RPM hunting - Not idle speed control valve

[Spetz]

Hi guys,

I've recently had the top end of my Mitsubishi 6A12 MIVEC engine apart.
So intake manifold, Y manifolds, cams, cam trays, valve springs, timing belt, fuel rail, injectors etc were all out.
Heads remained on the block

I've assembled everything and tried to start the car however the rpm are hunting.

I've disconnected the idle speed control valve and it made no difference.

What could this be?
Would a misaligned crank or cam sensor cause such issues?
Besides the hunting the engine is smooth and sounds fine (I did a tappet adjustment as well).

I don't think a vacuum leak could cause such hunting could it?
There is a slight injector leak as well where the o-ring didn't seat properly but it is very minor

 

[xnicke]

It could absolutely be an air leak!

 

[Spetz]

It has a MAP sensor though so there would be no un-metered air entering if it is an air leak?
Wouldn't rpm just go up?

I listened for an air leak and couldn't hear one but might need to test it somehow

 

[Astro14]

Vacuum leak. And you've already said that it's got a leak in an injector...

 

[spasm3]

Map sensor or not, a vacuum leak can do this. So can a bad iac valve if this engine has one.

 

[Drew99GT]

My corolla is map sensor based and it surges if you pull a vacuum line, so it most likely is a vacuum leak.

 

[3800Series]

The throttle position sensor can cause high rpm and low power. The cam sensor will cause rpm surging at idle and lose of power as well. A good bit of the time you can tell if its the can sensor as the rpm will just rise and drop constantly.

It could also be a bad plug or even a frayed line. A bad MAF can cause the vehicle to die off a lot but the sensor can be cleaned.

 

[SHOZ]

My Gen Coupe with an electronic throttle started the idle speed hunt. It was a dirty throttle plate/TB.

 

[eljefino]

Originally Posted By: 3800Series
The throttle position sensor can cause high rpm and low power. The cam sensor will cause rpm surging at idle and lose of power as well. A good bit of the time you can tell if its the can sensor as the rpm will just rise and drop constantly.

It could also be a bad plug or even a frayed line. A bad MAF can cause the vehicle to die off a lot but the sensor can be cleaned.


Yup. If the TPS is flaky the computer won't know it's at idle and that it has to fight to keep an idle. Won't throw a code either.

But I vote vacuum leak.

 

[Trav]

+3 Air/vacuum leak. Repair any air or vacuum leaks you can find, you know of one already, any unmetered air will scramble the ECM's egg.

 

[chrisri]

I agree with vacuum leak, you can diagnose it with carb or brake cleaner- but be absolutely careful, it's highly flammable chemical.

Second, some cars need TPS relearn, either via OBD or by disconnecting negative terminal and relearn procedure.