1993 1500 O2 sensor shutting off engine, why?

[first64]

Hello, I'm a newbie here but I did some searching and didn't find what I was looking for.

Here's what I have,
1993 H vin 305 that I swapped with a very mild 350, the truck has 177k miles on it. When I was trying to do the initial cam break in the engine would turn off after three minutes of running. I've checked for vacuum, plug wiring issues and verified the correct firing. So after reading here and other places I decided that it appeared the system was going into closed loop around the 3 minute time frame and shutting off. So I though I would try unplugging the O2 sensor to see what happens since it is referenced in closed loop.

Well the engine will run all day long (I've let it idle for 30 mins) without issue. I bought a new O2 sensor and it didn't change the shutdown problem. If I plug in the O2 sensor it will shut the engine off within 30 secs. I'm not getting a CEL at all with it plugged in or unplugged.

Any ideas? I was thinking about running a wire from the plug to a ground with the engine running to see if the connection is grounding out when the O2 is plugged in, other then that I have no idea.

I don't have any codes currently. Sorry for the long first post

[ChevyTech]

It is either going to rich or to lean.

When the oxygen wire is connected, is the exhaust rich, black, or stinky, before it kills?

Does it have headers?

Does it have a complete exhaust system?

Does it have any exhaust leaks?

Did you switch to 350 injectors?

What do you mean by mild? Does it have any lope at idle?

Test the vacuum to the MAP sensor and let us know how much vacuum there is at idle.

[first64]

With the O2 in the exhaust is somewhat stinky, not black though.
No headers, complete exhaust, no leaks.

I did not switch to 350 injectors since this is just a casual use truck and it doesn't need to run like a scalded dog.

[ChevyTech]

It should set a oxygen sensor code when the vehicle is run with the sensor unplugged.

Are you sure you are checking the codes correctly?

[ChevyTech]

Originally Posted by first64

With the O2 in the exhaust is somewhat stinky, not black though.
No headers, complete exhaust, no leaks. ...

It could be to rich.
Is the cam big enough to have some lope at idle?

Originally Posted by first64

I did not switch to 350 injectors since this is just a casual use truck and it doesn't need to run like a scalded dog.

Don't expect it to run well, especially in open loop, which is not were it is having trouble now.

[first64]

I did the paperclip terminal short and got 1 flash followed by two flashes. When I ran it at idle unplugged for almost 30 mins I never had a CEL. I would think it would flag the O2 sensor not being present. I know the bulb is good...

[first64]

No the cam is mild, no lope. I will look into switching the injectors out. Is there any relation between the injector and when the system goes into CL?

[ChevyTech]

Originally Posted by first64

No the cam is mild, no lope. I will look into switching the injectors out. Is there any relation between the injector and when the system goes into CL?

No

Thing is, it is having problems when it goes into closed loop operation, which is where it should run better, and be able to compensate for the wrong injectors.

[ChevyTech]

Originally Posted by first64

I did the paperclip terminal short and got 1 flash followed by two flashes. When I ran it at idle unplugged for almost 30 mins I never had a CEL. I would think it would flag the O2 sensor not being present. I know the bulb is good...

This doesn't seem right.

I am going to go look at a 1993 emissions manual and see what the criteria is to set a code 13.

[first64]

Originally Posted by ChevyTech

This doesn't seem right.

I am going to go look at a 1993 emissions manual and see what the criteria is to set a code 13.

I thought 1 followed by 2 flashes meant no codes.

13 = Oxygen sensor voltage stays between 0.35 and 0.55 volts for 60 seconds. Engine was at normal operating temperature and had been running for at least 2 minutes after start with a throttle angle above 5%.

I will double check the flashes again tonight to be certain...

[ChevyTech]

Here is the criteria for a 1993 small block V8 with an automatic transmission.

Code 13 criteria

Engine coolant temp greater then 158 degrees F = 70 degrees C
At least 2 minutes of run time
Signal voltage steady between .35 and .55 volt
Throttle position signal above 5%
All conditions must be met for 60 seconds


Let me know what transmission the truck has.

