Early 90's Corolla has increased idle speed for some reason

Maintenance/Repairs
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Over the past month I’ve notice the idle speed increasing. Yesterday I had to turn the idle adjustment screw to lower the idle speed, as the high idle speed was making it difficult to drive in stop and go traffic. Especially if I took my foot off the gas while in gear, it would jerk rather than slowing down smoothly as it used to. I think the jerking was related to how the ECM stops the injectors completely when slowing down in gear, to improve mpg, then before the rpms gets too low, fire the injectors back up again. Anyway, lowering the idle speed with the adjustment screw fixed the jerking problem, for now.

But in 20 years, I’ve never had this occur before. The only time I’ve fiddled with the idle adjustment screw was during tune-ups, and then only had to adjust it 5-15 degrees. This time I had to turn it in 45 degrees. I expect there is unwanted air getting into the intake manifold somehow. And it will probably get worse.

Here’s some ideas – in order of most likely to least likely – I’ve had what could be causing it. What do you think? Am I on the right track? Anything I’m missing?

  • Throttle valve sticking in throttle body and needs a cleaning.
  • Throttle linkage needs a lube.
  • Cannister purge valve is open during idle and allowing fumes from the canister into the intake manifold, when it shouldn’t be.
  • Throttle position switch has failed.
  • Vacuum hose or device leaking.
  • IAC valve (part of throttle body in this car) is sticking.
  • ECM is bumping the idle speed through the vacuum controlled actuator (VSD). Unlikely, b/c the idle speed still increases when I turn the headlights on or I turn the steering wheel, both of which use this mechanism (I think).
  • Thermostat – unlikely as it happens even when the coolant has reached 195 deg F (when the fan turns on).
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I recently had this exact problem with my '88 Supra. For some reason, the ECU kicked the idle up from it’s normal 750 RPM to 1300 RPM at idle. Before I could take time out to run through some diagnosis, I had to disconnect the battery to do an emergency repair. After reconnecting the battery, idle is back to normal. I guess the ECU just needed a reboot.

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First of all, there is no throttle adjustment screw on this car. There is a throttle stop screw and while it can affect the idle, its purpose is to protect the throttle body from the throttle plate digging into it.

You could have a dirty throttle body but usually you will feel the throttle sticking as you step on the gas from idle. A good throttle body cleaning never hurt anything.

The throttle position switch will give you a one trip DTC if it fails, however, it the throttle plate is not returning fully because of gunk, then it will thing that you are still pressing on the gas so the computer won’t take over and control the idle speed.

The shaft of the IAC could also be gunked up, but a thorough cleaning of the throttle body (remove the TB for cleaning) should address that too.

Vacuum leak, my first choice. Could be a vacuum hose or the intake manifold gasket. The manifold gasket is common on Saturns, have not heard of it being a problem on Toyotas, but it is easy to check, just spray something like throttle body cleaner or even WD-40 around the manifold gasket and if the idle is affected in any way, the gasket is breeched. Also spray around the throttle body gasket unless you clean the throttle body, in which case you would put on a new gasket anyway.

And the ECU might just need a reboot.

I think that Toyota’s use a separate ECTS for the computer so the gauge on the dash may say 195 but the computer might be seeing 95. The only way to verify that would be either to fully warm up the engine and then pull the connector off the ECTS for the computer and measure its resistance or if you get a check engine light, look at the freeze frame data. Running the engine for 2 minutes at high idle (over 950), twice (two trip DTC) should give you a P0507 code. Once should give you a pending code without a CEL and you can get freeze frame data from that.

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Good ideas. I tried giving the ECM a “reset”, didn’t work. I’ll give the others a try. Thanks.

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@keith … fyi, this particular year of Corolla does have an idle adjustment screw, routinely used during a tune-up. 4afe engine. There are also two other screws – which, like you say, are not supposed to be monkeyed with – which are used to mechanically position the stop for the throttle plate. So far I’ve been able to keep the idle down by turning the idle adjustment screw, but I’ve had to do it twice now. Whatever the problem, it appears to be getting a little worse week by week.

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Early 90’s Corolla, 4afe engine. Have done a little diagnosis, here’s results so far. Comments appreciated.

  • Throttle valve sticking. Haven’t yet cleaned, but ECM diagnostic indicates problem with throttle idle switch. Measured w/DVM, and switch works, closes at idle. But play – the distance the switch remains closed off the stop screw – is out of spec. Play should be 0.03 inch, measured is 0.015. Wondering if this could be caused by gunk in throttle body. Still don’t understand why ECM diagnostic on throttle switch is failing, since switch works and appears to be powered up.

  • Throttle linkage needs a lube.: Done, maybe a slight improvement.

  • Cannister purge valve is open during idle. No change w/tube clamped.

  • Throttle position switch has failed. See above.

  • Vacuum hose or device leaking.: Brake booster, power steering idle up tube, MAP all hold vacuum to 20.

