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Discussion Starter #1
Z14XEP. Symptoms:

Has to crank longer than usual to start, (hot and cold) but starts OK
Intermittently while driving the engine cuts out for a second - EML and Spanner both come on, but go out again. One weird thing though, as it cuts out the speedo drops to zero, then returns to normal.
This is all over in a second, nothing else is affected.

I've had P0136 this week, and cleared them till I can get ANOTHER rear O2 sensor; the one was new in August. Rescanning now shows two P0136 pending. Have cleared again.
Now I've had several of the rear O2 sensor faults over the years, but never had the cutting out/speedo drop problem with it before. Does seem coincidental though....

Any ideas, guys?
 

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cutting out/speedo could be the connection behind the dash instruments is a bit loose

need to take it out & check, guide is in the tutorials
 

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That would not affect engine operation, as ipc data is canbus driven and ecu does not use what it puts on canbus itself.

rpm data is hardware filtered and software processed all inside ecu and no other external component is allowed to interfere with the signal.
Design considerations on safety applications.
 

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Discussion Starter #5
I did consider the crankshaft position sensor, but would that affect the speedo in this way?

No, won't be a connection behind the dash - that wouldn't cut the engine.

I've though as possibilities -

  • the rear O2 sensor, though unlikely? As it's only 6 months old, it's throwing codes may be symptom rather than cause
  • crankshaft position sensor. Would that affect speedo? Wouldn't it be logging a code with everytime the engine cuts
  • EGR valve - but would that affect the speedo?
  • Coil pack - but would that affect the speedo?
  • wheel speed sensor - but would that cut the engine?
  • gearbox speed sensor (if there is one) - again, would that cut the engine?
  • water in an engine compartment connector, after the recent waterlogged roads.
I feel the key is that the speedo cuts with the engine, buit not having experienced a fault like this ever, I'm kind of in the dark!
 

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The ecu gets its rpm signal from the cranksensor and transmit it to the ipc. So if cranksensor stops working the rpm needle will be on zero or erratic.
 

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Discussion Starter #7
RPM needle isn't erratic though. Drops slightly as the engine cuts, but immediately returns to the correct speed, so seems to be accurately reporting engine speed. It's the speedo that's affected.
 

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If it were a Dti diesel its sounds like the classic solder issues in the engine ecu but im not familiar with where the petrol ecu's are mounted.
 

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Discussion Starter #10
Well, I changed the rear O2 sensor for an old one that I had, and went out and did 61 miles on all sorts of roads, at varying speeds. Very wet, so that was a good test for a problem exacerbated by water. It started on first turn of the key, and I thought it was cured, but 30 miles into the trip it did it again, and once more at 33 miles. That's far better than yesterday, when it did it a lot so plainly not cured. No codes, and none pending. Will give it a week or so and see it it improves further.

Meanwhile if anyone has any more ideas......
 

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Rear O2 sensor has only one purpose in life. To signal if the catalytic converter does it’s work and if vehicle complies with the emissions law. Blaming it for driveability issues is no point.
 

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Discussion Starter #13
Not really, no. I have a smooth and even tickover, cold or hot, and no P0335 or similar code. I'll repeat my question - would the crankshaft position sensor also kill the speedo for the duration of the miss?
 

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Discussion Starter #14
Rear O2 sensor has only one purpose in life. To signal if the catalytic converter does it’s work and if vehicle complies with the emissions law. Blaming it for driveability issues is no point.
And what does the ECU do to compensate when it receives out-of-tolerance readings from the sensor?
 

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Discussion Starter #18
Yes. I've been meaning to order a new O2 sensor - will probably change the Crank Position sensor too, even though I doubt it's the problem.
 

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Discussion Starter #19
Well, changed the Crank Position Sensor at the weekend (didn't fix it), cleaned the ECU earth, cleaned the multiplug connection behind the dash, cleaned the ABS sensor and ring on the n/s front wheel (speedo signal comes from there.) We'll see if that has any effect, but I doubt it will.

It ran OK, for 10 miles, but that's not a real indication of whether the problem is there or gone. Then I started it, and got an immediate DTC - EGR Valve. I cleared it, and it hasn't yet come back, but I suspect that's a red herring as far as the actual problem is concerned.

One noticeable thing - when it's taking a few more cranks to get it going, there's a strong smell of petrol. I did pull a plug to see if it was running rich, but it was a good colour, and not sooty.

Will change the O2 sensor in the next couple of days.
 

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Discussion Starter #20
Also checked the BCM for wetness, was fine. Have ordered a new ABS sensor to see if that helps. It will have the fault (but never a code) wet or dry, any speed, any gear, any throttle position. Sometimes 30 miles and it won't do it, then at other times 5 times in a minute....
 
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