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Reducing NOx emissions- '92 Golf

I have a '92 Golf (1.8L gas engine, Digifant II control, no EGR) with less than 60,000 miles. The car has passed many emissions tests over the years. The car runs well, with lots of power and good fuel economy. It does not ping under any normal load. On the last ASM2525 test I got the following readings under load:

HC 0 CO 0.01 NOx 764

The NOx is 20% over the allowed limit here. The HC and CO readings seem suspiciously low. I'm wondering if anyone has suggestions for why the NOx might be so high, and how to get it under the limit. The following have occurred to me:

  1. I was using 10% ethanol blend fuel- probably not a good idea if NOx is a problem.

  1. It's been suggested air leaking into the catalytic converter could give these results. There don't appear to be leaks, but the exhaust system is using cheap clamps, not the proper VW pipe connectors. I plan to tape the joints before a retest.

  2. I'm using 60%glycol/40%water coolant (winters here get pretty cold). Switching to 40%water/60%glycol for the test should lower the head temperature a bit.

  1. Yes, I should check the timing. I'm still looking for a 27mm hex head to get the plug out of the inspection port.

  2. I'm running some commercial fuel system cleaner (Redline) through the system to try and clean the injectors.

Any other ideas would be greatly appreciated.

Thanks in advance,

Garry

Reply to
Garry Tarr
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Steady state it should not matter, as the fuel system is supposed to close the loop on O2 in the exhaust gasses. (BTW, did you get the test results for O2 content?)

Doubtful, the exhaust system is under pressure, so any leak would go the other way.

You may want to re-read what you wrote there!

Low CO and HC with High NOx is typically a sign of the engine running too lean. Perhaps an O2 sensor problem, although from what I read this usually causes a rich condition. Could also be that you get some un-metered air entering the intake manifold through a leak somewhere. There are some good Digifant fuel system pages on the web, see what you can find with Google.

Reply to
Randolph

Maybe engine is running too hot or maybe there are some deposits on the valve stems sucking up the fuel (due to possibly poor fuel and not enough long distance driving) or maybe fuel pressure low or maybe a good tune up or engine service (test the CO at the plug, richen the CO and that should help)

let us know! later, dave Reminder........ Before you criticize someone, you should walk a mile in their shoes. That way, when you criticize them, you are a mile away from them, and you have their shoes. Frieda Norris

Reply to
dave

The test print out doesn't give O2. There is something called "dilution" reading 15.3. This sounds a lot like air/fuel ratio to me.

Oops, switching from 60%glycol to 40% glycol is what I meant!

Yes, failed O2 sensors are supposed to force the micture to the rich end.

Could also be that you get some

Possible- when I tapped the manifold vacuum at the hose leading to the air cleaner temperature sensing valve, the reading was only 16 inches of Hg. Is there a better place to measure the vacuum?

Thanks!

Reply to
Garry Tarr

Does it have a EGR valve on that engine? CO and timing has an effect on NOx so I would check the timing and CO% before the cat converter for the correct settings. I think CO% before the cat should be 1% from memory. You may still have the emissions tag somewhere under the hood which should give the data.

Reply to
Woodchuck

It seems to me that the system is running lean. The extra air will reduce the HC and CO and increase the burn temperature causing the excessive NOx. The good fuel economy is consistent with this.

I doubt that an air leak into the exhaust is the problem because this would cause the system to run rich. Because they're relatively easy checks, I would confirm the fuel pressure is correct and that the inlet manifold isn't leaking and then point a finger at the lambda sensor. Rich shift is not a very common failure mode so I'd want to test the sensor (switching time and response to a propane flame on a bench) before condemning. If the sensor had been replaced recently I'd wonder what was killing it, perhaps a coolant leak.

Charles

Reply to
Charles DH Williams

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