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Old 04-21-2013, 06:36 AM
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1 years worth of an annoying code... (4.3 TBI)

Last year I went to Texas. As soon as I crossed the TX line, my check engine light comes on. It was code 13. Let me give you the specs on code 13, then my motor specs before I continue...

Code 13: Oxygen sensor circuit - Oxygen sensor voltage stays between 0.35 and 0.55 volts for 30 seconds when the TPS signal was above 0.55 volts. The engine had been running for at least 40 seconds with an engine temperature above 110? F.

My engine: 4.3 bored over .060, Mild Comp Cam, 350 flattop pistons, MSD Ignition and HEI distributor.

I have searched the internet over for a solution to this annoying problem and cannot find a thing!

I have ruled out bad O2 sensors by replacing 3 times in this year. I have ruled out TPS, because I replaced that as well. I have since replaced EGR, and sealed all exhaust leaks. No vacuum leaks whatsoever. I have also ruled out a bad ECU wire by following instructions in this excellent write up at: ECM Pinouts I bypassed the factory O2 circuit wire in case there was something going on with the wire. The ECM pin looked fine.

The engine did well for 3 or 4 months after the build with no trouble codes whatsoever. It only happens when I drive it. It comes on after normal operating temp is reached. And ONLY during driving. I could let it warm up and idle all day long with no code. But once the engine is under a load, it never fails to come on. I am leaning to TPS issues because of this fact.

I have also ruled out bad ECU because I installed my other ECU with the same results. I have added an Edelbrock chip, venom injectors, and injector spacers since thinking it would help with fuel, IF that were an issue, and nothing.

If ANYONE here has some advice, I would love to hear it so I could put this trouble code behind me, that would be GREATLY APPRECIATED! My truck runs great, however, that damn light is buggin the crap outta me!

I have no catalytic converter on the system, but I figured if that were an issue, I would had gotten the code when the build was first completed, and during the first few months of driving.

