Open loop.
- Thread starter Paul Higg
- Start date
jmccloud
Well-known member
Open Loop
Paul,
To evaluate whether it is open loop that is causing your 'cold' poor running, try disabling the closed loop and drive the car after warm up. One easy way to do this is by unplugging either O2 sensor; that is either one that is ahead of the cats. The one behind the cats won't preclude closed loop.
In other words, unplug one of the forward O2 sensors (and tie it up so the leads don't get burned on the exhaust) then drive the car beyond the 140F where it usually gets better. Yes, this will throw a code but that's easy to evaluate and erase. But by remaining in open loop when warm, you will know whether something in open loop is causing your problem or if it's 'cold' related.
Good luck.
PS The FSM describes the 'closed loop' sensors but I can't find it. Let you know if I do.
Paul,
To evaluate whether it is open loop that is causing your 'cold' poor running, try disabling the closed loop and drive the car after warm up. One easy way to do this is by unplugging either O2 sensor; that is either one that is ahead of the cats. The one behind the cats won't preclude closed loop.
In other words, unplug one of the forward O2 sensors (and tie it up so the leads don't get burned on the exhaust) then drive the car beyond the 140F where it usually gets better. Yes, this will throw a code but that's easy to evaluate and erase. But by remaining in open loop when warm, you will know whether something in open loop is causing your problem or if it's 'cold' related.
Good luck.
PS The FSM describes the 'closed loop' sensors but I can't find it. Let you know if I do.
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- #3
jmccloud
Well-known member
O2 sensors
They're in the exhaust between the engine and cats, sort of beside the rear of the oil pan. By disconnecting, I meant only the electrical connector. Removing the sensor from the bung is very difficult.
Only remove (and tie up away from the exhaust) the electrical connector.
Good luck.
PS Don't mess with the one behind the right cat, disconnecting it will not keep the system out of closed loop.
They're in the exhaust between the engine and cats, sort of beside the rear of the oil pan. By disconnecting, I meant only the electrical connector. Removing the sensor from the bung is very difficult.
Only remove (and tie up away from the exhaust) the electrical connector.
Good luck.
PS Don't mess with the one behind the right cat, disconnecting it will not keep the system out of closed loop.
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- #5
Paul Higg
Well-known member
Thanks again for that info. I am going to try to do it as soon as I can but I have a lot on my plate right now. I might try to do it tonight. I have a hernia that needs surgery so I don't really look forward to jacking it up. My wife can help me jack it I guess. But I GOTTA troubleshoot this, it's driving me nuts not knowing and I LIVE to troubleshoot!
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- #6
Paul Higg
Well-known member
Okay jmccloud, I have found the info. The description and operation of the Open Loop/Closed Loop mode is in two places. 6E3-C2-2, "Run Mode" and in the description of the O2 sensor, 6E3-C1-5.
I also got the drawing of the location of the O2 sensors. It looks like I will disconnect just the left forward one. I didn't realize that they were so delicate. They even have to be removed at a temperature above 120 degrees F. I learned a lot. It seems the only components in the Open Loop mode are the ECT (changed last summer, OEM), MAP, MAF, and TP (replaced a couple of months ago but never had trouble with it). Not to mention the PCM. I am going to take a look at the voltage from the TPS which should be .5 volts fully closed on my 94 LT1.
I will be interested to see what happens when I force it in open loop mode. If the ECT reads 140 degrees and it has been five minutes after start and it runs poorly then I suspect (I may be wrong) that it COULD be the MAF.
What do you think?
Paul
I also got the drawing of the location of the O2 sensors. It looks like I will disconnect just the left forward one. I didn't realize that they were so delicate. They even have to be removed at a temperature above 120 degrees F. I learned a lot. It seems the only components in the Open Loop mode are the ECT (changed last summer, OEM), MAP, MAF, and TP (replaced a couple of months ago but never had trouble with it). Not to mention the PCM. I am going to take a look at the voltage from the TPS which should be .5 volts fully closed on my 94 LT1.
I will be interested to see what happens when I force it in open loop mode. If the ECT reads 140 degrees and it has been five minutes after start and it runs poorly then I suspect (I may be wrong) that it COULD be the MAF.
What do you think?
Paul
jmccloud
Well-known member
MAF?
One of my kids has a Chevy Express with a 350 and it suffered a failed MAF. It was not cold exclusive but ran as you describe all of the time.
You might check the FSM for MAF diagnosis.
Good luck
PS You don't have to remove the O2 sensor, just disconnect the electrical lead and tie it up so it doesn't touch the exhaust.
One of my kids has a Chevy Express with a 350 and it suffered a failed MAF. It was not cold exclusive but ran as you describe all of the time.
You might check the FSM for MAF diagnosis.
