Sorry to disagree, but there's a conflict of data. You can't have an O2 sensor reading rich, and a BLM adding fuel. That's why I asked for the coolant sensor to be disconnected, to see what the readings are, and to see if it sets a code. Based on what he's seeing, that ECM should set a lean code with the current info, but again, that O2 sensor reading indicates rich. There's something going on other than that O2.. The scan tool is revealing what the ECM thinks it sees, which might not be the actual readings.
One other check would be to disconnect the O2 sensor to see what the voltage reading is on the scanner, which should be .450 volts. And the ECM should go into open loop.

 

I use a Snap On scanner myself and I have not seen numbers stay high like that one is, it should be oscillating and it is not. My scanner shows them going back and forth very quickly. As oxygen sensors "age" they start to lag behind on their readings and then it will start making the signals narrower and less accurate. ANY C4 built before 1996 NEEDS to have the Oxygen sensor replaced every 50k miles or 24 months. If you have an older Oxygen sensor then replace it and enjoy the performance the car was meant to deliver. OBD2 cars can go 100k miles or 5 years on their (4) Oxygen sensors. Oxygen sensors are a part that degrades over time and requires regular replacement.

A 34 year old OBD1 car with the original Oxygen sensor?

Is it possible as KyleF mentioned above that the new replacement O2 was poisoned by some anti-freeze or other contaminants? I personally keep the cap covering the sensor on the O2 until I install it. I also don't buy older O2's that have been sitting on a shelf for years. To get the maximum out of your C4 Corvette you want a fresh O2 sensor at least every two years!

What about the knock counts he has on the scanner? Where would they come from? If they are real then again it would point to the EGR as it is a prime cause for detonation but that should not be occurring at 217*F. It would be very useful to see the results from an emissions test on the vehicle in question. Not having a functional EGR can make some real problems for L98 C4's. They are a PIA to replace but when they work it is good for the C4 and the environment.

The CTS being unplugged might make the car go into "Open Loop" or "Limp-back" mode. What will that show or indicate? His CTS was 217* when he did the screen shot and I was curious if it was accurate. Knock sensors are known to fail and cause issues of their own like keeping the car's timing retarded. Those are easily tested and replaced if needed.

Thanks for keeping us in the "Loop"!

Best regards,
Chris
I have never seen a O2 sensor read .450, when they are working they are constantly oscillating back and forth from.1 to .9 for a new sensor. On a hand held scanner I have it constantly goes up and down so fast you can barely read what the sensor is telling you. Disconnecting the O2 should put the car in "Limp Back" mode like it did to my 1990 when the O2 finally gave up the ghost. It is barely drive-able in that mode. I had to get off the highway and find a place to park, the car was going very slow and was clearly in limp back mode.

 

It’s my understanding some knocks are recorded during start up. This is normal right?

I’ve ordered a new o2 sensor on amazon for $8. I also used anti freeze on o2 sensor I put in 2 weeks ago. I’ll be hooking up the snap on mt2500 red brick scanner here shortly and run the tests

 

Anti Freeze or Anti Seize? Most all O2's come pre-treated with a high temp anti-seize. IF ANY of the anti-seize got on the sensor that would be enough to hurt or kill it. This is why they taught us not to add anti-seize on the O2 sensor.

Please be Wary of inexpensive parts, especially important parts. I bought 4 from RA and they sent me a bad one in the batch, when I bitched about it they said "no returns".

I go to a very Busy car parts place and buy the O2's from them as they have a fast turn over in their stock. A local NAPA franchise has a warehouse near my home and they go through a lot of O2's every month.

The " Red Brick" scanner is the one I have also, it is great with the older C4's and works on most every vehicle I have other than my 1968 C3. They make a Bosch ABS diagnostic cartridge that I want as it helps dealing with the early ABS like my 1988 has.

I am not sure about the knocks being recorded during start up. My car would not break traction at all and my neighbors 1987 C4 could run circles around mine UNTIL I changed the knock sensor. When it fails the car's ignition retards to protect the engine from any damage.

Good Luck 85c4z51!

Chris

P.S. No More anti-seize or anti-freeze on the O2's. Just screw them in with an O2 socket and your done!

 

So unfortunately i am unable to get the scanner tonight.

However, i did disconnect the CTS from the intake manifold. This sensor works as verified on my scanner.

However the ECM does throw a code 15 immediately on start up. for what its worth...

New o2 will be going in tomorrow as well.

 

Did you disconnect the O2 sensor, and read the voltage signal on the scanner? Should be about .450mv.

 

Not yet, i wasnt able to get the scanner tonight. It is a friends who is a mechanic and he needed it. I went ahead and bought a complete set Snap on MT2500 on ebay today so i will have my own here soon.

hoping to get the scanner tomorrow if possible. Just wanted to keep the thread update. As stated earlier, i did use anti seize on the o2 i may have fouled it.

 

OK! I doubt the anti seize did any harm unless you put so much that it got into the end of it. But try one thing before you replace it. Disconnect it while it's cool, then read the O2 sensor voltage on the scanner. You should see about .450mv, which is the bias voltage in the ecm. Before you start replacing parts, let's just get an idea of where we are. I just want to make sure the ecm is OK.
Good luck. And it was a good idea to buy your own scanner. MT2500 is not bad.

 

I think the aim of this is to make sure the ECM responds appropriately. I believe with no CTS it will go into limp mode and not Open Loop as IIRC, Open Loop operates off MAF and CTS on set tables.

It is a fair question. CTSs have caused some interesting running issues, but the CTS being off wouldn't explain the behavior of the O2 sensor.

