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Old 06-23-2014, 07:02 PM
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spark knock

What causes spark knock?

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Old 06-23-2014, 07:40 PM   #2
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By spark knock do you mean detonation? Detonation is when the fuel pre-ignites before the piston reaches the intended ignition event.

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Old 06-23-2014, 07:53 PM
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It happens when I'm heading into a strong frontal wind or when I step on it. Not during normal easy driving.
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Old 06-23-2014, 08:17 PM   #4
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That's under load. So what's the maintenance history insofar as engine related items and the mileage?
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Old 06-23-2014, 08:21 PM
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I am original owner. Has 97 k miles. Its time for new plugs etc. Change coolant.
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Old 06-23-2014, 08:32 PM   #6
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Depending on the year and the motor there are different tune-up items but I'd start with spark first. If your plugs are really gone, I could see the hesitancy and lack of ignition causing issues similar to pre-ignition. Get plugs, gap them properly, replace the air filter, check (or replace wires if you have them), distributor cap and rotor (if you have them), and clean the throttle body and IAC sensor with throttle body cleaner.

I'd do that and then see what's what.
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Old 06-23-2014, 08:41 PM
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Sounds like a plan
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Old 06-23-2014, 09:03 PM   #8
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Knock with no pinging? Do you tend to drive very conservatively and baby the engine?
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Old 06-23-2014, 09:25 PM
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Yes. I don't drive it hard. Only when I take it off road.
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Old 06-23-2014, 09:33 PM   #10
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Hot carbon build up can be an issue

Otherwise typically an compression vs octane vs timing issue none of which you can do much with on your jeep as timing not adjustable, compression already pretty low and standard octane is ok for your engine
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Old 06-23-2014, 11:40 PM   #11
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Carbon buildup is common in engines that are driven too conservatively. Carbon builds up inside the combustion chamber which reduces the available volume which increases compression pressures. Increased combustion chamber pressure can prematurely ignite the air-fuel mixture. That causes pinging and in more serious cases, spark knock.

There are multiple possible causes of spark knock like discussed at What causes spark knock and how do you get rid of it but carbon buildup from ultra-conservative driving would be my guess.

There are multiple methods used to get rid of carbon buildup but let's try the free and easiest method first called laughingly an "Italian tune-up". When safe (and no police around) spend a few days driving extra aggressively... wide open throttle accelerating to near redline RPMs repeated as often as you can over a couple days. This can often be very effective at blowing the carbon out.

Corvette mechanics have to do this to higher performance models owned by drivers afraid to drive them aggressively enough to prevent carbon buildup. They often do this without driving it, just repeatedly revving the engine hard to near redline in the dealer's service lot.

Try the Italian Tuneup over the next couple days and if it doesn't help there are other methods. The next one is free too which involves nothing more than plain tap water. I'll cover that a little later.
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Old 06-23-2014, 11:49 PM
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Old 06-24-2014, 12:34 AM   #13
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Quote:
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k
k?
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Old 06-24-2014, 07:27 AM   #14
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Additionally the carbon buildup can act as an ignition source causing both knock and on a carbureted engine the hot carbon can keep the engine running briefly after key is turned off
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Old 06-24-2014, 10:51 AM   #15
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Detonation, also known as pinging, pinking, or knocking, is a serious motor destroying event. It sounds different on many cars but it can sound like slapping metal wires against the motor, a rattle in the dash, shaking popcorn kernels in a coffee can, and when it’s real bad it sounds like marbles in a coffee can. It has a rhythmic sound much like a diesel (technically its very similar to a diesel). When you hear detonation GET OFF THE GAS IMMEDIATELY! Detonation is not the same as pre-ignition although they often occur at the same time. Detonation can be caused by too much spark advance, high IATs, lean mixture, dry air, high ECTs, low octane, etc. It occurs when the spark ignited flame front compresses the reaming air fuel mixture till it reaches critical temperature and pressure to auto ignite the mixture on the other side of the cylinder. The two flame fronts then collide with a giant explosion causing the audible knock sound. The cylinder temperatures and pressures increase drastically beyond the normal combustion temperatures and pressures causing burned holes in the pistons, bent or broken rods, damaged crankshaft bearings, and complete and total destruction.



Pre-ignition occurs when the mixture ignites on some hot spot (spark electrode, spark plug threads, a sharp point, carbon build up, etc) before, or at the same time the spark fires. The flame fronts do not always collide, the destruction comes from extended burn time in the cylinder. Pre-ignition does not produce the classic knock or ping, more of a dull thud, but it is often caused by detonation and therefore associated with the pinging sound. Pre-ignition is more dangerous than detonation because it occurs earlier in the engine cycle causing more temperature build up due to the extended burn times. It can also cause “runaway advance” where the hot spot gets continually hotter causing the explosion to occur earlier and earlier in the engine cycle. It can also be caused by detonation events heating up a hot spot to the point where the detonation then turns into pre-ignition to cause some serious destruction. This is why its very important to get off the gas when you hear or feel anything “funny” at WOT.



