stuck in closed loop but runs like its not

[texas tough]

restoring a 93 chevy c1500 with 4.3 stock tbi engine. engine was completely rebuild as well a trans.
engine starts, idles and revs fine,, hooked my autel obd2 scanner up to check things out, NO DTCs.
measured coolant temp sensor resistance/ temp corellation and within spec.. engine had new 195 F thermostat and new AC DELCO heated o2 sensor,. will not go into closed loop at idle, and havent driven it yet , as it needs tires badly.
I tried grounding the old O2 sensor signal wire to the ecm to ground and the scanner showed it went into closed loop, so I assumed it was the o2 sensor to be the problem. so we installed an new one and it still doesnt go into closed loop.. I have read that some trucks have closed loop timers on them, but cant find what they are.. truck sat at idle, at normal operating temp for abt 5 minutes, so I would think it would go closed loop by then,., but it didnt
the o2 sensor in in the manifold, and there is abt 6 inches of pipe left after the sensor, because the pipe was rusted thru so we chopped it off, to put a new exhaust system on it.,,
whats odd is the fuel trims are in control, short term 128, long around 16. o2 sensor is oscillating at around 450 to 510 MV
Truck doesnt run rich, or lean that I can tell. I unplugged the coolant temp sensor plug to see if the ecm would recognize a fault and throw a code, and it did.. the truck was throwing a knock sensor code, because when we rebuilt the engine, we got the wrong sensor,, out truck takes a 3900 ohm resisitance and they sold us a 100.000 ohm resistance knock sensor,, so we are very close to getting the truck running 100 percent perfect, just have to get it to go into closed loop.. any ideas appreciated.. I can give any scanner data requested.. thanks

 

[texas tough]

a little more info, when we grounded the signal wiire in the O2 sensor circuit, and it went into closed loop, this tells me the wiring intergrity to the ECM is good, and the ECM is capable of recieving the signal and closing the loop. im wondering if the exhaust being chopped off 6 inches past the heated o2 sensor is not leting the o2 sensor get hot enough, but on the same token, its a heated o2 sensor so it should compensate for this.. we read that the o2 sensor gets the power from ignition circuit to heat the sensor up. is this on its own fuse? we checked all of the fuses in the panel and they all checked good.,
I believe when the ignition switch is turned on, there should be voltage going to the heater circuit in the 3 wire o2 sensor, but dont know what the voltage should be .

 

[texas tough]

UPDATE.. may have figured it out,, was looking thru wiring diagrams and the power feed to the o2 sensor heater circuit either comes from the ignition on and thru the GAGES fuse, or the 4X4 fuse.. well my truck is 2 wheel peel only. so.. there was no fuse in the 4X4 fuse slot.. I installed a fuse and now in getting a 4 volt reading at the o2 sensor heater circuit.. I believe the heater circuit in the sensor is not a very high amp draw heater, being it ohms at 3900 mega ohms and this may be the reason a lower voltage is used to activate it.. have not found any literature on exactly how the system is supposed to work, just trying to figure it out by reading wiring diagrams. will test the system soon to see if it goes into closed loop..

 

[Jimmy D]

I found with my 91 the temp gauge looked to be in the normal operating range fully warmed but it wasn't warm enough to go into closed loop sitting in the driveway. It needed just a few more degrees to make the switch which, in my case, only happens after driving for a few minutes in warm weather or longer in cold weather. I logged mine on a drive cycle to see just when it switched and it was right there at the 195* thermostat temp. It dropped out of closed loop at 192*. My truck is all stock with a 350 and a stock AC delco fan and clutch.

 

[PlayingWithTBI]

The O2 sensor has to get up to ~400° before it'll give useful feedback. It's heated from the exhaust unless you have a heated sensor. The engine won't go into CL until water temp gets above somewhere around 160°

 

[texas tough]

yes I have the 3 wire o2 sensor.. the problem was there was no fuse in the 4X4 fuse slot,. tracing the wiring, one wire the pink and white I think, goes to the transmission harness connector plug, not sure why.. but now the truck goes into closed loop very quickly. before I installed the fuse in the vacant slot, I was getting no voltage to the o2 sensor heater circuit. this is the LAST thing I would have suspected to be the problem,, thanks to those who posted the service manuals on this site, had I not been able to trace the wiring diagrams down, this problem could have went on for a LONG time..

