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.

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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:
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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:

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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.