so you will have more detonation if the EGR system isn't functioning. This is more of a concern at higher ambient temperatures, so the fact that it's cooling off some is in your favor. The ECU can detect spark knock through the knock sensors (if they're working) and protect the engine some by retarding the timing advance - but it can only do so much. So you do want to fix this. Your car won't blow up tomorrow... but it's something that needs to function for the long term health of your engine.

Heath

[Unnatural1]

And to answer your question about "what happens if I don't fix this" - the EGR valve introduces exhaust gases into the fuel mixture to slow combustion. Slowing combustion helps prevent spark knock (much like high octane fuel) - so you will have more detonation if the EGR system isn't functioning. This is more of a concern at higher ambient temperatures, so the fact that it's cooling off some is in your favor. The ECU can detect spark knock through the knock sensors (if they're working) and protect the engine some by retarding the timing advance - but it can only do so much. So you do want to fix this. Your car won't blow up tomorrow... but it's something that needs to function for the long term health of your engine.

Heath

More accurately, the Exhaust Gas Recirculation (EGR) valve introduces inert gas (exhaust gas) to "dilute" the air/fuel mix with in the combustion chamber. This inert gas (does not burn) simply takes up space in the combustion chamber that would otherwise be filled with the air/fuel charge. This lowers combustion temps at higher cruising speeds to reduce the formation of Oxides of Nitrogen (NOx).

Loss of EGR when the ECU expects it may cause detonation. But, the MAIN purpose of the EGR system is to reduce NOx emissions. There are other ways to prevent detonation at cruising speeds, such as adding more fuel to the mixture. But, obviously this result in lower fuel mileage. Manufacturers, for this reason, lean out the air/fuel mix at cruising speeds, but lean mixtures burn hotter and create NOx above 1600*C. The EGR is then used to reduce combustion temps, formation of NOx and can keep detonation at bay.