Bucking in first and second at startup
Aug 5, 2004, 10:10 PM
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Bucking in first and second at startup
I've notice something odd w/ my Evo upon startup. When going from a stop to first gear and then to second, my car bucks. No matter what I do it bucks. This only happens when I first start the car up and only in first and second. Anyone know why this happens? 
Aug 6, 2004, 08:44 AM
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Hmm...I havent tried just letting the car sit n warm up. I don't know why the engine would be cold on a warm summer day though. I can't imagine how it will be in the winter. 
Aug 6, 2004, 08:54 AM
#4
Its because when you first start the car your in open-loop fuel control.. Kinda like taking an old car out (with a carb) and hit the gas and it just cuts out..
When you start the car (Whether its warm or cold) it runs in open loop for as long as 30 seconds.. If the coolant temp is low, it will run in open loop for as long as 5 minutes.. once it begins to warm up (and tests the O2 sensor) it switches to closed loop and the stumbling goes away..
Now.. Why does this do it?
1) Cold engine means the fuel doesnt vaporise well
2) open loop uses airflow to determine fuel, there's also an enrichment (choke) function therefore more fuel in the system plus a raised idle speed.
3) cold engine means cold plugs, and cold plugs tend to misfire a little more
4) Some fuel blends have alcohol in them, which also don't vaporise well in a cold engine.
combine all three and you get hesitation and bucking with a manual transmission.. Its not as obvious with a torque converter (automatic) since the driveline isn't as directly attached and its absorbed by the fluid in the converter... but EVERY car does it to some extent when cold.
When you start the car (Whether its warm or cold) it runs in open loop for as long as 30 seconds.. If the coolant temp is low, it will run in open loop for as long as 5 minutes.. once it begins to warm up (and tests the O2 sensor) it switches to closed loop and the stumbling goes away..
Now.. Why does this do it?
1) Cold engine means the fuel doesnt vaporise well
2) open loop uses airflow to determine fuel, there's also an enrichment (choke) function therefore more fuel in the system plus a raised idle speed.
3) cold engine means cold plugs, and cold plugs tend to misfire a little more
4) Some fuel blends have alcohol in them, which also don't vaporise well in a cold engine.
combine all three and you get hesitation and bucking with a manual transmission.. Its not as obvious with a torque converter (automatic) since the driveline isn't as directly attached and its absorbed by the fluid in the converter... but EVERY car does it to some extent when cold.
Aug 6, 2004, 02:16 PM
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I always make it a habit to let the temp needle rise over the cold mark a little. In the summer, it's about 3 minutes here in Vegas. In the winter, 5 minutes before I put it in gear and I drive out of boost until my oil temp comes up.
Aug 6, 2004, 03:38 PM
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Originally Posted by MalibuJack
Its because when you first start the car your in open-loop fuel control.. Kinda like taking an old car out (with a carb) and hit the gas and it just cuts out..
When you start the car (Whether its warm or cold) it runs in open loop for as long as 30 seconds.. If the coolant temp is low, it will run in open loop for as long as 5 minutes.. once it begins to warm up (and tests the O2 sensor) it switches to closed loop and the stumbling goes away..
Now.. Why does this do it?
1) Cold engine means the fuel doesnt vaporise well
2) open loop uses airflow to determine fuel, there's also an enrichment (choke) function therefore more fuel in the system plus a raised idle speed.
3) cold engine means cold plugs, and cold plugs tend to misfire a little more
4) Some fuel blends have alcohol in them, which also don't vaporise well in a cold engine.
combine all three and you get hesitation and bucking with a manual transmission.. Its not as obvious with a torque converter (automatic) since the driveline isn't as directly attached and its absorbed by the fluid in the converter... but EVERY car does it to some extent when cold.
When you start the car (Whether its warm or cold) it runs in open loop for as long as 30 seconds.. If the coolant temp is low, it will run in open loop for as long as 5 minutes.. once it begins to warm up (and tests the O2 sensor) it switches to closed loop and the stumbling goes away..
Now.. Why does this do it?
