engine and drivetrain Engine rattle only at 3100 rpms...

[Ferret2002]

Okay, here we go again . I can't figure this one out at all, hoping for input. My 75 2002 runs fine, engine has full power (or as much power as it had before the rattle began) and is acting 100% normal. EXCEPT for when I rev to 3100 rpms to about 3200 rpms, something in the engine bay is causing a rattle that actually leads to a slight vibration felt in the cabin. It's a pretty constant rattle, same noise no matter how long you keep it there...But above and below 3100-3200 rpms, the engine is unaffected....I've tried : Adjusting the mechanically advanced distributor, checked plugs wires and coil (all new)..and poked at the carburettor. (Literally poked..nothing else)...Any input?

PS: It happens both out of gear and under load.

It's a 75 with a 32/36 weber, the advanced ignition, and a head from an e21 if that helps any. Thanks for your time.

[c.d.iesel]

...what do the motor/trans mounts look like ?

got broken subframe steering/motor mount arm on the drivers side?

[AndyH]

I have a Miata that has IDENTICAL symptoms to what you describe. Mine is due to a loose heat-shield on my cat and since I don't have the torch to cut it off, I just live with it.

The best explanation I can offer is some sort of resonance at that RPM range. I realize your '02 probably doesn't have a cat or heat shield (mine doesn't) but I would give your exhaust a thorough examination, especially where your headers or exhaust manifold bolt to the motor.

[InkaSteve]

I have a Miata that has IDENTICAL symptoms to what you describe. Mine is due to a loose heat-shield on my cat and since I don't have the torch to cut it off, I just live with it.

The best explanation I can offer is some sort of resonance at that RPM range. I realize your '02 probably doesn't have a cat or heat shield (mine doesn't) but I would give your exhaust a thorough examination, especially where your headers or exhaust manifold bolt to the motor.

I agree with Andy. If you car still has the sheetmetal heat shield attached to the exhaust manifold, it may have a stress crack radiating from one of the mounting bolts. These are hard to see, but buzz like crazy at certain RPMs. Weld it up or drizzle a little JB weld on it to quiet it down for a while.

Good luck.

[FunElan]

Check the heatshield as mentioned, if it's not that check the connection between the downpipe and the resonator.

[JohnP]

voting for a bad motor mount...

[Napes]

I had the same thing @ 3000rpm and it was the air cleaner housing barely touching the underside of the hood brace. I had 3/32" milled off the bottom of the alum. riser that goes under the stock air cleaner housing and it solved the problem.

Bob Napier

[nbristow01]

crawl under the car and check the drive shaft to see if it has a ring around it. sounds like it could be the bottom of the shifter hitting the drive shaft at that RPM. I had this exact problem and was told by jeremy at IE to check this. he was correct and i ordered a new trans brace from BLUNT. Problem solved

[tjones02]

I have seen loose/missing bellhousing bolts cause a very similar rattle. Ask me how I know.

[RandyMolson]

My OEM aluminum exhaust manifold heat shield rattled like that when it had lost one or more mounting bolt. Long gone and replaced with the e21 gasket/shield assembly.

[Ferret2002]

Im sorry, I read over my initial post, realizing I gave a bad explanation. When I stand over the engine bay, and have it revved to 3000, the noise seems to be coming from either inside the engine or is muffled by something...Its very...how do I say this...rotationally based, as in it like flows with the engine and is a constant. Its more of a grindy noise...No hesitation, no performance loss, nothing. And it goes away clear after 3100 rpms..Not much vibration coming from the motor mounts, even tried bracing the engine by the intake manifold with my hands just in case(to make me feel better ofcourse...haha)..I took a few snapshots of my tranny mounts, and couldnt snag a focused one of my subframe...The exhaust is fine, altho thats not where the sound is coming from.

edit. attachment failed, not that you really need to see the pix i took of the mounts..

Thanks for your time and input.

edit part 2: I'm sorry. This is rediculous. I'm basically not providing enough information online, since its hard to describe the noise / problem. I'm gonna regroup my hopes tomorrow, try to narrow it down to a spot in the engine, and if I can't figure it out, ask around town. I wish I could get an audio clip, but the camera I have creates nothing but static and engine noise...Can't tell a damn thing.

[supark]

what area have you localized the noise? Could be alternator mount bushings that have gone south. Mine are toast on my 72, and allowing too much fore/aft movement at higher rpms. Try twisting the throttle while holding the alternator stationary and see if the rattle goes away.

[TobyB]

The dizzy? I had a mech advance come apart and do exactly this.

I replaced a lot of things before I found it.

If that's no help, try a mechanic's stethoscope.

Those things work great for the $15 they cost..

hth,

t

[c.d.iesel]

.....yer timing chain tensioner is sheit .

[fwardell]

Oh yeah, sounds like one I had a while back. Turns out it was the aluminum heat pipe from the exhaust manifold to the intake cold weather valve, you know looks like a 2-inch diameter metal clothes dryer vent hose. A couple of tie wraps took care of it.