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Old 08-28-2020, 12:56 AM   #1
hessam69
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Default 1990 240 pinging, dieseling, running not the best

I have a 1990 240 with 125,000 miles and since I have owned it it has shown signs of slight pinging under load, running on for a second after shut down and not having the same performance of my other 1990 240.

On one very warm day, it continued to run for 5 seconds after shut down. There are no fault codes. Can the knock sensor go bad? I have not seafomed this car, I'm not sure if seafoam is available where I live. It runs well otherwise, does not overheat, but I'd like to know what is causing this issue.

Thank you
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Old 08-28-2020, 11:04 AM   #2
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Is it carbureted? Have you verified ignition timing/changed the plugs recently?
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Old 08-28-2020, 12:13 PM   #3
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Fault codes pretty much means that this car is fuel injected. Dieseling / run on is usually not an issue / possibility with fuel injected cars because shutting off the power to the ECU means the injectors should be closed. No fuel = no run on (unlike carburetor equipped cars). If you truly ae getting run on then it likely that you have a leaking injector(s) which are supplying fuel into the engine until the fuel pressure in the fuel rail drops to 0 because the pump is not running.

In order for an engine to diesel after shut down, in addition to fuel it has to have a source of ignition. This is typically heavy carbon deposits in the combustion chamber. The definitive fix is mechanical decarbonizing which requires removal of the head to allow cleaning of the combustion chambers. I have no experience with Seafoam or the other snake oil decarbonizing treatments. Misting water or a water methanol mixture into the engine air intake while it is running will decarbonize the engine (turbo engines that spray meth run remarkably free of carbon), although it might be a somewhat slow process requiring multiple applications to see any results. Also, misting into the intake on a MAP equipped car is a non issue. With an AMM / MAF equipped car you are misting upstream of the sensor and I expect that cold water / meth hitting the hot wire anemometer might not be a good thing.

So, check for leaking injectors first and see if that makes the dieseling go away. You can try some snake oil in the gas to see if that helps with decarbonizing which also contributes to pinging. Yes, knock sensors can fail with age (along with the wiring to the knock sensor). You can test the resistance of the knock sensor to see if it has had a gross failure. However, even if the resistance appears to be in spec that is not a guarantee that sensor has not become hard of hearing.
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Old 08-28-2020, 04:56 PM   #4
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Thanks.

So there is no sure fire way to test the knock sensor?
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Old 08-28-2020, 05:10 PM   #5
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Originally Posted by hessam69 View Post
Thanks.

So there is no sure fire way to test the knock sensor?
Sure fire - if you measure the resistance of the sensor and its resistance is greater than 10,000 ohms I would say that it is sure fire dead. If you measure 0 ohms I would also say that is sure fire dead. If you measure a resistance in the normal range around a couple thousand ohms, it is not sure fire dead; but, can still be non functioning. If you are looking for more guarantees, replace it because the cost of replacement is less than the cost of the test equipment (portable oscilloscope).
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Old 08-28-2020, 07:12 PM   #6
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Sure fire - if you measure the resistance of the sensor and its resistance is greater than 10,000 ohms I would say that it is sure fire dead. If you measure 0 ohms I would also say that is sure fire dead. If you measure a resistance in the normal range around a couple thousand ohms, it is not sure fire dead; but, can still be non functioning. If you are looking for more guarantees, replace it because the cost of replacement is less than the cost of the test equipment (portable oscilloscope).
Piezo elements are high impedance. It will read as very high resistance or open circuit when in working condition. You can test it if you have a meter that can do AC millivolts, connect it up and tap the sensor and you will get a small voltage spike. Resistance values here are meaningless. Don't toss a working sensor. The crap that comes from parts stores these days are not as good as what is probably on the car
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Old 08-29-2020, 01:50 AM   #7
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Piezo elements are high impedance. It will read as very high resistance or open circuit when in working condition. You can test it if you have a meter that can do AC millivolts, connect it up and tap the sensor and you will get a small voltage spike. Resistance values here are meaningless. Don't toss a working sensor. The crap that comes from parts stores these days are not as good as what is probably on the car
You are correct. In my careless typing I was out by a factor of 1000 (probably more) in my high resistance test value. Piezo sensors do exhibit internal leakage resistance values that are well into multi mega ohm territory. Differentiating between an open circuit (a sure fire failed) sensor and the normal leakage resistance of a non failed sensor with a budget multi meter would be a challenge. Zero resistance would be much more conclusive - a short which would also be a sure fire failure.

