revs dropping at idle?
Discussion
Just been out in the car. Got to a junction after driving about 5 minutes and the revs suddenly dropped to about 500rpm and the engine was a bit juddery.
Drove for about another 10 minutes and the same thing at a junction.
Not sure if its my imagination, but the exhaust sounds different, like its blowing slightly, but when I blocked the end of the exhaust to check I couldn't find any obvious holes
It's not done it since then and when running it seems fine
Any ideas?
Drove for about another 10 minutes and the same thing at a junction.
Not sure if its my imagination, but the exhaust sounds different, like its blowing slightly, but when I blocked the end of the exhaust to check I couldn't find any obvious holes
It's not done it since then and when running it seems fine
Any ideas?
First thing would be to check and reset the base idle speed I would think:
http://www.miata.net/garage/isc.html
http://www.miata.net/garage/isc.html
JimSuperSix said:
First thing would be to check and reset the base idle speed I would think:
http://www.miata.net/garage/isc.html
Give this a go first as it's free. Sounds like it could be the issue.http://www.miata.net/garage/isc.html
but had the same thing on the way to a beach, it cut out about 5 times when revs dropped at junctions. Spent an hour on the beach and it worked fine after that and for the next 500 or so miles before I sold it.. I'd be taking the clutch switch apart as thats what happened to mine and I compensated by adjusting the idle speed, seemed to do it more after a hot restart which was odd.
Basically there is a neutral switch attached to the gearbox that when you shift to neutral it grounds a circuit to the ecu telling it maintain the engine idle speed / inject fuel etc
the other one that does this is the clutch switch (uses the same circuit if I remember right) when the clutch pedal is up the circuit is broken and ecu can see the car is running normally driving along and it won't attempt to correct the idle when you push the pedal down the switch is activated and it grounds the same circuit as the neutral switch and this tells the ecu, ok i need to maintain the idle speed now and inject fuel as apparently when you lift the throttle above certain speeds (over-run) the ecu stops injecting fuel which you would probably do when coming up to a junction, unless at Mcdonalds.
Thats why it stalls as the ecu just sits there drinking tea because it thinks the momentum of the drivetrain? will keep it running but when you clutch down you've knackered that idea!
So to fix it all you do is unplug the connector in the footwell going into the switch, use a 17mm spanner i think it is to undo the nut, twist the switch anti clockwise to remove it (its threaded into its bracket and the nut locks it into place) once its out get a small flat blade screwdriver and prise the metal tabs that hold the switch together until the swtich comes apart, then remove broken spring, clean any contacts that look crap and replace the spring with a clicky pen spring! nice and cheap! then bend the tabs down again and refit! i tried to align mine so that the switch was fully pressed and depressed when the pedal was up and down, you'll know what I mean one you see the little rubber pad the pedal contacts or just remember where yours was before you take it out.
That should hopefully solve it! Although I haven't actually tried driving mine again yet as I'm fixing other things but I'll let you know when I do unless you do try it first. its all over miata.net and other 5 forums anyway.
don't buy the clucth switch from autolink part n.o B001 or whatever it is, its the damn brake light switch! (think it might be a clutch switch in the bongo) I think its LA01 you want if you can't be bothered taking it apart and finding a spring from a pen
The neutral switch rarely fails from what I read and its easy to test as I remeber when out in mine I would shift to neutral when coming to a stop to stop the revs dropping or sometimes even stalling under hard braking! Only realised that after I bought one though and saw it was a bit of a pig to replace and thought sod it!
There is also a way to test it with an LED light in the diagonstic box on early 5's from what I've read, by placing it between MEN and GND i think it is and then when you press the clutch or put it in neutral the LED should light up showing the switch works
Basically there is a neutral switch attached to the gearbox that when you shift to neutral it grounds a circuit to the ecu telling it maintain the engine idle speed / inject fuel etc
the other one that does this is the clutch switch (uses the same circuit if I remember right) when the clutch pedal is up the circuit is broken and ecu can see the car is running normally driving along and it won't attempt to correct the idle when you push the pedal down the switch is activated and it grounds the same circuit as the neutral switch and this tells the ecu, ok i need to maintain the idle speed now and inject fuel as apparently when you lift the throttle above certain speeds (over-run) the ecu stops injecting fuel which you would probably do when coming up to a junction, unless at Mcdonalds.
