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ericw10

Off-Idle Ping/knock

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I have '90 LS400 with 197K miles. Engine runs great with excellent power, good smooth idle, and no other problems that I am aware of. But about a month ago, it started to have a pretty significant knock when accelerating from a stop for only the first second or two, then quiet. I mean it will knock/ping from idle to about 1.5-1.8K RPM, then nothing. I can be on the freeway and nail it full throttle and it just goes like hell and smooth/quiet as can be.

We use nothing but the highest octane grade available in California (91) and I have run several polyether amine based fuel system cleaners through it (RedLine and Gumout Regane)but no difference in the knock/ping at low RPM. I had a muffler shop check the EGR pipe to see if it is cracked or leaking and he said no.

Any of you veteran LS gurus have any ideas? I was hoping for a simple fix but doesn't look like it's going to be the case here.

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Is CEL light on? If so go dump the codes. If the knock sensor is working it could be sending an error which might be a clue. If not it could be a faulty knock sensor. If the ECU doesn;t get feedback of a knocking condition it might make improper fuel/timing decisions since it isn't seeing the knocks.

I would search this forum regarding knock sensor and see what you come up with.

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Is CEL light on? If so go dump the codes. If the knock sensor is working it could be sending an error which might be a clue. If not it could be a faulty knock sensor. If the ECU doesn;t get feedback of a knocking condition it might make improper fuel/timing decisions since it isn't seeing the knocks.

I would search this forum regarding knock sensor and see what you come up with.

Thanks for the reply. Nope, no CEL or any other error. But may just have to take it in to have it scanned for codes. Will probably be $100 just to hook it up, at least will have an idea where to start looking.

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Since you can only get 91 octane, I would buy some octane booster at the store and try that in the tank. See if it makes any difference at all. It isn't the end-all solution but might give you some clues as to whether it is an octane issue.

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