closed vs open loop

Stewart Way · 06-02-2016, 09:32 PM

Not race related but hopefully someone can answer my question. Have a 2001 Merc Marquis that was a 3rd car just to have incase. About 100,000 miles on it. It developed a problem that 3-4 miles after driving it will start to hesitate and finally die. If you are going 45 you slow to 35 then 25 then it quits. Let it sit a few minutes it starts back up and in a mile or so it dies again. Hooked a fuel gage up and the pressure stays around 46 so I ruled out fuel. No codes that we can find. My son is a mechanic and he has played with it off and on for months. Nothing. I think when it is warmed up and goes from open to closed loop something is getting a bad signal and shuts down. My son thinks there may be a short in the wiring that is supplying a sensor with the 0-5 reference voltage. I don't know enough about the EFI stuff to help.
Question is, short of solving the problem, can the system be fooled somehow to stay in open loop to see if it will stay running past 3-4 miles? Maybe fool the temp sensor if that is what tells the system to got to closed loop. If this can be done would it hurt anything to leave it that way other than mileage?

Thanks

Alan Roehrich · 06-02-2016, 10:27 PM

If you create high resistance across the temperature sensor, you can make the ECM think the car has not warmed up, and it will not go into closed loop. Measure the resistance of the sensor with the engine at ambient temperature, then go get a resistor to match that.

What will happen if it runs continually in open loop cold enrichment mode for a long time is a multi answer question. Eventually you'll have gasoline in the oil, fouled plugs, and other problems of that nature. You can over heat and melt the catalytic converter guts.

Stewart Way · 06-02-2016, 10:46 PM

Sounds like I will try the resistor if only to see if it is a problem when it goes from open to closed loop. Doesn't sound like a long term good idea. Thanks for the resistor idea and I will try it next time I am at my sons house. The car is resting over there.

countrypuppy4865 · 06-02-2016, 11:13 PM

Not familiar with this particular car. However if the O2 sensors are unplugged it should stay in open loop and use a default fuel map.

Stewart Way · 06-03-2016, 09:57 PM

Thanks guys. I may try to get it home tomorrow. I have AC in part of the shop here instead of working on my sons driveway. I will likely put the jumper on the water temp since it is easier to reach. I need to get this running. Wednesday someone ran a stop sigh and my 2500 diesel dodge is likely done. T-boned an explorer at about 50 and pushed the side in about a foot. No one hurt but the truck is about a foot shorter. I was on the wait list (#431) to get the killer airbag replaced in the Dodge but for some reason it didn't go off.

Painter · 06-04-2016, 06:53 AM

Stewart glad you are ok! I too have a Ford for a spare. 1992 model efi, so it is not obd 2. It was really doing strange things. I concluded computer. Advance auto parts has reman units for these things for 80-100 bucks. I wouldnt fool with it long, stick a computer in it.

Ed Wright · 06-04-2016, 04:55 PM

Your son's scan tool should tell him if it is in open or closed loop. Only way that would effect it is if in closed loop the O2 sensors (It has an O2 sensor for each bank) has output/signal voltage pegged every high, causing it to pull all the fuel it can to try to lean it down, or signal voltage stuck very low, causing the computer to pour the fuel to it to fatten it up where it belongs. Both sides do this is very unlikely. One side would make it run very rough, both side's O2 sensors doing the same like all of a sudden is not likely at all. A scan tool would tell you in a heart beat, if you know what to look for.

If he can't see anything on the scan tool, unplugging the O2 sensors, and it no longer shutting off, would tell you if the problem is in that area. It may be an issue of something electrical getting warm before it fails, not whether it's in open or closed loop.

Simply watching the scan tool would tell you easier. O2 milli volts should toggle above and below 450 mv, several times a second. Scan tools up date too slow for you to see it all, but the voltage is up and down all the time. Switching something like 60 times per second.
Takes a very fast digital 'scope to see it all. Staying up or down will prevent going into closed loop. Voltage staying above .450 indicates to the ECU that it's over rich, staying below .450 indicates lean to the ECU.

When it dies,check for spark, fuel pressure and injector pulse.

My experience with parts store reman ECUs is not good at all. I would not just throw a reman ECU at it. I learned to buy ECUs at the Ford dealer, GM dealer, etc.

