Engine Knocking in Cold Ambient Temps?

[wrangler]

Is it possible to get engine knocking in cold ambient temperatures instead of warm ones? My reasoning is that during cold temps the cylinder takes in more air during the intake cycle (air is denser when cold; same PSI but more mass). During compression when the air warms up the cylinder ends up with a higher PSI than it would have during warm ambient temps. Isn't this how intercoolers work?... If so wouldn’t that make it more plausible to get detonation?

The reason I ask is that during the colder months of the year I get a rattle from the engine during accelerations. And it gets really loud when I nail the gas. Initially I thought it was a cracked exhaust manifold but I can’t see any cracks and it ONLY does this every September to about March or April… It is driving me insane because I can’t figure out what it is. I think it gets better when I increase the octane… It’s hard to tell because it’s an intermittent thing.

 

[eljefino]

What vehicle and what throttle position?

Anything with a mass airflow sensor should very accurately meter fuel & timing to match.

Speed density systems have an intake air temp sensor which will get them pretty darn close, if working properly and if the car is unmodified.

Maybe you have something mechanical that has tolerances that change enough when it gets cold?

 

[Gary Allan]

If it's his jeep ...not mass airflow ...the air temp sensor must be in the manifold ...nothing above the throttle body ..just plastic air box ..no wires.

I've never had cold ambient temps cause knocking ...typically it is exactly the opposite. The higher the temp the more I knocked.

Now during open loop ..you are on a premapped fuel curve (no O2 sensor per se) ..but this should be worse in warmer weather I would think.

For about $25 you can have your PCM flashed at the stealership. If this is truly a ignition problem, this may fix it. I would first explore the exhaust further (both the possibility of a cracked manifold and just plain heatshield "buzzing").

 

[n8wvi]

Only until warmed up? Or all the time? If it's only for the first few minutes, I'd say piston slap.

Dave

 

[wrangler]

quote:

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Only until warmed up? Or all the time? If it's only for the first few minutes, I'd say piston slap.

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It’s more pronounced during warm up but not restricted to it...

How does a piston slap sound and how does engine knocking sound? I have never heard of either one so I'm guessing here...

Gary, have you ever checked the compression of your engine? I did and for some reason I ended up with 180PSI. That cannot be right.

 

[wrangler]

During high RPMs (>4K) the knocking is VERY loud. Its sounds like a balloon popping, which makes me think that it is not a heat shield. It’s either a broken exhaust gasket/exhaust manifold it engine knocking.

 

[Gary Allan]

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Gary, have you ever checked the compression of your engine? I did and for some reason I ended up with 180PSI. That cannot be right.

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No, I've never had cause to check it. The FSM spec's for only 3 compression strokes for a reading of 120-150. If you did 4

I'm still banking on the exhaust ..this would flush with the warming up thingie. This is a common problem for this engine ...but not so much in the newer models

 

[wrangler]

I also checked with the FSM and it states that all accelerations and WOTs are all Open Loop modes. So it doesn't even check the input from O2s to figure out the F/A mixture.

How can I tell if it is an exhaust issue? And how come it doesn't do it all the time. It’s intermittent so its really hard to narrow down.

By the way I checked with the dealer and they want $50 to re-flash the PCM. I might just take it in tomorrow so they can hook up the Chrysler computer and see if anything comes up. I'm kinda hopping its an exhaust issue and not an ignition one. I've been dying to get the Borla header anyway.

 

[Gary Allan]

quote:

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I also checked with the FSM and it states that all accelerations and WOTs are all Open Loop modes. So it doesn't even check the input from O2s to figure out the F/A mixture.

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Yes, you're in open loop. The 02 reading doesn't mod the A/F mixture ..but is NOT ignored. If it sees too many lean or rich conditions while in open loop it will increment or decrement the injector pulse width while in open loop. This is how it compensates for a weakening fuel pump ..or clogging injectors.

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How can I tell if it is an exhaust issue? And how come it doesn't do it all the time. It’s intermittent so its really hard to narrow down.

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Have someone put a rag over the tail pipe ..you listen at the manifold. You could also introduce a smoking agent (ATF or something) in the throttle body and if smoke appears in the engine bay ..you've got a leaking manifold.

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By the way I checked with the dealer and they want $50 to re-flash the PCM. I might just take it in tomorrow so they can hook up the Chrysler computer and see if anything comes up. I'm kinda hopping its an exhaust issue and not an ignition one. I've been dying to get the Borla header anyway.

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Depending on the year you can grab any codes from the odometer. It works on 2000+ TJ ..just do the key on-off-on-off-on and look at the odometer. It will show any codes (P4045 ..something like that) and then show "done". I'm uncertain if this is how the 97 works ...it may be the odo or the CEL flashing ..the 99 doesn't have this feature ..dunno about the 98.

$50 still isn't bad for the flash. Why not have them diagnose the problem? If it is an exhaust ..let me refer you to Class action. Btw-some MOPAR headers for this engine come with a lifetime warranty. Learn how to put a zipper on it and you're in for a lifetime of quiet exhaust.