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Old 9th July 2008
hkayssi hkayssi is offline
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Default Knock sensor bad?

How would one know when the knock sensor is bad? symptoms? How to test it?

The car is a 1997 NG900 N/A with an auto box and 104 K miles on it

TIA
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Old 9th July 2008
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Ok, I know this isn't for our cars but I remembered it from a Nissan forum a while back. The information is still sound.

Quote:
Originally Posted by Nissan Maxima Forum
Q) What does a knock sensor do?

A) A knock sensor is a specially designed listening device for car engines. It listens for engine knock, or detonation (pinging). Detonation can be very bad for an engine and is the result of the fuel/air mixture exploding too quickly instead of burning evenly and rapidly. This is due to either bad fuel, ignition timing which is too high, or built up carbon deposits in the engine (which increases the compression ratio). The sensor is small and consists of a piezoelectric sensor that listens for knock by detecting pressure. It is very sensitive and can be considered worthless if dropped. The vibrational pressure is then converted into a voltage and sent to the ECU for evaluation.

Q) How do I know if my knock sensor is bad?

A) Symptoms of a bad knock sensor include a sluggish engine, poor acceleration and poor fuel economy. Knock sensors rarely fail outright and more often get “soft” over time and cause false signals to be sent to the ECU, which thinks the engine is knocking when it’s really not. Thus, the ECU will reduce the ignition timing to the engine. The knock sensor on the VE30DE is prone to corrosion of the terminals and harness connection. This is due to the harness weather seal getting brittle and cracking, allowing moisture to seep into the harness from the rear and corrode the terminals. This creates a poor connection and faulty voltage readings to the ECU.



This next part may be Nissan specific (I know the harness location is) but you can give it a try.


Quote:
Originally Posted by Nissan Maxima Forum

Q) How do I test my knock sensor?

A) The knock sensor voltage can be checked through an electrical harness located right above the thermostat housing (where the lower radiator coolant hose attaches to the engine). It is a gray connector with an orangish red cap with two wires. Unplug the connector and attach a multimeter (or voltmeter) to the lower harness (the male connector). The wire for this connector should go left and underneath the intake manifold. Check the front most terminal (see pic) and ground out the negative point. You must have a multimeter than can measure more than 10 M ohms. Continuity should exist. This method is per the Factory Service Manual. NOTE: Knock sensors should be considered regular maintenance items. If you have over 100K miles and have never replaced the knock sensor, you can probably consider it bad.

Last edited by Frank_Drebin; 9th July 2008 at 12:41 PM.
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Old 9th July 2008
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Where is this knock sensor on a 2.3 '96 900S ? According to eEuroParts, Saab does use one , about $90 now for a Bosch. I have yet to see one; I think I have seen everything on this engine..

I asked eEuroParts, and should receive an answer later on..

Maybe the Saab units are better than the Japanese ones, but I do have over 150K and the MPG (23) could be better, even for an automatic....I think the performance is OK, but she has never seen a dyanometer...
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Old 9th July 2008
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The Motronic controlled normally aspirated cars use the traditional knock sensor:




which is really just a piezoelectric microphone mounted to the engine block near the cylinder walls. Part no. 4300729.
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Old 9th July 2008
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PHP Code:
Back of the engine between the head and intake
Courtesy of eEuroParts

I suspect the knock sensor is hidden behind the oil dipstick tube..
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Old 9th July 2008
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Do T5's have these as well, or is it part of the DIC?
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Old 9th July 2008
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Quote:
Originally Posted by demonking
Do T5's have these as well, or is it part of the DIC?
It's part of the DIC
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Old 9th July 2008
mocne mocne is offline
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[QUOTE=Frank_Drebin][b]Ok, I know this isn't for our cars but I remembered it from a Nissan forum a while back. The information is still sound.


I had the same type Nissan, so I read tons of info about this. Many people including myself found that the knock sensor, when bad would retard timing even if it wasnt needed. I never replaced mine, but people either replaced it which was $140 or they bypassed the knock sensor. In my car the car ran great when cold since it was using the map, but after warming up and using data from the sensors it became horribly sluggish.

It retarding the timing is a bit safer than lets say not doing anything at all and this can be the case also.
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Old 9th July 2008
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Quote:
Originally Posted by hkayssi
It's part of the DIC
Thought so .
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Old 9th July 2008
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[QUOTE=mocne]
Quote:
Originally Posted by Frank_Drebin
[b]Ok, I know this isn't for our cars but I remembered it from a Nissan forum a while back. The information is still sound.


