Has anyone experienced spark knock / pre-detonation?

Rubicon John

Rubicon John

TJ Enthusiast
Joined
Mar 12, 2020
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Location
Atlanta
Hey guys, so when I bought my Jeep it would “ping” under load up hills at lower RPMs. I did a carbon treatment and it basically stopped doing it completely. 187k miles on the odometer, I wasn’t surprised.

Recently I’ve noticed that it does it again. It’s not awful but it’s definitely there. I went ahead and did another chemical treatment but it didn’t do much. Now I’ve got a couple of suspicions but I wanted to post and see if anybody else has experienced this.

I have noticed that sometimes the Jeep has a long crank since this pinging has started back up. It has a newer Delphi fuel pump in it and priming it doesn’t necessarily have an effect. The long crank issue really seems to be hit or miss. Today I ran into the house with the engine warm and came back out 10 minutes later. Long crank. Doubt the fuel pressure dropped that quickly.

Which leads me to think that perhaps my crankshaft position sensor is failing. What I’m wondering is if anyone has experienced spark knock or pinging due to a bad CKP. Seems if the timing is pulled back a bit this could lead to it.

I have an auto trans with 33s and 4.10s so I am also dealing with the engine working harder than it needs to especially at lower rpms. Regearing is in the near future.

Interested in your guys thoughts.
 
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When trying to climb a hill or grade with the engine at too low of an rpm that can cause the engine to lug which can cause problems like that. Engines don't like low rpms when asked to do extra work like climbing a hill or driving up a long grade, help it by turning the Overdrive off. Your 4.10 gearing with 33's and your 42RLE transmission is adding to the problem, 4.88 or 5.13 gears would help a lot of things very quickly.

Carbon build up in an engine is often caused by driving an engine too conservatively, as in never letting it rev to higher rpms so it can get rid of carbon buildup.

No need for a paid carbon treatment, you can get rid of carbon for free by one of two methods. 1) Doing an "Italian tuneup" (Google it, it's real) by taking it out onto the highway/freeway in a safe area and repeatedly accelerating it HARD to high rpms. This might take several days depending on how badly it is carboned up. Driving it aggressively once in a while can prevent carbon buildup. 2) Remove the air intake tube from the top of the throttle body, then start the engine. Then while holding the engine rpms up with the throttle lever, SLOWLY (!!!) start trickling 12-16 oz. of water into the throttle body. Slowly enough that it will take a 1-2 minutes to drain the container. Maintain total control over the water container so you never let it do more than slowly trickle out, accidentally dumping too much water in at once can cause hydrolock. This causes mini-shockwaves inside the combustion chambers which breaks up the carbon so it can be blown out the exhaust. YES THIS IS SAFE WHEN DONE AS DESCRIBED. Even Mopar's CCC (combustion chamber cleaner) is mostly water, yes that is true
 
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Jerry Bransford said:
When trying to climb a hill or grade with the engine at too low of an rpm that can cause the engine to lug which can cause problems like that. Engines don't like low rpms when asked to do extra work like climbing a hill or driving up a long grade, help it by turning the Overdrive off. Your 4.10 gearing with 33's and your 42RLE transmission is adding to the problem, 4.88 or 5.13 gears would help a lot of things very quickly.

Carbon build up in an engine is often caused by driving an engine too conservatively, as in never letting it rev to higher rpms so it can get rid of carbon buildup.

No need for a paid carbon treatment, you can get rid of carbon for free by one of two methods. 1) Doing an "Italian tuneup" (Google it, it's real) by taking it out onto the highway/freeway in a safe area and repeatedly accelerating it HARD to high rpms. This might take several days depending on how badly it is carboned up. Driving it aggressively once in a while can prevent carbon buildup. 2) Remove the air intake tube from the top of the throttle body, then start the engine. Then while holding the engine rpms up with the throttle lever, SLOWLY (!!!) start trickling 12-16 oz. of water into the throttle body. Slowly enough that it will take a 1-2 minutes to drain the container. Maintain total control over the water container so you never let it do more than slowly trickle out, accidentally dumping too much water in at once can cause hydrolock. This causes mini-shockwaves inside the combustion chambers which breaks up the carbon so it can be blown out the exhaust. YES THIS IS SAFE WHEN DONE AS DESCRIBED. Even Mopar's CCC (combustion chamber cleaner) is mostly water, yes that is true
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I’m well aware of the issues with my gearing as well as the carbon issues, I work on Rovers for a living 😁. I keep the overdrive off all the time.

I am really just curious why it came back after seemingly going away. I did also get newer tires that are about an inch taller than the previous set since this has come back.
 
Skylinessuck said:
A bad crank position sensor advancing the timing could certainly cause it although I've never heard of that in our application.
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Me neither. I’m beginning to think that the engine is simply lugging hard like Jerry suggested. It seems to be fine when I get the rpms higher and get some more horsepower.
 
Try repeating whatever your preferred carbon removal procedure is again and again. I personally drive like an ass hole everywhere I go do no issues 😁 What plugs are in it btw? Does running 93 octane help? Oil contamination of the combustion chamber can cause pre-ignition as well (worn valve seals/rings/etc.)
 
Skylinessuck said:
Try repeating whatever your preferred carbon removal procedure is again and again. I personally drive like an ass hole everywhere I go do no issues 😁 What plugs are in it btw? Does running 93 octane help? Oil contamination of the combustion chamber can cause pre-ignition as well (worn valve seals/rings/etc.)
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I have the Autolite Iridiums, the good ones recommended here. Pretty new also. Running 93 helped the first time around but I haven’t tried it this time. Oil contamination is always a possibility but I’m surprised the issue went away and came back.
 
