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Lean Fuel Condition [maybe] triggering O2

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1989 K1500 RCLB 5.7 TBI


I'm posting this on this site as the GMT400 site hasn't turned up much help so i'm counting on you guys! 


Code 13 will set when:
Engine at normal operating temperature
At least 2 minutes engine time after start.
Oxygen sensor signal voltage steady between .35 and .55 volts.
Throttle position sensor signal above 4%.
All conditions must be met for about 60 seconds. If the conditions for a Code 13 exist, the system will not go "Closed Loop".


This is exactly when the light comes on [typically will trigger at any cruising speed [30-80] But only while cruising ( i assume to meet the TPS sensor above 4% for 2 minutes). It triggers more 'easily' if you can imagine what i mean, at higher speeds [80mph]. The light will turn off if i go WOT for a few seconds, if i idle for a few minutes [at a stop light], or if i'm driving around town. The light wont turn on when driving around town. 


Another note: The DTC flowchart asks to probe the O2 Harness Side to Ground - should read 0.3v to 0.6v. 


Mine is reading 1.48v so i checked the "open at 413" error and the ground connection is good, ground strap was cleaned and terminals were cleaned so the only other error it says is that the ECM is faulty, which i just simply don't believe. The truck runs absolutely beautifully except for this code. 


Small note the truck kind of feels like its surging at high speeds [80mph]. Not down shifting but just like its missing or something funny is happening, doesnt happen at low speeds <70mph. 


All the obvious things have already been cleaned/replaced/rebuilt: 

New top end gasket job 4 weeks ago [no leaks]

O2 sensor replaced --> 3-wire O2 sensor modification

Fuel pressure 12 psi with a new ACDelco fuel filter

EGR Solenoid Replaced - Quick and dirty EGR valve check by pressing on it while running and it will stumble - the port was cleaned while doing the top end gasket job. 

ACDelco plugs gapped to spec, new high quality wires, new distributor, timing set to 0*


Other things within the last 12 months

Rebuilt Trans

Brakes all the way around

All new suspension

New alternator

New water pump

New power steering pump/pulley, new steering gear 

New TPS sensor

New IAC valve

New Knock Sensor

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  • 2 weeks later...

That is what i'm starting to believe, but i'm in denial, haha. 


There is one other weird issue that i'm having with the truck. 


When i'm going pretty fast (80mph), it will sometimes happen at slightly lower speeds but what happens is that the truck will kind of 'miss' or hesitate, like it drops power for just a micro second, very random pattern, i cant predict when its going to miss but it happens on and off every second, every 3 seconds, twice a second, every 10 seconds, very random. I can tell its happening because the truck kind of lurches and surges, but really high frequency, like 0.25- 0.5 second cycle between the lurch and surge. I was thinking fuel delivery but like i said i kind of proved out its not that. (it will usually throw the O2 code when this happens, could be a coincidence thou)


Any input on whether the EGR vavle test i did was good enough to tell that its working properly, or not? 

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This problem occurred with it in its original state.


I made the switch to a 3-wire because i have headers with an open true dual setup (free flowing). The idea is that because the exhaust does have back-pressure from the cats, y-pipe, and single tube exhaust that the temperature wont be as high (the O2 sensor needs like 600*F degrees or so to operate properly and produce its own voltage). 


I was sure to get an O2 sensor that delivers the same voltage when its up to temperature on the signal wire. The other 2 wires in the 3-wire sensor are simply for 12v to power a small element to heat the sensor. I have mine on a switch so I can 'convert' it from a 1 wire sensor to a 3-wire with the flip of a switch. My O2 issue exists in both heated and non-heated scenarios. 


I was thinking that the heated element still may not be enough to heat the sensor to operating temps. Maybe header wrap would help keep the temperature up and produce the right voltage??? 

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Back in the '80s and '90s we used to change our O2 sensors very often to keep the engines running in top form.  They did not like the additives we used and would certainly fail using unleaded race gas after a few passes.  They old 1-wire sensors worked just fine on pump gas with our gutted catalytic converters.


Does yours still cycle up and down quickly once the engine is up to temperature?

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