Help: can't pass emissions - NOX too high

My biennial proiblem is back - can't make it through the Ohio e check with my '88 9000 turbo. For the past two cycles I have taken it in,
it flunks, then without doing anything to it, I'll take it back the next day, and it passes (in 2002 they flunked it because they said they couldn't get a reading on it).
However, I've a feeling it won't pass this time - the tech told me they just started measuring NOX, and while I'm way under on everything else, the NOX is high.
Any suggestions? FYI: this is the 2.0 litre turbo, re-chipped (SAAB issue) to bump it to 175 bhp (from 160). The oxygen sensor was replaced aproximately 4 years ago, and the MAP sensor was replaced ~3 years ago. This car is still a rocket - my 2.3 litre HPT CSE can only keep ahead of it from dead stops, but then any lead is lost on high-speed rolls.
I'm wondering if premium fuel might be the culprit - i.e. 93 octane Amoco (re-chipped, SAAB recommends nothing less than 91 octane). Any suggestions?
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<cs> wrote in message

High NOx levels are related to the EGR system in most cases (disconnected or plugged for some reason). Changing to 89 or 87 octane is unlikely to give an improvement. The amount of NOx formed is directly related to the combustion temperature in the cylinders. The EGR system feeds exhaust gas back into the combustion chamber. Since exhaust gas is inert (or is supposed to be) it doesn't do anything to promote combustion and reduces the combustion temperature. My Haynes manual simply says that if there is a fault in the EGR system, a fault code will be stored in the LH-Jetronic ECU and a warning light is lit (as usual, not much help). Since you have a pre-DI model, maybe you can retard the timing by about 5 degrees to see if that would help.
Here is another suggestion. The Haynes manual (it may be of help this time!) says that a vacuum advance diaphragm is used on your model year to retard the timing when the turbocharger is in operation. These things go bad - particularly after 16 years. Maybe this is the cause of your problem. In Illinois we have the same type of test where the car is run on a dynamometer at various speeds up to about 60 mph. If your test works the same way and this diaphragm is broken, this could be your problem. They used to just measure the emissions at idle; that kind of test wouldn't have revealed this problem. Was a dynamometer test performed on your car?
The EGR and the distributor vacuum advance are both operated by engine vacuum; a check for cracked or loose vacuum hoses would also be beneficial. I hope this helps.
Walt Kienzle 1991 9000T
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Walt, thanks for the reply. I'm not sure my car even has an EGR - not all of these cars did, but then I'm not even sure what an EGR valve looks like for this car, or where it is located As for octane, I am running 93 - would changing to 87 make a difference?
As far as the test procedure and/or dynamometer test, while they don't tell you 'what up,' it appears that Ohio runs the car in the 25 MPH range for several minutes (in adddition to taking a sample from the exhaust).
Your comments, re vacuum hose integrity are noted - while I have replaced some vacuum hoses several years ago, it might be time to replace them all, given the age of the car.
I'll report back if/when I'm sucessful in getting the car passed.
wrote:

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Changing to a lower octane is unlikely to make a difference, particularly since the car is designed to run on premium. Looking back on my recommendations, the vacuum "advance" (or in this case, retard) mechanism on the distributor is the first thing I would check. It is easily tested using a hand-held vacuum pump and vacuum gage available at any auto parts store. I even saw one at a Wal-Mart once for about US$30. If it is broken (doesn't hold a vacuum), it should be inexpensive to buy and easy to replace.
Walt
<cs> wrote in message

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>cs wrote:

I have an '88 9000 turbo as well, UK spec if that makes a difference. The EGR is a thin pipe going from the cylinder head to the throttle body via a one-way valve. Under vacuum (part throttle) exhaust gases are sucked in, basically to improve emissions like Walt said. Under hard acceleration, the valve stops boost pressure from going back into your crankcase.
There is also a thicker pipe going from the same place back to the turbo inlet - this is the crankcase breather.
Anyway, the valve in the thin pipe could be blocked or the wrong way round, or the pipe itself could be disconnected.
Also, is the engine overheating or pinking? Running too hot won't help, and pinking makes things worse.
Finally, an engine tune-up by a specialist might help.
Regards, Alan
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