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failed smog again! just NO(ppm)

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so my 76 passed everything but the NO(PPM)

what di i need to do?

i know it is EGR or cat converter related but that is the extent of my knowledge. where do i start for diagnosis and fixing it?

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Excess NOx happens when the combustion temp is too high. Have you verified that the EGR valve is operating? I've seen a simple check for that but can't remember where; something to do with diddling the vacuum lines to the EGR valve. Check the owner's manual and Repair Manual. DO you know if the mixture is correct.

It would be helpful if you posted the complete test results including limits.

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Checking the '74-79 BMW EGR System for operation and opening RPM

Tools needed...

Vacuum pump

Vacuum Gauge

Misc Vacuum hose

Vacuum "T"

7mm wrench

Small screw driver

EGR Valve operation ......

The EGR Valve introduces inert exhaust gases into the intake charge displacing some intake charge volume at and above around 1650RPM. It is there to reduce combustion temperatures to below the NOx producing 2500degree level. Without the EGR system actually operating the combustion can easily soar above 2500deg and produce NOx.

Overly advanced Ignition Timing, low octane fuel and carbon buildup in the combustion area can also produce NOx in the form of pre-Detonation of the combustion mixture... I always use 91 or higher octane gas in my and customer cars when sending out for the smog test, to help prevent pre-detonation....

1. Check EGR valve, static vacuum test.

Engine NOT running... remove each vacuum hose from the valve and connect pump one at a time... Pump to see if that EGR Valve port holds vacuum. Repeat on the other EGR port. IF either port on the EGR Valve does not hold vacuum then the valve is bad. IF they both hold good vacuum then move onto step 1.1.

1.1 Check EGR Valve, Idle Vacuum test.

Engine at Idle... repeat above test... this time checking if EGR opperation properly effects Idle quality. Engine should stumble badly when vacuum is applied to either EGR Valve vacuum port. If there is no effect on Idle then there may be carbon build up somewhere between the exhaust manifold and the EGR hose port at the intake manifold. OR, the Valve itself may be jammed up with carbon buildup not allowing the valve to move. The EGR Filter under the intake manifold is one prime culprit of being plugged up while one of the fittings in the valve itself is another. The system needs to be disassembled and cleared out.

2. Exhaust Leak...

Pretty self explanatory... Is there a leak in the EGR piping anywhere from the exhaust manifold through the valve into the intake manifold? The EGR Flex pipe below #4 intake port and the hose from the valve to the intake are both very suspect here. The former will be an exhaust leak with the valve in closed operation turning into an intake vacuum leak with the valve open. Thus the air that the EGR introduces is combustible and not inert.

3. EGR system operation...

Check all EGR system vacuum hoses for proper connections using either the corresponding vacuum diagram for the original Solex 2bbl carburetor or for your year of California smog legal Weber CARB exempt diagram...

Solex diagrams here...


Weber diagrams here...


4. Checking and adjusting EGR system opening RPM...

Connect vacuum gauge inline with blue output hose at the Vacuum Transducer on the firewall on the driver side. At engine Idle there should be no vacuum reading on the gauge. As RPM climbs past 1650-1800 some vacuum should be reading on the gauge if not.... There is a set screw with a 7mm lock nut on the vacuum transducer... Loosen the lock nut while not letting the screw turn. To lower the EGR opening RPM turn the screw in. To raise the EGR opening RPM turn the screw out... Remember! you do not want EGR at Idle! Meaning NO vacuum from this connection at Idle.

5. Once the EGR is opening somewhere between 1650 and 1800 RPM the NOX emissions will be reduced.... how much I cannot say!

If any of this is confusing... :) Or y'all have any other questions about the smog systems on BMWs feel free to email me through the FAQ (I'm listed... :) for my phone # and I will walk you through it...

Just pulled this out of my noggen... based on years and years of experience getting BMWs to pass California SmogII testing.


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Contributors to excessive NOx readings are EGR (as mentioned), fuel mixture (as mentioned) and timing in addition to other things. The '76 Ca. model is timed at 0 deg. TDC at 700-900 (idle) RPM. I don't know where I got that number and it conflicts with the spec. quoted above but if you run the engine with that much advance, it'll burn too hot increasing the emissions.

Check your timing. During the test, the vacuum advance is disconnected and the hose is plugged. (This is explained in the footnote in the smog examiner's handbook that is provided by the people that make the emission test eqt. Every station has it. Look up BMW 2002 1976 is he questions this.)

While I've heard that a really clean burning car will pass even if this step is overlooked, the lack of advance during the test helps with your particular emissions issue.

Your valves have to be set COLD at .008, and your mixture right on. A lean mixture, I've been told, increases NOx. Be certain that there are no solvents or carb cleaner type additives in the fuel. The alcohol that they put in as a knock retardant is not a problem....I've heard. Good Luck.

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Just get a new VIN plate off of a car that is not going to see the road again and get new plates. Problem solved.

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