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.

06-02-2016, 09:32 PM	#1
Stewart Way Member Join Date: Aug 2003 Location: Atlanta Posts: 476 Likes: 20 Liked 70 Times in 22 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 Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,111 Likes: 1,570 Liked 1,815 Times in 413 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: 476 Likes: 20 Liked 70 Times in 22 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: 476 Likes: 20 Liked 70 Times in 22 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 162 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: 980 Likes: 17 Liked 228 Times in 70 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