If it has a manual transmission which has a 160 Baud rate C3 ECM that the 1992 TBI trucks had.

An automatic in a 1993 should be an 8192 BAUD PCM if it is a ½ ton C/K truck

[ChevyTech]

Originally Posted by first64

I thought 1 followed by 2 flashes meant no codes.

...

No

It will flash out a code 12 first, followed by any other codes. It will flash out a code 12 again after it has flash out all other codes, and keep repeating the codes sequence over and over until you stop the code test.

[ChevyTech]

When you are checking codes with a paper clip, it will flash out each code three times before it moves to the next code.

Test the voltage on the wire that the oxygen sensor plugs onto with the sensor unplugged. Let me know what you find.

[first64]

Originally Posted by ChevyTech

Here is the criteria for a 1993 small block V8 with an automatic transmission.


Let me know what transmission the truck has.

It's an auto, 4L60E

Originally Posted by ChevyTech

When you are checking codes with a paper clip, it will flash out each code three times before it moves to the next code.

Test the voltage on the wire that the oxygen sensor plugs onto with the sensor unplugged. Let me know what you find.

I will double check the code readouts soon. Do I check for V with just the key on or does the engine need to be running?

[ChevyTech]

Originally Posted by first64

Do I check for V with just the key on or does the engine need to be running?

Key on, engine off.

[first64]

The V at the O2 connector is .4v

The codes if I'm counting correctly are 33, 35, 42. That was with me sitting there awhile making sure I was counting correctly.

[ChevyTech]

33 = MAP sensor – Check and make sure there is vacuum getting to the MAP sensor – make sure the hose fits tightly too.

35 = IAC – If the IAC (Idle Air Control) does not work causing a very low idle, this could cause the stall and the code 33.

Code 42 can be set be unplugging the set timing connector to check the timing.

My first guess is – the IAC is bad.

When you replace, remove and clean an IAC, or unplugged the IAC with the key on, the ECM may need to relearn the IAC position. This will not happen without driving the truck and meeting specific conditions or taking other steps the cause it to relearn the idle.

Have you driven the vehicle at highway speeds after doing all the work? Specific criteria must be met for the system to self relearn the IAC position, unless you manually invoke a relearn.

Did you clean the TBI with carburetor cleaner when you did the engine work?
The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

You should clear all the trouble codes from the memory.

[first64]

I tried to take it for a spin around the block and it didn't go well. After getting to maybe 15 mph the truck surges and sputters then runs a little then wants to stall. When I got it to run a short distance I went to stop and the engine stalled. I was able to plug in the o2 sensor and force the engine to stay running but it didn't improve drivability. I also unplugged the CTS and that seem to keep the engine running with the o2 sensor plugged in, however it did stall and was very hard to restart.

Back to your question, yes when I did the engine work I did clean the TBI with carb cleaner and didn't protect the IAC. I'll pick up a new one and I check for vacuum at the MAP and get back to you.

This is my first go round with OBDI, how do you invoke a manual learn for the IAC?

[first64]

update:

I replaced the IAC, that cleared the 35 code.

I checked the vacuum to the MAP, 18 cm of Hg (that's what the multipurpose vac gauge says)

I still have the 33 though.

Tried to drive it, still sputters/hesitates that's with the o2 unplugged.

[ChevyTech]

Originally Posted by first64

... I also unplugged the CTS and that seem to keep the engine running with the o2 sensor plugged in, however it did stall and was very hard to restart.

If unplugging a sensor improves the way the engine runs, usually it is because the sensor is giving the computer bad readings.

If the grounds at the front of the intake are not clean, tight, and all connected, the coolant sensor reading may be wrong. The coolant sensor ground wire terminates at the front of the intake. The computer uses the coolant sensor reading to determine how much fuel to inject & what the idle speed should be.

