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K Series cutting out


chris whitlow

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I have a 1.6 K Series and it’s started to cut out at around 6.5k revs. 
I think it must be something electrical, I’ve checked the HT circuit and that there are no leads near the crank sensor, but otherwise I’m at a complete loss as to what the problem is.
Has anyone suffered this before so can give me some ideas.
Thanks in advance.

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Any recent work or changes?

Has it been drowned?

...

Does the idle also sound rough and erratic?

If you hold the engine speed constant at say 5,500 rpm does it keep running smoothly?

In that clip did it stop by itself after going haywire or did you switch off?

Does it restart immediately after that?

...

... I’ve checked the HT circuit...

And the relevant earths and the battery connections?

Jonathan

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I've had something similar with mine. It was cured simply by re-routing the CPS cable, but it was difficult to find. I discovered that the ECU sent 8k rpm to the VBOX datalogger at 6k which made me check the sender. I even replaced it without luck.

The Stack dash didn't register the 8k rpm, so filtering there must be better than the VBOX.

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Thanks for the replies and apologies for taking so long to respond.

I changed the camshafts recently but it has run perfectly up until now.

It's not been drowned.

The idle sounds normal and at 5,500rpm it runs smoothly.

In the clip it stopped by itself but then starts again immediately after.

The earths and battery connections all appear ok.

This morning I've fitted a new Crank Position Sensor and also swapped the Throttle position senor but to no avail.

I've also run it with spark indicator lights on the plugs and when the problem starts there's no spark when the rev counter drops to zero so I'm wondering if it could be an ECU or coil issue.

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Mmmmm... answers and facts. :-)

...

Does the problem occur at the speed at which one of the shift lights is activated? Try removing the signal to that unit?

...

And, much more crudely, measure the battery voltage as the problem occurs to exclude something taking down the whole system.

...

Jonathan

 

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It would not affect the rev counter if it was something on the HT side.

Is the car EU2 or EU3? And do you know where from the rev counter get its signal?

I'd probably check the wiring to the ECU for faults.

 

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The way the tacho is swinging (in sync with the sound), it looks like the ECU believes the the engine is not turning, so its not initiating sparks (or injection).

If it were intermittently loosing sparks on the HT side, then the tacho feed would not be so erratic. The tacho looks to be suggesting that the speed is dropping very low, which it clearly isn't.

It could be a bad connection somwhere in the loom that is only affected with vibbration a certain frequencies.

It may be worth untying the loom and waggling it around. It could also be as far back as the fuse box.

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To my mind, if it shuts off at the same sort of RPM every time, that's too much of a coincidence to be a loose wire vibrating; yes I get resonance but it doesn't ring true to me.

I'd look very carefully at the ECU earthing.

One possible scenario:

As the RPM increases the number of coil pack pulses per second increases. Same pulse duration so the effect is that the average current draw of the coil packs increases roughly in proportion to the RPM. If you have a bad ECU earth, the ECU earth voltage will then also rise in proportion to the current (Ohm's law through the resistance in the earth lead). From the ECU's perspective this will be seen as the voltages at other terminals falling as the ECU measures everything relative to the earth it sees. At some fairly accurately fixed RPM, the voltage seen at the ECU "Ignition Sense" will fall to the point where the ECU thinks the ignition has been turned off. Engine stops firing, stops supplying pulses to the tacho, everything shuts down abruptly. Earth voltage then recovers , ECU turns back on again. The "Ignition Sense" input will be smoothed and damped to stop the ECU responding to noise pulses, so you will get a slight time delay and therefore you see the ECU cycling on an off as appears to be happening in the video.

Just a guess, maybe way off, but makes sense and worth a check!

Measure the voltage between the earth pins on the ECU (black wire, Pin 59) and the ignition sense pin (green wire with pink stripe, Pin 11). Measure this right at the ECU connector if you can (you can slip fine metal needles into the connector alongside the wire to make contact with the specific terminals inside) and see if it falls as you rev the engine.

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Thanks for all the replies.

I’ve checked the voltage between the earth and ignition voltage pins on the ECU and it remains constant when the problem occurs. I used pin 29 instead of 59 as there’s only 36 pins and 29 was black.

Also fitted a coil that I borrowed but that also made no difference.

I’m going to try swapping the ECU st the weekend.

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