2.7 high idle when warm

[eurasiaoverland]

Hi guys

I've got my truck back on the road finally after a full engine rebuild and a whole lot more. I have just one niggle left with the engine. My truck is a 1996 Hilux Surf (JDM 3rd Gen 4Runner) 3RZ with no EGR (early JDM 3RZs do not have an EGR).

When I start it from cold, the revs are high, around 1500 rpm for a couple of minutes, I assume this is normal.

If I just leave it idling, once it has warmed a bit the idle drops down to 700 rpm, which is correct according to the FSM.

However, if I drive the car, even after half an hour or so, the idle stays high, at around 1000 rpm, sometimes a bit less, maybe 800 rpm. With the engine at normal temperatuire, if I am running and knock it out of gear and take my foot off the throttle, I see the idle dip down to 700 momentarily, then jump back up to 1000 or so.

Since this is an intermittent fault, I do not suspect that the idle stop screw is incorrect, because it will sometimes idle at 700 rpm. And it is definitely not a sticking throttle cable as I can manually turn the throttle bellcrank under the hood and confirm that it is sitting correctly on the stop.

I have no CEL showing. Aside from the high idle, the engine seems perfect, no hesitation and fuel economy seems about right, perhaps a little high though I have lifted the truck 3", so this might explain that).

I have replaced the following as part of the rebuild:

-New ISCV (OEM)
-New crank position sensor (OEM)
-New oxygen sensor (OEM)

I have also cleaned the MAF and throttle body.

Only things I can think of which might be affecting this are:

-Air temperature sensor (have a new one on the way)
-EFI engine coolant temperature sensor (have a spare)
-Fuel pressure regulator (seems fine, no fuel coming through the vacuum tube)
-MAF is bad
-Throttle position sensor (though I would imagine that this would affect the engine throughout the throttle range)

I'm beginning to think that the MAF must be bad... which would suck as it's a €400 part here plus tax and shipping

Has anyone else had this symptom? Could the MAF be bad without giving a code?

Thanks,

EO

 

[IBallEngineer]

Hi guys

I've got my truck back on the road finally after a full engine rebuild and a whole lot more. I have just one niggle left with the engine. My truck is a 1996 Hilux Surf (JDM 3rd Gen 4Runner) 3RZ with no EGR (early JDM 3RZs do not have an EGR).



EO

In order for the idle speed to change, air has to get in to the plenum, this can be a vacuum leak, an open throttle, or an open ISC. Excessive fuel would not have this effect.

I know you addressed the ISC, but that is a common culprit. The vacuum leak theory is also an option, seeing as you rebuilt everything, but that can be very hard to pinpoint. I encourage you to to some thorough searching on this forum to see what others have found, but my money is still on the ISC/vacuum leaks.

 

[eurasiaoverland]

Solved

Thanks for the reply.

I didn't want to fork out hundreds for a new MAF, so I decided to swap both the air temperature sensor and EFI coolant temperature sensor with new ones (OEM of course) and that has fixed it. Now, coasting down to a standstill with the engine running, I see the engine revs drop to 700 - 800 as the voltage drop across the MAF wires drops down.

I tried pulling vacuum tubes to the valve on the steering pump, and to the brake servo, and both caused the engine revs to drop (I think it could have stalled if I didn't plug the hole with my finger, so no vacuum leaks there.

Anything upstream of the plenum has no effect.

Now, I'm on the road to Russia with a nicely idling engine

Cheers

EO

 

[desertegal7]

Hi,

I had the same issue, i did everything to pin point what was wrong, had no CEL.
Last time i ever had it was when i drove for a long distance uphill, the day next i started the car up and heared a rattling eventually fro catalytic converter.
I removed it and pour out some little pieces of the catalyst then put back.
My conclusion is that it was partially clogged creating back pressure on the EGR then into the manifold thus getting high RPM.

Other theory is i used to fill it up to the neck since we had fuel crisis. So the charcoal canister definitely got its fair share, causing high RPM.

Also i experienced fuel smell during warm up only from the exhaus + engine run hotter since i could clearly hear the fan during those random occasions.


Never experienced it againg since the catalyst story and couldn't really tell what it was.

 

[desertegal7]

Also i rememberd something.
Once i floored gas pedal then got this high RPM back... But this time escalated to 2500 then 3500 which is undrivable... And slowle decreases to 1800-2000 but never less.
I discovered that the wire from the gas pedal to cruise control unit was shredding and now it is more. Replaced it and never had this high RPM... Thus i cannot tell it was it or the catalyst.

Also Google gave some logical result that throttle plate was wearing out by half millimeter allowing more air in while closed, you may start with this.