1991 ZR1 engine knocking on idle

[Hib Halverson]

It's too bad the mic was not down lower closer to the exhaust and also closer to the engine. That is...two different recordings.

I've played that footage a couple of times. Once with the volume as loud as my computer could go. It's really hard to tell but I think I could hear a "dead miss" in the exhaust in sync with that bad rattle from the engine.

Without better audio or a stethoscope session, I'll make a wild-assed guess that a valve lifter(s) have failed. The engine has a stuck, bent or broken valve.

It would be great to find out of the rattle is coming from the engine or the clutch housing and if from the engine, which bank.

[evski]

Here is an update. I changed the plugs and #8 plug seemed to be different than others (wet). New plugs didn't change anything. Then I checked spark on each cylinder using spark gap tool. No spark on #8 cylinder but nice long blue spark on #5. Also pulling #8 wire made no difference on idle roughness unlike pulling the others.
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I checked resistance on wires 5/8 1/6 2/3 4/7 and all measured 8.1-8.3 kOhm so there is an electrical connection from #8 plug to 5/8 coil. I'm aware of a high voltage there so my measure doesn't eliminate connection as a posiible cause.

Also I read a different opinions whether it's possible or not to have no spark on only one cylinder caused by the coil in a wasted spark system. Any experience with that?

What brand poduced yellow coils for ZR1?

Now I'm preparing for plenum pull, already got new coils and wires prepared and new gaskets from Jerry are on the way to Poland.

[A26B]

Originally Posted by evski

Here is an update................................

Also I read a different opinions whether it's possible or not to have no spark on only one cylinder caused by the coil in a wasted spark system. Any experience with that?

Yes, it can happen. Replacing the coil will correct the miss.

What brand poduced yellow coils for ZR1?

Accel brand

[XfireZ51]

I had a spark knock issue which was at least partially caused by ignition coils that presented cracked housings. You can see carbon tracking on the coil surface.
Since you are going to the trouble of a plenum pull, I would also recommend having the injectors flow tested and rebalance the fuel delivery by matching up the injectors properly. You?ll be pleasantly surprised at how much smoother the engine runs.

[WARP TEN]

Originally Posted by XfireZ51

I had a spark knock issue which was at least partially caused by ignition coils that presented cracked housings. You can see carbon tracking on the coil surface.
Since you are going to the trouble of a plenum pull, I would also recommend having the injectors flow tested and rebalance the fuel delivery by matching up the injectors properly. You?ll be pleasantly surprised at how much smoother the engine runs.

I'll toss out a thought from a long, long, long time ago. Back in the late '60s or early '70s, I had an intermittent miss on my '68 HEMI Roadrunner that I could not diagnose. Late one afternoon, in a fairly dark place, I had the hood up and the engine running. Jiggled the plug wires a bit and saw lots of arcing from the wires to the engine block when they got close to it. Could not see the arcing in bright daylight. OEM wires about 3 years old. Replaced them and everything was fine. I see you replaced your plug wires about a decade ago; maybe a possible culprit by now? --Bob

[XfireZ51]

Did that one also Bob on my 72 Coupe.

[evski]

I pulled the plenum and have few more questions.
What could be that black powder that came out from the vacuum line on the passenger side?
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Should I be concerned with the amount of oil on the case ventilation cover?
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Why I don't have a temperature senson on my throttle body extension? Is it a common mod?
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