Opel Astra F knock sensor

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Opel Astra F knock sensor

Postby cristi0001 » Tue Apr 22, 2014 9:26 am

Hi everybody,

I have a problem with the an Opel Astra F, x16sz engine, 1.6 petrol, 1995. Hard start especially when engine is cold and hesitates when in load. When the engine is hot, it runs well. I exclude a fuel supply issue because the car behaves the same on petrol and on LPG.

I checked the sensors with the oscilloscope (engine coolant temperature , throttle position sensor , manifold absolute pressure sensor , crankshaft angle sensor and knock sensor, EGR valve is cancelled with a thin sheet of copper) . All signals are looking good ( compared with those from Autodata , Vivid Workshop and your website database) except the knock sensor. The knock sensor is a one wire type. I have no errors stored in the ECU. The car behaves much better with the knock sensor disconnected (hard start is still there but much better in load conditions).

With the knock sensor disconnected I get approximately 4.5V at the ECU input, and with the sensor connected I get 2.5V (ok, maybe that's why I didn't get a DTC related to knock sensor) .
This is a capture of the knock output when engine is running:
knock.jpg
Knock sensor output


It can be seen the the knock output is biased to 2.5V.

When key on engine off and I lightly knock in the engine block I get similar waveforms but with slightly lower amplitude.

From what I've seen on the net about the signals generated by other knock sensors, their oscillations were much longer than what I have - other sensors had an oscillation duration of a few milliseconds and I have only a few microseconds (1000 times less).

On my engine, beside the knock sensor, the system also includes a knock processor, which, they say, amplifies and filters the knock signal so that the detonation is easily recognized by the ECU .

This knock processor, when there are no detonations, for example when ignition is on and engine off is should output about 10V (according to Autodata and Vivid Workshop ), that's what I have, all ok until now. The moment it detects a detonation, the processor output should pull to ground - 0V (according to Vivid Workshop ) and is easily read by the ECU. In my case, when the engine is running the processor output stays at 10V for the whole engine running time, it never pulls to ground. The wiring seems ok , knock sensor signal reaches the processor and processor output reaches ECU.

Maybe the knock processor doesn't work correctly because the knock signal is invalid? (oscillation duration too short in my case?)

How should a good knock signal look like? Do you think I'm dealing with a defective sensor?
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Re: Opel Astra F knock sensor

Postby cristi0001 » Wed Apr 23, 2014 10:14 am

I'm coming back with more info. This is the sensor output when key on engine off when I hit the engine block with a lever.
knock_lever.jpg
Knocking the engine block with a lever close to the sensor

I get this only when I hit the engine block close to the sensor, if I hit the other side of the engine block, I get nothing. In the trace it can be seen that the knock processor is also working, so I think that I have a functional knock detection system (sensor and processor), the thing is that I thought that when the engine is running I would occasionally get a knock, and when I didn't see it I thought that I have a defective sensor or processor. So the problems of my car are apparently not caused by the knock sensor, other ideas? I cleaned the IAC and sealed the fuel canister purge valve output to the intake manifold, I thought maybe it's leaking. I'm yet to see whether the problem persists or not.
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Re: Opel Astra F knock sensor

Postby arjen » Wed Apr 23, 2014 4:48 pm

Did you looked at the mechanical side? Especially with an engine running on lpg you can have a compression / valve problem.
If the basics look good then you can check the ignition.
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Re: Opel Astra F knock sensor

Postby cristi0001 » Thu Apr 24, 2014 8:52 am

I changed the ignition coil, plug wires and plugs and the problem still persists. The problem is there only when motor cold until it warms. The severity of the problem variaes randomly but is present only when motor is cold. Sometimes it takes more time(extra 10minutes) for the problem to dissapear even if the engine coolant temperature has reached 80 degrees Celsius.

The problem is like this: hard start, hesitation under load and even explosions on the exhaust if accelerator pedal is pressed more.

I haven't checked the mechanical side, i'm not a mechanic and don't have the tools and place to do this.