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Old 04-21-2013, 08:23 AM
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You didn't say what year ECM you were using, or if you did I missed it. Anyway looking at the 1985 ECU pinout list it calls for an oxygen sensor(D7) voltage of 0.45V average. I see just above it on the list an oxygen sensor "Grn'd" (D6) should be 0(zero) volts.
If there is any corrosion on the "ground" circuits terminals either at the sensor or at the ECU(or the wire itself) it can add a slight resistance to the circuit. If there is any resistance it will cause a voltage increase for current to flow. Similar to attaching your multi-meter to the battery's ground terminal post and the engine block. Any voltage indicates a resistance(there will be a very slight voltage drop, even if its just millivolts, to overcome the resistance of the wire. The longer the wire the more of a drop).
That resistance(voltage) will be added to the voltage that is dropped across the oxygen sensor from the ECM perspective. For instance if the voltage drop across the oxy. sensor is 0.45 volts, and the "Grn'd" circuit has enough resistance to cause a 0.2V drop, the computer if properly grounded would see a 0.65V signal on the oxy. sensor terminal(D6).
Have you tried measuring (w/DVM)the "Grn'd" (D7) voltage by back probing the connector at the ECM?
Just a thought.
FWIW
ssmonty
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Old 04-21-2013, 08:30 AM
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Quote:
Originally Posted by ssmonty View Post
You didn't say what year ECM you were using, or if you did I missed it. Anyway looking at the 1985 ECU pinout list it calls for an oxygen sensor(D7) voltage of 0.45V average. I see just above it on the list an oxygen sensor "Grn'd" (D6) should be 0(zero) volts.
If there is any corrosion on the "ground" circuits terminals either at the sensor or at the ECU(or the wire itself) it can add a slight resistance to the circuit. If there is any resistance it will cause a voltage increase for current to flow. Similar to attaching your multi-meter to the battery's ground terminal post and the engine block. Any voltage indicates a resistance(there will be a very slight voltage drop, even if its just millivolts, to overcome the resistance of the wire. The longer the wire the more of a drop).
That resistance(voltage) will be added to the voltage that is dropped across the oxygen sensor from the ECM perspective. For instance if the voltage drop across the oxy. sensor is 0.45 volts, and the "Grn'd" circuit has enough resistance to cause a 0.2V drop, the computer if properly grounded would see a 0.65V signal on the oxy. sensor terminal(D6).
Have you tried measuring (w/DVM)the "Grn'd" (D7) voltage by back probing the connector at the ECM?
Just a thought.
FWIW
ssmonty
I have not checked that yet. I did replace the wire on the ECU (original ECU was 89, replacement from 86 I believe but same number 1227165) So corrosion on wire is out of the question. As for the ECU being grounded. Is ground achieved through the harness, or do I actually have to bolt the ECU back into place? Dumb question I know, but I figured if grounding were an issue, I would be seeing a flood of issues with other computer controlled components?
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Old 04-21-2013, 09:26 AM
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I feel your pain, it happened to me too. How is your gas cap? Sometimes if the gas cap doesn't seat properly or isn't tight enough it will give you that false check engine light. Happened to me on a 2003 Pontiac Sunfire. Replaced the cap and solved the problem. Look at the easiest fix first, right?
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Old 04-21-2013, 10:02 AM
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Can't say for certain whether or not ECU has to be bolted in for ground. I wouldn't think so since there are gnd terminals(A12, D1, D3, D6, D10) in the connector.
Just for elimination I would measure all of them to make sure there is a good ground on each one. I don't think there is an actual wire in the harness to gnd for each(I could be wrong, I haven't seen a schematic). If there were I think you may be able to just connect the black lead of the DVM to the neg. batt. post and ohm out each terminal to gnd. without back probing after unplugging the connector.
I think you may have to measure for voltage at each by back probing with engine running(including the sensor return line D2). There shouldn't be hardly any if I'm not mistaken.
You said you replaced the wire so there is no corrosion. There could be a bad crimp that causes problems. Just saying.
Any chance that an add on(sound system,GPS device,ect) could be increasing the current through the ECU ground circuit that could cause an increase of voltage on the line that could change relative readings.
FWIW
ssmonty
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Old 04-21-2013, 10:51 AM
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Just a thought...... Maybe the exhaust isnt well grounded?
Most systems are hung on rubber hangers to lessen vibration.At the manifolds they have the spring/bolt kits.If they are rusty in that area the sensor may not have a good ground path if it is a one wire sensor.
Also if the voltage is hanging inbetween .35 and .55 as you said with the TPS greatr than .55v .......then that would kinda rule out a mixture problem as it sounds like the O2 sensor reading was going into default.
Make a ground strap for the exhaust. Be sure the connections are absoluely clean and tight.See what happens.
And the computers wiring has all the grounds for the sytem built in so be sure they are clean and tight. Dont overlook the engine to body and engine to frame grounds either. Seems to me there is one at the back of the engine on the passenger side that corrodes away or gets left off. That one causes some ******* to go on right there.
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Old 04-21-2013, 11:45 AM
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Quote:
Originally Posted by DanTwoLakes View Post
I feel your pain, it happened to me too. How is your gas cap? Sometimes if the gas cap doesn't seat properly or isn't tight enough it will give you that false check engine light. Happened to me on a 2003 Pontiac Sunfire. Replaced the cap and solved the problem. Look at the easiest fix first, right?
Gas cap is good. Have gone thru two actually, as I thought about that possibility. That is a notorious problem with the new GMC/Chevy trucks
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Old 04-21-2013, 11:48 AM
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Quote:
Originally Posted by ssmonty View Post
Can't say for certain whether or not ECU has to be bolted in for ground. I wouldn't think so since there are gnd terminals(A12, D1, D3, D6, D10) in the connector.
Just for elimination I would measure all of them to make sure there is a good ground on each one. I don't think there is an actual wire in the harness to gnd for each(I could be wrong, I haven't seen a schematic). If there were I think you may be able to just connect the black lead of the DVM to the neg. batt. post and ohm out each terminal to gnd. without back probing after unplugging the connector.
