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Tach dropping to 0 on highway, stalling at stops.

48K views 11 replies 5 participants last post by  aquanaut20 
#1 ·
Hi Everyone,

A couple of days ago, my 2012 Kia Rio 5-door (LX+) stalled for the very first time while in stop-and-go traffic.

The ignition struggled to turn at first, but when I got the car back up and running, the tachometer was sitting at 0 and remained there until I parked the car, turned the vehicle completely off and then started it again. There were no issues with acceleration or deceleration.

I thought this was a one-off, but then the next morning when I was driving on the highway to work, I felt a bit of engine hesitation, and the tachometer dropped to 0. The speedometer continued to work normally. The car continued to accelerate and decelerate as normal. When I parked at work and turned the car off and on again, the tachometer came back to life.

In both situations, all electrical elements of the car remain on (lights, radio, dash, etc.).

No check engine light comes up, but I have had one time where a picture of a car with a key in it was illuminated on the dashboard for about a minute after restarting from a stall.

I have had the car stall 3 more times since then and have had the highway power/tachometer drop 4 times.

I took the car into a repair shop that was unable to replicate the issue and found no error codes. The shop said they found no issues with the car.

The shop says to keep driving the car until the check engine light comes on, but as you can imagine I am concerned that by the time the light does come on, I may be faced with a dead car or a dangerous situation on the roads.

Has anyone had this issue before?

I am reading some things about a cam shaft sensor and a crankshaft sensor, but the shop said they don't want to start replacing components based on a "hunch" as repair costs could quickly spiral upwards.
 
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#4 ·
Sorry to hear of your issue.

Sounds like a random problem, did you noticed happening after acceleration, descel, or hitting a pothole (loose sensor, ignition or PCM wire maybe?)

You maybe on the right path, the CPS maybe the culprit. But maybe you need a better mechanic, if we amateurs can figure that out, he should had too.

MV
 
#5 ·
the shop said they don't want to start replacing components based on a "hunch" as repair costs could quickly spiral upwards.
Has your mechanic not been trained to diagnose and test, the Tack signal is generated by the CAM sensor, which produces a digital square wave... Generally tested with a scope on the signal wire..

Oh! I get it, he only knows how to change part as dictated by code reader....!

.. Philip
 
#7 ·
It is possible that the mechanic didn't know what was going on when he checked it...

But when I spoke with the Kia Service Dept., they said that if the engine is not throwing off any codes, they would not necessarily be able to figure out what what the problem is, either.

I can see how testing the cam sensor would be smart, but considering the problem is intermittent, I would imagine any issues would not show up if the problem is not happening (especially considering there are no error codes in the computer after having this happen a few times).

You could very well be correct. I am not sure which diagnostics they performed when I took it in.

Thanks for your advice!
 
#6 ·
Thanks for your advice, MV!

Heading into and out of Toronto, there are potholes galore. I have hit one or two in my day, so that could be a culprit, but I would hope that the technician would have caught it at the shop! I also have to drive over speed bumps quite regularly, but do so at a slow speed.

It happens while coasting at highway speed (110 km) and also happens when completely stopped (at a red light, 0km). Generally speaking, it is happening on flat terrain. It does not happen with any more frequency in bumpy roads.

The other day I was trying to reverse into my driveway, and when stopped and shifting from Drive to Reverse, the car stalled out completely.

The interesting thing, to me, is that the engine does not stall when on the highway. It just has a little drop in power with the tach dropping to zero.

It only comes to a complete stall when the car is stopped.

I called the Kia service center and they told me that if no error codes are presenting themselves, that they will likely not be able to find a problem either.

I wonder if I drove the car in the state where the tachometer is at 0 if their diagnostic test would show something different.
 
#10 ·
The crankshaft position sensor is an available part at the local auto parts stores here. I don't know if that's so in Canada. Haven't changed one on a Kia but I have on a couple of other vehicles and it wasn't too difficult job.
 
#11 ·
I'm finding it hard to understand how the PCM can't make enough sense of the CKS signal to produce the RPM display, but OTOH it's good enough to run the vehicle. Perhaps there's programming in the computer to use only the CPS signal if the CKS is not completely good, but still within a certain margin of error. That would also explain not running at all once the CKS failure is more severe.

There's a part of me that's thinking PCM issue (messing up a good CKS signal), even though I'm usually advising others that the computer is last on the list of potential issues. In any case, interesting information to file away for future reference. Thanks to the OP for the follow up, and hopefully he will also confirm that the new CKS did indeed resolve this issue.
 
#12 · (Edited)
When I spoke of signal degradation, this is what happens (CAM/Crank), the signal is a pull down circuit, as it degrades the signal(12 or 5vdc dependent on Mfg spec) is only partially pulled (not down to 0vdc). The code is not set until critical.. the ECU may provide spark/fuel.

The method of diagnostic is to use a scope, a MM does not sample at a rate high enough to capture. The OBDII will not help until past critical..

Had one recently with a 12vdc circuit which gave an issue at 4vdc (12/4 instead of 12/0, square wave).

... Philip
 
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