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63 Posts
Discussion Starter · #1 ·
I've read through several threads here about engines stalling but none of them were identical to what our RAV is doing.

We have a 1997 4 door 5MT AWD with about 160K miles.

The engine runs very well -- smooth and strong (well, as strong as that little 2.0L engine gets) pulling hills at WOT. No stumbling, no hesitation, just full power.

It also cruises smoothly at any speed.

There are no codes, no CEL.

I've replaced the spark plugs but the problem persists.

Plug wires seem fine -- no arcing even when wet.

A couple months ago I cleaned the idle air control (IAC) valve and throttle body. That made no difference.

All vacuum hoses look good.

The VSV for the EGR valve was replaced a few months ago.

I've applied vacuum directly to the EGR valve -- it's working as it should, not sticking.

I disconnected the EGR and plugged the vacuum hose -- no change (other than an expected 0401 code).

Battery and alternator are good.

One potential clue: The engine usually idles normally upon start-up (regardless of coolant temp or ambient temp) -- at around 1,400 rpm when cold, falling to ~800 rpm when warmed up. However, periodically the idle speed will increase to 2,200 to 2,300 rpm. It will remain at that speed for anywhere from about 1/2 second to several seconds, and then return to 800 rpm (or whatever the idle speed was before it increased).

The more serious problem is that when stopped at a light (for example) the idle speed will slowly decrease and the engine will stall. Feathering the gas has no effect. When the ignition is turned to 'start' it fires right up and runs normally -- until the next time...

It does not seem to be caused by a brake booster vacuum leak.

All I can think of is that although the IAC is clean, perhaps it is either a) getting erratic signals from the ECU, or b) it has failed and is randomly moving freely from open to closed (and anywhere in between).

Question #1: Does the IAC valve flow enough air to cause the engine to stall -- if the ECU does not increase fuel delivery to compensate for the additional air? The IAC opening ahead of the throttle plate is very small -- I wouldn't think that enough air could flow through there to kill the engine. Also, I'd think that feathering the gas would get it going again before it dies.

Question #2: What is the test procedure for the IAC valve? If possible, I'd like to confirm whether it's good or bad before replacing it.

Question #3: Any other ideas as to what might be causing this behavior?

Thank you.

63 Posts
Discussion Starter · #3 ·
Replace the IAC valve assembly, and it it doesn't come with one, replace its special oil-ring material gasket (dealer part). That's how I solved this problem.
Thanks LugNut.

Just to be clear, were your symptoms the same as what I described?:

* Starting normally -- no extended cranking or rough idle, etc.
* Occasional, intermittent surges to ~2,200 rpm when idling, lasting for a split second to maybe 4-5 seconds before returning to normal idle speed.
* Full power.
* Smooth cruising.
* After driving for a while, engine will occasionally stall when idling at a red light or stop sign, but restart instantly and run fine.

I just did some more testing:

* Started engine (cold). Idled at 1,400 rpm (normal speed).
* Disconnected IAC valve. RPM dropped to 1,200.
* Reconnected IAC, rpm went back up to 1,400.
* RPM surged to 2,200 for a few seconds.
* Then rpm dropped to 500-600. Disconnected IAC and rpm went up to ~1,000, steady and stayed there.
** I noticed that when disconnecting and connecting the IAC, there was a soft 'thump' (like the sound made when tapping on a plastic tub used for margarine or cottage cheese. Like a small bass drum). In addition to the thump I could hear air -- maybe a vacuum leak -- when I reconnected the IAC.
* There were several times when connecting the IAC caused the engine to stall, or would have if I hadn't quickly disconnected it. Every time, I heard the thump coming from the front left corner and what sounded like an air leak from the same area.

I was thinking that perhaps there is something tied into the IAC, like an EVAP cannister purge, but the underhood diagram just shows the EGR system and EVAP system.

My Haynes manual isn't much help -- it just has R&R instructions for the IAC.

I just checked Amazon and prices for the IAC are all over the map -- from $50 at "Well Auto"(?) to ~$225, with most around $200. So while it seems like the IAC is the culprit, I'm trying to eliminate everything else first, before replacing it.

Again, there are no codes -- except when I disconnected the EGR valve (P0401) or the IAC valve (P0505). So the OBDII system seems to be working, yet it thinks everything is A-OK.

Question: When you were having problems, did you get a CEL and code P0505, or at least a pending code 0505? Any other codes?

I wish there were a way to test the IAC.

63 Posts
Discussion Starter · #4 · (Edited)
Testing IAC valve

Google is your friend...

I searched for a test procedure earlier but found nothing.

I just searched again and found this:

| Repair Guides | Components & Systems | Idle Air Control Valve |

Looks straight-forward:

Connect the positive (+) lead from the battery to terminal +B and negative (-) lead to terminal RSC, and check that the valve is closed.
Connect the positive (+) lead from the battery to terminal +B and negative (-) lead to terminal RSO, and check that the valve is open.
If operation is not as specified, replace the IAC valve.

Click image to see an enlarged view.


[There is a helpful illustration on the AutoZone page. The only problem is pins #1 and #3 are both labeled "RSC"].

"B+" is center (#2) of three (3) pins.
It doesn't really matter whether #1 or #3 is RSC or RSO. Grounding one should close the valve and grounding the other should cause it to open.

I'm going to give that a shot.

