Rough Hot Idle - 1992 2TZ-FE - The Saga of Njordi

[Boerderij_Kabouter]

Hi Everyone, first task with my new Previa is to figure out why I am having bad idle once hot. I started by taking out the throttle body, EGR, and EGR modulator. Two culprits found.

PXL_20250112_231837200.jpg

All the vacuum hoses to the modulator were cracked and/or fully blocked. When taking apart the modulator the filter was completely solid and caked with carbon. I tried cleaning it out but I think it is a total loss. I just ordered a new one (Toyota part number 25870-76010).

The other problem I found was that the intake tube to the throttle body is falling apart and was fully cranked off at what I believe is the crank case vent that connects between the intake hose and the oil fill. It is cracked almost completely off where shown here:
PXL_20250112_215232899.jpg

So I ordered a new Dorman intake hose and hopefully that will be sorted (Toyota part number 17881-76050).

The throttle body was really quite clean and the IAC was also smooth and clean. I gave both a thorough cleaning anyway, but I very much doubt either of those are my problem.

Njordi goes into my mechanic on Wednesday for a block test.

[Boerderij_Kabouter]

I found this great thread detailing the EGR modulator: https://www.toyotavantech.com/forum/...ulation-Previa

All the symptoms of a stuck open EGR align with Njordi 's symptoms (bad gas mileage, stalling at idle). Fingers crossed this fixes my issue this weekend.

[Boerderij_Kabouter]

The stoke is high!!!! Block test came back negative! No blown head gasket (for now). List of things after taking a close look up on the lift and running diagnostics:

1. bad mass air flow sensor
2. bad coolant temp sensor
3. leaking valve cover
4. leaking front main seal
5. leaking seal on the manual transmission
6. super small seeping on the front brake calipers

And that is about it!!! So once I get the idle figured out, I have a few projects, but nothing urgent.

[Boerderij_Kabouter]

Installed the new intake hose and new EGR modulator. Rechecked all vacuum hoses. And fired up Njordi...

Complete victory. Runs smooth as butter and idles at 800 (or whatever that mark is on the tach). Dead even, easy power, feels WAY better to drive.

Feeling very happy.

[timsrv]

Awesome!

[John Kaufmann]

I second Tim's assessment; I liked your troubleshooting (and Jonny's, on the EGR operation).
So to summarize: you think the running problems were due to the EGR modulator and a leaky intake hose?

[Boerderij_Kabouter]

That is correct. If the EGR modulator diaphragm is ruptured, the EGR valve will always be open, leading to a super lean air/fuel mixture and make it impossible for the engine to idle smoothly. The lean mixture causes a very low and inconsistent idle. Similarly, vacuum blockages or leaks cause the EGR system to function inconsistently. Here is the EGR function diagram from the factory manual:

PXL_20250117_172227735.jpg

Similarly, the broken intake hose was allowing air to enter the throttle body after the mass airflow meter on the airbox. So the computer has an inaccurate indication of how much air is entering and therefore the air/fuel mixture will again be lean and inconsistent.

[John Kaufmann]

Nice.

[Jonny]

When the modulator diaphragm is ruptured, the modulator unit is unable to create the necessary pressure differential between lower and upper halves for the modulator to function, so it is unable to effect change on the position of the EGR valve diaphragm. A functioning EGR valve is closed by default and most EGR valves work even at our vans' old ages. A faulty EGR valve might stick open, but this doesn't seem like it would happen very often, and a non-functional modulator would not be the cause.

[Boerderij_Kabouter]

I see that I was wrong there. So maybe my hot idle issue was more related to the leak in my intake hose and not really the EGR modulator. Either way, I am glad to have the EGR system working again and my hot idle is still going great. At least this is one thing off my project list! Thanks @Jonny !

[Boerderij_Kabouter]

The rough hot idle has returned. Makes me think it is actually to IAC. I cleaned it and it worked for a bit, but has regummed up??? Seems plausible. I think I will try cleaning it again and see if that does it. Then if the issue returns I bite the bullet and but the new one for $260. Oddly, the idle flattens out at 700 rpm if you let it idle for about 3-5 minutes. I posted up a YouTube video of the tach during the issue. I would love to hear any ideas about diagnosing the issue.

[Jonny]

I watched the video. Are you regulating the idle with the throttle to stop it stalling or do the revs oscillate like that on their own? Your note on the video makes it sound like it does that all by itself, but just wanted to clarify.

FWIW, I had a low idle when hot (~ 300 rpm - practically stalling), bought a used IAC (or ISC if you have one of the early year previa repair manuals) from the scrap yard... it solved the problem and has worked fine since. However, my problem was a constant low idle which never came back up, and my hunch is that your problem is different. A scrap yard IAC might be a cheaper way of testing whether it's the IAC, but of course the scrap yard one could be bad too (but my guess is that the chances are fairly low).

The idle oscillation seems like it might be the ECU compensating for some problem it is detecting. Maybe? Any codes?
I have a friend who has an old Toyota truck which does the oscillating idle rev thing. I don't know much about it other than he has a major exhaust leak.

[man_btc]

Did you make any measurements of the IAC ("ISC" in Toyota-speak) as in the shop manual? Here's a write up that's sort of a rework of the same info - can't remember where I sourced it from.

Idle Air Control Maintenance Practices.pdf

[Boerderij_Kabouter]

I am not touching the gas pedal or throttle at all in the video. That is the Previa all on its own hunting for a steady idle. I did not take the measurements when I first took the IAC off and cleaned it. I will be under the van again shortly to replace the front end seal, oil pump seal, and filter housing seal. When I am under there I will remove the IAC and give it a thorough going over and testing. Thanks for the tips!!!

[Boerderij_Kabouter]

It has been really busy lately, but I just took a minute to go check codes and I am storing a 41 code for open circuit at the throttle position sensor (TPS). None of the other codes that I used to be getting. I checked the wires going to the TPS sensor and the sensor itself and everything was in spec. so I really doubt that is my problem. Also, those who report the wire failure at the TPS connector have issues with throttle under load; that is not my issue. I think I will clean out the IAC valve first and see if that fixes it. Then, if that fails, I think I will take the EGR valve apart again. I never really tested the EGR valve itself after cleaning it. The EGR loop is the primary thing that changes between cold engine and hot engine, so this seems like a likely problem point.

[John Kaufmann]

It has been really busy lately, but I just took a minute to go check codes and I am storing a 41 code for open circuit at the throttle position sensor (TPS). None of the other codes that I used to be getting. I checked the wires going to the TPS sensor and the sensor itself and everything was in spec. so I really doubt that is my problem. Also, those who report the wire failure at the TPS connector have issues with throttle under load; that is not my issue. ...

Maybe, but I would not just discount an open-circuit code as spurious: the whole aggravation of the TPS harness problem is its intermittence, characteristic of fraying wires. If there is a correlation with engine load, that probably reflects mechanical stress on the harness as the engine rocks (vibrates) around its crankshaft axis. With frayed wire, it might not take much mechanical displacement to make a measurable electrical difference. FWIW, when I checked mine, I pulled the harness to see the problem, using an analog meter.