Weird emissions failure - need to resolve (1 Viewer)

Joined
Mar 7, 2006
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So here's a new one. Last year, the truck sailed through emissions. Literally almost no NOX to speak of, low CO, etc. Thought I was doing great.

This year, I get failed at two stations. The idle test is fine, but the dynamic load test (rolling road) keeps blowing the error "VMass flow error" and shutting the test off.

The first shop says it's in the carb. I'm running too rich and heating up his equipment too much (he says the test pipe is melting from the heat). His prescription is for a carb rebuild.

The second shop says a VMass Flow Error means there's not enough volume of air flowing through the equipment, and the machine thinks I'm trying to cheat by not having the hose hooked up properly. Their diagnosis is a blocked exhaust system or clogged cat.

Have any of you had this happen, or know what it means? I find it strange that two shops could have such different interpretations of a single error.

I need to get this resolved before I leave, so any help at all is appreciated.
 

inkpot

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Howdy! Could be that the Cat is failing (plugging up) which would reduce the exhaust flow, and it might also get really hot, as the cat is nothing but a blast furnace anyway. Does the rig seem like it is getting a bit sluggish compared to last year? Find a "Good ol Boys" type exhaust shop and get a hi performance unit welded in. Bout $150 around here. John
 
Joined
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NYC, NY
Hey. Thanks for the quick response. The cat is a hi flow unit, and was installed no more than a year and a half ago. I'm hoping to figure out the error itself before fixing/replacing/opening parts I don't need to. Is a year and a half enough time for the cat to f itself?
 

MJM

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Feb 26, 2003
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Next to the Falls in OH
 
 
 
Hey. Thanks for the quick response. The cat is a hi flow unit, and was installed no more than a year and a half ago. I'm hoping to figure out the error itself before fixing/replacing/opening parts I don't need to. Is a year and a half enough time for the cat to f itself?
Do they check under hood ie. visual inspection in NY?
 
Joined
Aug 2, 2004
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San Francisco Bay Area
 
 
 
The first shop saying you need a carb rebuild does not make any sense.

Maybe you have an exhaust leak so the error is real and just not seeing enough flow at the sensor in the tailpipe. Sounds like the error is real since you got the same one at two different shops.

You can use a vacuum gauge to check for a restricted exhaust. Slowly raise the engine speed with the gauge connected to good manifold vacuum. The vacuum should quickly recover after the throttle is closed. If not, there may be a restriction in the exhaust.
 

inkpot

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Howdy! OK< it's probably not the cat. It should be good for 5-10 years easy. So, like NoCal said, it may be a restriction. Have you smashed the tailpiipe somewhere? Also, check your vacuum advance/retard diaphram on the dizzy. If it leaks, or the dizzy base is stuck on the inside, it will not be adjusting the timing as the engine revs up and runs under load at speed. Also, make sure the centrifical weights under the rotor are loose. I found mine had rusted shut last year. John
 

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