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Unread 02-11-2013, 12:24 AM   #1
Stingray427
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4.0 Detonating/Knocking While Accelerating...Help!

My Jeep runs great thankfully but I have noticed that I am getting a slight detonation/knocking sound when accelerating. I know these things are computer controlled so I guess you can adjust the timing but it sounds like it may be doing some damage so I am wondering what to do.

Everyone says premium fuel is a waste of money in these things but I am wondering if the lower octane fuel is part or all of the problem, any thoughts?

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Unread 02-11-2013, 12:38 AM   #2
Michaelgoesrawr
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This post has been deleted by Michaelgoesrawr. Reason: Wrong info. Apologies.
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Unread 02-11-2013, 01:40 AM   #3
Stingray427
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Quote:
Originally Posted by Michaelgoesrawr View Post
From my little experience I'd say you need to check your timing.
Can you adjust the timing on a 1993 4.0 YJ? I was under the impression that the computer controlled everything.
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Unread 02-11-2013, 04:35 AM   #4
95yj0645
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A 93 has a distributor. You can adjust timing. And as far as premium fuel goes I wouldnt say its a waste, I will have to run it after my 4.0 stroker is built due to high compression. I can promise you I would know if I was running 87 or 93 after that haha. Try setting ignition timing and post your results. I can get instructions for you off of mitchell or my haynes manual if you need it.
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Unread 02-11-2013, 05:29 AM   #5
TSEJEEPERS
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Quote:
Originally Posted by Stingray427 View Post
Can you adjust the timing on a 1993 4.0 YJ? I was under the impression that the computer controlled everything.
No you can not. The distributer has a tang under the hold down clamp that is cast into the distributer. It keeps it in one spot.
I suppose if you cut off that tang then could, but I would not recomend it.
I would start with giving it a tune up.
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Unread 02-11-2013, 06:10 AM   #6
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Quote:
Originally Posted by TSEJEEPERS View Post
No you can not. The distributer has a tang under the hold down clamp that is cast into the distributer. It keeps it in one spot.
I suppose if you cut off that tang then could, but I would not recomend it.
I would start with giving it a tune up.
x2

And keep standard 87 octane gas. The higher octane is for higher compression ratios and if you use it you will probably have some ping.

Does the "knock" go away after a certain rpm? My 4.0L sounds like a diesel at idle but goes away when the rpm's increase. I've heard it's due to piston slap but I'm not sure.
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Unread 02-11-2013, 07:55 AM   #7
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You are either running lean or too much advance. You need to check your fault codes. The biggest input for advance is your map sensor. A higher octane gas will fix preignition but it shouldn't be necessary.
http://www.jeepforum.com/forum/f12/h...codes-1257145/
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Unread 02-11-2013, 11:41 AM   #8
jsawduste
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Without going to a big long explanation.

Timing is controlled by the notches in the flywheel and the crank sensor/crank sensor location. In open loop the ECM reads algoritms cal`ed into the ECM. Once operating temp is achieved it reverts to closed loop and the ECM reads primarily off the O2 sensor with additional inputs from the MAP, Coolant, TPS etc. These inputs are what the ECM uses to adjust timing and fuel mapping. At 75/80% throttle the ECM goes back into open loop and reads primarly off the output voltage of the MAP sensor. It adjusts the injector duty cycle and timing based again on ca-led in algorithms within the ECM.

The dizzy does essentially nothing and should not be a consideration in your case.

More likely is the fuel blend. Refineries continually adjust the fuel blends to change the volatility based on customer needs. Hot vs. cold at it`s simplest. % of additives % of ethanol etc.

I can watch my A/F ratio gauge and see immediate changes when filling up. After a few miles I can see the ECM readust it`s mapping and the A/F ratios come back to normal. Most cases a fill up that has ethanol will cause the engine to run lean.

Higher octane fuel are not a waste. However feeding an engine higher octane then it needs is. In your case I`d try a different brand of fuel and if that doesn't work then a bump in octane.

Overly simplified answer with plenty of holes for you guys to shoot through. But the basics are there and they are correct.
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Unread 02-11-2013, 12:53 PM   #9
EABiker
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You may also have some carbon build-up in the combustion chamber which will bump up your compression a bit and cause hot spots which lead to pre-combustion, (pinging). Give it a Seafoam treatment down the throttle body to clean things up and rule that out as a cause.
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Unread 02-11-2013, 01:15 PM   #10
Stingray427
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I have recently done a tuneup as far as new plugs and wires, air filter, etc.

I also did Seafoam twice through the vacuum line and one through the tank. I also did one in the oil and changed the oil shortly thereafter.

Didn't change the distributor cap so don't know if that is part of the problem.

When i changed the plugs the old ones didnt look too bad.

As for premium fuel or not, I have a quick question. As I have posted in other threads, for some unknown reason my Jeep is real fast. I don't know if it is a stroker or not but this thing really moves and hitting close to 100 MPH is not out of the question. I recently drove from So Cal San Diego to Nor Cal and driving up through the mountains I was hitting 85+ through the mountains so I dont know whats up with my Jeep but maybe whatever is under the hood needs the higher octane fuel? Thoughts?
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Unread 02-11-2013, 01:24 PM   #11
EABiker
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Since you are not sure of what's in the engine as far as compression goes, I would go ahead with the higher octane; it certainly would not be a waste to use in an engine that requires it!
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Unread 02-11-2013, 01:44 PM   #12
Stingray427
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Quote:
Originally Posted by EABiker View Post
Since you are not sure of what's in the engine as far as compression goes, I would go ahead with the higher octane; it certainly would not be a waste to use in an engine that requires it!
Yeah I was just hoping there was a less expensive solution to this pinging on acceleration... cause have you seen the price of premium in California...Yikes!!!
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Unread 02-11-2013, 02:28 PM   #13
jsawduste
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I run a 5.0 liter stroker. It`s fed premium but it has the timing bumped up 5/6 degrees.

You might want to try moving the IAT to the airbox instead of the intake manifold. Fools the ECM and adds a little more fuel.
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Unread 02-11-2013, 03:02 PM   #14
Stingray427
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Quote:
Originally Posted by jsawduste View Post
I run a 5.0 liter stroker. It`s fed premium but it has the timing bumped up 5/6 degrees.

You might want to try moving the IAT to the airbox instead of the intake manifold. Fools the ECM and adds a little more fuel.
How do I move the IAT?

Also what the heck is an IAT?
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Unread 02-11-2013, 04:12 PM   #15
jsawduste
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Intake Air Temp sensor. Single wire sensor by the throttle body on top of the intake manifold.

Moving it from the hot intake manifold to the cooler airbox tricks the ECM into thinking the incoming intake charge is cooler. The ECM algorithm reads the sensor signal then pumps more fuel by increasing the injector duty cycle........Time the injector is open.........

I`ve seen folks simply drill a hole in the airbox and epoxy the sensor in. I turned an aluminum bung on a lathe then welded it to my cold air intake tube. Guessing your set up is a little different though.

http://www.angelfire.com/my/fan/IAT_sensor.html

http://markism.net/jeep_IAT_relocate.html
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