Rpm drops and raises quickly

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Janus
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Rpm drops and raises quickly

Post by Janus » Mon Nov 12, 2018 7:39 pm

I have 1988 BX 19 gti and problem occurred today with rpm dropping when I press accelerator quickly/hard. Rpm raises around 2000 and then quickly drops to 1000-0 rpm after that it raises quickly back to 2000. Sometimes engine shuts off completely. Today I checked ignition and fuel pump. Both suth off completely during rpm drop. If I press accelerator lightly there is no problem. I also checked throttle position switch but it seems to be working.

This is my first BX and first petrol car so all advises are welcome :D

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Tue Nov 13, 2018 8:59 am

Oh I forgot to mention that engine fault light is on during the rpm drop.

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Kitch
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Re: Rpm drops and raises quickly

Post by Kitch » Tue Nov 13, 2018 10:01 am

Air flow meter?

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Tue Nov 13, 2018 11:18 am

I have to chekck that today. Can I read the fault codes from ECU without any special equipment? I think my BX has jetronic ECU.

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Tue Nov 13, 2018 5:46 pm

Kitch wrote:
Tue Nov 13, 2018 10:01 am
Air flow meter?
Thanks for advice! I cleaned it and problem seems to be away now.

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Re: Rpm drops and raises quickly

Post by Kitch » Thu Nov 15, 2018 9:35 am

Glad to hear it!

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Thu Nov 15, 2018 10:44 am

Kitch wrote:
Thu Nov 15, 2018 9:35 am
Glad to hear it!
Unfortunately the problem came back. I did some testing yesterday and cleaned some connections and grounding point for ignition module. Biggest suspect is ignition module because I have heard those tend to break sooner or later. Ignition coil and coils lead are possibilities too.

It seems that voltage is fine in the primary side of the coil during rpm drop. Secondary side in the other hand is completely dead about half a second.

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Fri Nov 16, 2018 2:17 pm

Quick update. I changed ignition module and ignition coil to new ones. didn´t change a thing. TDC sensor would be my next guess.

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Jaba
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Re: Rpm drops and raises quickly

Post by Jaba » Fri Nov 16, 2018 6:39 pm

I seem to remember that when I had similar erratic symptoms 20 plus years ago I was able to check the flow meter's output, with engine not running, while moving the flap slowly and measuring the output voltage. I think I used both an oscilloscope as well as a voltage meter.
Does the rev counter drop to zero during the malfunction indicating a loss of ignition pulses ?
There is a way of reading the ECU fault codes. It is clunky and often inconclusive and involves looking at the error light flashing sequence.
Then when you have exhausted checking the ignition side what about fuel filter, dirty fuel, fuel pressure.
The Joy of BX with just one Citroën to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Fri Nov 16, 2018 7:14 pm

Jaba wrote:
Fri Nov 16, 2018 6:39 pm
I seem to remember that when I had similar erratic symptoms 20 plus years ago I was able to check the flow meter's output, with engine not running, while moving the flap slowly and measuring the output voltage. I think I used both an oscilloscope as well as a voltage meter.
Does the rev counter drop to zero during the malfunction indicating a loss of ignition pulses ?
There is a way of reading the ECU fault codes. It is clunky and often inconclusive and involves looking at the error light flashing sequence.
Then when you have exhausted checking the ignition side what about fuel filter, dirty fuel, fuel pressure.


I checked the ignition with stroboscope and during malfunction it is completely off. Rpm drops to zero (or near it) even when i am driving the car. So I think it can´t be fuel related problem or at least it is improbable. I have read that there is safety relay that cuts out fuel pump if engine rpm signal is lost. That would explain fuel pump turning off during fault. correct me if I´m wrong but what i can understand Jetronic ECU can´t affect ignition anyway. It only gets rpm signal from ignition module.

I have to do more testing tomorrow but unfortunately I can´t get any parts during weekend if needed. Even worse fact is that I need to get new isnpection done next friday :D Well I think this is part of the hobby. I bought this car about a month ago and as I said earlier this is my first BX. So there is still alot to learn. Thankfully this is not my first Citroen :D

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Jaba
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Re: Rpm drops and raises quickly

Post by Jaba » Fri Nov 16, 2018 8:15 pm

I am not sure of the mechanism used but with this ECU system, when working normally, while slowing down with the throttle closed the fuel is cut off and maybe ignition too until the revs reach about 1500 when it starts burning fuel again. I used to be aware of this because I felt a very slight surge at 1500 revs which made me realise that this was what was happening.
Have you been able to get hold of the Bosch diagnostics for your jetronic system ?
The Joy of BX with just one Citroën to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.

Janus
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Re: Rpm drops and raises quickly

Post by Janus » Sat Nov 17, 2018 11:33 am

Jaba wrote:
Fri Nov 16, 2018 8:15 pm
I am not sure of the mechanism used but with this ECU system, when working normally, while slowing down with the throttle closed the fuel is cut off and maybe ignition too until the revs reach about 1500 when it starts burning fuel again. I used to be aware of this because I felt a very slight surge at 1500 revs which made me realise that this was what was happening.
Have you been able to get hold of the Bosch diagnostics for your jetronic system ?
Maybe I need to dig deeper to that. Even though this problem occurs while I accelerate. Maybe throttle position switch could cause this after all. I have not been able to get bosch diagnostic tool. where can I find some article about reading the fault codes via engine fault light?

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Re: Rpm drops and raises quickly

Post by Jaba » Thu Nov 22, 2018 12:44 pm

Janus wrote:
Sat Nov 17, 2018 11:33 am
Maybe I need to dig deeper to that. Even though this problem occurs while I accelerate. Maybe throttle position switch could cause this after all. I have not been able to get bosch diagnostic tool. where can I find some article about reading the fault codes via engine fault light?


I think I have the jetronic diags somewhere if you need them I can mount a search. They might be online here though: http://citroen.tramontana.co.hu/en/electrics
The Joy of BX with just one Citroën to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.

Janus
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My Cars: 1988 BX 19 gti 8 valve

Re: Rpm drops and raises quickly

Post by Janus » Fri Nov 23, 2018 11:37 pm

Jaba wrote:
Thu Nov 22, 2018 12:44 pm
Janus wrote:
Sat Nov 17, 2018 11:33 am
Maybe I need to dig deeper to that. Even though this problem occurs while I accelerate. Maybe throttle position switch could cause this after all. I have not been able to get bosch diagnostic tool. where can I find some article about reading the fault codes via engine fault light?


I think I have the jetronic diags somewhere if you need them I can mount a search. They might be online here though: http://citroen.tramontana.co.hu/en/electrics
Thankls for the link. The problem have vanished again tough. I had ordered the TDC sensor before that so I might as well change it when it arrives. If this problem occurs again I have to shake wires in engine bay. It might be bad connection somewhere.