Lean Condition Open Loop Driveability
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Senior User
Joined: Nov 2004
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From: Lancaster, PA
Lean Condition Open Loop Driveability
I'm curious if anyone else has dealt with a lean cold stumble when first shifting into first gear and letting out the clutch? I have a 1997 4.0L 5spd and it will do this only once in the morning if the engine is not warmed up for atleast a minute. Just curious on thoughts..
I have smoke tested the engine and the intake with no leaks found. The vehicle is properly tuned seeing as I am a technician myself. I plan to diagnose the barometric sensor and MAF sensors but I need to know if anybody else has experience with this problem? New IAC was just installed.
I have smoke tested the engine and the intake with no leaks found. The vehicle is properly tuned seeing as I am a technician myself. I plan to diagnose the barometric sensor and MAF sensors but I need to know if anybody else has experience with this problem? New IAC was just installed.
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From: Lancaster, PA
The O2 sensors arn't part of the circuit on a cold engine. I know its lean because I'm getting great gas mileage. And with my experience in automotive repair I've become familiar with many lean conditions.
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Joined: Nov 2004
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From: Salt Lake City, Utah
To determine if it lean, check the fuel trims with a scanner. I would suspect that the O2 sensors need to be replaced. They are possibly lazy and giving a slow response. A bad MAF will do the same thing but to a lesser extent. The TPS can cause a hesitation as well.
When the engine is cold, it runs rich. How are you measuring the fuel economy when it is cold? My guess is it gets good fuel economy because it is running properly when it warms up, but is running rich when cold.
I can't really diagnose too much with theh information I have available. But I would suspect that a sensor is not giving some critical information and that is why it is stumbling.
When the engine is cold, it runs rich. How are you measuring the fuel economy when it is cold? My guess is it gets good fuel economy because it is running properly when it warms up, but is running rich when cold.
I can't really diagnose too much with theh information I have available. But I would suspect that a sensor is not giving some critical information and that is why it is stumbling.
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From: SW Va
Lets think about this some more.
He's saying that cold weather mpg is good & that he has a cold stumble, only at under one minute of warm up, while it's still in open loop operation, before the O2 sensors should be online, so the engine should be running on it's canned open loop rich mixture program, but it still stumbles when you open the throttle, which suggests the air/fuel mixture is going lean.
So if the O2 sensors aren't online, that seems to leave us looking at the TPS, MAF & maybe a vacuum leak, seeing as how the mixture seems to go lean & his cold weather mpg is so good.
So how about something like a stuck open PCV valve, wouldn't take much moisture & some cold weather to freeze it up, or depoits to make it stick open, or maybe a cracked vacuum line, something not too gross, but just enough to lean the cold open loop mixture!!!!
Just some more thoughts for pondering.
Will be interesting to hear what you find.
He's saying that cold weather mpg is good & that he has a cold stumble, only at under one minute of warm up, while it's still in open loop operation, before the O2 sensors should be online, so the engine should be running on it's canned open loop rich mixture program, but it still stumbles when you open the throttle, which suggests the air/fuel mixture is going lean.
So if the O2 sensors aren't online, that seems to leave us looking at the TPS, MAF & maybe a vacuum leak, seeing as how the mixture seems to go lean & his cold weather mpg is so good.
So how about something like a stuck open PCV valve, wouldn't take much moisture & some cold weather to freeze it up, or depoits to make it stick open, or maybe a cracked vacuum line, something not too gross, but just enough to lean the cold open loop mixture!!!!
Just some more thoughts for pondering.
Will be interesting to hear what you find.
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From: Lancaster, PA
Your with me PawPaw !! The vaccum leak idea is great, however all intake components have been smoke tested for leaks... My idea of the Baro sensor seems to keep me intrigued because any slight sensitivity to the air density could be enough for a cold engine dealing with friction to be undercomponsating fuel for air flow. The MAF is possible as well and will be tested when I get a chance. A lazy MAF sensor can be hard to track down.. Seeing as it only stumbles once and is done, its really not a huge deal, just makes me mad
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Joined: Jan 2004
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From: SW Va
Well I'm leaning more toward a sticking, frozen, or somehow stuck open PCV valve, or leaking PCV valve hose or fitting, as in that mode, it isn't compensated for, in the canned open loop cold operating computerprogram & thus amounts to a unmetered vacuum leak, that would likely mess with & lean out the air/fuel mixture, untl the heated O2 sensors come online & help the computer compensate for it. WHEW!!!!! lol
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Joined: Jul 2006
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From: Denver
Most likely candidate to me is an intake air temp sensor that isn't reading how cold it is. If you can get a scanner or datalogger on it, you can check for that as well.
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