spluttering and stuttering under full throttle

[spookey]

My g40 has just started to splutter under full throttle. The car drives as normal under any other part of the throttle.
what do you guys reckon the problem is? boost leak or something more serious? anyone had this prob before?

Many thanks
Luke     

[PeteG40]

probably an ignition component breaking down.

How old are plugs, leads, rotor arm and dizzy cap?

[spookey]

ow dear. tbh I don't know how old the leads, rotor arm, dizzy caps are. I haven't replaced them within my time of owning the car. the sparks were all replaced a few months ago.

   

[GR40]

Well the throttle body could be to blame as well. It has a sensor that detects full throttle position! and prety much nothing else! could have gone bad or something in the loom gone bad! Worth a look around!

[spookey]

okay thanks for that gr40. Is there any tests i could do to prove it is the full throttle sensor?
Many thanks

[Alex]

Mine did this earlier in the year. It was the coil pack. Only five years old too- changed it just before the conversion. The car's been better than ever since!

[hardchargin40]

Well the throttle body could be to blame as well. It has a sensor that detects full throttle position! and prety much nothing else! could have gone bad or something in the loom gone bad! Worth a look around!

Its just a microswitch, same as the idle one.  Tells the ECU to use 'WOT' settings.  Ive disconnected it before and gone driving while going through the motions to find a misfire i had years back.  Doesn't really affect anything bar no WOT.

[hayesey]

okay thanks for that gr40. Is there any tests i could do to prove it is the full throttle sensor?
Many thanks

Like Paul said, it's just a switch you can test it with a multimeter or a test lamp without the engine running, it should make contact just before the throttle is fully open as you are turning it.

[Andy]

Well the throttle body could be to blame as well. It has a sensor that detects full throttle position! and prety much nothing else! could have gone bad or something in the loom gone bad! Worth a look around!

Its just a microswitch, same as the idle one.  Tells the ECU to use 'WOT' settings.  Ive disconnected it before and gone driving while going through the motions to find a misfire i had years back.  Doesn't really affect anything bar no WOT.

The full-throttle switch applies a correction factor to the fuelling value it has read from the fuel map. This correction factor varies with rpm.

If your full-throttle switch is disconnected, you'll still get full throttle and boost - but you'll only get the base amount of fuel from the map and run lean!

[Turbodaily]

Check the switch for resistance aswell ideally, as the switch can still 'switch' but even the slightest bit of resistance will confuse the ecu and generally give it the hump.

[Andy]

That's a bit miss-leading to be fair. If the resistance is something like 10kΩ then it'll start to cause problems, but 0-100Ω or so should be fine.

[hardchargin40]

Well the throttle body could be to blame as well. It has a sensor that detects full throttle position! and prety much nothing else! could have gone bad or something in the loom gone bad! Worth a look around!

Its just a microswitch, same as the idle one.  Tells the ECU to use 'WOT' settings.  Ive disconnected it before and gone driving while going through the motions to find a misfire i had years back.  Doesn't really affect anything bar no WOT.

The full-throttle switch applies a correction factor to the fuelling value it has read from the fuel map. This correction factor varies with rpm.

If your full-throttle switch is disconnected, you'll still get full throttle and boost - but you'll only get the base amount of fuel from the map and run lean!

Yeah, thats a better explanation, cheers Andy.

[Turbodaily]

That's a bit miss-leading to be fair. If the resistance is something like 10kΩ then it'll start to cause problems, but 0-100Ω or so should be fine.

A couple of years ago my car used to hunt at idle, at any opertunity it could. When i checked the idle switch, i measured 0.06 ohms when switched on. Checked a known good tb, it measured 0.00. Fitted another tb, problem was cured.
Dont know if it work the same as the idle switch, but i assume it does.
I think the resistance causes an implausable signal, as it is neither on or off? if that makes sense? lol

[Andy]

That's a bit miss-leading to be fair. If the resistance is something like 10kΩ then it'll start to cause problems, but 0-100Ω or so should be fine.

A couple of years ago my car used to hunt at idle, at any opertunity it could. When i checked the idle switch, i measured 0.06 ohms when switched on. Checked a known good tb, it measured 0.00. Fitted another tb, problem was cured.
Dont know if it work the same as the idle switch, but i assume it does.
I think the resistance causes an implausable signal, as it is neither on or off? if that makes sense? lol

The full-throttle switch input to the micro-processor on the ECU will be close to 5V when the switch is open, and close to 0V when the switch is closed - as the switch shorts the input to ground. It'll probably interpret >4.5V as open, and <0.5V as switch closed (full-throttle), and anything else inbetween as a fault.

When the switch is open, the microprocessor input is "pulled-up" to 5V via a resistor to ensure that there's a stable input and the micro continues to work reliably. This pull-up resistor is likely to be at least 1kΩ in value. Your full-throttle switch has a value close to 0Ω, and pulls the microprocessor input down to ground.

If our throttle switch (and wiring) has a resistance of 0.06Ω then the voltage the microprocessor will see at its input is as follows:

V = 5V x (0.06/[0.06+1000]) = 0.0003V

i.e. the ECU won't know (or care!) about whether the throttle switch is 0.06Ω or 0.00Ω.

If we get up to the 100Ω I talked about, it gets very marginal:

V = 5V x (100/([100+1000]) = 0.45V

[hardchargin40]

Huh?   Am I losing the plot... what equation is that in electrical theory?