emission test
 

1980 ford with cat failed the test as follows:

Driving test:
HC ppm (maximum allowable/my reading) 199/178
CO % 1.3/.47

Idle test:
HC ppm 355/485 *** fail
CO % 4.03/2.08

There seems to be a slight misfire when idling but not noticable while
driving. All new plug wires, rotor, cap, plugs.

Could an adjustment to the idle mixture screws reduce the HC to passable
levels?

HC is high in both idle and driving test. How is this significant?

Thanks

>Martik wrote:
>1980 ford with cat failed the test as follows:
>Driving test:
>HC ppm (maximum allowable/my reading) 199/178
>CO % 1.3/.47
>
>Idle test:
>HC ppm 355/485 *** fail
>CO % 4.03/2.08
>
>There seems to be a slight misfire when idling
>but not noticable while driving. All new plug
>wires, rotor, cap, plugs.
>Could an adjustment to the idle mixture screws
>reduce the HC to passable levels?

The idle screw(s) would more directly adjust CO by way of richening or
leaning the mixture at idle. Extreme leaning would cause lean misfire
and send HC skyrocketing. How's your spark timing? How's compression?
Cylinder leakdown?


>HC is high in both idle and driving test.
>How is this significant?


It could point to slight valve seat leakage. In a simple textbook
situation, not considering all of the real world variables, a burned
exaust valve can cause exaust gas to be sucked in on the intake stroke.
This technically "richens" the mixture because it displaces some of
the available fresh air volume that would normally come through the
intake. Also it can cause a partial cylinder misfire, which would
account for higher than normal HC.

If you had a vacuum leak at idle I would expect to see really low CO,
but CO looks pretty normal for that engine. Plus, if that were your
only problem the numbers would improve dramatically at cruise.

Toyota MDT in MO

"Comboverfish" > wrote in message
oups.com...
>
>>Martik wrote:
>>1980 ford with cat failed the test as follows:
>>Driving test:
>>HC ppm (maximum allowable/my reading) 199/178
>>CO % 1.3/.47
>>
>>Idle test:
>>HC ppm 355/485 *** fail
>>CO % 4.03/2.08
>>
>>There seems to be a slight misfire when idling
>>but not noticable while driving. All new plug
>>wires, rotor, cap, plugs.
>>Could an adjustment to the idle mixture screws
>>reduce the HC to passable levels?
>
> The idle screw(s) would more directly adjust CO by way of richening or
> leaning the mixture at idle. Extreme leaning would cause lean misfire
> and send HC skyrocketing. How's your spark timing? How's compression?
> Cylinder leakdown?


I have not checked these yet, will let you know. Steady vacuum reading of 20
at idle.
>
>>HC is high in both idle and driving test.
>>How is this significant?
>
>
> It could point to slight valve seat leakage. In a simple textbook
> situation, not considering all of the real world variables, a burned
> exaust valve can cause exaust gas to be sucked in on the intake stroke.
> This technically "richens" the mixture because it displaces some of
> the available fresh air volume that would normally come through the
> intake. Also it can cause a partial cylinder misfire, which would
> account for higher than normal HC.
>
> If you had a vacuum leak at idle I would expect to see really low CO,
> but CO looks pretty normal for that engine. Plus, if that were your
> only problem the numbers would improve dramatically at cruise.


Thanks for your help.

"Martik" ) writes:

> I have not checked these yet, will let you know. Steady vacuum reading of 20
> at idle.

you mentioned misfire at idle. should show up as small fuxuations in
needle on vacuum guage.

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