  • IAC valve (part of throttle body in this car) is sticking. Haven’t tested yet.

  • ECM is bumping the idle speed through the vacuum controlled actuator (VSD).: No, this appears to be working correctly. Slight chance of leak where it seals to intake manifold I guess.

  • Thermostat - Unlikely, as idle speed remains high at 195 deg F.

Edit

  • Engine coolant temp sensor - Haven’t tested.
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New measurements today. Discovered idle speed returns to normal when ECM is put into open loop test mode. And timing is spot on at 10 Deg BTDC in open loop mode. When then returned to closed loop mode (the normal driving mode), idle speed increases and timing advances from 10 to 15 Deg BTDC. Not sure what it means, but it is something anyway.

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Are you switching modes by inserting a jumper wire in the test connector? This is how you negate the timing advance, and idle is supposed to drop slightly. Idle speed is confirmed with the jumper out. The IAC is supposed to control the idle speed. It may be dirty.

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@BustedKnuckles … thanks for the comments, yes, I performed the closed-loop to open-loop test by inserting the jumper. Ok, I see what you mean, maybe that all is a red herring and the ecm is just doing what it is supposed to do.

Re: IAC. The IAC on this engine (4afe) is not a separate part that can be removed and re-installed or replaced and is not electrically controlled. It is integral to the throttle body. Controlled by the coolant circulating in some tubes connected to the throttle body. The valve itself works sort of like a thermostat, thermo-wax based, when cold the valve allows a little air to flow bypassing the throttle, to boost the idle rpm, then as the coolant heats it up, due to the thermal sensitive material it is made of, it closes, returning the idle speed to normal. That’s how it is supposed to work, and the idle speed does in fact decrease as the engine heats up so it seems to be generally doing its thing. It’s just that when the engine is fully warmed up, the idle speed is still too high.

But the IAC could be gunked up like you say. Have you ever cleaned this version of Toyota IAC? Would I need to remove the throttle body from the engine to clean it?

Like I say above, there’s a diagnosis code (actually a lack of code 51 displayed) indicating a problem with the throttle idle switch which I think may in fact be due to a dirty throttle plate where it seals in the throttle body being gunked up. 20 year old car and all. So I’m planning to buy some throttle body cleaner and clean the main throttle plate area this weekend.

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I have a '92 Celica with the same engine.1.6L 4A-FE. 375,000 miles on it. The best way to clean the throttle body is off the car. Especially if it has not been done in a long while. In my case, the build-up was so thick, I needed two cans of throttle body cleaner and some very thin gauge wire to clean out the vacuum ports and get the EGR working again to pass emissions. It is difficult to clean the backside of the throttle plate on the car. A word of warning, tho. Check with your local parts store for a throttle body gasket. It may need to be ordered. I got lucky at a NAPA that had one in stock, but I did have to drive 10 miles out of my way to get to it.

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Thanks for the heads-up. I’ll get the gasket in hand before removing the throttle body.

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New progress to report today on my poor early 90’s Corolla’s high idle speed problem. 4afe engine. Problem started about 10 weeks ago and has gradually gotten worse each week.

Summary: I removed, cleaned, and re-installed the throttle body, and the idle speed increased even more!!! Oh my!! Must not have been a dirty throttle body causing the problem. I’m thinking the gunk was keeping the idle speed from going even higher. Visually checked the IAC and didn’t find anything unusual, but couldn’t figure out a way to test it for air leaks when warm either. Also adjusted the throttle position switch to spec.

Details: Removed throttle body. There was some blank gunk all right in the throttle plate area, but not extremely gunked, for being there 20 years/200K I mean. Cleaned the throttle plate area and whatever else I could with Berryman’s throttle body cleaner, seemed to clean up ok, nice and shiny. After cleaning, and I look through the long section of the throttle bore (the side normally toward the air cleaner), with the other side toward the sky, I could see a little daylight around the edges of the throttle plate. The throttle plate seems to be exactly centered, but a tiny bit of light could get in around the edges.

Does that seem normal? Or should the throttle plate completely seal the bore at idle?

I also removed an access plate to inspect the IAC piston. (The IAC on this engine is directly controlled by coolant temp, no electrical or ecm involvment.) Seemed pretty clean in there. I heated that area up with a hair dryer and the piston moved out as the temp increased, like you’d expect I guess. I couldn’t quite figure out how to tell if the IAC was leaking any air through that path under warm engine conditions though.

The throttle position switch adjustment was slightly out of spec. I adjusted it to spec, about a 0.75 mm transition point off the hard stop. The transition point before adjustment was less, about 0.45 mm. Didn’t seem to change anything symptom-wise.

After doing all this, the idle speed is even higher. I think cleaning the gunk is no allowing more air around the throttle plate edges. I’ve had to turn the idle adjustment screw all the way in now, to keep the car drivable. There’s no air at all presumably going through the idle adjustment screw path. Before – when things were normal – the idle adjustment screw is turned out about 1 1/2 to 2 full turns.

