1995 K1500 TBI 350 Running Rich

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If you have no scan tool, try disconnecting each individually your coolant temperature sensor, your intake air temperature sensor, and your MAP sensor. Disconnecting the sensor forces the PCM to use the base map with no modifiers. Maybe you'll get lucky and simply find a failed sensor. It doesn't seem likely but I can't think of anything else that could improve after warmup; Once conditions are met to use the O2 sensor in closed loop, the PCM corrects fuel as necessary so you need to test while quite cold.

I sadly don’t have a scan tool just yet, but I will do some testing when I do get one.

I’ve replaced the coolant temp sensor twice, but I’m not a fan of shotgunning parts at it.

 

[pressureangle]

I sadly don’t have a scan tool just yet, but I will do some testing when I do get one.

I’ve replaced the coolant temp sensor twice, but I’m not a fan of shotgunning parts at it.

Not replace, disconnect. If one of the sensors is bad, you may reveal it by removing it's input.

 

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Not replace, disconnect. If one of the sensors is bad, you may reveal it by removing it's input.

Sorry, I misread your comment. I’ll try disconnecting the sensors and see if anything changes and I’ll post the results.

 

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Sorry, I misread your comment. I’ll try disconnecting the sensors and see if anything changes and I’ll post the results.

I noticed something. The O2 sensor is lazy when it’s hot and idling as well, but not when it’s driving. Is that a symptom?

 

[Schurkey]

Common especially for non-electrically-heated O2 sensors. Many vehicles drop out of closed-loop at idle when not moving, and then go back into closed loop by the time the vehicle is half-way across the intersection after a stop light.

Our trucks are not SUPPOSED to drop out of closed-loop at idle...but lots of 'em do. A sign that the O2 sensor is failing/getting lazy.

 

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it’s a ‘95, so it should be heated. Cross counts are two at idle in 10 seconds and 4 when driving in ten seconds. I jumped pins an and b that’s how I’m getting the cross counts without a scan tool.

 

[Schurkey]

it’s a ‘95, so it should be heated.

OK.

Cross counts are two at idle in 10 seconds and 4 when driving in ten seconds. I jumped pins an and b that’s how I’m getting the cross counts without a scan tool.

That's all-new to me.

The older OBD, like my '88, was set up so that when the engine wasn't running, you could jump pins to get codes. If the engine was running, jumping those same pins could tell you rich/lean and open/closed loop. But I don't think that rich/lean was indicating cross-counts.

 

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OK.


That's all-new to me.

The older OBD, like my '88, was set up so that when the engine wasn't running, you could jump pins to get codes. If the engine was running, jumping those same pins could tell you rich/lean and open/closed loop. But I don't think that rich/lean was indicating cross-counts.

Hmm. I could be wrong. It could be just rich and lean. I’ll have to research that again. It would tell you open and closed loop as well?

 

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I read up on it a little more and it turns out I was just looking at open loop, closed loop like you said Shurkey. I’m just guessing until I get a scan tool.

 

[Schurkey]

I have not actually done this in so long (30+ years) that I don't remember it well. It's listed in the service manual, but I'm too lazy to look it up.

The light flashes with the engine running. The amount of time the light stays "on" versus "off" tells you rich vs. lean. The speed of the flashing tells you open loop vs. closed loop. Slow flashing is one way, fast flashing is the other, and I don't remember which is which.