12A Rattle on Cold Start
 

'85 S, when it's cold (as in not-operating-temperature, it'll still do it if it's 85 degrees outside and the car's been sitting) and I start the engine, it's got an awful sounding metallic rattle for anywhere from a second or two to maybe 10 seconds. Seems to be random how long it lasts. It feels like the engine smooths out when the rattle fades out, but that could be an illusion.

When it's hot the engine fires right up on the first turn and runs great. It pulls hard and otherwise seems to be running very well.

Any ideas what this might be? I'd like to do something about it.

Exhaust? Engine mounts?

I doubt it's the exhaust, as it did it on the factory system as well as the RB system I put on it (this isn't a new rattle, it's just getting old).

Would engine mounts quiet down after a few seconds? Seems sort of unlikely to me. With the sound only lasting a few seconds twice a day (in the morning, then leaving work in the evening) it's tough to get an ear in the engine bay to narrow it down.

Oil pressure comes up strong quickly - even with the laziness of the gauge to show it, the noise continues long after oil pressure is established so I don't think it's a drainback/starvation issue, either (if this were a piston engine I'd almost think it was a main bearing).

I'll hazard a guess that it could be the waterpump. Try removing the belts when
you start it next time and see if it goes away. Also, could be the air pump if you have one.

could be your Alternator also but TG's suggestion will be your test

Maybe the clutch fan.

No air pump, and I changed the fan recently (there were some chunks missing from it - I was hoping that was the sound, but in either case it was a ticking time bomb). Would be curious about the waterpump, though I'm not sure how to start it up completely cold without belts and not either flood it (from turning it off cold) or overheat (from no waterpump/fan). No signs of leaks from the waterpump, and the temp never makes it past just-under-halfway.

To add detail (now that I've posted this thread I've been paying more attention to the sound), it starts a second or two after the engine starts, it's not right away.

Just watch temps closely and turn it off when it gets to temp on gauge.

Have you checked the body mounted heat shields under the car above the exhaust? Check them to make sure they're secure at all mounting points.

Do you have an aftermarket inline fuel pressure regulator?

​​​​I have one that looks like the below (for now), and the top dial will make a metallic rattle at idle and high revs.

https://cimg3.ibsrv.net/gimg/www.rx7...a810a176e9.jpg

That FPR is a very poor design and doesn't work very well. Better to get a good quality holley low pressure regulator with a gauge attached to it to monitor the pressure.

Thanks for all the input - while I've been listening to the sound, I have been doing much diagnostic since I'm trying to drive as much as possible these last nice days before winter finally arrives. I might have time soon to try a couple things though.

When I did the exhaust system all the heat shields on the body seemed to be secure, the rattle never comes back after it stops 3-10 seconds after startup, so I doubt it's those. I have the stock fuel system as well. I'd be curious if it might be something in the alternator; based on the voltage gauge it doesn't start charging until a while after the car starts (though its not correlated to the rattle starting or stopping) - not sure if that's normal on these cars or not. Might pull the belt off just that some time and see if it does it (and if I can feel anything odd turning the alternator by hand).

Don't spin the alternator by hand! If you do, you'll brun out the voltage regulator diodes. The alternator needs to be grounded and havve the field powered up to work without damaging it.

The fan clutch is temperature regulated and could result in a squeal or rattle on cold starts because the silicone fluid inside the fan clutch must circulate to lock up the clutch to the blades. This can take several seco ds if the fluid has leaked out over time, and a bad fan clutch bearing could allow the fan blades to rattle excessivley until the fluid circulates. To test this, remove the 4x 10mm bolts which hold the fan assembly onto the water pump, secure the blade assembly in the shroud and start the car. Do not run it this way for an extended period of time, or you'll overheat the engine.

I've worked on many a car and have never heard of an alternator being damaged from turning it by hand. Do these cars have some special super-sensitive regulator in them that taking the fan belt off and turning it would damage it? Wouldn't that mean it gets damaged if you were to turn the engine by hand, or while installing/removing belts? Is the field energized while cranking over the engine?

The unbolt-the-fan-clutch test does sound like it would be worthwhile some cool morning. I can aim a large fan in the car to help keep it cool until it's up to temp (keeping an eye on the gauge to shut it down once at temperature of course; remember this only happens from completely-cold starts).

My guess was fan clutch back in post 6. Could also be the alternator bushing but that would/should happen all the time.