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Old 10-08-2009, 02:42 PM   #1
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Check Engine Light - Diagnostic Code 43

I have a 98 Jeep Wrangler with a 4.0L engine and Automatic transmission. Recently the Check Engine Light can on. I checked in to the codes and it registered code 43 which indicates "multiple misfires or misfires in one of the 6 cylinders.

Can someone provide some advice as to things I should check to correct this issue? I just put in new plugs about a month ago.

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Old 10-09-2009, 05:57 PM   #2
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Were the plugs gapped to specs? Check your distributor cap and rotor contacts. How do they look?

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Old 10-09-2009, 06:20 PM   #3
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There is an official Jeep TSB (technical service bulletin) specifically for this problem, though there's no guarantee of a complete fix if you perform the TSB. Some early 4.0L engines in '97 and early '98 came with a bad (weak) set of valve springs from a bad batch which is what causes the misfire for a good percentage of those getting the 43 DTC. The TSB instructs that the valve springs be replaced and the engine be de-carbonized afterwards.

My '97 often even had multiple 43 codes at once (like 12 43 43 43 55) and before Jeep came up with the TSB, I fought the problem for years to the point that Jeep engineering actually contacted me after reading of my pleas for help on the various Jeep forums. The engineer who called me (none other than THE Jim Repp, "father" of the Jeep Rubicon) sent me a care package of components to swap into the Jeep one-by-one but none of them helped. The kit had all new fuel injectors, fuel injector wiring harness, PCM (engine computer), clockspring, and a few other odds & ends. I had already replaced everything in the ignition system including the plugs, ignition wiring, distributor cap, and rotor.

Then later, the TSB was issued and I replaced the springs which cut my repeated 43 codes by probably 80%. I suspect I had the 43 code occurring for so many years that the valves or valve seats might have burned a bit which is why I still get a very occasional 43 code code but it's rare compared to how it was before I replaced the valve springs. A friend had the same issue and a complete valve job with new valve springs completely cured his TJ and he hasn't had a 43 code in probably five years now.

Note that a TSB is not the same as a Recall so you will have to pay for any work performed under a TSB. Good luck with it.
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Old 10-09-2009, 06:34 PM   #4
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Jerry -
I wonder why the clockspring? Maybe it was suspected it was momentarily shorting power to ground?
Did he give any indication as to why?

While you were talking to him, did he say specifically what the PCM is monitoring that can set a misfire code?
It can't be monitoring plug/coil voltage, as that will be the same whether it misfires or not. If it was monitoring purely electrical voltages, then the valve springs shouldn't affect it. I know low compression does too.

Maybe by instantaneous RPM change? (The increase in RPM when a cylinder fires.)

If we knew exactly what sets it and how it sets, it could help track down what to fix.
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Old 10-09-2009, 06:52 PM   #5
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The clockspring can cause voltage spikes that gets fed back to the computer and can cause strange hard-to-diagnose problems. At the time, we were leaving nothing to chance so he included one. I have no idea how the PCM picks up on the misfire well enough to generate the 43 code, possibly from an erratic signal from the crankshaft position sensor as the engine misfires but that's just a guess.

Anything can cause a misfire, all the 43 code is saying is there's a misfire and the TSB addresses that when none of the usual suspects (plugs, rotor, etc.) help. Leaky valves from weak valve springs can certainly cause misfires and in my case, multiple misfires.
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