Kevin in Atlanta

Will a disconnected EGR valve cause high nox?

In addition to a gross leak under the manifold that results in an high idle (~1000), I noticed there was no vacuum line going to the EGR valve.

Read the High Idle Suggestions thread last night. Mandatory reading in my opinion. Great stuff. Will have to pull the intake anyway.

Want to pass emissions so I can the car in for paint.

Kevin

dr bob

There is no EGR valve.

Fix the vacuum leaks. Those alone will easily cause the high NOx numbers. Plus you are doing the engine no good by running it so lean.

Look for a couple recent threads on high NOx for more hints if fixing the vacuum leaks doesn't solve the issue.

Kevin in Atlanta

Then I guess I have to pull the intake. Feh. I guess I must have been referring to the carbon canister valve.

Bmw635

Failing MAF could also run lean, cause NOX- it was my case.

worf928

Or air pump.

mark kibort

I don't thnk you can be too lean to do any damage to the engine at light part throttle or expecially idle. NOx, that is a totally different story. bad for the environment.
infact, the leaner the better for gas mileage...

dr bob

Not worried about light or part throttle damage. But realistically, only you and I drive the cars gently. And I'm not so sure about you...

Even at idle and just-off-idle loads, even small vacuum leaks can be enough to give hot-spots and high local flame temperatures. The high NOx reading is a telltale of high combustion temps.

"The leaner the better for gas mileage" holds up to the point where you have lean misfire. It may be a once-in-a-while misfire, and at low loads you probably won't even notice it. There's poorer cylinder scavenging on the misfiring cylinder, so the next cycle has enough fuel to make the mixture rich enough to support full charge combustion.

mark kibort

well, I think im talking about low load levels and idle, as at WOT, a vacuum leak doesn't provide enough air flow to do much of anything I would imagine. (disconnected vacuum line, or lines)

I don't think you will see any hot spots with even a stoich mixture, as generally, at near idle or part throttle , that's where the mixture is designed to be.... its the most efficient combustion point, but does provide the highest of temps. that's why I would think that a leak would bring you further from that point and cool things down. high NOx seems to be increased as you go leaner, not necessarily hotter. in fact the coolest EGTs you will see will be way lean of stoich. (just before misfiring) And that is the best gas mileage as well, but highest NOx levels.

123quattro

While this is true, there is a no-mans-land in the middle which is VERY prone to detonation and high in-cylinder temps under load. You can't go through that area. You have to jump to it.

Lambda 1.2+ ratios make for cool combustion charges by diluting the intake mixture with room air. Lambda 0.75 mixture make for cool combustion charges by physically cooling the cylinder by evaporating excess fuel.

mark kibort

I never understood how just lean of stoich can produce cooler egts, and have more NOx, as what I thought Bob said to be true for NOx being primarily caused by high combustion temps.
but, if you have a vacuum leak, and and a bad one at that, I would think NOx would go down. maybe the leaks we see are just enough to put it in no mans land. Plus, we are talking idle and low power throttle settings so knocks are not as likely I would imagine.

dr bob

So NOx is produced where ANY part of the flame exceeds somewhere in the neighborhood of 2300ºF Might be a very small portion of the total combustion chamber volume, might be more depending on charge density and mixture. And chamber shape. At light loads, the total mass of air and fuel passing through is pretty small, so having a hot spot in the flame path won't contribute much to higher EGT's. The knocking from the detonation is inaudible, often "soaked up" by the surrounding metal without being massive enough to ring through to outside the block. At that some low load and low total charge mass, the intake manifold is also at a low pressure, so a small leak brings in a large mass of unmetered air proportional to the total charge. So CO numbers are small with the lean mixture, the HC from the misfire is pretty easily diluted, but the NOx spikes trigger the FAIL on the smog test.

For diesel applications where detonation is the normal combustion mode, NOx is mitigated by adding an amine (ammonia-like) liquid to the exhaust, and the mix is passed over a titanium dioxide-coated ceramic honeycomb, but only after the particulates are filtered out. Sounds like something that we ought to be using with petrol engines, except we really don't want to have the combustion temps that make NOx at any time.