1987 GMC V2500 TBI 350/TH400 Thermostat Help!

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'77, '78, '79, '84, '88
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K5 thru K30
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350-454
How do you know if open or closed?

Sometimes you can tell just by they way they are running, if you are familiar with that particular engine. But sometimes it's nearly impossible just by feel and sound. If you put a jumper wire across pins A and B in the diagnostic port (like checking codes) while the engine is running, the SES light will flash faster when in open loop.
 

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