I don’t know what is wrong, but I think these are important clues:

  • From a cold start, the idle speeds seems about right at first, but it doesn’t go down w/temp as much as expected, and after it does go down, it seems to drift back up again as the engine reaches full operating temp.

  • If I park the car and leave it for an hour, after reaching full operating temp before parking, when I start it up, the idle speed seems normal, but returns to an overly fast idle as the engine runs for 10 or 15 minutes.

  • From an experiment before today’s effort, the idle speed seems about normal when the ecm is put in open loop mode.

  • edit: When engine is warm, at idle, and I gas it a bit to increase idle speed, then release, the idle speed drops way below what it was, almost seems like it wants to stall, but returns to high idle speed within a few seconds. This is definitely not normal.

Any ideas what to do next?

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Try this

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And this…

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@casper has some good ideas. I’d also suspect the coolant temp sensor (CTS). If it is sending the ECM bad info, like the coolant temp is cold when it is warmed up, the ECM will react by stepping up the idle and enriching the fuel mix, mimicking the action of the choke on older, carburetor cars. The fact that the idle behaves when the engine is cold is a key factor.

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New results today. Good news! I think I found the source of the air leak. The idle air control is the culprit. Part of the throttle body unit, inseparable, coolant runs through it. It’s operation is similar to a thermostat, as it moves w/temperature, it controls idle air flow into the engine, cold vs warm. Hard to test on the bench. But I came up with an idea to test it with the throttle body installed on the engine.

What I did was put a piece of tape over the inflow hole to the IAC (in the inlet of the throttle body bore). At that point the car wouldn’t start. Seems like a bad thing, but actually good. Once I returned the idle adjustment screw to its nominal position – I had it screwed all the way in just to make the car drivable with the air leak – then some air could get in, and the car started and now idles fine, when warm at least. This is a kluge fix, as the downside is the engine doesn’t idle fast enough when cold and is a bit balky then. (And the tape could come loose and get sucked in and stuck on an intake valve, hope that doesn’t happen.)

I’m not sure if this is fixable with a new part to replace the old IAC gadget – it is something that appears to screw into the IAC cylindrically shaped chamber – or if I need to replace the entire throttle body. If the latter, could be an expensive fix.

Thanks to Casper and Busted Knuckles above for yesterday’s good guidance. I did check the CTS today and it is ok. As is the Intake air temp sensor. And the EGR is holding vacuum and its movement appears to be working. The Throttle position checks out ok. As does the throttle position switch. The MAP sensor appears to hold vacuum, but its sensor operation is hard to verify by bench testing as it appears to be something more complicated than just a variable resistor controlled by vacuum pressure. It must use one of those fancy silicon pressure sensitive electronic devices.

Anyway, thanks to all for the excellent guidance. This has been the toughest diagnosis for me on the Corolla in 20 years! If you have any thoughts about the best way to fix this problem, a new throttle body or replace the IAC gadget thingy — which I haven’t been able to find a source for online — I look forward to reading your ideas.

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@GeorgeSanJose, there are tons of these 4A-FE engines in junkyards across the country. I had a pick-up coil puke it’s guts all over the rotor shaft of the distributor in my wife’s '92 Celica with that engine. A reman was going to cost over $200, on an old car with 365,000 miles on it. At my favorite parts yard, I found a Corolla with the 4A-FE and only 136,000 miles with the complete distributor, and it cost me $25 for it. The throttle body in mine is the original, and I’ve never had idle issues, so I think this part is pretty robust and a junkyard replacement may be reasonable.

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Here’s a crazy kludge:

Run a small vacuum hose from a manifold vacuum port (add a tee) into the passenger compartment.
You can manually uncap this to introduce extra air for fast idle.
You’ll need to put an adjustable valve somewhere in line to limit the amount of air.

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Good idea @circuitsmith. For now it’s not that difficult to just press on the gas pedal for 30 seconds until it warms up enough to idle ok, but when it gets colder that will prove more problematic. I was thinking to use a choke cable as an improvement to my kluge, somehow to hold the throttle open with a manual lever in the passenger compartment, sort of like the choke on the 62 Ford I had as a teenager, but the vacuum hose/valve idea would be much easier to implement.

@BustedKnuckles … Helpful comment. Your idea is probably the best in the long run. Secure a junk yard throttle body. Should be no problem to find like you say, w/so many 4afe’s out there. The only problem might be that my Corolla is a manual xmssion, and the throttle body may be slightly different manual vs automatic. I have a sense replacement parts for the air control inner workings are not made. There are some Utube videos about the 4afe throttle body air valve posted by others with the same problem, and suggest in some cases it can be fixed by removal and cleaning of the air valve inner workings, so I’ll look into those ideas too. From what I can tell, to remove the inner workings is going to require a special tool. Of course!

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Tried posting this earlier but who knows where it landed.
Try this…