Rev range 'bounce' ?

[Cattwooduk]

Ok I've just discovered a new confusing issue with my engine to keep me on the learning curve.
I just had a play and the problem seems to be; if I slowly rev the engine up as soon as I reach 1650rpm it suddenly drops to normal idle then bounces back up if I keep my foot in the same position, then hits 1650 again and drops etc etc. If I slowly edge more onto the power it suddenly pops up to around 2000 rpm and is fine from there upwards. Just seems to not like the 1600-1900 range at all. If I stick at 2000 and gently come off the power it just plops down to idle and bounces around until I get to about 1550 then it's fine again.
I've had problems with the usual wax-stat related idle before and just upped the warm idle to about 1000-1100 with the idle screw and seems to have solved the problem, but this seems unrelated.

Any ideas oh wise ones?
Thanks
~Chris

[Danthrax]

I had a problem where mine was running rough in that rev range which was a little bit of water got down by one of the spark plugs when i cleaned the engine, sprayed it with a load of WD40 and that sorted it. Prob not the same problem your having but thought i'd mention it just in case.

[MartG]

Could be a problem with the Throttle Position Sensor giving the ECU an incorrect reading at certaing throttle positions

[JMR_AW11]

I'm not totally clear what you are doing but you may be invoking the ECU into fuel cut on a very fine throttle setting.
i.e. if your throttle switch (IDL microswitch) is still closed (badly adjusted?) with a whiff of throttle then you can cause the ECU to cut fuel if you are able to rev it too high with the IDL switch not yet open?

I've forgotten the revs at which it fuel cuts and I've forgotten the rules for fuel cut but it WILL cut the fuel if you are able to rev it with the IDL switch closed. Once the revs drop to a lower level the ECU puts the fuel back on.
I'm pretty sure the fuel cut is somehere around 1600-2000rpm on a warm engine and it restores at lower revs.

If this is your problem then the engine will hit a precise rev threshold and the engine will cut fuel and the revs will collapse. It will be fine and stable until you cross the rev threshold stored in the ECU (IIRC there's a fuel cut map for overrun rpm trip vs coolant temp)

Then once the engine falls to a lower rev range the fuel comes back on and the revs will rise again. If you don't move your foot then the whole cycle will keep repeating with revs rising to the trip point and falling again etc etc.

Maybe you should check your IDL switch adjustment...

[LimeyMk1]

Same thing used to happen to my Mk1 when I left the manual choke on when warming the car up for an oil change.

As mentioned it's probably the TPS that needs adjusting.

[JMR_AW11]

Also, with a trained ear you can hear a healthy mk1 MR2 hit fuel cut if you blip the throttle and let the engine resettle. The revs shoot up to about 3000rpm and once you lift off the throttle you can hear the engine revs drop 'quickly' with no combustion noise and you can hear the engine firing the fuel again just as it hits idle.

You have to know what to listen for but once you do, the fuel cut action is obvious...

Also, I don't know how much range there is on the idle adjuster screw but you can probably replicate this 'problem' by simply revving it too high with the idle screw. (or with a manual choke, see post above )

A persistent person could probably find a sweet spot where they can get the engine to rev without activating the IDL switch and they will get these symptoms.

Are you able to find this symptom easily or are you deliberately coaxing the car into this state with a carefully pressed throttle?

[JMR_AW11]

Same thing used to happen to my Mk1 when I left the manual choke on when warming the car up for an oil change.

I'm pretty sure you were seeing fuel cut because you are effectively revving the engine at idle without tripping the IDL switch.

So the ECU sees this as an opportunity to improve economy and it will cut the fuel if you rev it beyond the threshold in the ECU map table.
It doesn't cut fuel when cold because the ECU knows not to do this.

The car cuts fuel a LOT during normal driving as well. Especially if you drive at varying urban speeds with your foot constantly on/off the throttle.

[Cattwooduk]

Also, with a trained ear you can hear a healthy mk1 MR2 hit fuel cut if you blip the throttle and let the engine resettle. The revs shoot up to about 3000rpm and once you lift off the throttle you can hear the engine revs drop 'quickly' with no combustion noise and you can hear the engine firing the fuel again just as it hits idle.

You have to know what to listen for but once you do, the fuel cut action is obvious...


Are you able to find this symptom easily or are you deliberately coaxing the car into this state with a carefully pressed throttle?

