Smog fail on California dyno for 1999 6 cyl. all ok except high NO reading (worse at low speed) Grand Cherokee Laredo.

B

billthepilot

Member
Need help, I don't believe that there is an EGR on this engine. Passed barely on 25 MPH run, but failed the 15 MPH-even worse than the 25 reading.
What is the problem likely to be? My first guess would be an air leak into the intake manifold- bigger leanout at lower manifold pressure (higher vacuum) Failed smog with this Jeep a few years ago with same problem, but retest passed with good numbers. Am looking for work orders etc.
Bill
 
Adding to my earlier message.- To lower NO you need an EGR valve- None on 6. Retard ignition timing? Can't its computer controlled. Same for richening mixture. Lower engine compression- By adding Carbon cleaner in fuel, but will this hurt Cat in converter? Thicker head gasket? (maybe later) Something added to the gasoline to reduce combustion temp? (Note that my oxygen sensors don't show any codes.) Different brand of gas? Maybe use a cooler spark plug? New or no air filter. The only thing left seems to be to reduce engine (Cylinder Head) temperature with a cooler thermostat and to be sure the radiator fans are running during the dyno part of the test.. ANY Ideas? Maybe my questions will help someone down the line.
 
The cooler thermostat may be enough bandaid to get you past emissions, maybe. Is there any possibility the intake or exhaust mans have a leak? I had a GC years ago with a leaky exhaust manifold. It did weird things, intermittently. Otherwise I dunno??
 

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