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agk

Stalling when slowing down at times

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Greetings to All, 

This happens to my S4s only occasionally, not at every deceleration: at times, during a ride, when engine is warm, when slowing down after going at least 40 mph, say coming to a stop (despite switching the tranny to neutral), the engine would stall. I would then start it without a problem and then dive normally. Upon subsequent slow-downs it wouldn't repeat. Normally it's harder to replicate when driving on public roads; when driving around the block, speeds are less than 30 mph, so repeated coming down to a stop doesn't produce engine stalling. 

I just changed IAC valve (original) to a new one. Idle seems to be at normal 1,100-1,200 when cold, and around 900 when warm. No Check engine light. I did disconnect battery a few times recently. One of the reasons I can think of is that maybe I didn't let the car go through the ECU learning cycle correctly (if somebody can give a correct one to me, I'd appreciate it). 

No vacuum leaks (which I check from time to time turning ignition into ON position but without starting the engine: I wait until the vacuum pump shuts off - which it promptly does). Other than this, the car drives fine. 

I did notice that it started happening more in the colder weather, if that matters. 

I will appreciate any advice on this, thank you in advance. 

Anton 

'95 Esprit S4s

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Are you absolutely certain you replaced it with the correct one?

you need to plug in a laptop and reset the blm. Then start up from cold and leave until the fans kick in.


Only here once

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The vacuum leaks you should check for, are between the engine and sensors (MAP etc). This is a completely different circuit from the vacuum system used to control the ventilation flaps, throttle jack and EBPV (and brake booster on older cars). So you might want to check all the lines to and from the engine, there should be a diagram in the owners manual and possibly on the inside of the boothlid.

Filip

 

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I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Barry, 

Yes, I replaced the IAC with a new one from JAE (correctly spec'd, I'm sure; part # A910E6942F as per parts manual). I will try to plug my PC with a serial port (through which I run Freescan) and reset BLM (I checked that I can do it via Freescan), and then, as you advised, to start up from cold and wait until the fans kick in. 

Filip, 

I'll double check all vacuum lines (to MAP etc) (noted it's a different circuit). I checked it recently, but I'll check it again (yes, I have the Engine Management manual with all the diagrams). 

Thanks very much for your help, I'll let you know then. 

Anton. 

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If you can get a datalog of when it stalls that may help with figuring out what is going. A datalog from Espritmon or TunerPro would be preferable to one from Freescan.

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Hi All, 

I finally got to try it. As I don't have a laptop, but only a PC, I could connect it and run Freescan while the car was stationary. So I've reset BLM via Freescan - and, while doing it, got a datalog (ignition on, engine off, before cold start). From the log, I noticed 2 things: 1) BLM switched from 128 to 126 (what does that mean?), 2) Des Idle changed from 1,450 to 1,250. 

I also followed the ECU reset procedure: with ignition on, engine still off, floored the gas pedal. Then started the engine. 

After that I let the car idle and engine warm, as it was chilly outside, even after idling for 10 minutes, the fans didn't kick in. So I drove the car for 10 minutes at various speeds, slowed down and stopped multiple times, trying to imitate the conditions during which the idle would drop all the way. It didn't. The car drove normally all the way and would never stall (after a slow-down). 

I don't know what this is indicative of. I'll keep an eye on how the car drives. What I take out of this is at least this: lately, I disconnected the battery a few times, but didn't do ECU reset and didn't reset BLM, so perhaps the ECU didn't go through a proper learning cycle - well, at least re: the idle rpm. 

I will appreciate your opinions on this, thank you. 

Anton. 

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12 hours ago, agk said:

BLM switched from 128 to 126 (what does that mean?)

This occurred because the once the engine was started the BLM cell was taken from a different RPM vs MAP location in the BLM table. The value of 126 means the ECU is adjusting the fuelling by reducing it by 1.5%.

12 hours ago, agk said:

I also followed the ECU reset procedure: with ignition on, engine still off, floored the gas pedal. Then started the engine.

Having the throttle does not reset the ECU but it will inhibit the injectors during cranking (i.e. it's a clear 'flood' mode).

The ECU may have updated its IAC leaned position values which is why the stalling is not occurring at the moment. Hopefully it will stay that way.