Thats why it stalls as the ecu just sits there drinking tea because it thinks the momentum of the drivetrain? will keep it running but when you clutch down you've knackered that idea!
So to fix it all you do is unplug the connector in the footwell going into the switch, use a 17mm spanner i think it is to undo the nut, twist the switch anti clockwise to remove it (its threaded into its bracket and the nut locks it into place) once its out get a small flat blade screwdriver and prise the metal tabs that hold the switch together until the swtich comes apart, then remove broken spring, clean any contacts that look crap and replace the spring with a clicky pen spring! nice and cheap! then bend the tabs down again and refit! i tried to align mine so that the switch was fully pressed and depressed when the pedal was up and down, you'll know what I mean one you see the little rubber pad the pedal contacts or just remember where yours was before you take it out.
That should hopefully solve it! Although I haven't actually tried driving mine again yet as I'm fixing other things but I'll let you know when I do unless you do try it first. its all over miata.net and other 5 forums anyway.
don't buy the clucth switch from autolink part n.o B001 or whatever it is, its the damn brake light switch! (think it might be a clutch switch in the bongo) I think its LA01 you want if you can't be bothered taking it apart and finding a spring from a pen
The neutral switch rarely fails from what I read and its easy to test as I remeber when out in mine I would shift to neutral when coming to a stop to stop the revs dropping or sometimes even stalling under hard braking! Only realised that after I bought one though and saw it was a bit of a pig to replace and thought sod it!
There is also a way to test it with an LED light in the diagonstic box on early 5's from what I've read, by placing it between MEN and GND i think it is and then when you press the clutch or put it in neutral the LED should light up showing the switch works
That's a big mix of information and misinformation 
If you lift off the throttle while in gear over 1500rpm then yes, the ecu will not fire the injectors at all. However, as the revs fall below 1500rpm the ecu will start firing the injectors again. You can hear this when driving, lift off but leave it in gear at around 3000rpm and let the revs drop - as it drops below 1500rpm you'll hear the exhaust note change/start "popping". I believe this will happen regardless of being in gear/clutch state though whether it does or not doesn't matter in this case as it will always be injecting fuel under 1500rpm.
From OPs original description it sounds more like a misfire to me and the most common cause for that is plug leads.
If you lift off the throttle while in gear over 1500rpm then yes, the ecu will not fire the injectors at all. However, as the revs fall below 1500rpm the ecu will start firing the injectors again. You can hear this when driving, lift off but leave it in gear at around 3000rpm and let the revs drop - as it drops below 1500rpm you'll hear the exhaust note change/start "popping". I believe this will happen regardless of being in gear/clutch state though whether it does or not doesn't matter in this case as it will always be injecting fuel under 1500rpm.
From OPs original description it sounds more like a misfire to me and the most common cause for that is plug leads.
Sounds a lot like the idle speed control thingy to me. my advice would be to first spray a load of injector cleaner through the throttle body.
you can adjust the idle speed but it will just keep on creeping down until you cant adjust it any more.
I have had exactly the same symptoms as the OP and the injector cleaner was the only thing that fixed it long term. another added plus is that the car ran better afterwards as well.
Of course if that doesnt work then by all means change plugs and leads.
Edit: just a note that if you have (or someone else has) already adjusted your idle speed before using the injector cleaner you will need to reset it at its lower (original) speed. also the car will struggle to start and will run rough and smokey for the first few minuets afetre you have done this.
you can adjust the idle speed but it will just keep on creeping down until you cant adjust it any more.
I have had exactly the same symptoms as the OP and the injector cleaner was the only thing that fixed it long term. another added plus is that the car ran better afterwards as well.
Of course if that doesnt work then by all means change plugs and leads.
Edit: just a note that if you have (or someone else has) already adjusted your idle speed before using the injector cleaner you will need to reset it at its lower (original) speed. also the car will struggle to start and will run rough and smokey for the first few minuets afetre you have done this.
Edited by Pete Franklin on Thursday 31st March 16:37
Gassing Station | Mazda MX5/Roadster/Miata | Top of Page | What's New | My Stuff