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  #1  
Old 09-28-2014, 02:14 PM
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knocking engine detection

Hi , I own a 1999 jeep grand Cherokee 4.0 6 cyl.
not while ago a knocking sound began in the engine along with strong fuel smell, hard to start engine and check engine code (bank 1) lean something(I cleared the code already)
I use a stethoscope all around the engine and the only place with the knocking sound came from the bottom of the throttle body manifold.
please any inputs will help me a lot.
Robert.

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  #2  
Old 09-28-2014, 07:08 PM
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Re: knocking engine detection

This could be a faulty O2 sensor, but also a vacuum leak at the intake manifold or any loose hose or duct.
Do you have a scanner which can show live data? (can be an android phone with a bluetooth adapter and Torque app).
You will need to check if the O2 sensor voltage will be going from 0.2V to 0.8V to 0.2V, and so on. A faulty O2 sensor can have a steady voltage output (in your case you will have to look at the 1-1 sensor)
I had this happening on my 4.7, the O2 sensors where faulty and giving a constant lean signal to the PCM, causing the PCM to try to compensate for it with a max positive fuel trim of +30.
It is always a good idea to write down any fault code you see, you maybe will need it later.
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Old 09-28-2014, 08:26 PM
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Re: knocking engine detection

O 2 sensor replaced month ago due to faulty voltage. no leaks found too.
what else can be?
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Old 09-28-2014, 08:37 PM
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Re: knocking engine detection

Knowing the exact fault code(s) will help a lot. If there is a too lean fault code, but you can smell raw fuel, then for some reason the O2 sensor is not reading the real situation.
Why did you change the O2 sensor before? What was the faulty voltage?
You do not have a scanner or can not get one for loan?
Did you check for vacuum and exhaust leaks?
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Old 09-28-2014, 09:25 PM
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Re: knocking engine detection

I own a scanner , what is the first step that you need me to do ?
the old o2 sensor (1) got stuck on the voltage. When I replaced it the truck stopped shaking which was the first problem that I had. The code it gave me was too lean. I hope for your instructions thank you.
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Old 09-29-2014, 10:55 AM
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Re: knocking engine detection

You will need to hook up the scanner and run the engine. It should be in closed loop, which means it will use the O2 sensor output.
Check the voltage of the 1-1 sensor ( and depending on the configuration also 2-1 sensor). Their voltage should change from 0.2-0.8V rapidly. The faster this change goes, the better. Also check the 1-2 sensor (and 2-2 sensor), they should have a more or less steady voltage output.
Then check the fuel trim figures: STFT (short term fuel trim) and LTFT (long term fuel trim) and let me know what you see.
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Old 09-29-2014, 07:11 PM
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Re: knocking engine detection

Hi Frank, O2 sensor 1/1 fluctuation from 0.2 to 0.8 is correct and rapidly.
sensor 1/2 steady 0.240. (*small fluctuation)
STFT fluctuation of -2.0 -3.5 -4.0 -5.5 -6.2 -7.1 etc mostly minus.
LTFT 21.9 ALWAYS. I am uploading a video on youtube
with scanner hook in live data . as soon is ready upload you can see it.
I give you the link now anyway please check if you can see it.
thank you and waiting instructions.

YouTube
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Old 09-29-2014, 07:24 PM
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Re: knocking engine detection

On the job now and i can not access youtube from here.
The O2 sensor voltages look ok.
The STFT looks normal, but the LTFT is much too high ( single digit figures are considered normal, but as soon as it goes to double digits, its not). And that is strange, because the LTFT is just an average of the STFT over a certain period.
The STFT shows a reduction of fuel injection pulse compared to the basic preprogrammed schedule. But the LTFT is positive double digits, which means that a longer fuel injector pulse is used compared to the basic schedule. (so extra fuel injected).
These measurements where done at idle with engine at operating temperature?
Did you check during driving as well?
I will have to look further into this to come up with some sensible answer on what can be wrong.
Can you have a look as well what the MAP sensor pressure is?
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Old 09-29-2014, 07:35 PM
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Re: knocking engine detection

measurements were taken at idle between first 10 minutes from I started the vehicle. I will check now at driving temp. and let you know. oh! and MAP sensor too.
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Old 09-29-2014, 07:41 PM
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Re: knocking engine detection

O, and check what the LTFT and STFT and MAP do when you raise the rpm to 2500. The result from this could point towards a certain problem.
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  #11  
Old 09-29-2014, 09:06 PM
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Re: knocking engine detection

First thing I want to tell you O2 sensor number 1 is not fluctuating anymore on a 20 minute drive at normal operating temperature. This is the 3rd O2 sensor that I have changed in less than 1 year. LTFTRM is 23.4 at 2500rpm. STFTRM is 20.0 at 2500rpm. MAP is 26.9 at 2500rpm.
The O2 sensor number 1 got stuck at 0.080 and 0.060.

LTFT while I was driving not on 2500rpm was 0.0,7.0,14.5,17.0,20.0
STFT while I was driving not on 2500rpm was 0.0,4.3,18.0,26.2,32.1.
MAP ("HG) 17.1,20.0,14.0,16.0

I was also wondering what the knocking sound had to do with any of this. Thank you I'm waiting for you response with much anxiety.
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Old 09-29-2014, 10:48 PM
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Re: knocking engine detection

I was in the bus heading home, while i made a story, but then my phone screen went black and i lost it all.
Lest start all over again.
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