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Discussion Starter #1
Hi,

My '90 SPG failed its California Smog Check. Any ideas on what may be wrong are appreciated.

I did not spend enough time preparing the car, as it seemed to be running well and it has always passed before.

This car can routinely return 32-34 mpg on the freeway, so I imagine it can't be off that much.

The techs always say to change the cat, but I did that on my other Turbo and it made little difference, especially with NO.

274222


Thanks
 

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NOx are a result of "high" combustion chamber temps. Excess NOx can be caused by things like lean running or carbon deposits that prevent the cooling system from working well. The primary way of fighting NOx under normal circumstances is EGR, which we don't have. The only other way to directly affect them is ignition timing. After that, it's on the cat.

High HCs are the result of incomplete combustion. That could be a rich mixture, poor ignition, or something like a false lean condition due to pre O2 sensor leaks that suck in air and fool the O2 sensor into thinking the car is running lean when it's not. When all that is working right, the cat cleans up the excess.

High HC and high NOx at the same time suggests the cat, but there is a whole system in play here and everything has to be dialed in. Your fuel economy is quite high - to me that sounds like it may be running a little lean. How is boost? When was the last tuneup? How many miles? History on the injectors? History on the cat?
 

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Discussion Starter #3
I recall having put new plugs in a few months back. At that time I cleaned up the cap and rotor also. I had to replace one of the injectors with a used one when I damaged it. I have had suspicions about the condition of that particular injector.

Boost is something that needs work, not like the other SAAB, which has always had gobs. Maybe something with the wastegate - not sure.

The cat is probably original. A friend told me about a product called Cataclean - not sure if that would help... If necessary, I can put another cat on it, but I have already had two cat replacements on my '85 and it made little difference. The first one to the embarrassment of the smog tech at the time. He got it to pass by taking it out and driving it, came back and it went from a gross polluter to passing.

Also, I'm wondering if the breather system can be a source of vacuum leaks. That doesn't get talked about much.

Is the air inlet flap supposed to seal up when the engine is off? Could the the vacuum tank leak? Both my 900s do not seal the flap when the engine is shut off.

I will try to test the O2 sensor to see if that cycles. I do have a new one if need be. And I will measure the NTC sensor to see if that is even close.
 

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The HVAC system is SORT OF separate from the engine vacuum system. There is a single vacuum hose that feeds the reservoir which is sealed with check valves. You only need to worry that the reservoir holds vacuum from the engine side.

That and the PCV are two of many possible sources of leaks. The ONLY way air should get into the engine is through the air filter, across the MAF, and into the throttle body. Any other air getting in there is a problem and needs to be addressed. The sole exception to this is the evap system ... Fumes are drawn in a metered way from the tank... But when the purge valve is closed the system should not leak.

In order for the cat to work right you need the right amounts and combination of gassed getting in. Anything too far out of spec will prohibit the cat from doing it's job. Because there aren't many emissions components on a c900 and because we don't have modern levels of diagnostics, it's really a pretty manual process ensuring everything is operating properly. And of course we have the MAF itself, which is not only critical to the system working right but a total black box in terms of knowing whether it's working or not. My experience is that any MAF with more than 100k on it is suspect, and any MAF with more than 150k is probably shot. The degradation is slow and the effects are often overlooked, but nonetheless. Modern cars trash MAFs every 150k or so... There's no chance one made 30+ years ago will fair better.
 

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Discussion Starter #5
Are the Cardone MAFs OK? What about the Chinese units seen on Alibaba and probably eBay?

I did measure the NTC sensor with the engine "cold". It was 1957 ohms at about 83 to 84 degrees F. I suppose I should take another reading when the engine is "hot".
 

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Cardone is probably fine, but I would probably try to send mine out to Injections Labs for a personal experience. I wouldn't trust a Chinese MAF. Just too much that can go wrong to bet on parts with no QA process.
 

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Discussion Starter #7
Saw their website and looks like they re-engineer the entire sensor. I will have to inquire about pricing and turn around should that prove to be the culprit.Thanks, jvanabra.
 

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They've redone a few for me, and I've recommended them to lots of people for lots of years. IME they do good work, and you know what you're getting back is hand-tested. I like that aspect... On my XR4Ti I've bought Cardone and other reman VAMs (vane airflow metal, like a MAF but mechanical) and fully 50% are outright defective out of the box. Really makes me wonder about the 50% that seem fine, and it makes me really hesitant about a MAF... if you can't get s simple potentiometer right, I can't believe you can get a hotwire right!
 
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