ThreeSheds Posted August 10, 2009 Share Posted August 10, 2009 Has anyone tried running an EFI hotwire engine without the air bypass valve? Mine is giving trouble (yet again) and I am wondering what would happen if I simply block it off and up the base-idle speed to 750rpm or thereabouts? The vehicle is an off-road toy so I do not need the every-day civility of having it maintain my tickover when I turn on the air-con .. Also, does anyone know if any of these thoughts are correct? The air bypass valve (abv) is not an enrichment device, it is merely the equivalent of holding the throttle a little bit open. When tthe ECU detects that the vehicle is moving (speed transducer) it will hold the abv open a little - presumably to preven full-on engine braking? Removal of the speed transducer will prevent the above. Removal of the abv from the circuit will cause a fault to register in the ECU. Quote Link to comment Share on other sites More sharing options...
Boothy Posted August 10, 2009 Share Posted August 10, 2009 When Ifirst put an 3.9Efi hotwire in my trialling truck it was a complete pain in tha arse for constantly cutting out when I dropped off the throttle especially when braking, this was diagnosed as not having the bypass valve connected viathe speed sensor on the rear of the box and when braking I was totally losing the manifold vacuum, connected it up hey presto problem solved. Now I've a bigger motor with the auto and Megasquirt and I haven't got it connected although its still on the back of the Plenum chanber, but I have no cutting out etc issues, so answer is don't know. Quote Link to comment Share on other sites More sharing options...
Aragorn Posted August 10, 2009 Share Posted August 10, 2009 The ECU controls the ABV and will richen up the fuelling to coincide with it being open. If it thinks its open and its not, i'd imagine it would still add the extra fuel, which may make the running even worse. Quote Link to comment Share on other sites More sharing options...
Aragorn Posted August 10, 2009 Share Posted August 10, 2009 hmm, what i said there really depends on wether its goal is idle control or just coldstart, and i'm not familiar with the system to know for sure. If its idle control then the fuelling shouldnt be a problem, but the ECU might get grumpy with it missing. Quote Link to comment Share on other sites More sharing options...
Boothy Posted August 10, 2009 Share Posted August 10, 2009 This is just a guess or an unfounded theory from memory, but I thought it was airbypass and airbypass only, the idea is that when you take your foot of the throttle on stopping or slowing is that the throttle butterfly shuts, restricting air to the inlet but the engine can still be running at quite some revs, this is hard especially in an auto and even more so when using the servo to regain any sort of manifold equilibrian and therefore the airbypass opens to allow an amount of air to enter just to stop the dip and let it fall evenly to tickover, this is governed or sensed by the pulses to say the vehicle is in motion from the reed switch attached to the speedo housing. This sensor is also what is used as a speed limiter to restict the top speed of the vehicle to 112mph (I think) on the classic RR's, and if I remember righly the old racers to remove the switch and open the butterfly slighly toovercome the effect. This could be b******t but I sure its at least half accurate, please don't call me for it. Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted August 10, 2009 Author Share Posted August 10, 2009 hmm, what i said there really depends on wether its goal is idle control or just coldstart, and i'm not familiar with the system to know for sure. I must admit that I thought that fueling was controlled by various temperatures, the TPS and the hotwire - I thought that when the bypas valve opens it causes an increase flow through the hotwire and hence more fuel, and that cold start was merely due to temperature(s), but I don't know. If its idle control then the fuelling shouldnt be a problem, but the ECU might get grumpy with it missing. Yes - I was concerned about that so I intend blocking the pipe off, not removing the actual valve... Quote Link to comment Share on other sites More sharing options...
Boothy Posted August 10, 2009 Share Posted August 10, 2009 Just as a quick add on to attempt to shed some light, Megasquirt doesn't need it, so on my MS I have just left the pipe in place to the ABP (to idle to blank it off) and thats where it stays. Good luck Quote Link to comment Share on other sites More sharing options...
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