Forums/Z31P/z31-repairs-forums engine-mechanical/(Urgent help please) Engine braking is killing engine

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ComeAlive!: (2019-02-07 20:05:19)

Hey guys, I already have an thread about my 85 300zx NA 5speed’s high idle and misfire when cold, but this is a new issue that came up while I pulled into work so It’s kinda urgent- while I was engine braking in 4th or 1st gear (not all the way, like as soon as I let off the throttle) the car would sound like the RPMs would suddenly drop in a loud misfire like noise, but it was fine if you kept on the gas. Does anybody have any idea what happened or why? I don’t want to risk driving it if you guys think it’s snything major but I’d appreciate any insight, thank you guys

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ComeAlive!: (2019-02-07 20:28:10)

PS this might (probably is) be related to when If I would drive the car cold it would act like the engine was restricted, like a misfire-y not running on all cylinders restricted kind of deal, then the engine would finally “open up” and it would drive decently with little to no miss- again thanks guys any ideas appreciated

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Z_Karma: (2019-02-07 22:47:44)

{{#if:ComeAlive!|ComeAlive! wrote:|Quote:}}

PS this might (probably is) be related to when If I would drive the car cold it would act like the engine was restricted, like a misfire-y not running on all cylinders restricted kind of deal, then the engine would finally “open up” and it would drive decently with little to no miss- again thanks guys any ideas appreciated

84-85 z31's have a lean start issue that is inherent to the ECU. In my experience with my 84T, When it is cold it acts restricted and will misfire if accelerate from a stop and i don't keep the RPM's up. Once the digital temp gauge gets to the 3rd bar, it's like a switch is flipped somewhere in the ECU and it suddenly runs fine. I've tried replacing the CHTS and harness and the old mod where you switched the CHTS signal to the FTS, but it didn't alleviate the issue.

{{#if:| wrote:|Quote:}}

https://dev.z31library.com/xenonzcar_archive/z31/ECUdifferences.html

There was a service bullitin that addressed this where they added some helper module beneath the battery tray. My 84T has this, but still has the same issues. The agreed upon fix for this is to run a newer ECU that is compatible with your particular car.

http://redz31.suomiz.net/pages/fuel.html

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ComeAlive!: (2019-02-07 23:01:33)

Thanks for the input on lean starts, although the car would run rough for minutes then increase RPMs and drive, not just 30 seconds. It’s always done the fast idle at first start, then kick down to a lower RPM, but won’t be drivable until, like you said, a switch is flip and it’ll do its best to get up to speed.

I guess the all-of-a-sudden strong misfiring with engine braking is someyhing im gonna have to test on the drive home, hopefully the car doesn’t explode or anything

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ComeAlive!: (2019-02-08 02:57:29)

It survived to Autozone and back, but something definitely isn't right-I'm gonna put the symptoms in my main high idle thread, thanks guys