I think a few here have hit on a key point. I often post about trying to diagnose issues without scan data is hard because you can't see what the ECM sees. I am going to say the opposite here though and agree with what has been posted. You are seeing what the ECM is seeing, but that is only half the story. I do believe it's time to get some readings with a multi-meter of what the CTS and O2 sensor are outputting. Do they agree?

 

Correct! I want to see how the ecm reacts by disconnecting the O2 and the Coolant sensors. And also see what the bias voltage is when it's disconnected. I just want to make sure we have a properly operating ecm before we start replacing parts. Especially since we have conflicting data.

 

I agree KyleF about seeing what the ECM sees. I use a handheld scanner made by CREADER and a Snap On MT 2500 with multiple cartridges for the GM cars. It is very important to see what the ECM sees to be sure the correct data is being fed into the ECM. I also use Fluke meters designed for mechanics when working on my cars. I am a big believer of having the right tools. I also start testing with my meters before I bring out the scanners. I like testing the parts individually like the CTS. Many years ago I bought a kit from Mid-America that has testing jigs for the various fuel injection components. It allows me to have a meter reading what is being sent to the ECM. I use these tools frequently as they let you test everything while the engine is running.

I agree with you about not going to Open Loop mode when the CTS is disconnected. The car should go to limp mode on older OBD1 cars. The same for an O2 dropout, my car went immediately into the Limp Mode.

The temperature of the engine showed it fully warmed up, if the CTS was bad and the engine was in fact cold then the ECM would make the fuel mixture leaner, not richer.

I am really big on old Oxygen sensors because I have seen most people ignore this important part until it fails. These parts are designed to be replaced on a schedule. I am amazed at people driving around with a 30+ year old oxygen sensors. This part has a serious effect on the performance and economy of the C4's. I have seen so many cars with old O2's that people claim are fine. As long as it doesn't break down on the road it is pretty much ignored. From my perspective it is a vital part that needs to be in perfect shape to get the most out of your Corvette.

"Automotive O2 sensors determine in real time if the air fuel ratio of your vehicle's engine is lean or rich. ... Driving with a bad O2 sensor is like avoiding going to the dentist. You will cause damage to your car, just as you would cause damage to your teeth by avoiding the dentist.
O2 sensors that are failing tend to read lean, which causes the fuel system to run overly rich to compensate. The result is increased emissions and fuel consumption. ... The same goes for any O2 sensor that has a bad internal heater circuit. O2 sensor failures can be caused by various contaminants that enter the exhaust.

O2 sensor failures can be caused by various contaminants that enter the exhaust. These include silicates from internal engine coolant leaks (due to a leaky head gasket or a crack in a cylinder wall or combustion chamber) and phosphorus from excessive oil consumption (due to worn rings or valve guides)."
Copied from the web

Best regards,
Chris

 

The temperature of the engine showed it fully warmed up, if the CTS was bad and the engine was in fact cold then the ECM would make the fuel mixture leaner, not richer.
I believe the strategy is just the opposite. The colder the engine, the longer the injector pulse width. If the ECM leans out the mixture on a cold engine, you won't drive very well.

 

Why we have the wonderful 9th injector originally. Colder = More fuel.... If CTS reads Hot but engine is in fact cold... the 9th injector works independently of the ECM. Extra fuel should be added in via the 9th injector... but this should all be occurring during open loop... talking a bit in circles it seems, but let's explore this.

The Scantool says he is in Closed loop. For this to occur...
1. Coolant above 105*. OK
2. Oxygen Sensor is warm enough to return varying output voltage. This is questionable
3. A specific amount of time has passed (Pre-determined in the ECM)... questionable because we don't know how long he had to car running

Let's play it this way. He is not driving the car so it will take longer to warm up. So, while the ECM is ok with seeing some varying voltage from the 02 Sensor, the threshold time has passed, and it sees high enough temps it enters closed loop... but the engine isn't actually warm. This would create poor running conditions when actually the 02 isn't warm enough, the engine is actually lean for the conditions but nominally rich and the computer is trying to get enough fuel to run properly. Could it be as simple as the CTS is bad and the 02 Sensor just isn't hot enough yet to return expected readings???

I would definitely be working to confirm the CTS. This is a tough one.
Is there a combination where the 9th injector isn't working?

 

The ECM has to see more than a time out to enter closed loop. It has to see CTS and O2 voltage change above and below .450mv.

 

cts is working. It’s new. Previous video it read 217 while dash gauge read 220. Here is some new info after plugging in scanner.

if cts u plugged it reads -39c
block learn has returned to 128 after battery reset

this pic is KEY ON IGNITION OFF AND o2 still plugged in

 

Ok. Ok this scanner o2 sensor is moving from .2-.8 mv while blm is dead at 128. Integrator is around 126 or so.

This was all with cts unplugged- it does say closed loop... Going to try and upload another video.

 

Also. With o2 Unplugged

O2 unplugged
reading moves from 443 to 447. That’s it

 

Something else to add

the o2 sensor stays high most of the time. Around .700-.760. Indicating the rich condition I am smelling. I mean my clothes stink after working or being around the car while it’s running

it appears I’m getting too much fuel vs air.

im still suspecting injectors at this point.

 

good call. I will check this out. The o2 wire harness after the exhaust mani is bundled up and around with that ground wire strap. I’ll check this out. Thank you

 

Have you done a pressure decay (leak down) test... didn't read back. Sorry if you have. This starts running in circles though, it says rich and smells rich but BLMs are high.

You other post with the CTS unplugged, sounded excellent.

Also never hurts to check grounds as mentioned.