Piston Slap: TPS, IAC, VTV…WTF?

by Sajeev Mehta

Published: December 20th, 2019

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

Sajeev answers:

Thank you for the detailed query! While having a VTV is a good idea for livability (and they probably aren’t pricey), I doubt it is causing this situation. In my experience with 1980s Fords, a failing throttle position sensor (TPS) causes this issue. So I would do the following in this order:

Clean the idle air control valve (IAC), it could be coked up and the mechanism is jammed. I reckon that would only cause stalling issues, but whatever, this is a free fix (if you already bought the $5 can of throttle body cleaner) so just do it.
Replace the TPS with a good quality aftermarket part or a legit Toyota bit.
Add the VTV and do it with vacuum diagrams in hand to ensure you do it right. But I suspect the problem will be solved in the previous step.

What do you think, Best and Brightest?

[Image: Toyota]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

#PistonSlap #ToyotaMR2 #VTV

Sajeev Mehta

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2 of 14 comments

Dukeisduke on Dec 20, 2019

I just checked online at the Toyota dealer I order parts from, and they still list the TPS for the supercharged MR2. The Toyota part number is 89452-20050, and it goes for $99.55 (list is supposedly $135.73). I wasn't able to look up the VTV, but then I'm not sure what category it falls under. Also, I went to look at factory service manual info for the MR2 on TIS (Toyota's Technical Information System): https://techinfo.toyota.com/techInfoPortal/appmanager/t3/ti?_nfpb=true&_pageLabel=ti_home_page But they only go back to 1990. Looking on eBay, I found hardcopy manuals for '89 - a service manual by itself for $107.99, or the service, EWD (electrical wiring diagrams) and NVH (noise-vibration-harshness) manuals for $144.95. There are probably others further down the list. The pictures for the $144.95 set show the table of contents for the service manual, and it includes a chapter on the supercharger system.

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-Nate on Dec 20, 2019

I'd first look at the SAS screw, the F.I. cars from that time that I worked on, all had a blunt nosed screw for setting the idle speed via a controlled vacuum leak and they'd gunk up over time along with the aperture they fit in . Easy to clean the nose of the SAS screw and usually the tiny O-Ring that seals it . Then look at the idle vacuum by pass valve (I'm forgetting it's proper name) ~ this controls how quickly the idle speed drops to reduce trailing throttle emissions . _NEVER_ use carby cleaner to find leaks because it creates them by attacking and rotting rubber and plastics . -Nate

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