Piston Slap: TPS, IAC, VTV…WTF?

Jeremy writes:

Hey Sajeev,

Long time reader, first time writer; I finally have an issue I can’t seem to figure out and was hoping you can help.

I have a supercharged 89 Toyota MR2 I bought a couple years ago but only really started driving a couple months ago, and while the car runs and drives about as well as any peak-era Toyota from time to time I would notice the revs would hang around 2,000 rpm when I came to a stop. I never really cared that much since I could stab the throttle once or twice and they would dip down to ~1,000.

However, I recently had a bunch of preventive and routine maintenance done, which has caused the rev hanging to become much more prevalent (I originally thought it was a stuck throttle cable, but the mechanic said it wasn’t; I also Seafoamed it before it went in the shop, but I don’t think that’s behind the rev hang worsening).

Now 2,000 rpm is the norm rather than the exception when I’m at a stoplight, with 3,000 rpm a not-uncommon occurrence, and when I stab the throttle the revs don’t really drop down anymore.

The mechanic said he couldn’t find a vacuum leak, but the car wouldn’t die when he shut the idle control valve completely; he also said something called a VTV was missing in the vacuum circuit. I have a feeling this is why the supercharger engaging feels like VTEC is kicking in, yo. The mechanic also said he couldn’t get the timing set dead perfect after my work.

The mechanic thinks the issue behind the rev hang is either in the missing VTV or perhaps something lurking in the air bypass valve, and I was hoping for a second opinion as well as ideas on where to look next.

Give all my best to Sanjeev,
-Jeremy

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