Originally Posted by first64

This is my first go round with OBDI, how do you invoke a manual learn for the IAC?

You can try this.
Using a watch or timer to make sure you hit the threshold times on step 3 and 4:
1) Disconnect battery for at least ten seconds. (with the Key OFF)
2) Reconnect the battery.
3) Start engine letting it run for five seconds.
4) Turn key off for ten seconds.
5) Then start it and see how it idles.

[ChevyTech]

Originally Posted by first64

... I checked the vacuum to the MAP, 18 cm of Hg (that's what the multipurpose vac gauge says).

Good

Originally Posted by first64

I still have the 33 though.

The sensor could be bad.

MAP sensor testing TBI

Manifold absolute pressure sensor

A normal Map sensor reading on the wire that connects to terminal “B” (possibly Lt green) would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

NOTE: if it runs poorly it could have much lower vacuum at idle which would cause a higher voltage reading. But at 18 inches the voltage should fall between 1 and 2 volts.

Terminal “A” (possibly Gray) is voltage supply to the Map sensor. (Should be close to 5 volts)

Terminal “C” (possibly purple) is the ground for the Map sensor. (Should have no voltage)

Note: If I remember correctly the MAP and coolant sensor share the same ground to the front of the intake. Test the ground to these sensors.

[first64]

I tested the MAP sensor, I think you might have switched the A & C terminal btw. The Haynes manual says c is 5v and A is ground, none the less I had 5V and 2ohms and zero volts on the ground. So i decided to unplug the MAP and try to drive it...well it worked, idling coming down from a cruising speed was still an issue though.

I bought a new MAP and installed it, took a quick spin around the block and it seemed to run okay. So I decided to drive it on the road to try and cal the IAC, well during the test drive it ran fine until something happened in the engine and I had to limp the truck home.

Well, I broke an exhaust valve spring...I got that replaced and the engine back to running okay at idle, however now it's running like it was before I replaced the MAP and I am still showing a 33.

I was going to replace the MAP again since it's under warranty and try again. Hopefully the truck will stop fighting me...

What type of highway driving is needed for the IAC to relearn? I got it to 50-60mph cruising before the spring broke, maybe 5-10 mins of run time.

[ChevyTech]

I think you might have switched the A & C terminal btw. The Haynes manual says c is 5v and A is ground, none the less I had 5V and 2ohms and zero volts on the ground.

You are correct. I did have A&C switched. Sorry about that. I copied and pasted it from my notes, which have been posted many other times and you are the first to catch the error. Thanks for bringing it to my attention.

Well, I broke an exhaust valve spring...I got that replaced and the engine back to running okay at idle, however now it's running like it was before I replaced the MAP and I am still showing a 33.

I was going to replace the MAP again since it's under warranty and try again. Hopefully the truck will stop fighting me...

Keep checking the vacuum supply to the MAP sensor. If you get a problem like an EGR valve sticking open at times it will cause low vacuum, the code 33, and make it run poorly.

You could disconnect and plug the hose to the EGR valve and take it for a test drive.

… What type of highway driving is needed for the IAC to relearn? I got it to 50-60mph cruising before the spring broke, maybe 5-10 mins of run time.

I can't remember all the criteria but I believe the speed the vehicle must reach is 45 MPH, but I am sure other conditions must be met.

If when you start the truck, the idle speed flares up and then starts slowing down, the IAC is working. The IAC is what gives it a fast idle when started cold.

How is the idle speed now?

[first64]

Sorry its been some time since I made it back on here.

I did get a new MAP sensor, and replaced the low temp thermostat with the correct temp one.

When I drive it the SES light clears and when I get back to idle sitting in park the SES light comes back on. Still showing a code 33 I believe, I been doing other things to get the truck through inspection so I have lost my reference point at the moment.

I plan to connect a scan tool so I can monitor some things when it's running since I think I don't have the timing set where it should be. Once I get the tool I will try to set things correctly and see where I end up. I'll check back after that.

[first64]

double post