I think you may have to measure for voltage at each by back probing with engine running(including the sensor return line D2). There shouldn't be hardly any if I'm not mistaken.
You said you replaced the wire so there is no corrosion. There could be a bad crimp that causes problems. Just saying.
Any chance that an add on(sound system,GPS device,ect) could be increasing the current through the ECU ground circuit that could cause an increase of voltage on the line that could change relative readings.
FWIW
ssmonty
Okay, well, I have a 500W system, but had that originally as well, so I never had any issues before. I'm gonna have to be walked through the back probing procedures as I am not an electrical kinda guy. lol I can do alot with the motor, but have not trained myself properly in electrical diagnosis, and testing. Simply the basics.
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Old 04-21-2013, 11:50 AM
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Quote:
Originally Posted by LATECH View Post
Just a thought...... Maybe the exhaust isnt well grounded?
Most systems are hung on rubber hangers to lessen vibration.At the manifolds they have the spring/bolt kits.If they are rusty in that area the sensor may not have a good ground path if it is a one wire sensor.
Also if the voltage is hanging inbetween .35 and .55 as you said with the TPS greatr than .55v .......then that would kinda rule out a mixture problem as it sounds like the O2 sensor reading was going into default.
Make a ground strap for the exhaust. Be sure the connections are absoluely clean and tight.See what happens.
And the computers wiring has all the grounds for the sytem built in so be sure they are clean and tight. Dont overlook the engine to body and engine to frame grounds either. Seems to me there is one at the back of the engine on the passenger side that corrodes away or gets left off. That one causes some ******* to go on right there.
Thanks, I will look into that. What would be your suggestion into the proper way to ground the exhaust to the frame? I do have a ground strap in the back, underneath the driver side tailgate that has been broken for quite sometime.
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Old 04-21-2013, 12:01 PM
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weld a small metal tab on the exhaust. Be sure it has a small bolt hole in it. Bolt a ground strap on it, then bolt that to the block. Be sure the areas under the boltdown are clean and free of rust and paint.
After all, we are talking about a voltage of 1 volt or less. Any resistance on the return circuit will have negative effect.
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Old 04-21-2013, 12:59 PM
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Originally Posted by LATECH View Post
weld a small metal tab on the exhaust. Be sure it has a small bolt hole in it. Bolt a ground strap on it, then bolt that to the block. Be sure the areas under the boltdown are clean and free of rust and paint.
After all, we are talking about a voltage of 1 volt or less. Any resistance on the return circuit will have negative effect.
Any suggestion as to placement on the actual exhaust? Closer to the O2 sensor the better? Just curious. I did fix the broken ground strap in the back, and will go ahead and secure the ECU again sometime this week. Will update as I go. Thanks for everyone's suggestions.
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Old 04-21-2013, 06:04 PM
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S10BlazerBuilt,
Back probing is reading the voltage/signal on a connector that is connected to its mate(plugged in) when you can't get to the otherside of the circuit. In otherwords you insert the test leads into the connector from the same direction as the wire from the harness, instead of probing the terminals that make a connection when the connector is plugged up.
Most test leads of a DVM are too large to get into the "backside" or wire entry side of a connector. You make an adapter of sorts to interface with the test lead and the connector terminal. For instance you can use a thin diameter wire that is strong enough not to bend too easy(paper clip), and twist it around the end of the test lead three or four times(sort of like a small spring around a nail), so that it makes a good connection, and bend it, so that it acts like a small diameter extension of the test lead, if you follow.
You can insert it into the back of the connector(wire side) while the connector is plugged up to your computer, and read the voltage/signal that the computer "sees" while its operating.
It may be difficult to back probe a connector if the ECM is mounted in its normal position. Not having it mounted may be a stroke of luck right now. All of the "Grn'd" terminals that I mentioned earlier should measure nearly zero volts when the engine is running.
You might want to measure the continuity of the ground terminals to the negative battery post by unplugging the connector(while engine/key is off of course), and probing the connector from the ECM side of it? Easy enough to do.
Good luck.
ssmonty
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Old 04-21-2013, 06:07 PM
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Small sewing needles make a good backprobe.
Pomona makes some good probes too.
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Old 04-22-2013, 05:09 AM
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S10BlazerBuilt,
I should have mentioned that it would be easier and quicker to just remove the connector and ohm between the ground terminals(A12, D1, D3, D10,ect).
Just attach one test lead to say A12 and measure the resistance to each other terminal with the other test lead. They should all measure nearly zero ohms between them.
I was most concerned with terminal D6 as it was the "grn'd" for the oxygen sensor according to the 1985 ECM pinout list. Looks like it was changed to just "Grn'd" on the 86-89 ECM were the other terminals were changed to "System grd". I still think its the return for the oxy. sensor and if its resistance is higher than the others it could cause problems IMO. I think the designers realized the exhaust wasn't a reliable ground because of corrosion or whatever, and changed the sensor to a two wire type.
I would think that if it were a two wire type that it wouldn't make any difference if the exhaust was grounded, as it would be redundant, but I could be wrong. When trying to measure such a small voltage accurately just a slight difference in resistance can make a big difference!
ssmonty
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Old 05-12-2013, 12:32 PM
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SOLVED!!!!!!!!!!!!!!!!!! I think.... Well, I looked into every possible avenue for why I was getting this code, and took every step trying to trouble shoot it... Someone mentioned my stereo amplifier, so I got to Playing around where my remote wire for the amplifier was tied to, in regards to my fuse box. Today I switched the wire over to the wiper fuse and so far, no check engine light. Previous fuses I have had it tied to were ignition, and ecm. So I take it the amp was enough to rob the o2 sensor of it's proper voltage readings, therefore causing the code 13. Anyways, Got my fingers crossed and hope this is the solution. No check engine light in the last 40 miles! WOOT!
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