The hammer test may work if you have a digital multi meter with a fast up date rate or a sensitive analog meter (does anybody use those anymore?). With a slow up date rate on a digital meter you may miss the hammer tap completely. Also, the hammer test will likely only have a chance of working if your sensor has a flat frequency response. Some knock sensors have a narrow tuned characteristic that typically matches up with the resonant frequency of the cylinder, typically above 3000 hz. Inexpensive multimeters generally lack the sample rate to measure in those frequencies so may measure nothing. Even the mid range Fluke meters only list a frequency response up to about 1 kHz in the AC millivolt range.

Bosch definitely makes flat response sensors and I believe they make tuned sensors. I don't know what is used on a 1990 vintage 240.
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Old 08-29-2020, 08:34 PM   #8
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I guess I could swap the knock sensor from my other to test it
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Old 09-05-2020, 02:11 PM   #9
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good idea. But make sure you torque the bolt to spec.
The piezo knock sensor is part of a feedback loop. If it's truely broken i think you should be able to see an error code in the EZK-diagnosics .
The more torque you apply to the bolt, the sooner it will sense pinging, the sooner that EZK will start pulling timing advance.
Leave the bolt too lose and EZK will continue to advance timing as far as it can, possibly too far resulting in pinging.
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Old 09-13-2020, 01:44 PM   #10
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Piezo devices use a wafer of ceramic or quartz with metallic contacts, much like a capacitor. There is no dc current flow through the device, thus a working one will show nearly infinite resistance. On the other hand a low resistance will actually mean a part failure.
Seems to me that if a knock sensor is reporting a problem, you will hear it easily, or perhaps by using a listening tube. What do your plugs look like? Dieseling is usually caused by hot carbon buildups inside the cylinder. Have a friend shut off the ignition and then quickly pull the coil wire. If it still runs, I would say carbon build up.



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Old 09-13-2020, 05:49 PM   #11
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Yes you can hear the ping very easily. I'll swap another knock sensor in.

So what would I do next? Pull the head?
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Old 09-13-2020, 08:24 PM   #12
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I must have misread your post. You are saying that the pinging is easy to hear? And you want to replace the knock sensor??? Not exactly logical. Before yanking the head look at your plugs, and confirm that the cam and distributor are timed correctly. Consider that engine knock is almost always something expensive in the block such as main bearings, rod bearings, wrist pins, etc. If you want to save the car, look for a rebuilt engine. Nothing much in the cylinder head will produce a knocking noise.

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Old 09-13-2020, 09:28 PM   #13
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Dood brah how rich is it running/how glowing horribly hot are the plugs/cylinder head/how leaky are the injectors if an EFI car is dieseling?
Punctured FPR diaphragm/vac line stink of gas?

Is the ignition switch sticking on or something?
LH2.4+? Flywheel advanced 45 degrees ?

Points ignition crab iron head dinosaur I'd understand.
Alum head, EFI, electronic ignition, something has to probably be hooooooooribly wrong to cause that.
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Old 09-14-2020, 01:54 AM   #14
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Did you verify timing of crank gear(not pulley), cam gear(and lobes) and distributor(rotor aligned to notch on distrbtr) ?
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Old 09-14-2020, 02:27 AM   #15
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Yes, all that is correct. The engine runs quite well save for the pinging and occasional (only in very hot weather) dieseling. It has only done 125,000 miles.

So I'm wrong to assume a faulty knock sensor causing pinging? It's the sensors job to detect detonation and retard the timing correct? What makes it more confusing is the lack of fault codes.
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Old 09-16-2020, 09:40 PM   #16
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From what you've described I couldn't really say your knock sensor is the problem, but for $18.25 you won't be set back much and you might at least eliminate one possible issue/or doubt as you try to diagnose the problem at hand ?

https://www.fcpeuro.com/Volvo-parts/...&b=5&d=34&v=11

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Old 09-19-2020, 01:52 PM   #17
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Normally an engine is set up with a timing curve that will not cause detonation. The knock sensor function is to detect detonation which may be caused by poor quality fuel or other changes such as carbon build up in the combustion chamber that can cause detonation. If you engine has 'other issues' a faulty knock sensor may result in no knock control in the ECU and replacing the sensor may cause the knock to go away because you have restored the knock control function.

Your 1990 is pre OBDII. I don't know whether the Bosch pre OBDII ECUs have a knock sensor failure error code. You would have to check the service manual to see if there is an applicable code. That said, knock sensor failure can be hard to detect. If the ECU is sophisticated enough to monitor for the presence of a noise signal on the sensor it may be able to detect a sensor failure. Other wise it may be reduced to checking for shorts which could miss the more likely failure modes.
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