By on December 20, 2019

Toyota MR2 AW11, Image: Toyota Europe

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:

  1. 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.
  2. Replace the TPS with a good quality aftermarket part or a legit Toyota bit.
  3. 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 [email protected]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. 

Get the latest TTAC e-Newsletter!

14 Comments on “Piston Slap: TPS, IAC, VTV…WTF?...”


  • avatar
    FerrariLaFerrariFace

    It’s been a while since I had my AW11, but this sounds similar to the “bouncing idle” that is common on these cars. Caused by an air bubble trapped in the coolant lines at the throttle body. Be sure to research and follow the special method for bleeding the air from the coolant and this may go away. TPS is a good bet, too, but be prepared to change it a few times. Even “good quality” and “OEM replacement” sensors have been known to be bad right out of the box.

    This guy also has some good ideas: https://www.driving4answers.com/fix-idle-related-issues-mr2-mk1-4a-ge-cars/

    (Since the radiator is up front and the engine is in the back, the coolant system needs to be bled in a particular way. It’s weird. But make sure you do it right or you’ll be chasing your tail forever with problems like this. And any time you disconnect the coolant hoses from the throttle body, it’s a good idea to bleed it to avoid the aforementioned air bubble)

    • 0 avatar
      Featherston

      Re: your third paragraph, aren’t two of the Fiero’s vulnerabilities (for lack of a better word) improper coolant system bleeding and pinched coolant lines from improper jacking?

  • avatar
    teddyc73

    “as well as any peak-era Toyota from time to time” Huh? Peak-era? So 1989 was the peak of Toyota? Who writes like this?

    • 0 avatar
      ToolGuy

      The adults are diagnosing a vehicle problem. Can we hold off on the writing critiques?

      Jeremy, if you suspect a vacuum problem (at all), I highly recommend a good old fashioned vacuum gauge. T it in and watch what it’s doing (I’d be tempted to rig it up so I could watch it while driving).

      [It sounds like your problem is getting progressively worse, which would be consistent with hose degradation or some other mechanical component going bad.]

      • 0 avatar
        SPPPP

        “The mechanic said he couldn’t find a vacuum leak, but the car wouldn’t die when he shut the idle control valve completely.”

        Well, even Toyotas don’t magically run without air. It’s getting into the engine somehow.
        – Did he try blocking off the throttle body? If the car still ran, that’s a dead giveaway that there’s a leak after the throttle plate. Either a bad hose or a leaking intake manifold gasket.
        – If the car dies with the throttle body blocked off, then the high idle might be just an IAC problem. Did the mechanic physically remove and block off the IAC valve, or just disable it electrically? This may be important because some IAC valves go bad and leak internally (not sure about the MR2 style one).

        • 0 avatar
          dukeisduke

          True, it sounds like it could be a vacuum leak. He could spray carb cleaner around different places to see if it’s getting drawn into a leak (the engine should speed up as the stuff is burned in the engine). An old mechanic friend of mine would use a propane tank with a hose connected, and a nozzle on the end that would meter out a small amount of propane. It worked better than the carb cleaner, as it was less messy, and it was a gas rather than a liquid.

    • 0 avatar
      Featherston

      @ teddyc73 – I’m not sure if you’re trolling, but I’ll take your question at face value.

      #1, even if it were bad writing, Jeremy’s the person submitting the question, not the professional writer answering it. Save up that bile for the next time Steph stumbles on subject-verb agreement.

      #2, it’s not bad writing. Peak Toyota is a pretty generally accepted concept referring to the quality of Toyota’s late ’80s and early ’90s offerings. I’m not sure when and with whom it originated, but Jeremy didn’t invent it out of whole cloth. Case in point: My family’s ’93 Toyota had better paint than our ’15 Lexus does. Some of that, I’m guessing, is due to environmental/legislative hurdles but some, I’m also guessing, is due to Toyota’s cheapening the product. Off the top of my head, HVAC is the one realm in which Toyota has upped its quality relative to the contemporary market over the past 30 years.

  • avatar
    StudeDude

    I had the same problem with my 1991 Stealth SOHC with 177K miles. It was idling low and roughly even after a full tune up. After cleaning the throttle body thoroughly, the idle varied between 1500-2000 RPM. Installing a new IAC made it worse. After rereading the service manual, I noticed the existence of the SAS speed adjusting screw on the bottom of the throttle body on the firewall side. It’s a basic adjustment screw similar to a carb
    idle adjustment. Using that I was able to bring the idle speed down to 700-800 RPM. Check out whether your MR2 has that same adjustment.

  • avatar
    DenverMike

    Does pedal hang up if you disconnect at the throttle body? What about the throttle body with the pedal disconnected? Does it have mechanical cruise control?

    I’d replace the cheapest stuff first. Notice your mechanic start with the most expensive always.

  • avatar
    DenverMike

    As a side note, it’s incredible this is the same Toyota that signed off on this little go kart for the street.

    We need another trade embargo with Japan.

  • avatar
    dukeisduke

    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.

  • avatar
    -Nate

    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

Read all comments

Back to TopLeave a Reply

You must be logged in to post a comment.

Recent Comments

  • Jeff S: Much easier to disparage something in hindsight. At the time PLCs were popular and very good cars for the...
  • tankinbeans: To be clear, I’m not generally trying to activate it at low speeds, but I get the idiot light....
  • tankinbeans: I came along just as this car was starting to shuffle off into hell or wherever it was to ultimately end...
  • Lou_BC: LOL. That’s why I prefer motorcycles.
  • Lou_BC: @el scotto – well said. Germany clued in post WW2 that any Nazi symbolism was bad and rendered illegal....

New Car Research

Get a Free Dealer Quote

Who We Are

  • Adam Tonge
  • Bozi Tatarevic
  • Corey Lewis
  • Mark Baruth
  • Ronnie Schreiber