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Discussion Starter · #1 ·
I have a '91 toyota 4x4 3.0 with Downy air intake, headers, cat. converter, and free flow exhaust. All components are certified with CARB. It's a manual transmission and according the the vehicle report the timing is 10 degrees BTDC.

My results were:

15mph:

1725 RPM
%CO2: 13.6
%O2: 0.9
HC measured 149 (127 max)
CO measured 0.54 (0.78 max)
NO measured 1943 (1031 max)


25mph:

2713 RPM
%CO2: 14.1
%O2: 0.4
HC measured 36 (108 max)
CO measured 0.07 (0.99 max)
NO measured 368 (891 max)

My speedometer broke a few days ago and it caused my truck to throw a 42 code: vehicle speed sensor. I took the cable off the transfer case and found it broke. Could the speed sensor cause emissions problems?

Any help is appreciated, thanks.
 

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Georgian Pickup Drifter
1991 Toyota Pickup
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933 Posts
It's possible, but more likely an oxygen sensor. If the speed sensor is sending the wrong signals to the computer, it might be over/under compensating for something. Judging by your results though you're pretty low at 25 mph. Might wanna replace the speedo cable anyway.
 

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YOTA Master
'93 2wd P/U
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2,434 Posts
I'm going to say it could be an issue with dirty injectors with a bad spray pattern creating a partial lean mixture/misfire on several cylinders. It would not be uncommon for something like this to clear up as engine speed increases.
 

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Discussion Starter · #4 ·
I was thinking faulty EGR valve but I think the NOx would of been high on the 25mph test also. So I agree, it must be lean at part throttle to cause high combustion temperatures. I'll ohm out my O2 sensors just to be sure they are ok.

I guess I should just run some injector cleaner and Chevron Supreme gas throught it and then retest. I'll fix the speedo first though, to get rid of the 'check engine' light. I was thinking of retarding the timing a couple degrees too.
 
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