I found with my 91 the temp gauge looked to be in the normal operating range fully warmed but it wasn't warm enough to go into closed loop sitting in the driveway. It needed just a few more degrees to make the switch which, in my case, only happens after driving for a few minutes in warm weather or longer in cold weather. I logged mine on a drive cycle to see just when it switched and it was right there at the 195* thermostat temp. It dropped out of closed loop at 192*. My truck is all stock with a 350 and a stock AC delco fan and clutch.

I can see how that could happen.. I have a 195 stat, but on the scanner the CTS reads around 193DF max never gets to 195 idling in park... . I thought that might have been the issue, but wasnt,, what I did, I have an extra CTS , so I unplugged the connector from the one on the engine, plugged in the extra sensor and heated it up with a heat gun, while watching the temp on scan data.,. I heated it up to 210F and it still wouldnt go into closed loop.. so thats when I started looking at the O2 sensor circuit. it seems that the o2 sensor , the CTS and run time are the mechanisms that decide closed or open loop but mostly the o2 sensor..

 

[texas tough]

I found with my 91 the temp gauge looked to be in the normal operating range fully warmed but it wasn't warm enough to go into closed loop sitting in the driveway. It needed just a few more degrees to make the switch which, in my case, only happens after driving for a few minutes in warm weather or longer in cold weather. I logged mine on a drive cycle to see just when it switched and it was right there at the 195* thermostat temp. It dropped out of closed loop at 192*. My truck is all stock with a 350 and a stock AC delco fan and clutch.

do u have a 3 wire heated o2 sensor? if so it may not be the CTS and its really the O2 sensor heater circuit.

 

[Jimmy D]

No, mine is stock.

 

[Sabinoerc]

whats odd is the fuel trims are in control, short term 128, long around 16. o2 sensor is oscillating at around 450 to 510 MV

trims are just set to some default mapping unless in closed loop - so can’t really say anything about time in control until in closed loop

I have a 93 5.7 and have messed a bit with this but definitely not an expert like those who tune.

there are several qualifiers for getting into closed loop, time is one, temp is one, the ECM will command a change in fuel and expect to see a certain response in the O2 in a certain time, perhaps some other stuff- hard to get clear specifics but you can google around and find some info on it. If you have a tool like TunerPro you can see closed loop qualifier flags and the like.
Once in closed loop the O2 should vary between something like 0.4v to something like 0.8v As the ECM changes from trim up/down. If O2 changes too slowly, it will not stay in closed loop - google “lazy O2”

 

[texas tough]

took the truck on a spin last night, running very good. scanner reports closed loop now shortly after start up.. after the first moderate run, when we came to a stop the engine stalled, but cranked back up and ran good,, I think one of the injectors mat be a little lazy from lack of use for a long time, so im going to pour a little Superzilla in the tank .,, amazing stuff,,,, I have to break the engine in as we put new cam and crank bearings in , and reused the cam. this engine has the roller cam set up, so not too woried abt that, but want to seat the rings and crank bearings in . im running break in oil and my oil pressure is 30 psi at idle and 50 psi driving ,.most v6 engines shake alot, even with the balance shafts, but this one is very steady, with no balance shaft so im very happy abt that. it has a mild cam upgrade in it and was surprised how torquey the engine is. this has been a 2 year project, rebuilt the engine and transmission and it shifts great.. put a longer slip yoke on the driveshaft, an 8 inch from summit racing, and 1310 u joints as it had the odd ball 1330BC joints and the original slip yoke wasnt going far enough into the trans to engage the shaft seal. going to do the little bit of body work it needs and paint it in the fall.. the truck should be good for another 30 years no problem, rust free, interior mint, drives like new , no rattling, shimmy or shake, we built the truck to flip, but we have so many hours and quite a bit of money in it, it will be hard to let go.. we used AC DELCO everything, ARP head bolts, reinforced flywheel, high volume oil pump.. the heads were ported and polished, intake matched, the throttle body was rebuilt and mirror finish polished. we have around 1500 in parts in the transmission alone with a new high quality torque convertor, and all of the upgrades. engine bay was cleaned and repainted, new brake master cylinder, differential drained, checked and refilled. next we need to replace the exhaust, going with a magnaflow 40 series, pull the old tint off the windows and re tint, get a new set of tires, and do the A/C conversion as it still has the r12 system,,