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What happens when the accelerator is floored?

Posted by Dave_G 
OK, yeah, I know the obvious answer: you go fast. smiling smiley

But I'm asking in particular when you're starting up the engine. I still have an issue with hard starting in cold and wet weather. But if I hold the accelerator to the floor as I turn the key, it starts up pretty quickly. So what exactly is happening, mechanically or electrically, when I hold the pedal to the floor? And why would that make the engine start easier?

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Dave
'91 325iX
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Dave_G
OK, yeah, I know the obvious answer: you go fast. smiling smiley

But I'm asking in particular when you're starting up the engine. I still have an issue with hard starting in cold and wet weather. But if I hold the accelerator to the floor as I turn the key, it starts up pretty quickly. So what exactly is happening, mechanically or electrically, when I hold the pedal to the floor? And why would that make the engine start easier?

Could be a number of things but probably caused by a too rich condition. Things to look at are the coolant temp sensor, the TPS, the FPR and potentially leakinjector(s).
Pushing the throttle to the floor opens the throttle body wide so lots of air can be ingested; it puts the TPS into WOT mode so that fuel mapping is defaulted and all sensors are ignored. This combination is what gets the engine started.

It's not good to have to do this because you usually can't back off the throttle fast enough when the engine catches and the cold engine reaches red line.
rkj
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Archeo-peteriX
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Dave_G
OK, yeah, I know the obvious answer: you go fast. smiling smiley

But I'm asking in particular when you're starting up the engine. I still have an issue with hard starting in cold and wet weather. But if I hold the accelerator to the floor as I turn the key, it starts up pretty quickly. So what exactly is happening, mechanically or electrically, when I hold the pedal to the floor? And why would that make the engine start easier?

Could be a number of things but probably caused by a too rich condition. Things to look at are the coolant temp sensor, the TPS, the FPR and potentially leakinjector(s).
Pushing the throttle to the floor opens the throttle body wide so lots of air can be ingested; it puts the TPS into WOT mode so that fuel mapping is defaulted and all sensors are ignored. This combination is what gets the engine started.

It's not good to have to do this because you usually can't back off the throttle fast enough when the engine catches and the cold engine reaches red line.

The leaky injectors usually so themselves on hot starts though, an hour or so after shutting down.

Does your condition show itself more in damp conditions?, cold conditions don't usually have any affect on starting, other than if the oil is too thick. Dave, you use synthetic, right?

Rick
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rkj
Does your condition show itself more in damp conditions?, cold conditions don't usually have any affect on starting, other than if the oil is too thick. Dave, you use synthetic, right?
Yup, it's cold and dry is a bit of a problem, but it's worst when it's cold and damp.

I am using 10W/40 synthetic.

I've already replaced the coolant temp sensor with no effect, and the whole AFM. What it really needs is a mechanic with diagnostic software that knows old BMWs. There's one about an hour away but I haven't made the journey yet.

__________
Dave
'91 325iX
rkj
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Dave_G
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rkj
Does your condition show itself more in damp conditions?, cold conditions don't usually have any affect on starting, other than if the oil is too thick. Dave, you use synthetic, right?
Yup, it's cold and dry is a bit of a problem, but it's worst when it's cold and damp.

I am using 10W/40 synthetic.

I've already replaced the coolant temp sensor with no effect, and the whole AFM. What it really needs is a mechanic with diagnostic software that knows old BMWs. There's one about an hour away but I haven't made the journey yet.

Do you have a meter? If so start checking a few things; juice at the coil when the switch is turned on and things like that. Over the years these connections, like the key switch, can become dirty and lose voltage making for hard starting. Could also be a weak coil, I would do a fuel pressure check too. My bad start in damp conditions turned out to be spark plug wires.

You would think plugging the car in to a dealer machine you would find out everything, sadly, that's not always the case...

I know, in my case, when I found the bad part, my car sang a different tune smiling bouncing smiley
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rkj
Do you have a meter? If so start checking a few things; juice at the coil when the switch is turned on and things like that. Over the years these connections, like the key switch, can become dirty and lose voltage making for hard starting. Could also be a weak coil, I would do a fuel pressure check too. My bad start in damp conditions turned out to be spark plug wires.
I do have a meter. Maybe this weekend I'll try poking around the coil with it. I also have a used replaced fuel pressure regulator that I haven't installed yet, but I'll give that a go too.

__________
Dave
'91 325iX
rkj
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Dave_G
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rkj
Do you have a meter? If so start checking a few things; juice at the coil when the switch is turned on and things like that. Over the years these connections, like the key switch, can become dirty and lose voltage making for hard starting. Could also be a weak coil, I would do a fuel pressure check too. My bad start in damp conditions turned out to be spark plug wires.
I do have a meter. Maybe this weekend I'll try poking around the coil with it. I also have a used replaced fuel pressure regulator that I haven't installed yet, but I'll give that a go too.

One caution Dave, instead of swapping out parts in hopes of a good return, test the system/s you have in place first. I should write up the fuel pressure test that I do in the shop here, its a most complete way of doing it.

I don't think the fpr is your starting problem but, anything can happen. Usually a faulty fpr will show itself on hard acceleration.
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rkj
I should write up the fuel pressure test that I do in the shop here, its a most complete way of doing it.
That's something I would read. I have no idea even what I would use to measure the pressure.

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I don't think the fpr is your starting problem but, anything can happen. Usually a faulty fpr will show itself on hard acceleration.
Hmm. Acceleration is just fine, so if that's the case you're probably right.

I had forgotten to check the plug wires too. That's probably where I'll start next.

__________
Dave
'91 325iX
rkj
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Dave_G
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rkj
I should write up the fuel pressure test that I do in the shop here, its a most complete way of doing it.
That's something I would read. I have no idea even what I would use to measure the pressure.

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I don't think the fpr is your starting problem but, anything can happen. Usually a faulty fpr will show itself on hard acceleration.
Hmm. Acceleration is just fine, so if that's the case you're probably right.

I had forgotten to check the plug wires too. That's probably where I'll start next.

Plug wires are one thing you migfht have to change out (and don't forget to check the cap for hairline cracks, although you can't always see them!), I don't think there is a test for leaky ones.
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rkj
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Dave_G
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rkj
I should write up the fuel pressure test that I do in the shop here, its a most complete way of doing it.
That's something I would read. I have no idea even what I would use to measure the pressure.

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I don't think the fpr is your starting problem but, anything can happen. Usually a faulty fpr will show itself on hard acceleration.
Hmm. Acceleration is just fine, so if that's the case you're probably right.

I had forgotten to check the plug wires too. That's probably where I'll start next.

Plug wires are one thing you migfht have to change out (and don't forget to check the cap for hairline cracks, although you can't always see them!), I don't think there is a test for leaky ones.

Leaky plug wires can be tested in two simple ways; there is the touch test which will hurt if they are leaking and the night time test where you open the hood in the dark and look for arcing while the engine is running.
A resistance measurement can also sometimes tell if the wires are bad...anything over 6000ohms is suspect.
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Archeo-peteriX
Leaky plug wires can be tested in two simple ways; there is the touch test which will hurt if they are leaking and the night time test where you open the hood in the dark and look for arcing while the engine is running.
A resistance measurement can also sometimes tell if the wires are bad...anything over 6000ohms is suspect.

I recall from the old forum that there is also a night time test where you spray water mist over the plug leads and you will see them arc if they are busted. Worth doing, I reckon, since you say it's worse when it's wet.
I had a similar starting problem until I had my injectors cleaned. I took them to Witchhunter here in the Puget sound area. One day turn around. He also does UPS and Fedex one day turnarounds.

Bob in Everett
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