Truck won'T pass Ma. state inspection.


Old 09-24-05, 06:55 AM
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Truck won'T pass Ma. state inspection.

1988 Dodge dakota 3.9L automatic.I have failed 3 times for NOX reading.The reading first time 6.54.I changed EGR valve and the reading 2nd time was
3.87. The limit is 3.50.I changed cat converter and o2 sensor and 3rd time the reading stayed at 3.87.Each test cost $29.00. Any ideas what else to try?
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Old 09-25-05, 01:26 PM
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NOx is formed when combustion chamber temps are too high (IIRC, 1220 degrees or so). I don't know if ignition timing is adjustable on your truck or not.... if it has a crank sensor separate from the dietributor, it is not. Even then, it may help to have cam synch checked - some unwary folks will try to adjust the Dodge distributor in efforts to adjust ignition timing.

Make sure the cooling system is up to snuff... insufficient coolant flow through the cylinder heads can have this effect.

Hope my little bit of input helps...
Old 09-25-05, 01:39 PM
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I would run a carbon treatment. GM sells upper cylinder treatment which works well.

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