Detonation is affected by many things including ECT, IAT, octane, timing, compression ratio (or boost), RPMs, and AF. Increased coolant temps or air intake temps will increase the chances of detonation. Too much advance can also cause detonation resulting from increased burn times. Too much compression will cause detonation that can be eliminated with increased octane or in the case of forced induction by lowering the boost and or intake air temps. Octane is the easiest factor to control detonation. Octane is the fuels resistance to auto ignition (detonation and pre ignition). Sufficient octane will solve nearly all minor detonation problems. High octane fuel burns slower than low octane, and high octane is much more resistant to higher pressures and temperatures. Engines are much more prone to detonation while “lugging” when at high loads and low RPMs. This is mostly due to the extended burn times at low rpms and usually a motor will not detonate under the exact same conditions, but with a higher RPM. Lean AFR mixtures are also very prone to detonation and rich mixtures can sufficiently cool the cylinder to reduce or eliminate detonation. A motor might run great at 11.5:1 and detonate like crazy at 12.5:1 on the same octane. The goal to eliminating all detonation is to keep things cool and use sufficient octane for your appropriate boost and timing requirements. Water methanol injection can drastically reduce detonation by cooling the chamber as well as adding octane.
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Old 06-24-2014, 10:54 AM   #16
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If you believe detonation is occurring where do you start to fix the problem?
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Old 06-24-2014, 10:56 AM   #17
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I'm curious to see a picture of the plugs. Carbon buildup is as good a theory as any I reckon and while I have definitely never had to worry about it in any engine I've owned I guess some folks actually do putt around enough to cause it.

Cleaning it with high revs or water, or supposedly seafoam (never touched the stuff) is probably a good thing, but if the ignition parts are garbage, I'd still swap those out if it were me, in fact is do it before I ran down the road at high RPM if it were me.
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Old 06-24-2014, 11:10 AM   #18
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Based on his 'k' response to my earlier response, I got the impression the OP was no longer interested in help.
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Old 06-24-2014, 11:12 AM   #19
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Well I'm interested in help! Trying to figure out if this is what's wrong with my jeep.
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Old 06-24-2014, 11:40 AM
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I am interested. I posted that k response at midnight last night. tired I guess. Thank you guys for the great info. seems all at once things are needing attention. I guess that is what happens when a vehicle hits almost 100k miles. I have a coolant leak also to deal with. time to address some maint. issues. I am the original owner of my jeep and its been a great jeep. wanna keep it going
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Old 12-17-2014, 10:51 PM   #21
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My 97 TJ has the same problem, I had it tested today on the computer and it was not throwing any error codes, the timing was on the button and when we deliberately made it pink the readings were all still Ok. All readings were where they should be, they did however reset its memory settings ?????

I like your advise Jerry, and would like to hear you other suggestions.

PS: I do drive mine conservatively
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Old 12-17-2014, 11:01 PM   #22
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My 97 TJ has the same problem, I had it tested today on the computer and it was not throwing any error codes, the timing was on the button and when we deliberately made it pink the readings were all still Ok. All readings were where they should be, they did however reset its memory settings ?????

I like your advise Jerry, and would like to hear you other suggestions.

PS: I do drive mine conservatively
Read and take advantage of the info in post #11 above.
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Old 12-21-2014, 10:20 PM   #23
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Worked a treat, no more pinking, thanks for the advise.
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Old 12-21-2014, 10:33 PM   #24
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I'm not sure if I have the same issue, but my 1999 6cl with 180000, is coughing when in 5th gear and going up hill at 50. There doesn't seem to be any issues at HWY speeds. I don't get to the HWY very often. I was thinking about replacing the injectors?

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Old 12-21-2014, 11:10 PM   #25
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Sounds like the engine is simply lugging. Going uphill in 5th gear is not something I'd expect the engine would be happy being asked to do. Have you tried downshifting to 4th when going up hills?
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Old 12-23-2014, 10:35 AM   #26
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Thanks for the answer Jerry.

Is this unusual or is this something I need to chase down?

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Old 12-23-2014, 10:44 AM   #27
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Is the need to downshift for hills unusual? No, that's just a basic thing that you should be doing. Are you new to owning a vehicle with a manual transmission?
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Old 12-23-2014, 03:26 PM   #28
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Yup, that's me, the "FNG".

To me it seems like something more. It has been years since I've had a standard. However, I figure if your tacking at 18 to 22 rpms, I shouldn't have to downshift on basic hills. Now that being said, I am the "FNG", so invite all ideas.

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Old 12-23-2014, 03:29 PM   #29
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Taching 1800-2200 RPMs may be enough for a V8 or some small cars but it's not enough to climb grades in 5th gear which is an Overdrive ratio in a Wrangler which is heavier than the average car. Automatics will downshift out of Overdrive for grades, manual transmission drivers should too.
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Old 12-23-2014, 06:13 PM   #30
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Thanks Jerry. Even at this age I can learn something new.

Merry Christmas to all.

Michael

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