Pvt Parts · 06-05-2016, 11:18 AM

Quote:

Originally Posted by Ed Wright

Your son's scan tool should tell him if it is in open or closed loop. Only way that would effect it is if in closed loop the O2 sensors (It has an O2 sensor for each bank) has output/signal voltage pegged every high, causing it to pull all the fuel it can to try to lean it down, or signal voltage stuck very low, causing the computer to pour the fuel to it to fatten it up where it belongs. Both sides do this is very unlikely. One side would make it run very rough, both side's O2 sensors doing the same like all of a sudden is not likely at all. A scan tool would tell you in a heart beat, if you know what to look for.

If he can't see anything on the scan tool, unplugging the O2 sensors, and it no longer shutting off, would tell you if the problem is in that area. It may be an issue of something electrical getting warm before it fails, not whether it's in open or closed loop.

Simply watching the scan tool would tell you easier. O2 milli volts should toggle above and below 450 mv, several times a second. Scan tools up date too slow for you to see it all, but the voltage is up and down all the time. Switching something like 60 times per second.
Takes a very fast digital 'scope to see it all. Staying up or down will prevent going into closed loop. Voltage staying above .450 indicates to the ECU that it's over rich, staying below .450 indicates lean to the ECU.

When it dies,check for spark, fuel pressure and injector pulse.

My experience with parts store reman ECUs is not good at all. I would not just throw a reman ECU at it. I learned to buy ECUs at the Ford dealer, GM dealer, etc.

Yes, for critical parts that have to be right, OEM (Purchased from the dealer) are the only way to go!

Stewart Way · 06-06-2016, 10:33 PM

My son drove the car over to my place Sunday AM. Made the trip without shutting off (about 15 miles) but there are 2 bridges with rough entries and it stumbled at each bump. We checked as many wire connectors as we could find and unwrapped a few locations looking for a chafed wire and found nothing. Time to take I to ford and see what they can find. The fuel pressure does not change when it dies and the only code it has shown is vehicle speed sensor. When it dies, the temp light comes on but not the check engine light.
Thanks again for the info.

Ed Wright · 06-07-2016, 02:24 PM

If wiggling the harness won't make it stumble, or die, try "the slap test". Take the computer out of it's mount, hold it in one have, and slap it with the other, while the engine is running. If it stumbles' misses, etc, you have a broken trace on the printed circuit board. Not uncommon at all.

06-02-2016, 09:32 PM	#1
Stewart Way Member Join Date: Aug 2003 Location: Atlanta Posts: 475 Likes: 15 Liked 70 Times in 21 Posts	closed vs open loop Not race related but hopefully someone can answer my question. Have a 2001 Merc Marquis that was a 3rd car just to have incase. About 100,000 miles on it. It developed a problem that 3-4 miles after driving it will start to hesitate and finally die. If you are going 45 you slow to 35 then 25 then it quits. Let it sit a few minutes it starts back up and in a mile or so it dies again. Hooked a fuel gage up and the pressure stays around 46 so I ruled out fuel. No codes that we can find. My son is a mechanic and he has played with it off and on for months. Nothing. I think when it is warmed up and goes from open to closed loop something is getting a bad signal and shuts down. My son thinks there may be a short in the wiring that is supplying a sensor with the 0-5 reference voltage. I don't know enough about the EFI stuff to help. Question is, short of solving the problem, can the system be fooled somehow to stay in open loop to see if it will stay running past 3-4 miles? Maybe fool the temp sensor if that is what tells the system to got to closed loop. If this can be done would it hurt anything to leave it that way other than mileage? Thanks __________________ Stewart Way 2424 SS

06-02-2016, 10:27 PM	#2
Alan Roehrich VIP Member Join Date: May 2006 Location: Murfreesboro TN Posts: 4,915 Likes: 996 Liked 1,042 Times in 274 Posts	Re: closed vs open loop If you create high resistance across the temperature sensor, you can make the ECM think the car has not warmed up, and it will not go into closed loop. Measure the resistance of the sensor with the engine at ambient temperature, then go get a resistor to match that. What will happen if it runs continually in open loop cold enrichment mode for a long time is a multi answer question. Eventually you'll have gasoline in the oil, fouled plugs, and other problems of that nature. You can over heat and melt the catalytic converter guts. __________________ Alan Roehrich 212A G/S

06-02-2016, 10:46 PM	#3
Stewart Way Member Join Date: Aug 2003 Location: Atlanta Posts: 475 Likes: 15 Liked 70 Times in 21 Posts	Re: closed vs open loop Sounds like I will try the resistor if only to see if it is a problem when it goes from open to closed loop. Doesn't sound like a long term good idea. Thanks for the resistor idea and I will try it next time I am at my sons house. The car is resting over there. __________________ Stewart Way 2424 SS

