90 Chevy k1500 no closed loop at idle

Joe Technician Benicia, California Posted   Latest   Edited  

Have a 1990 Chevy k1500 5.7 TBI that I’m not sure if the issue is actually just normal operation or not and searched all over but couldn’t find a straight forward answer. Basically the Vehicle will not enter closed loop at idle when at operating temp. I had already found the ECT inaccurate, replaced sensor and now reads correctly. the temp will hit right about 190F. just off idle the vehicle will enter closed loop, as soon as it returns to idle it falls back into open loop. Customer had already replaced the o2 sensor (single wire) and I’ve verified it’s operation off idle. when It returns to idle the o2 sensor voltage falls and switches around 190-300 mv. If the secondary air is disabled ( it stays on in open loop) the sensor reads rich. The grounds have been checked that are near the t-stat housing. I’ve read that the v6 and the 454 will go into Closed loop at idle but does not mention the 350. its In for a smog failure and just need to know if this is normal operation or not. I’m probably just missing something simple here so I apologize in advance if that’s the case

0
Scott Owner/Technician
Claremont, California
Scott Default
 

Hi Joe, I’m pretty sure what you’re seeing is normal. I own an 89 with the 350 “K” engine and this is how it behaves.

+6
Default Ð Bounty Awarded
Agree
Helpful
Joe Technician
Benicia, California
Joe Default
 

Thank you sir!

0
Default Ð Bounty Awarded
Sean Owner
Cornville, Arizona
Sean Default
 

What are the o2 crosscounts at idle? Pretty sure it won't go into closed loop if the sensor is too slow. Those do go into closed loop at idle, they are just stubborn about it.

+1
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Default
 

at Idle the O2 range is 190-300mv. From what I remember when testing an O2 you bring it slightly off idle in which case it acts normal. This does have a new sensor in it also

0
Default Ð Bounty Awarded
Cory Owner
Wichita, Kansas
Cory Default
   

New sensor or not it has to cross 450 before it will count as a cycle and you don't have any Cycles if it's between 190 and 300 new sensor or not it's getting cold pretty common though

0
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Default
 

Right.. which is why I was concerned and trying to figure it out

0
Default Ð Bounty Awarded
David Owner/Technician
Red Springs, North Carolina
David Default
 

These wouldn't go into closed loop with a scanner connected unless it had an option for test drive,

0
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Default
 

It does have an option for test drive and that was the data stream i was using . The other data stream would raise the idle and it would go into closed loop

0
Default Ð Bounty Awarded
Dave Mobile Technician
Ironton, Ohio
Dave Default
 

This is normal. The O2 is not an electrically heated unit and due to the placement of the sensor it will cool off at idle to the point that it really is inoperable.

0
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Default
 

Makes sense. Just weird that in the small amount of info I found on all data stated this was normal on v6 and the 454 but no mention of this particular engine

0
Default Ð Bounty Awarded
Ray Diagnostician
North York, Ontario
Ray Default
 

What is the issue with the smog failure? High CO and HCs at idle? Is the EGR partially open at idle? What is the engine vacuum?

0
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Default
 

The test is an ASM on a dynamometer for this vehicle. so there is no idle test for it but the failure is high hc but didn’t fail by much. Also had elevated but passing co and nox. There was o2 present as well. I was more or less confirming this before continuing testing. The customer had pulled out his shotgun prior to bringing it to us and an egr valve was another component replaced already…

0
Default Ð Bounty Awarded
Gary Owner/Technician
Buena Vista, Colorado
Gary Default
 

Absolutely normal, Joe'' You have to remember that this '90 K1500 is indeed your Fathers's Oldsmobile :0) The single-wire O/2 must be at least 600-725 degrees F. before it will generate a reliable signal. Increasing the idle speed to about 1,500 rpm will usually restore operation until the exhaust manifold again cools down...

0
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Default
 

Haha thanks Gary. Just not used to the old stuff I guess :/

0
Default Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia
Martin Default
 

As others have alluded Joe, the o2 sensor on that old truck is too far downstream and cools off, resulting in open loop. On V6 equipped engines of those years with the same issue, the O2 sensor was relocated closer to the engine. KOEO the sensor should have a bias voltage of 0.45v. Switching should be like any other 1.0v zirconia sensor in closed loop.

0
Default Ð Bounty Awarded
Rob Owner/Technician
Nanaimo, British Columbia
Rob Default
 

As others have confirmed it's a completely normal condition.

0
Default Ð Bounty Awarded
Joe Technician
Benicia, California
Joe Resolution
 

Thanks to everyone for the help. Appears to be normal operation

+1
Resolution Ð Bounty Awarded
Stephen Educator
Bethel, Ohio
Stephen Default
 

- I worked at a chev dealer back then and this is a normal operation of the sensor. These vehicles are known to do this because the o2 sensors are far enough away from the exhaust manifold they don't stay hot enough at idle. i has serious doubts your will be able to do much to change it.

0
Default Ð Bounty Awarded
Glenn Owner/Technician
Houston, Texas
Glenn Default
 

FYI. We were able to get a lot of these to pass the dyno test and improve fuel economy by installing a heated O2 sensor and if needed place .1 mfd capacitor across the injectors.

0
Default Ð Bounty Awarded
Scott Diagnostician
Merritt Island, Florida
Scott Default
 

That’s normal. As others have said, the O2 sensor cools off at idle and it goes open loop You can install a 3wire O2 sensor with the heater wires grounded through a switch of some sort. (IGN feed for power and ground side switched through a throttle micro-switch, maybe). I considered doing that on mine, but blew the transmission again before I got around to it.

0
Default Ð Bounty Awarded