I had the same type Nissan, so I read tons of info about this. Many people including myself found that the knock sensor, when bad would retard timing even if it wasnt needed. I never replaced mine, but people either replaced it which was $140 or they bypassed the knock sensor. In my car the car ran great when cold since it was using the map, but after warming up and using data from the sensors it became horribly sluggish.

It retarding the timing is a bit safer than lets say not doing anything at all and this can be the case also.
How can I adjust the timimg on the car (1997 900 N/A)??? I thought it wasn't adjustable on our cars
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Old 9th July 2008
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Thats the point. The knock sensor tells the ECU to retard the timing when it senses a knock. If it goes bad it may permanently retard the timing causing the car to be sluggish.
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Old 9th July 2008
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Oh, but it is , thru the devices(temp sensor,throttle body, anti-knock, CPS, maybe some others as well) to the cars ECU...

It would be nice to have the book on all of this ....
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Old 9th July 2008
hkayssi hkayssi is offline
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Ok I'm not sure what the problem with the car is then

I bought a non running 1997 NG900 N/A with 104K miles on it. The car came with absolutely no history. All I know is that it has the short belt mod and the engine was replaced a year ago. I did all the diagnosis and isolated the problem to the fuel pump. I replaced the fuel pump insert with a new walbro unit. The car fired right up and ran but shook and it still does. In the pump unit there was a hose that connected to nothing on 1 end in my pump unit
http://www.saabcentral.com/forums/sh...=119759&page=2

read post 21 and following.


Symptoms that I have are:

The car starts with a hunting idle between 1800 RPM and goes down to 1000 then up again to 1800 for about 10 times then stablezizes at around 1200-1400 and feels shaky. The CEL is on and it indicates a misfire on cyl 4 but there is compression, spark and fuel. Engine is shaking like if it was running on 3 cylinders. All 4 plugs look the same no more carbon, wet, oily or anything.

Spark plugs were replaced, vacuum hoses too. IAC was cleaned twice then swapped with the one from mt other perfect running car but no change.

I unplugged the hose to the evap valve and plugged the TB end leaving the EVAP venting to atmosphere; no change.

Did the water torture treatment and nothing changed.

I'm getting spark plugs wires to see if that improves things but I'm really doubtful as I have good spark.

Anybody has thoughts on this???
Intake temp sensor? CPS? Leaky intake gasket? cracked intake??? I'm really getting desperate here TPS, knock sensor...WHAT?

At least it's my project car not the daily driver

Last edited by hkayssi; 9th July 2008 at 09:23 PM.
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Old 9th July 2008
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Quote:
Originally Posted by hkayssi

At least it's my project car not the daily driver
Rub it in my face why don t'cha!

Lets see, it it FEELS like it's misfireing, yet you have fuel, spark, and compression. Well, you could have a weak spark, but that would affect a single spark plug (which are on their way to Canada right now). You don't have a DI so it isn't a bad coil. SO that goes to fuel. If it is misfireing, yet you have fuel and spark. I'm thinking dirty injector. The injector may not be totally clogged, but could not be giving enough fuel to fire. Check that out! If you are skeptical, thow a can of Seafoam in the fuel tank and see if she gets any better.
Good luck man!

Tom =]
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Old 9th July 2008
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  • Get an old-school timing light and verify spark on each wire
  • Do a compression/leak down test on each cylinder
  • Check the distributor cap for cracks
  • Rebuild/replace distributor
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Old 9th July 2008
hkayssi hkayssi is offline
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Quote:
Originally Posted by Frank_Drebin
  • Get an old-school timing light and verify spark on each wire
  • Do a compression/leak down test on each cylinder
  • Check the distributor cap for cracks
  • Rebuild/replace distributor
Thanks Frank
I have a spare distributor, I'll try it on tomorrow. It's not the cap or rotor I replaced them with good spares and no change.

I don't have a timing light but will check that at a garage somewhere close by. But would the timing be adjustable?

I'll check the compression again in all cylinders and compare I checked just the one with misfire.

It may be my coil actually. maybe giving some weak spark and then goes back to OK??? I thought they failed outright.

BTW thanks Tom for the wire set I hope to test it right after I get it.