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Hi

i had this problem when I used to disconnect the battery often in an attempt to not let the battery flatten ( it was an old battery ) 

the ECU was going through the relearn process each time I did this and it stalled when coming off throttle to idle ( for about 30 minutes until the ECU learnt )

i replaced the battery and now use a maintainer and never disconnect battery and it never happens now.....probably not your problem but if it is then easy fix ! 

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Derek, Ajhep, 

Thank you for your replies. Indeed, I disconnected battery a few times lately. And in part, I thought it would save battery's life (as I drive the car once a week at most) even though it's a new one in my case. I'll try to use a battery tender from now on. 

However, I'd like to understand something here: do I get this right that this lowest possible idle (rpm), below which the engine would stall, is not 'hard-coded', but rather re-learned by the engine every time (during initial 30 minutes of driving) after the battery was re-connected? Please kindly explain. 

Anton.

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Describing it in very basic way would be to say that there is a target idle speed based upon the coolant temperature and the IAC valve position is constantly being adjusted to achieve this or be positioned in anticipation of a return to closed throttle. Because there are lots of influences on the idle speed one of the 'tricks' the ECU uses is to learn some adjustments and retain these in its memory. If these learned adjustments are lost then the ECU has to re-learn them.

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Derek, thank you. So if get this right, the lowest idle rpm is one of those parameters that ECU keeps dynamically re-learning, based on the ongoing/changing driving conditions? 

Just sharing my further thoughts on this: I also noticed that when the car was doing that - dropping the idle below the lowest mark, so the engine would stall, it was usually after a brisk deceleration, say when coming to a stop sign/traffic light (never mind the untimeliness/inconvenience of such a mishap - I'd have to re-start the engine approaching an intersection, with other cars usually before and after me :) ). I wonder why it was the case (pattern)... And it wasn't the first deceleration during a ride (before which the battery would be re-connected), but it could a first brisk one. I guess I'm saying the ECU already re-learned at least a part of parameters for slowing down, but for some reason - not yet for slowing down quickly. To learn that, I guess that scenario had to be repeated a few times.

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It is not the lowest idle RPM that the engine learns, it is information about the IAC valve position. 

When your engine is warm, with the A/C, fans and all electrical accessories off, you should check what the IAC position is when the idle is stabilised at closed throttle. It should be between 25 - 40 steps, if it isn't then this would suggest an underlying problem else where, e.g. throttle stop screw setting, TPS closed throttle voltage etc.

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Derek, if so, how do I check what IAC position is? In particular, in Freescan, what parameter corresponds to that? Among others, I see "IAC" there, is it the one? 

I take it, I should plug in Freescan to get IAC position when, like you said, the idle stabilizes at closed throttle. I'd say, it's pretty stable even right after a cold start. So should I take the measurement when the engine is cold, or when it's warm before a ride, or after a ride? Re: throttle stop screw setting, I made sure not to change it. 

Please advise, thank you. 

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In Freescan the IAC valve position is called 'IAC Position' on the 'Engine Data' screen.  The closed throttle IAC valve position check should be made when the engine is warm and the A/C, fans and all other electrical accessories are off.

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Derek, yes, thank you, I can see this value as 'IAC Position' on the Freescan's 'Engine Data' screen indeed. I haven't taken a fresh measurement yet, but I looked up snapshots of Freescan from my previous diagnostic 'probes' - usually after a run, when the engine is warm but off (and so are all electrical accessories), and I saw a typical value being around 170. This was from a few probes during past summer and fall months. You said it should be within 25 - 40 range. Trying to reconcile this with my setting, I'm checking Freescan pdf manual and I see that this IAC Position value should be: 

1) Engine on at idle, warm-up cycle: 170, will slowly drop down to about 70. 

2) Engine on, normal driving: should now be between 40 and 70. 

Sadly in my case, the only way I can test it with the engine running is when the car is idling, stationary. I wonder what the expected range should be for that? Please advise, thank you. 

 

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When the engine isn't running the IAC valve will be in its parked position which is 170 steps.

The value of 25 - 40 steps is taken from the Service Notes. I just looked at one of my datalogs and the IAC valve position was in the high thirties. I have datalogs from other cars showing some in range, one in the low 40's and a GT3 in single digits.

Checking when the vehicle is stationary is preferable. If the vehicle was moving you would probably be in gear and that would effect the engine speed.

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