I had a little play revving it and then coming off the power and I think after a while I can hear what you're talking about the firing kicking back in just as it hits idle speed... not completely sure but I think so

I initially decided to have a play yesterday because on my home from work I was coming up to some lights so I came out of gear and just rolled up to a stop, and noticed the idle was bouncing between 1000-1500 or so as I was slowing down (didn't noticed exact rpm as I was looking more at the car in front!) The car wasn't fully warmed up about 1/3 on the temp gauge) so was still idling a little faster than normal, so I'm not sure if maybe has something to do with the rpm dropping as it warmed up?
I was 'causing' I suppose, this rev bounce when I got home and was trying to see if it would still bounce around when I very slowly accelerated.
And apologies, I'm not particuarly mechanically minded/know a great deal about engines, still learning, and I'm not sure I follow what is being said about the IDL switch or what it stands for What does it do? I assume it's an open-close switch controlling fuel or something?
Thanks for replies, may not be any problem here at all by the sounds of it!?

[JMR_AW11]

Also, with a trained ear you can hear a healthy mk1 MR2 hit fuel cut if you blip the throttle and let the engine resettle. The revs shoot up to about 3000rpm and once you lift off the throttle you can hear the engine revs drop 'quickly' with no combustion noise and you can hear the engine firing the fuel again just as it hits idle.

You have to know what to listen for but once you do, the fuel cut action is obvious...


Are you able to find this symptom easily or are you deliberately coaxing the car into this state with a carefully pressed throttle?

I had a little play revving it and then coming off the power and I think after a while I can hear what you're talking about the firing kicking back in just as it hits idle speed... not completely sure but I think so

I initially decided to have a play yesterday because on my home from work I was coming up to some lights so I came out of gear and just rolled up to a stop, and noticed the idle was bouncing between 1000-1500 or so as I was slowing down (didn't noticed exact rpm as I was looking more at the car in front!) The car wasn't fully warmed up about 1/3 on the temp gauge) so was still idling a little faster than normal, so I'm not sure if maybe has something to do with the rpm dropping as it warmed up?
I was 'causing' I suppose, this rev bounce when I got home and was trying to see if it would still bounce around when I very slowly accelerated.
And apologies, I'm not particuarly mechanically minded/know a great deal about engines, still learning, and I'm not sure I follow what is being said about the IDL switch or what it stands for What does it do? I assume it's an open-close switch controlling fuel or something?
Thanks for replies, may not be any problem here at all by the sounds of it!?

Have you checked the coolant level recently?
The usual cause for a self bouncing idle on a mk1 MR2 is due to air bubbles in the coolant and this can make the engine rev itself as the coolant sensor alternates between seeing air pockets (cold) or coolant (hot)

This confuses the engine control unit and it causes the fueling and timing to fluctuate and this causes the revving.

The cure is to bleed the cooling system and make sure it is topped up.

However, your symptoms seem slightly different.

The IDL switch is a little switch that sits in the throttle position sensor. It's function is to let the Engine Control Unit know the driver has their foot off the throttle.

This IDL switch lets the ECU know when to run its idle program or to run the program for when the driver is pressing the throttle. There is a procedure for adjusting it and it should be linked somewhere on this site.

[PW@Woodsport]

The usual cause for a self bouncing idle on a mk1 MR2 is due to air bubbles in the coolant and this can make the engine rev itself as the coolant sensor alternates between seeing air pockets (cold) or coolant (hot)

This confuses the engine control unit and it causes the fueling and timing to fluctuate and this causes the revving.

Sorry Jeremy but lack of coolant to the EFI sensor is most definately NOT the reason for Mk1 idle bounce.

The bounce can be induced simply by having a stuck open waxstat and plenty of coolant in the system.The bounce is caused by the ECU seeing a shut IDL but the MAP sensor tells it throttle is being applied.... now on a cold engine it ignores the auxilliary bypass air, which is normal, but the ECU does not recognize the extra air on a hot engine and a shut IDL contact.

For proof try removing the rubber cap on the rear of the throttle body (used for AC idle up on USDM cars) on a hot engine with plenty of coolant in it, i guarantee it will bounce like mad = not coolant starving the EFI sensor as the cause.In fact the EFI sensor will be seeing heat from the surrounding head/housing anyway, i watched my EFI temp go up to 80* with no coolant present.

The only time lack of coolant can cause the bounce is when it starves the waxstat and that then refuses to close, this in turn means too much bypass air on a hot engine, it has nothing whatsoever to do with coolant not being present at the EFI sensor.

[JMR_AW11]

The usual cause for a self bouncing idle on a mk1 MR2 is due to air bubbles in the coolant and this can make the engine rev itself as the coolant sensor alternates between seeing air pockets (cold) or coolant (hot)

This confuses the engine control unit and it causes the fueling and timing to fluctuate and this causes the revving.

Sorry Jeremy but lack of coolant to the EFI sensor is most definately NOT the reason for Mk1 idle bounce.

The bounce can be induced simply by having a stuck open waxstat and plenty of coolant in the system.The bounce is caused by the ECU seeing a shut IDL but the MAP sensor tells it throttle is being applied.... now on a cold engine it ignores the auxilliary bypass air, which is normal, but the ECU does not recognize the extra air on a hot engine and a shut IDL contact.