1) Cold engine means the fuel doesnt vaporise well
2) open loop uses airflow to determine fuel, there's also an enrichment (choke) function therefore more fuel in the system plus a raised idle speed.
3) cold engine means cold plugs, and cold plugs tend to misfire a little more
4) Some fuel blends have alcohol in them, which also don't vaporise well in a cold engine.
combine all three and you get hesitation and bucking with a manual transmission.. Its not as obvious with a torque converter (automatic) since the driveline isn't as directly attached and its absorbed by the fluid in the converter... but EVERY car does it to some extent when cold.
Excellent info. I've always wondered why, but never thought of asking.
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Aug 8, 2004, 09:05 PM
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I posted about this a while back but a moderator had a power trip and locked the thread... It is a issue but I believe it is a MAF issue or more so a ecu issue.
bryan
bryan
Aug 8, 2004, 10:17 PM
#9
Possibly a MAF issue, but likely the car is running in open loop until it warms up.. These are typical symptoms when the car is cold.. especially if its sitting over night and its a bit colder out..
A thought though, if the car has a few miles on it (30k or so) its possible the O2 sensor is becoming worn..
I do occasionally get stumbles and misfires due to my colder plugs when the engine is cold.. once the engine is up to operating temp, the car idles smoothly and no longer stumbles at all..
I also get the "bucking" on occasion when the car is cold, specifically open loop operation.. I can quickly tell when datalogging when the car goes into closed loop.. In my case my MAF pipe causes my car to run LEAN in open loop before the engine is warm and it stumbles big time.. (with my UTEC) I added about 10% fuel at idle and a few low throttle positions so when the car is running open loop, I'm not getting as lean..
like I said earlier, there's a few flaws in the way the stock ECU operates, but its not surprising, nobody really wants to run their car when its cold and in open loop at low rpm's.. Its just not going to run well.. I don't know of any car that runs all that well when its cold and in open loop operation at part throttle driving, it has to make airflow assumptions in a less than optimum environment when the engine is cold.. you'll get fuel not atomizing well, very cold airflow (which in turn kills fuel atomization too) cold oil and engine parts (things not moving as easily) cold engine parts contributing to fuel puddling and misfiring, plugs that are fouled from raw fuel and carbonizing from incomplete combustion..
Think about the old school carb cars.. you have a choke and fuel enrichment, accelerator pump, fuel jets for primary and secondary operation, idle control circuits.. its a wonder that engines ran at all..
There's a reason why engines aren't "Smogged" at wide open throttle and cold start.. its also why most troubleshooting engine problems are done at operating temp..
A thought though, if the car has a few miles on it (30k or so) its possible the O2 sensor is becoming worn..
I do occasionally get stumbles and misfires due to my colder plugs when the engine is cold.. once the engine is up to operating temp, the car idles smoothly and no longer stumbles at all..
I also get the "bucking" on occasion when the car is cold, specifically open loop operation.. I can quickly tell when datalogging when the car goes into closed loop.. In my case my MAF pipe causes my car to run LEAN in open loop before the engine is warm and it stumbles big time.. (with my UTEC) I added about 10% fuel at idle and a few low throttle positions so when the car is running open loop, I'm not getting as lean..
like I said earlier, there's a few flaws in the way the stock ECU operates, but its not surprising, nobody really wants to run their car when its cold and in open loop at low rpm's.. Its just not going to run well.. I don't know of any car that runs all that well when its cold and in open loop operation at part throttle driving, it has to make airflow assumptions in a less than optimum environment when the engine is cold.. you'll get fuel not atomizing well, very cold airflow (which in turn kills fuel atomization too) cold oil and engine parts (things not moving as easily) cold engine parts contributing to fuel puddling and misfiring, plugs that are fouled from raw fuel and carbonizing from incomplete combustion..
Think about the old school carb cars.. you have a choke and fuel enrichment, accelerator pump, fuel jets for primary and secondary operation, idle control circuits.. its a wonder that engines ran at all..
There's a reason why engines aren't "Smogged" at wide open throttle and cold start.. its also why most troubleshooting engine problems are done at operating temp..
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