Slow throbbing at higher revs

[GheeBhow]

Hello all, I'm hoping for a bit of advice please.

My Minor 1000 has started to emit a slow, slightly rough throbbing once the revs get to something like 2500rpm. It pulses a bit faster than once a second, and seems to be coming from somewhere in front of the engine. I'm thinking it's probably the alternator (converted 30 years ago) or water pump, but is this something others have come across and resolved? It does it when stationary, so nothing to do with the running gear.

Thanks!

[les]

I once experienced this type of event, it was the fan blade, I had fitted a 4 blade fan.

[geoberni]

What, if anything, have you Added/Removed/Adjusted in the period between when this was not occurring and when it started....


I went out in my car for Drive It Day and there was a strange metallic rattle when car vibrating, like a nut or ball-bearing being shaken in a tin can....
Took me a few minutes to realise it was the button on the 'new' handbrake lever...
First time out on the open road, rather than a quick drive up and down the road to ensure the brakes were settled in....

Brake handle.JPG (46.8 KiB) Viewed 5356 times

[myoldjalopy]

" I'm thinking it's probably the alternator (converted 30 years ago) or water pump..."
Remove the fan belt temporarily and run the engine. If it is either of the two suspects you mention then the noise should disappear as the belt will not be driving them. Don't run for too long without the belt or the engine will start to overheat and the battery will not charge.

[GheeBhow]

Thanks for the quick replies both.

It still has the original 2-blade fan, so I guess it's not that.

I haven't changed anything. But it was left largely un-run over the winter, which makes me wonder if the alternator / water pump might have suffered from standing. When I first noticed it, I thought it might have been from the exhaust because I'd disconnected the middle section to get at the underside - but it's definitely not that.

[GheeBhow]

" I'm thinking it's probably the alternator (converted 30 years ago) or water pump..."
Remove the fan belt temporarily and run the engine. If it is either of the two suspects you mention then the noise should disappear as the belt will not be driving them. Don't run for too long without the belt or the engine will start to overheat and the battery will not charge.

That's a very good idea! Something for the bank holiday weekend...

[svenedin]

What, if anything, have you Added/Removed/Adjusted in the period between when this was not occurring and when it started....


I went out in my car for Drive It Day and there was a strange metallic rattle when car vibrating, like a nut or ball-bearing being shaken in a tin can....
Took me a few minutes to realise it was the button on the 'new' handbrake lever...
First time out on the open road, rather than a quick drive up and down the road to ensure the brakes were settled in....
Brake handle.JPG

I was also bothered by an annoying noise on Drive it Day. A high frequency buzzing. Seems to be coming from the passenger side glovebox area. Quite possibly a loose screw but I will need to go out driving with a passenger who can try to localise the sound. After all the work I have done it could be anything but I suspect the over-hard rubber engine mounts are causing the vibration. I once nearly lost the plot driving a very long way to Germany and being bothered by a rattle. I eventually realised it was the links on my watch band!!!!

Stephen

[GheeBhow]

What, if anything, have you Added/Removed/Adjusted in the period between when this was not occurring and when it started....


I went out in my car for Drive It Day and there was a strange metallic rattle when car vibrating, like a nut or ball-bearing being shaken in a tin can....
Took me a few minutes to realise it was the button on the 'new' handbrake lever...
First time out on the open road, rather than a quick drive up and down the road to ensure the brakes were settled in....
Brake handle.JPG

I was also bothered by an annoying noise on Drive it Day. A high frequency buzzing. Seems to be coming from the passenger side glovebox area. Quite possibly a loose screw but I will need to go out driving with a passenger who can try to localise the sound. After all the work I have done it could be anything but I suspect the over-hard rubber engine mounts are causing the vibration. I once nearly lost the plot driving a very long way to Germany and being bothered by a rattle. I eventually realised it was the links on my watch band!!!!

Stephen

That reminds me of someone who told me he'd been convinced he was being tracked by a UFO on the other side of a dark valley he was driving along, and much later on he realised it was the reflection of his ignition light on the driver's window...

[oliver90owner]

" I'm thinking it's probably the alternator (converted 30 years ago) or water pump..."
Remove the fan belt temporarily and run the engine. If it is either of the two suspects you mention then the noise should disappear as the belt will not be driving them. Don't run for too long without the belt or the engine will start to overheat and the battery will not charge.

Moj has got in one. If you were thinking hard enough, you would have thought of Moj’s suggestion. Get to it and report back. It might only take 5 minutes to check.

[GheeBhow]

Right, I've had the chance to unhook the fan belt and see what difference it makes. Without the water pump and alternator, the car runs very smoothly - so it was definitely one of the two. Or both contributing.

The surprise is that when I put the belt back on, the throbbing has stopped! Before starting, I noticed that the belt was a little slack - so my theory is that one or other of them was getting a bit old and loose, and the belt being a bit slack allowed a vibration to set up at higher revs. But tightening the belt is taking up the looseness that was causing the throbbing.

Does that sound plausible?? The alternator sounds a bit rattly, and I'm thinking that maybe its bearings have dried out - it's only done about 15k since it was fitted so I doubt the bearings have worn.

[oliver90owner]

I count three items. Alternator, drive belt and/or water pump.

15k miles is irrelevant if the belt tension had been too tight, or it was reconditioned with cheap bearings.

[GheeBhow]

Thanks. I'm going to watch and wait on this one to see if it returns.

[myoldjalopy]

OK, fair enough. No doubt you will hear the noise if it returns.
I discovered wear in the water pump on my car a few years ago by rocking the fan blades back and forth, which revealed quite a bit of play in the pump, despite it not making a noise. So I replaced it with a new one - no play at all in it.