For proof try removing the rubber cap on the rear of the throttle body (used for AC idle up on USDM cars) on a hot engine with plenty of coolant in it, i guarantee it will bounce like mad = not coolant starving the EFI sensor as the cause.In fact the EFI sensor will be seeing heat from the surrounding head/housing anyway, i watched my EFI temp go up to 80* with no coolant present.

The only time lack of coolant can cause the bounce is when it starves the waxstat and that then refuses to close, this in turn means too much bypass air on a hot engine, it has nothing whatsoever to do with coolant not being present at the EFI sensor.

The only time lack of coolant can cause the bounce is when it starves the waxstat and that then refuses to close, this in turn means too much bypass air on a hot engine, it has nothing whatsoever to do with coolant not being present at the EFI sensor

OK fair enough but the cause is low coolant level. (I guess an air leak would cause it too) It happened to me many years ago and the engine literally sits there and revs itself up and down. I cured it by bleeding the coolant.

So I guess what was most likely happening in my case was that it revs on the falsely open waxstat till it hits fuel cut and the revs drop due to lack of fuel and then the ECU refires the injectors once the revs drop and the whole cycle keeps repeating.

"For proof try removing the rubber cap on the rear of the throttle body (used for AC idle up on USDM cars) on a hot engine with plenty of coolant in it, i guarantee it will bounce like mad"

The next time you try this can you check to see if the injectors get cut at the peak revs and refire again at low revs? This would prove if it was fuel cut causing the bounce.

i.e. I don't think this issue has anything to do with the MAP sensor, or the ECU ignoring air etc, I think it simply hits the revs required for IDLE fuel cut on a warm engine.


Maybe all the symptoms described on this page are caused by this fuel cut.

I don't have any mk1 MR2 ECU stuff on this PC anymore so I can't easily check the ECU code but from memory the ECU has a map in it to set the idle/overrun fuel cut and I think this is what is happening here. The ECU is being fooled into preserving economy and it cuts the fuel and this causes the bounce etc. I didn't think the trip point for the fuel cut was as low as 1650rpm on a hot engine but maybe it is this low on some ECUs?

[Cattwooduk]

The bounce can be induced simply by having a stuck open waxstat and plenty of coolant in the system.The bounce is caused by the ECU seeing a shut IDL but the MAP sensor tells it throttle is being applied.... now on a cold engine it ignores the auxilliary bypass air, which is normal, but the ECU does not recognize the extra air on a hot engine and a shut IDL contact.

Been busy as hell over christmas thanks for the replies, had read all of them but not had time to really try to understand it all and reply.
So from what I understand I don't really have any issues at all, I've just discovered 'problem' which doesn't exist? Driving it seems absolutely fine so I guess I was inducing the symptoms myself.

Still seems a wee bit odd though, is it possible as mentioned earlier, that the IDL switch may be dodgy? ie, when it should be seeing the throttle being applied it's thinking it's actually idling too fast and so cutting the fuel? Or is it just preset to run the fuel cut cycle at that rev range?
I've tried comparing it to how it sounds when you rev the engine and let it drop down to idle before it starts to fire again, but can't hear any specific differences really... not a 'trained' ear I'm afraid
I'm not too knowledgeable with all these MAP, EFI and IDL sensors etc
Trying to follow Algosse09, but sorry to say I have almost no idea where or what any of these components are... besides the fuel sending unit!

[System-G]

Ignore the post by Algosse09. His account has been clasified as spam and has been dealt with by the Moderators.

[JMR_AW11]

I've just been to the garage to give my mk1 a warmup and charge the battery etc.

I ran it until fully warm then turned up the idle on the idle airscrew.

Sure enough the revs went up and hit a peak and went into bounce mode.

I'm pretty sure it is simply hitting fuel cut as it sounds like the injectors are being cut.

I didn't have any test gear with me to prove this (too cold!) but the cause of the rev bounce looks like fuel cut rather than anything to do with the ECU ignoring extra air etc.

It revs to the correct speed for fuel cut, then the ECU cuts the fuel, the revs drop and the ECU refires the injectors and the whole cycle keeps repeating.

So my guess is the OP is somehow invoking this response from the ECU and therefore might need to adjust the trip point for the IDL switch to prevent revving the engine with IDL closed.

The ECU is meant to cut the fuel if you lift off the throttle when coasting at lowish engine speeds and it obviously also cuts it if the idle gets too high when warm.

[Cattwooduk]

Ok that's interesting, as you say, it sounds like a normal occurrence, just that mine seems to be kicking in with my foot on the pedal when it shouldn't, though I have one question... what is the OP and how do I adjust the IDL switch?
And just to clarify as no matter how many times I re-read the posts, I can't pick out... is the IDL supposed to be open or closed when the throttle is applied?
Thanks again

[MartG]

OP = Original Poster i.e. you

Setting the TPS





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