Hi -- I have a '98 4-Runner SR5, 3.4L V6. It has developed a mystery noise that has my mechanic stumped.

I describe it as a hollow, thumping noise. It's at the front of the engine. My mechanic pulled off the front of the engine down to the timing belt and can't locate the noise. (He replaced the timing belt and water pump less than 6 months ago. The vehicle just hit 160,000 miles.) My mechanic suspects the noise may be in the cam area.

I recorded the sound using my iPhone. Visit the link below to give a listen!

What you'll hear -- I opened the hood and held the phone a few inches above the front of the engine. You'll hear the engine running. There's also some wind noise as the fan blows air across the phone's microphone. The mystery noise is the rhythmic, thumping sound. This was at idle speed. The rhythm speeds up as the engine RPM speeds up.

The sound is only heard when the engine is cold. Once it warms up -- say 5-6 miles in 70 degree weather -- the sound goes away. My mechanic verified this. He said they heard it when they started working on it, but as the engine warmed up the noise went away.

I'm supposed to take this vehicle on the road soon, so I'm kinda anxious to know if I need to rush to the Toyota dealer and open my wallet.

All thoughts are welcome!



Thanks!!
Steve

Here's the link to the sound file:

http://www.spatters.com/sounds/memo.m4a

 

Does the thump change speed with your rpms? I'm thinking maybe it's a sticky valve that gets unstuck after the engine warms up???

 

Yes. Noise speeds up as RPM speeds up.

 

Does the truck shake almost like it's missing when it's doing this?

 

I hear it and I don't like it.

I won't venture a guess at what it is.

Did your mechanic think it was silly to spend money on it to try to go after it and locate it? To me that would tell you how serious he thinks it might be...

let me send the link to my mechanical expert guy and see if he would venture a guess. Dang I hate annoying noises like that it can ruin your peace of mind. Hopefully it is nothing serious.

 

Sounds like it is about 1/4 to 1/6 your engine speed. Don't know if that helps diagnose it.

The only engines I've heard that had dirty/sticky valves and made thumping/ticking noises have been 4.7L Mopar V8's. That doesn't translate well to V6 toyota engines when trying to compare. To me it doesn't sound like valvetrain in the clip....but a clip is a clip....it's hard to get much besides the general sound and speed. Plus, the classic "dirty valve thump" does usually go away at speed, BUT it does not go away when the engine is warm. You can usually hear it at idle both cold and hot.

Possibly something broken in the water pump? You should have been able to hear that though if you took the whole front apart. Have you done the long screwdriver "stethescope" method yet? Try listening to every cylinder on the block with a long screwdriver and see if you can pinpoint it.

 

No shaking. Other than the noise all seems to be well.

 

My mechanic suggested I take it to another shop for more diagnosis. His words were, "I'd have it checked out."

(He didn't charge me a penny for the work he did. Said he wouldn't take anything since he couldn't help me. I've been using him for several years. Always seems fair and above board.)

He recommended a couple of other area shops he deals with. We also talked about taking it to Toyota. I'm okay with that as a last resort due to the potential cost.

 

My mechanic checked the water pump. He put it on about 6 months ago when he changed the timing belt. He said all that checked out okay.

 

my guy listened to the clip and advises having it checked out further before taking a trip with it. Good luck- hopefully it is nothing serious

 

I would take the fan belts off and start it cold.
No idea here

 

just a stab in the dark but maybe a wrist pin on number 1 or 2 pistons

 

Took it to Toyota. They did a lot of head scratchin', but three hours of labor charges later they determined that the noise was caused by a cam sensor that was out of place and was rubbing against the timing belt.

When I took it in I explained the problem to the service advisor. He called over one guy, who called over another, etc. At one point, as it was sitting in the service drive with the hood up and the engine running, there were 5 guys gathered around the front end all with their heads stuck under the hood listening and throwing out ideas. I heard ideas ranging from some washer being out of alignment, to the water pump, to a cracked fly wheel. I wish I had a picture!

I'm no mechanic myself, so bare with me as I try to repeat what the mechanics at Toyota told me they found.

Seems the mounting plate that the cam sensor is on was just a tiny bit out of place causing the sensor body to barely touch the timing belt. Theory is that over time the sensor was teased up out of the slot it rests in causing it to be more in contact with the timing belt. This would explain why it didn't show up until several months after the timing best was replaced.

Had I continued to drive it that way they felt that the sensor would have eventually come loose or come apart and tangled up with the timing belt. So, it's a good thing I took it in.

Once they reseated the cam sensor and realigned its mounting bracket the noise went away. There was a scratched area on the sensor body, but not bad enough to warrant replacing the sensor. Also, the timing belt wasn't harmed. So, no parts required, just labor costs.

Why did it only make the noise when the engine was cold? There are theories, but, ultimately, that part's still a mystery!

Thanks to all for your thoughts on this. Just wanted to pass along the results for future reference.

Best,
Steve

 

glad to hear you got it solved and did not ruin a trip over it. We all learned something.

Have a safe trip and enjoy!