I’m hoping I can explain my 4runner’s behavior well enough that someone out there can give me a diagnosis. A strange thing happens when I am on an incline, and 3VZE engine speeds pass 4000 RPM. The engine suddenly gets quieter, there is a significant increase in power, and it almost feels like the clutch is slipping for a few seconds (but I don't think it is actually slipping, and it never slips at any other time). Under those conditions, the 4Runner runs exactly how I think it should run, but it sounds and feels wrong the rest of the time.

I remember reading about something in the EFI sensor system that disengages at full throttle, but I can’t remember what it is. Any suggestions?

Bump. No responses, huh?

Here are a few more details that may help. I just finished adjusting the valve shims, repairing some vacuum leaks, and replacing a bad PCV valve. The engine suddenly has more power, and the RPM range where the engine gets quiet and has better torque is now below 3000 RPM.


The engine noise sounds like a low pitched whine that increases pitch with engine speed. But as the engine transitions into quiet/power mode, the whine falls in pitch until it disappears completely. Sort of sounds like a siren. Any ideas?

Can you get it on video with a phone? I've gotta hear this siren you speak of!!!

It doesn't sound right to me, the engine noise goes away as it gets more powerful? Is the rpm increasing when it does this? Could it be that it is slipping and the sound gets quieter just because its now at a different rpm that's not so resonant in the cab?

Does it sound like a engine with timing gears instead of a belt or chain? that wanna be supercharged sound? Lol
If so that's not good I know I head that on one of my engines right before the belt broke and shredded.
Check for anything rubbing on the serp or ronin belt. And make sure nothing got left in the valve covers.

Good idea! I'm going on a road trip on Wednesday, so I'll see if I can record it.

Bad_Vision, Yes! It does sound a bit like a supercharger. I'd doubt it has anything to do with the belts though. I replaced the timing belt a few months ago, and nothing else is rubbing. It has always made that sound since I bought it, but it never just faded out until I hit high RPMs. Now, after replacing the gaskets and PCV, it fades out at a lower RPM. I'm thinking it might be vacuum related. Maybe one of the vacuum diaphragms has a small leak.

The PO told me that it overheated, and there were definite signs of heat damage when I did the valves. The PCV grommet was cooked, some of the split loom was so brittle that it crumbled in my hand, and the plenum gasket was brittle as well. Is it possible that the intake manifold gaskets are bad too? PO also said something about the power steering pump, but I don't know if that could be related.

Do you think it could be the bearings in your alternator? Mine are starting to get a little noisy.

nicnmike brings up a good point here, it could be the belt slipping. Check the alt belt, see if its glazed over. You can also pull the belt off and take it for a VERY quick drive (only if its NOT the same belt that drives the water pump!), see if that changes the sound and/or power.

Also make sure you bring the belt with you, and possibly a battery jumper box. If that belt was slipping, its possible the battery hasn't been getting a full charge and may only last a few minutes on the road without being charged!

Or, the safer way, is to just do a little maintenance and change all the belts out right now! And if you just did that, check to make sure they all have proper tension on them!

Good points. When I get back home, I'll run through everything with bearings or belts. Unfortunately, the noise got worse driving across the state. More importantly, the noise no longer cuts out at any RPMs. Now it sounds like a supercharger all the time. Oh, the irony.

The Plot Thickens

I noticed that my radiator was a bit low (maybe a quart) on coolant when I checked after driving across the state again. Since I had drained and refilled it before leaving, I just assumed that I didn't quite top it off all the way. I filled it up again, and after one week of driving, it is low again (barely visible in the radiator), and my temp gauge never reaches temp. No sign of leaks, and the oil looks good (probably not a head gasket). The last time something like this happened was on my F250. The intake manifold gaskets were leaking and sucking in coolant. I sort of suspected the intake gaskets might be blown while trying to troubleshoot the supercharger sound. Any thoughts?

Does the antifreeze look clean or is there a light oily film floating at the top?

How about the rad cap? Any oil residue showing up on it?

Check the oil cap too, make sure its not milky inside the cap itself.

Did you do a full burp on that radiator when you refilled it? This requires the heater to be in full heat mode, front end up in the air as high as you can get it (I use ramps on an steeply-inclined driveway), and the engine brough up to normal temp with cap off. When you see the water start rushing past the opening, the thermostat is open and you can then watch to see if it drops down a bit.

I'm gonna say its a possibility that you didn't burp the system, and you probably got all the water out of the heater core in the cab, but when filling it, the antifreeze didn't make it back in until you started driving and you turned the heater on. That would make it drop about a quart of 'freeze.

If you did burp it, then its time to do a compression test, then if that shows good, do a full leakdown test to make sure its not a tiny leak somewhere in the heads.

Does it have a thermostat? I can't remember how to explain this in the correct manner, so excuse the vagueness: The engine has to reach it's proper running temp in order to run correctly (closed loop, or something like that). Without a thermostat, it will never properly heat up.

My 4runner will have a coolant dump if I don't use the heater for a long time, but, once topped off, lasts for awhile. I'm sure there's a leak somewhere. I just bypassed the rear heater. Hopefully that will fix it.

As far as rpm vs horse power goes, my engine really kicks in somewhere past 2500. It's pretty noticeable. On certain days the power band is pretty consistent and there is no "punch". I've been fiddling around with things trying to make it consistent all the time. I've replaced the coil and put the original air intake duct work back in and that has helped some. I just recently replace the temp sensor and found a potential vac leak where the pcv valve hose connects to the plenum. Haven't had a chance to run it but it seemed to start easier (might be all in my head, but isn't everything?) good luck!

Yes. I installed a new thermostat when I replaced the timing belt and water pump less than 6 months ago. That was my first guess as well until I saw coolant was missing. Even if the thermostat is stuck open, I shouldn't be losing any coolant.

I am going to start a new thread since this issue may not be related to the high rev symptoms.