A bad fuel injector is possible too
An air leak at the TB's lower gasket is possible too.
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Old 9th July 2008
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I don't think timing would be adjustable. The timing light has a clamp that goes on a spark plug wire and flashes each time current flows down the wire. If one is misfiring or dead you can see it. Remember the old Sun machines?
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Old 10th July 2008
mocne mocne is offline
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In that same nissan maxima with the bad knock sensor I had one fuel injector in the front bank go out and I simply replaced it, then one went bad on the rear bank and I had similar issues, meaning the car felt it was running on 5 cylinders....sometimes it felt better or worse and I figured it was clogged. Replacing it would be a pain, since I would have to drain coolant, take off the intake manifold and change the gasket, so I said hey let me try techron fuel injector cleaner. I drove it hard for a few days and the faulty injector started working again.

Obviously when one cylinder on a 6 cylinder is not running there's an imbalance that cause the rpms to go up and down because the ecu had a hard time controlling the engine.

Hope you get her running.
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Old 10th July 2008
hkayssi hkayssi is offline
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Quote:
Originally Posted by mocne
In that same nissan maxima with the bad knock sensor I had one fuel injector in the front bank go out and I simply replaced it, then one went bad on the rear bank and I had similar issues, meaning the car felt it was running on 5 cylinders....sometimes it felt better or worse and I figured it was clogged. Replacing it would be a pain, since I would have to drain coolant, take off the intake manifold and change the gasket, so I said hey let me try techron fuel injector cleaner. I drove it hard for a few days and the faulty injector started working again.

Obviously when one cylinder on a 6 cylinder is not running there's an imbalance that cause the rpms to go up and down because the ecu had a hard time controlling the engine.

Hope you get her running.
I just read your post mocne and I think you're dead on

Here's what I did

Swapped the distributor with another; no change so it's not it.

Measured the compression in all cylinders and the compression is equal in all cylinders at 180.

Taking off the #4 cyl wire does absolutely nothing (the #4 spark plug wire is bad but I fixed the ending on it until I get Tom's set any day now). There's plenty of spark touching the spark plug so it's not the wire nor is it the coil.

I suspected the spark plug. Swapped the #1 and #4 plugs and it stayed the same. Removing the cyl #4 spark plug wire won't do a thing and removing that of #1 would make the car shake violently.

So my guess is a restricted flow on the #4 injector???? Could it be something else?

I mewan there's plenty of spark, compression air is the same for all 4 cylinders, right? unless there's a problem in the intake or a leak in it.

What confuses me is that I hear faint backfires in the exhaust when the car is hot so I have no idea what that means. It seems to contradict the theorie of a clogged injector

I'll check Radare's link on fuel injector rebuild (posting #89)
http://www.saabcentral.com/forums/sh...9&page=5&pp=20
but will get Tom's old injectors if he wants to sell them. Tom, you got a PM

Any input is greatly apreciated

Last edited by hkayssi; 10th July 2008 at 03:29 PM.
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Old 10th July 2008
mocne mocne is offline
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Quote:
Originally Posted by hkayssi
I just read your post mocne and I think you're dead on

Here's what I did

Swapped the distributor with another; no change so it's not it.

Measured the compression in all cylinders and the compression is equal in all cylinders at 180.

Taking off the #4 cyl wire does absolutely nothing (the #4 spark plug wire is bad but I fixed the ending on it until I get Tom's set any day now). There's plenty of spark touching the spark plug so it's not the wire nor is it the coil.

I suspected the spark plug. Swapped the #1 and #4 plugs and it stayed the same. Removing the cyl #4 spark plug wire won't do a thing and removing that of #1 would make the car shake violently.

So my guess is a restricted flow on the #4 injector???? Could it be something else?

I mewan there's plenty of spark, compression air is the same for all 4 cylinders, right? unless there's a problem in the intake or a leak in it.
Actually, you are seeing the right results. If cylinder#4 is bad, then removing the spark wouldn't significantly change anything, since this is the bad cylinder. Where as if you remove spark from a good cylinder will show even worse operation of the motor. I had the same symptoms, check the resistance on the faulty injector. If its totally bad it would show infinity or open circuit (at least on my nissan). You can replace the injector with a known good one off your daily driver. This will atleast tell you if its the injector. Trying techron is a $10 possible fix. Swapping with a good injector will test this whole clogged/faulty injector theory.

Quote:
Originally Posted by hkayssi
What confuses me is that I hear faint backfires in the exhaust when the car is hot so I have no idea what that means. It seems to contradict the theorie of a clogged injector

Any input is greatly apreciated
I think the backfire might just be the sound of cylinder 1-3 working and then theres the gap in sound when 4 doesnt fire and then cylinders 1-3 again. You would here puffing noises maybe even the backfire.

I'm not an expert by any means, but swapping injectors would tell you if an injector is your problem.

Maybe Radare, can chime in, hes rebuilt his injectors. He might know something about it.
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