Idle problem
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Joined: Mar 2003
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From: Nashua, NH
Idle problem
Two days ago I suddenly started to have an idle problem associated with a stalling problem.
The car has a regular, about two-per-second idle variation ranging from minor to about 200 rpm. I'm guessing, because it's too frequent to show up on the tach. The car will occasionally stall at idle as well, but ONLY at idle. Initial acceleration feels a little ragged also, but once spooled up over about 1500-2000 rpm it feels smooth.
Does this sound like an idle controller?
Any other ideas?
The car has a regular, about two-per-second idle variation ranging from minor to about 200 rpm. I'm guessing, because it's too frequent to show up on the tach. The car will occasionally stall at idle as well, but ONLY at idle. Initial acceleration feels a little ragged also, but once spooled up over about 1500-2000 rpm it feels smooth.
Does this sound like an idle controller?
Any other ideas?
Nordschleife Master
Joined: Aug 2002
Posts: 9,010
Likes: 2
From: South Texas
O2 sensor.
If it gets cloged, it won't regester fast enough, and your computer will constantly be pushing the mixture way away from correct. You won't notice it at anything but idle.
The test, so I understand, is to get the car warmed up, and then pull the O2 sensor plug. Connect it to a high impedence analog volt meter. Get somebody to cause a large vacuum leak, like pull the tube for the brake booster. The O2 sensor should respond nearly instantly. The voltage should rapidly fall to near OV. You can do this again by causeing a rich condition, again the voltage should rapidly rise, with in a second, headed upward past like 1 volt.
The next puppy is the MAF, but you don't want to think about that.
If it gets cloged, it won't regester fast enough, and your computer will constantly be pushing the mixture way away from correct. You won't notice it at anything but idle.
The test, so I understand, is to get the car warmed up, and then pull the O2 sensor plug. Connect it to a high impedence analog volt meter. Get somebody to cause a large vacuum leak, like pull the tube for the brake booster. The O2 sensor should respond nearly instantly. The voltage should rapidly fall to near OV. You can do this again by causeing a rich condition, again the voltage should rapidly rise, with in a second, headed upward past like 1 volt.
The next puppy is the MAF, but you don't want to think about that.
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From: Southern NH
My 85 has a similar problem. The O2 sensor is new and when cold it hunts for an idle. Almost like a cold diesel engine. After it warms up its fine but will stall occasionally when you give it quick gas off idle. I recently noticed that if i hold it in 1st and get the rpms up (can't remember how up) and hold it there while driving I can feel it surging ever so slightly. My performance off idle is pretty crappy also. I haven't been able to really look into it yet but I have checked the idle valve which seems OK but who knows and again, the O2 sensor. I'll be watching this to see what comes up. My 85 starts immediately when cold but very difficultly when hot. I have a check valve to install but that shouldn't affect how the car runs. Good luck!
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Joined: Jun 2001
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From: Ball Ground, GA
One point to remember is how the oxygen sensor controls the mixture.
The oxygen sensor controls the fuel mixture by measuring the overall exhaust output - that is, an average of all cylinders.
If you have a vacuum leak that affects only part of the cylinders, the mixture will be set to the average output, so that the cylinders with the leak will still be too lean, while the cylinders unaffected by the leak will now be too rich.
If you have a cylinder that is not firing because of a bad plug wire or a bad injector, the oxygen for that cylinder will pass thru unburned, so the average sensor reading will be high. The mixture will be adjusted based upon the average, so the seven functioning cylinders will now be way rich.
It appears that if the engine is running well, the closed-loop oxygen sensor operation will tweak it to make it near perfect. If the engine has a problem, the closed-loop operation may make it run worse.
The oxygen sensor controls the fuel mixture by measuring the overall exhaust output - that is, an average of all cylinders.
If you have a vacuum leak that affects only part of the cylinders, the mixture will be set to the average output, so that the cylinders with the leak will still be too lean, while the cylinders unaffected by the leak will now be too rich.
If you have a cylinder that is not firing because of a bad plug wire or a bad injector, the oxygen for that cylinder will pass thru unburned, so the average sensor reading will be high. The mixture will be adjusted based upon the average, so the seven functioning cylinders will now be way rich.
It appears that if the engine is running well, the closed-loop oxygen sensor operation will tweak it to make it near perfect. If the engine has a problem, the closed-loop operation may make it run worse.
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From: Owen Sound Ontario Canada
Ernest...Do you think your temp problem and idle problem are related?
I think I'd still try a leak-down test on the engine and watch the coolant for bubbles...Good-luck
Bill
I think I'd still try a leak-down test on the engine and watch the coolant for bubbles...Good-luck
Bill
User
Joined: Jul 2001
Posts: 3,291
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From: Midland, Texas
Wally,
I'm sure someone has asked this before...I'll go ahead and ask again.
If you disconnect the the O2 sensor what happens?
Can you use a disconnected O2 sensor as a before and after kind of test?
Ern,
Are we gonna - hafta' flat bed your butt over to Blanco fer - tha' shindig?
At least there will be more helping hands and tools.
I'm fairly certain someone will bring a set of Maintenance Bible's.
Hang in there.
I'm sure someone has asked this before...I'll go ahead and ask again.
If you disconnect the the O2 sensor what happens?
Can you use a disconnected O2 sensor as a before and after kind of test?
Ern,
Are we gonna - hafta' flat bed your butt over to Blanco fer - tha' shindig?
At least there will be more helping hands and tools.
I'm fairly certain someone will bring a set of Maintenance Bible's.
Hang in there.
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Three Wheelin'
Joined: May 2001
Posts: 1,641
Likes: 0
From: Hudson Valley NY
John,
When a sensor that is part of a close loop system fails the ECU runs the car on a sensor failure map mode. Maps are operating set of instructions program into the EPROM. For any sensor that controls fuel the default is usually rich.
When a sensor that is part of a close loop system fails the ECU runs the car on a sensor failure map mode. Maps are operating set of instructions program into the EPROM. For any sensor that controls fuel the default is usually rich.
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