Hopefully the above will help someone else.

63 Posts
Discussion Starter · #6 ·
Got mine from RockAuto, and gasket ordered from local dealer.
Thank you.

Side note:

Buyer beware when it comes to Rock Auto. I've gotten some good deals from them, but I've also received several wrong parts.

I was working on my sister's 1991 Town Car (which is in amazing shape) and ordered parts from RockAuto. About half the parts were wrong.

RockAuto agreed that I had ordered the correct parts and said we could return them, but said that we would not get a refund for the original shipping charges and we would have to pay the return shipping!!

So they admitted that it was their mistake (or their supplier's) but expected us to pay the shipping both ways!

The RockAuto CSR was very rude and unprofessional and refused to help or put us in touch with a superior. I did some research and got the contact info for the owners and contacted one of them. He was apologetic and agreed to give us a refund and pay the shipping both ways.

So it ended well, but we had to go through a lot of bs over what should have been a routine return, and it was not easy to find the owners' contact info.

63 Posts
Discussion Starter · #7 ·
IAC valve test / Stump the chump

I followed the above procedure and the IAC valve tests *good*.

Go figure.

I cleaned the throttle body so it looks almost new, and I did what I could with the intake plenum.

Then I sprayed throttle body cleaner into the IAC intake opening and exercised the valve by repeatedly applying voltage both directions -- open and closed.

I measured the resistance of the plug wires -- they were all in spec, from 6K to 12K ohms (I had previously checked for arcing, none found).

Distributor cap and rotor look good.

Air filter is clean.

Vacuum hoses look good. No obvious leaks.

Reassembled everything and ... drumroll...

Now it runs worse! If not worse, it certainly isn't any better.

When I initially started it after putting everything back together it ran fine. Then within a couple minutes it started bumping the rpm up as high as 2,600 (previously it was ~2,200 rpm) for several seconds at a time.

Other than the occasional increase in rpm, it seemed to running fine. The low/stable rpm was correct for the engine temp -- ~1,400 cold dropping to about 800 warm. If I opened the throttle it would rev normally.

Then the rpm intermittently dropped as low as ~300. It would almost stall, then idle back up to about 800 rpm. When it was acting that way, I found that if I quickly floored the accelerator it would stumble a bit and then the rpm would increase and it would rev normally. However, if if would just crack the throttle, or 'feather' it, the engine would stumble badly and sometimes die.

Sometimes it would start right back up, but other times the engine would turn over without starting.

I'm not an ASE certified mechanic, but I am an electro-mechanical tech. I've been working on cars casually (family and friends') for 40+ years and I've never run into a problem like this one.

As a last resort, I dumped most of a 20 oz bottle of Techron concentrate ("fuel system cleaner") in the tank but it only remained running for a couple minutes after that. I started it several more times and each time it ran for a few minutes and then died.

I have to admit, I'm about out of ideas.

Any suggestions?

63 Posts
Discussion Starter · #8 ·
Problem solved!

I think I figured out the reason the RAV4 is stalling when warm -- not the EGR, not the IAC valve, but the engine coolant temp (ECT) sensor.

For those who don't know, the ECT is a thermistor -- resistance varies inversely with temp.

There are graphs that show the approx. resistance vs coolant temp and the acceptable range. I could not find one for our 1997, but the ones I saw all looked similar. Roughly, the resistance of the thermistor should start around 1,500 to 2,500 ohms at 70-80*F and slowly and smoothly decrease to around (say) 200 ohms at operating temp.

I was monitoring our ECT sensor with my meter and saw some wild changes in resistance. The changes should be very gradual, so that was clue #1.

In addition to stalling while idling when warm, the idle sometimes surges to ~2,500 rpm. I noticed that when I would unplug the ECT sensor while the engine was running fast the idle would return to normal. Clue #2.

Here's some more info:

Toyota RAV4 Service Manual: Engine coolant temperature circuit - Diagnostic trouble code chart - Sfi system - 2Az-fe engine mechanical

I haven't got the replacement ECT sensor yet. When it is replaced I will report back.

BTW -- I have not seen any codes at all the entire time I've been working on this problem, and no CEL (aside from when I disconnected something).

I know my code reader is working because the CEL will come on if I disconnect the EGR valve; IAC valve; or ECT sensor, and I can see the code. The code I saw with the ECT disconnected was P0115. As soon as I plugged it back in the CEL went out and the trouble code was gone.

I'm not sure how the ECT can act up enough for the engine to actually stall, yet not throw a code. According to the trav4 link above, the DTCs -- 0115/0117/0118 indicate an *open* or *short* circuit. I never saw infinite/open or zero/short -- just some wild fluctuations. Apparently the resistance jumping around isn't enough to throw a code.

Not having a DTC makes troubleshooting more difficult.

Hopefully newer cars recognize sudden changes in resistance as an ECT sensor failure.

63 Posts
Discussion Starter · #9 ·

The ECT sensor was the culprit.

The engine is running as it should.

A ~$23 part fixed the problem, but there was no code because it was not shorted or open -- the resistance was just intermittently fluctuating.

Hopefully this will save someone else time and effort.

Premium Member
'99 RAV4.1, 3MZ-FE, E250F 4x4, Torsen Dif
2,163 Posts
Youtube is your friend for diagnostcs, check out the Scotty Kilmer videos.
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