The problem si there on both gasoline and LPG. I made the conversion to LPG in january this year and I have until now around 5000km on LPG.

Do you think it could be a valve sealing problem? The engine has 250.000km.
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Re: Opel Astra F knock sensor

Postby Steve Smith » Fri Apr 25, 2014 1:55 pm

Hi Cristi and thanks for the information about the knock sensor and processor and nicely eliminated. I would have to agree with Arjen regarding the mechanical condition of the engine. Unfortunately LPG is a killer for the conventional internal combustion engine. I my experience with LPG, after just 10,000 miles I have seen rapid valve seat wear to the point of zero clearance. Your vehicle however uses hydraulic lifters and should compensate, however compression has to be suspect here and well worth pursuing from cold and hot. From a diagnostic point of view I would isolate the gas to enable the engine to run on the fuel it was designed to burn as when hot the system will switch over to gas and I suspect will run better with LPG?
This will prevent the goal posts from moving whilst diagnosing the underlying issue.
Thanks for the information to date, take care.....Steve
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Re: Opel Astra F knock sensor

Postby cristi0001 » Sun Apr 27, 2014 1:12 pm

Thanks for the answer Steve.

I tried to start on gasoline and it behaves the same. The problems I have with the car started almost one and a half years back when the car was running only on gasoline. The problems started with a slight hard start(start in 3 and quickly recover) but never had problems when in load when cold. As time passed the problem got worse. Harder and harder to start until the engine did not work correctly when cold and in load.

Can you please tell me what engines are suitable for running on LPG without "damage"?

If it is indeed a valve/seats problem with my engine, can I "upgrade" it to be more "tolerant" to LPG? (hardened valves and seats?)
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Re: Opel Astra F knock sensor

Postby Robski » Sun Apr 27, 2014 3:13 pm

i'm not a mechanic and don't have the tools and place to do this.


Seeing this more often now, purchasing a scope too :shock:

So by the comment i quoted, you won't have the WPS will you, or indeed understand the internal workings/events of a four stroke internal combustion engine ?
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Re: Opel Astra F knock sensor

Postby cristi0001 » Sun Apr 27, 2014 4:02 pm

@Robski: the scope I purchased was for electronics development (2204A), it's not an automotive one, I'm trying to learn about cars as a hobby, and hope that some day I'll be able to fix my car (electric and electronic faults).

What does WPS mean?
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Re: Opel Astra F knock sensor

Postby Steve Smith » Mon Apr 28, 2014 12:17 pm

Hi Cristi and thanks for the feedback. Regarding LPG for engines designed to run on gasoline, there are numerous additives and "valve seat savers" that are alleged to protect the engines from premature seat wear, but in my experience with Toyota/Lexus, none of these have worked. I am sure there are engine able to cope in one form or another which you can research, but again all my experiences have not been good. The WPS Robski refers to is the WPS500X pressure transducer, and well worth a look here https://www.picoauto.com/pressure-transducer-kit.html

I hope this helps, take care......Steve
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Re: Opel Astra F knock sensor

Postby Fat Freddy » Mon Apr 28, 2014 12:34 pm

If it is indeed a valve/seats problem with my engine, can I "upgrade" it to be more "tolerant" to LPG? (hardened valves and seats?)


LPG loves valves seats. What I have noticed is after the engine (hydraulic lifters) is revved the miss is more noticeable I assume until the lifters bleed down again. Speak to someone who reconditions engines.

I'm trying to learn about cars as a hobby, and hope that some day I'll be able to fix my car (electric and electronic faults).


I know what you mean. I'm trying to do it as a job. :lol:

What does WPS mean?


It means spending more money on a "Wonderful Pressure Sensor". https://www.picoauto.com/pressure-transducer-kit.html
Unfortunately this is the cost of fixing cars now. The kit isn't bought for fun. It helps avoid guess work. If your vehicle turned up at my door I would probably use the WPS but won't guess.

HTH
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