Topic: Check Engine Light With Questions

[Kurlee Daddee]

So I installed Downey headers and a high flow cat exhaust on my truck. Before the install, my O2 sensor was working fine. On the Downey exhaust system, the O2 sensor is located before the cat and farther downstream then the original location. I was getting CEL's cruising only at freeway speeds. I don't get any CEL's during city driving. I installed a 4 wire O2 sensor last night (1 signal, 2 ground, 1 power to the + side of coil.) to remedy the cold O2 sensor theory from the 1 wire sensor. I also installed new ground straps (one from the body to the negative side of the battery, the other from the body to the motor.) I sanded down all contact points to bare metal for better contact.

So this morning was the test to see if my mods would work. I got another CEL as soon as I reached 70mph as a cruising speed. As soon as I slowed off the exit of the freeway, the CEL turned off. I pulled the code and it flashed 5 times, which is for bad O2 sensor.

My question to you guys is, since the CEL's turns on and off only from freeway driving, will the ECU automatically throw the motor into SAFE mode? Or should I not even worry about the CEL since it turns on and off? I filled up my gas tank last night and I am going to track my MPG's to see if the CEL effects gas mileage.

Please let me know if you have any suggestions. Thanks.

[emsvitil]

Without the O2 sensor, the ECU goes into open loop mode.

Did you separate the heater ground from the sensor ground?    Having them together can affect the O2 signal

(especially if your power source for the heater is on the coil, since the voltage will vary with the ignition)   This variance will make it to the heater ground wire, and will overwhelm the O2 sensor signal output if it's grounded with the heater ground.  (the sensor ground will vary up and down by as much as 100 milliamps, which will cause the O2 sensor signal to vary that much; which will confuse the ECU if the output voltage goes up or down when it's expecting down and up)

[Kurlee Daddee [OP]]

Without the O2 sensor, the ECU goes into open loop mode.

Did you separate the heater ground from the sensor ground?    Having them together can affect the O2 signal

(especially if your power source for the heater is on the coil, since the voltage will vary with the ignition)   This variance will make it to the heater ground wire, and will overwhelm the O2 sensor signal output if it's grounded with the heater ground.  (the sensor ground will vary up and down by as much as 100 milliamps, which will cause the O2 sensor signal to vary that much; which will confuse the ECU if the output voltage goes up or down when it's expecting down and up)

Good point. I did ground both wires with the same bolt. I will seperate them and try again. Thanks.

[Kurlee Daddee [OP]]

Moved grounds away from each other and still get a CEL. All of my connections to extend the wires were soldered and have heat shrink over the top of them.

Any way of checking the O2 sensor with a volt meter?

[emsvitil]

I digital voltmeter will work at lower speeds.    It flip-flops to fast for the meter when you're going faster.

Did you pull the fuse to the ECU so it can start over with the relearning?

[Kurlee Daddee [OP]]

I digital voltmeter will work at lower speeds.    It flip-flops to fast for the meter when you're going faster.

Did you pull the fuse to the ECU so it can start over with the relearning?

I guess I can do that and try again. Negative battery terminal off for an hour or so will just as well, right?

[Kurlee Daddee [OP]]

Okay I have done everything I can think of including what is on this thread. I thought the ground wires weren't getting a good ground maybe because of a bad body ground. Extended them to the negative battery terminal. Pulled the Neg. battery cable to clear the CEL. Still happened. Yesterday I put a new gasket between the O2 sensor and the pipe it goes on since maybe there was an exhaust leak and causing the CEL. This morning I got more than 3/4 of the way to work and then the CEL came on. I can, however, decelerate and make the CEL go off. I know its not a big deal, but my gas mileage went from 23mpg to 19mpg since installing these headers. Once again, the CEL only comes on at freeway speeds.

Anyways my question is, does the ECU tell the EFI to dump fuel as soon as it sees the CEL and continues this route until you clear the code. Or does the ECU only tell the EFI to dump fuel when the CEL is illuminated? Anyone know?

[emsvitil]

I'd guess that when the CEL goes on, it's running in open loop mode because it thinks the O2 signal is invalid.   

Open loop is usually richer.

When the CEL turns off, it thinks the O2 sensor signal is once again valid, and runs in closed loop.  (leaner).

[Kurlee Daddee [OP]]

I'd guess that when the CEL goes on, it's running in open loop mode because it thinks the O2 signal is invalid.   

Open loop is usually richer.

When the CEL turns off, it thinks the O2 sensor signal is once again valid, and runs in closed loop.  (leaner).

Thank you Ed for answering my questions. I guess I can play with the truck while I am driving it to make the CEL go off. Just kind of annoying.

[emsvitil]

Is the heater in the O2 sensor good?    

What's the resistance across the heater wires?

What about an exhaust leak at the head/header flange?  
Bad weld in header?  
EGR holes in head/header flange?

[emsvitil]

Then there's those block off plates above the manifold for the 22RE.   (22r has air injection there)

[emsvitil]

Just thought about the EGR.......

Normally there needs to be backpressure for the EGR valve to open up and let exhaust into the intake.

But there are some modes in which EGR flow is forced by opening up the EGR valve anyway.....

What if the EGR valve is forced open, but instead of exhaust going into the intake,  the headers are working really good and are actually sucking intake air thru the opened EGR valve.....

[Kurlee Daddee [OP]]

Is the heater in the O2 sensor good?   

What's the resistance across the heater wires?

What about an exhaust leak at the head/header flange?   
Bad weld in header?   
EGR holes in head/header flange?

Haven't checked the voltage yet on the wires.
Header flange is nice and tourqed down. Check it with a torque wrench every week (Yes I am anal and don't want exhaust leaks.
Headers are in perfect shape.
Block off plates are nice and tight with no holes.

About the EGR. Wouldn't I get a different code other than Code 5 to tell me there is a problem with the EGR?

[emsvitil]

About the EGR. Wouldn't I get a different code other than Code 5 to tell me there is a problem with the EGR?

An 85 ECU isn't that smart............

In 85 the EGR was still controlled by vacuum and pressure lines, no interaction with the ECU.

[Kurlee Daddee [OP]]

CEL problem fixed!!!!

I changed back to the one wire O2 sensor. This time I stripped insulation about a half inch back on the signal wire harness side. Was careful to get all the metal insulation away from the wire connection. Soldered the wire from the O2 sensor to the harness signal wire and all is good. I also took off the sheild from the O2 sensor I had on there. I reset the ECU and it runs a lot better. I have been driving it for 2 weeks without another check engine light.

So I think the whole issue was I wasn't stripping back the metal insulation enough and it was grounding out the signal connection. So it was user error

Thanks Ed for all the advice. I really appreciate the help.