Engine Pinging Gs 400

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1998 GS 400.. 95,000 originl miles always using 91 octane gas and am now starting to hear engine pinging at slow speed in high gear on inclines. What's going on? Please help with any suggestions. Is this a sign of water in gas? Have read not to use any alcohol additives to remove water as alcohol will harm engines. Planning on using a bottle of Chevron Techron Concentrate Plus added to gas on next fill up to clean combustion chamber deposits. We don't have Chevron stations in western NY so always use Sunoco.

[1990LS400]

Bad gas is rare but a possibility. Departing from "common wisdom" I sometimes run my tank down until the low fuel warning light comes on. My view is that I might as well get rid of as much of the crap in the fuel as quickly as possible. You might try running the fuel down until the low fuel light comes on and switching gas stations/brands for the next tank full.

I know that the spark plug replacment interval for the 98 LS400 was 90,000 miles - is it the same for the 98 GS400? (The interval was increased to 120,000 for the 99 LS400 when iridium spark plugs were used.) Did you have the 90,000 mile service done per your maintenance booklet?

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Thanks for reply, plugs were changed at 60,000 mi with iridium plugs. Just had new starter installed which required removal of intake manafold . While that service was done I had the tech check the plugs and they were fine. Hope the additive Chevron Techron Concentrate Plus works on next fill up.