Mine did the same thing. I tried cleaning it with water,then seafoam and the drive the hell Out of it technique Nothing really helped. I replaced the MAP,ATS sensors and the fuel pump all at the same time and it fixed it permanently. I wish I would have done one at a time so I would know exactly what it was. My guess is that it was the fuel pump. It must have been getting worse a little at a time. Now it runs a lot better all around.It never had a CEL either.
 
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chad said:
Mine did the same thing. I tried cleaning it with water,then seafoam and the drive the hell Out of it technique Nothing really helped. I replaced the MAP,ATS sensors and the fuel pump all at the same time and it fixed it permanently. I wish I would have done one at a time so I would know exactly what it was. My guess is that it was the fuel pump. It must have been getting worse a little at a time. Now it runs a lot better all around.It never had a CEL either.
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I need to find a fuel pressure gauge to hook up that I can monitor while I drive.
 
Skylinessuck said:
What did the old plugs look like? Or pull these and check. Easy way to see if it's running lean.
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Haven’t pulled them. I’ll do the diagnosis necessary, just looking for opinions. And I’m finding that most opinions are things I’ve thought of. Looks like it’s time to dig a little deeper.
 
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Running an octane higher than 87 can stop the pinging but it's better to fix the problem causing the pinging. Carbon buildup or forcing the engine to work hard at too low of an rpm.
 
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I was just thinking of 93 as a trouble shooting method, not a long term fix. Even race gas can knock with oil getting in to the cylinders. If the timing is off a couple of degrees though, it could help for example.

Which also makes a loose timing chain pop into my head as a possible cause. Or would that cause retarded timing? Have to think through that more.
 
Skylinessuck said:
I was just thinking of 93 as a trouble shooting method, not a long term fix. Even race gas can knock with oil getting in to the cylinders. If the timing is off a couple of degrees though, it could help for example.

Which also makes a loose timing chain pop into my head as a possible cause. Or would that cause retarded timing? Have to think through that more.
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I do remember using some 93 the first go around and the pinging went away.

It may consume a bit of oil. I don’t drive it much so I’m not sure how bad it is but I feel like I’ve topped it off before. I suppose I could try a high mileage oil and see if it softens up the seals enough to diminish consumption, if it’s not past the rings.
 
Jerry Bransford said:
Running an octane higher than 87 can stop the pinging but it's better to fix the problem causing the pinging. Carbon buildup or forcing the engine to work hard at too low of an rpm.
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With a compression ratio of 8.8, a 4.0 should not be pinging with 87 octane. You have something else going on.
could be due to lugging the engine at lower rpms. Your tire size/ gear combo combined with the 42re trans.
Your profile shows your a technician, do you have access to a injector cleaner? you could try cleaning the injectors.
Running 92 octane is going to slow your combustion and prevent preignition, but it also has a negative effect on what little performance a 4.0 puts out.
My LJ will also detonate from time to time, but its usually after my 40 minute at 75-80 mph commute home.
 
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Higher octane does not degrade performance in a vehicle optimized only for 87 octane. It's an interwebz myth originating from an incomplete understanding of how octane and combustion chamber burn works.

OP sounds like not a newb and has already gone through most of what we are throwing at him. He's more bouncing ideas of everybody seeing if we come up with something he hasn't. I think he's on the right track with more mechanical digging and checks.
 
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If you are running an ethanol blend it can ping. Ethanol runs cooler. You have a situation allowing the ping to happen. A short term fix could be switching to a straight gasoline. There is often a ping problem up north as they switch over to winter fuel mix I'm not sure why, but it's been around since my Pops was young (he says).
Jerry's 2 solutions are good advice for every vehicle. Especially ones that don't typically get run hard enough. I like using a spray mister bottle to add the H2O through the throttle body. I tend to either pour too fast, or my sciatic kicks in too soon. If you get yourself a OBD2 reader that can stream live data you can get a better idea about which sensor is starting to fail.
New plugs may help if the current ones are carbonated up. That could be timing or coil energy, but some good sustained higher rpms under load could clean up the plugs as per Jerry.
 
williambmac said:
With a compression ratio of 8.8, a 4.0 should not be pinging with 87 octane. You have something else going on.
could be due to lugging the engine at lower rpms. Your tire size/ gear combo combined with the 42re trans.
Your profile shows your a technician, do you have access to a injector cleaner? you could try cleaning the injectors.
Running 92 octane is going to slow your combustion and prevent preignition, but it also has a negative effect on what little performance a 4.0 puts out.
My LJ will also detonate from time to time, but its usually after my 40 minute at 75-80 mph commute home.
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I don’t think we have an injector cleaning tool with the right adapter for the Jeep unfortunately. I have put a can of cleaner in the tank.

I am really wondering if it’s simply the lugging causing it like you mentioned. It definitely only occurs under 2000 rpm, the thing is that it’s super sensitive. The slightest little grade and the slightest bit of throttle in that scenario and I can get it to act up.
 
Skylinessuck said:
Higher octane does not degrade performance in a vehicle optimized only for 87 octane. It's an interwebz myth originating from an incomplete understanding of how octane and combustion chamber burn works.

OP sounds like not a newb and has already gone through most of what we are throwing at him. He's more bouncing ideas of everybody seeing if we come up with something he hasn't. I think he's on the right track with more mechanical digging and checks.
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Spot on, hoping someone chimes in and says “mine did that until I regeared it!” 🤣
 
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Rubicon John said:
Spot on, hoping someone chimes in and says “mine did that until I regeared it!” 🤣
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I have low-rev pinging and my plan is to re-gear. I've found that switching octanes doesn't do much if anything to improve that situation so its 87 for me. I don't have any other engine issues/symptoms.
 
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