Good luck
PS You don't have to remove the O2 sensor, just disconnect the electrical lead and tie it up so it doesn't touch the exhaust.
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- #8
Paul Higg
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Yeah, that was what I was going to do, tie it up and out of the way. I saw in the FSM that a bad MAF might throw a code, I THINK. I dunno know. My memory fails me. 
ad
I have to jack the car today to change the oil so I will tie back the plug today, I will also look at the output voltage of the TPS.
I'll keep you posted and thanks again for your help. I do appreciate it a lot.
adI have to jack the car today to change the oil so I will tie back the plug today, I will also look at the output voltage of the TPS.
I'll keep you posted and thanks again for your help. I do appreciate it a lot.
bradfordsvettes
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I would give the coolant sensor a look for sure it is a very direct input if it is telling the car it is very cold out it will spew tons of black smoke. it should read ambient temp when the key is turned on before starting Steve
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bradfordsvettes
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are we talking the same thing this is the sensor for the ecm input not thermostat . also this would need to be read with a scan tool and it may not set a code dependind on how bad it may have failed if it is changing at all the ecm may be happy and not set a code Steve
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Paul Higg
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I was talking about the ECT, the engine coolant temp sensor that is installed in the water pump. Was that what you meant?
I have never had any smoke whatsoever either. Would a bad ECT cause black smoke?
I replaced it in March and it has 6000 miles on it and I have never had a problem with it so far. It was OEM too. But wait, now that I think about it, I recently had the water pump replaced and they had to remove it and reinstall it. But that was on 30 August and 3000 miles ago. Still no problems with it. No codes. How would I tell if it was malfunctioning?
Paul
I have never had any smoke whatsoever either. Would a bad ECT cause black smoke?
I replaced it in March and it has 6000 miles on it and I have never had a problem with it so far. It was OEM too. But wait, now that I think about it, I recently had the water pump replaced and they had to remove it and reinstall it. But that was on 30 August and 3000 miles ago. Still no problems with it. No codes. How would I tell if it was malfunctioning?
Paul
bradfordsvettes
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Maybe I do not understand your whole problem but anect would give problems if it is out of range ' what are you chasing exactly Steve
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- #14
Paul Higg
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Since it has gotten cold here in Georgia and ONLY since it has gotten cold I have had this problem. When I first start the car and leave my subdivision I have to get up to 55 or 60 ASAP. When I get on that road and punch the gas it seems to hesitate and it sounds like it is backfiring but not out of the tailpipes, it sounds like that under the hood. As SOON as it reaches a temp of 140 degrees and about three minutes all is well. So I think the O2 sensors and the ECT are telling the ECM that is is okay to leave open loop and go to closed loop. Then it starts to work fine. So jmccloud's advice was to disconnect one of the forward O2 sensors to FORCE it to stay in open loop and see what happens when the engine warms up. What do you think?
Paul
Paul
bradfordsvettes
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how do you know it is rich ? sounds like you really need a scan tool . what you describe almost sounds like it make be lean . These cars were notorius for injector problems I would check all the injector resistance values and maybe a fuel pressure check. an injector fuel drop test coukd not hurt either . but do not know if this is something you could do Steve
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Paul Higg
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I have not checked the injector values but I did put some cleaner in the tank months ago that was recommended here on this site. Also, fuel pressure is great. I had them replace the fuel pump because it wouldn't start, pressure was zero. Now pressure is where it should be, I think about 44 pounds.
Does the FSM tell how to do a resistance check? I know how to troubleshoot with a digital meter as I did that for 25 years on aircraft. Can you give me a real short description? Also, I have never heard of a fuel drop test either.
Does the FSM tell how to do a resistance check? I know how to troubleshoot with a digital meter as I did that for 25 years on aircraft. Can you give me a real short description? Also, I have never heard of a fuel drop test either.
bradfordsvettes
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off the top of my head it seems 10 to 16 ohms is the value just remove the plug and check them at the injector the closer to the same values the better any dramatic differace it is junk high or low you would most likely see them low . a fuel drop is with the rail pressurized then a tool is used at the injector to pulse it and the drop of fuel pressure is measured the closer to the same values the better sometimes you pulse them 2 times and read the values both times it baicly tells you if you have partialy clogged injectors or an electrical issue
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bradfordsvettes
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it would be more evident cold ,with may be a slight rough idle warm if they are not to bad when it goes into closed loop the ecm can make adjustment that compensates for the problem to a certain degree a scan tool is a huge plus with problems like
this steve
this steve
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- #20
Paul Higg
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Well, I do not have a scan tool. If I'd have to connect it to the ODB II plug in my 94 it will be difficult as the plug is ODB II but the system is ODB I. I have tried at Advance Auto and other places and they even had an adapter and it couldn't read codes. Since then Advance Auto removed the adapter per management. Go figure. How would I specifically connect a scan tool to my car?
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