06-03-2016, 09:57 PM	#5
Stewart Way Member Join Date: Aug 2003 Location: Atlanta Posts: 475 Likes: 15 Liked 70 Times in 21 Posts	Re: closed vs open loop Thanks guys. I may try to get it home tomorrow. I have AC in part of the shop here instead of working on my sons driveway. I will likely put the jumper on the water temp since it is easier to reach. I need to get this running. Wednesday someone ran a stop sigh and my 2500 diesel dodge is likely done. T-boned an explorer at about 50 and pushed the side in about a foot. No one hurt but the truck is about a foot shorter. I was on the wait list (#431) to get the killer airbag replaced in the Dodge but for some reason it didn't go off. __________________ Stewart Way 2424 SS

06-04-2016, 06:53 AM	#6
Painter Member Join Date: Dec 2012 Posts: 227 Likes: 4 Liked 161 Times in 45 Posts	Re: closed vs open loop Stewart glad you are ok! I too have a Ford for a spare. 1992 model efi, so it is not obd 2. It was really doing strange things. I concluded computer. Advance auto parts has reman units for these things for 80-100 bucks. I wouldnt fool with it long, stick a computer in it.

06-02-2016, 11:13 PM	#4
countrypuppy4865 Senior Member Join Date: Nov 2003 Location: Donaldsonville, LA Posts: 966 Likes: 8 Liked 183 Times in 62 Posts	Re: closed vs open loop Not familiar with this particular car. However if the O2 sensors are unplugged it should stay in open loop and use a default fuel map. __________________ Jimmy "Cooter" Hidalgo, Jr. 4865 E/SA '04 GTO 4865 SS/GA '99 Firebird

06-04-2016, 04:55 PM	#7
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 389 Times in 168 Posts	Re: closed vs open loop Your son's scan tool should tell him if it is in open or closed loop. Only way that would effect it is if in closed loop the O2 sensors (It has an O2 sensor for each bank) has output/signal voltage pegged every high, causing it to pull all the fuel it can to try to lean it down, or signal voltage stuck very low, causing the computer to pour the fuel to it to fatten it up where it belongs. Both sides do this is very unlikely. One side would make it run very rough, both side's O2 sensors doing the same like all of a sudden is not likely at all. A scan tool would tell you in a heart beat, if you know what to look for. If he can't see anything on the scan tool, unplugging the O2 sensors, and it no longer shutting off, would tell you if the problem is in that area. It may be an issue of something electrical getting warm before it fails, not whether it's in open or closed loop. Simply watching the scan tool would tell you easier. O2 milli volts should toggle above and below 450 mv, several times a second. Scan tools up date too slow for you to see it all, but the voltage is up and down all the time. Switching something like 60 times per second. Takes a very fast digital 'scope to see it all. Staying up or down will prevent going into closed loop. Voltage staying above .450 indicates to the ECU that it's over rich, staying below .450 indicates lean to the ECU. When it dies,check for spark, fuel pressure and injector pulse. My experience with parts store reman ECUs is not good at all. I would not just throw a reman ECU at it. I learned to buy ECUs at the Ford dealer, GM dealer, etc. __________________ Ed Wright 4156 SS/JA

06-06-2016, 10:33 PM	#9
Stewart Way Member Join Date: Aug 2003 Location: Atlanta Posts: 475 Likes: 15 Liked 70 Times in 21 Posts	Re: closed vs open loop My son drove the car over to my place Sunday AM. Made the trip without shutting off (about 15 miles) but there are 2 bridges with rough entries and it stumbled at each bump. We checked as many wire connectors as we could find and unwrapped a few locations looking for a chafed wire and found nothing. Time to take I to ford and see what they can find. The fuel pressure does not change when it dies and the only code it has shown is vehicle speed sensor. When it dies, the temp light comes on but not the check engine light. Thanks again for the info. __________________ Stewart Way 2424 SS

06-07-2016, 02:24 PM	#10
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 389 Times in 168 Posts	Re: closed vs open loop If wiggling the harness won't make it stumble, or die, try "the slap test". Take the computer out of it's mount, hold it in one have, and slap it with the other, while the engine is running. If it stumbles' misses, etc, you have a broken trace on the printed circuit board. Not uncommon at all. __________________ Ed Wright 4156 SS/JA