I have a '90 RS 305 TBI and I have to run the A/C all the time to run my cooling fan so the idle surges every once in a while.

Anyway, when it comes down from a surge it levels at around 800 RPM at idle but it will occasionally drop to around 700 and when it does this you hear a scraping sound coming from the front of the engine.

Does anyone know what this could be? Harmonic balancer?

The weird thing is that it goes away as soon as it hits 800 RPM or higher.


Thanks to anyone that can help.

 

Could it be that the scraping sound is what is dragging the idle down? Maybe the A/C compressor?

 

I don't think it's that, it's difficult to hear it from above the motor, I usually always can hear it when I leave it running while I add air to the tires at the gas station, usually when I'm near the front left side of the car kneeling down putting air in the tire.

It's like a scraping or rattling.

 

your ac compressor is wore out

 

You can always do a bit of searching and add a fan switch to the dash. I need to get rid of the creeping temp when under 25mph which was my reason for adding the fan switch.

I'm with them on the compressor as well. Bearings can make different noises at different loads and RPMs.

 

Balancer is easy enough to tell from just a visual inspection- look straight down behind the water pump. If the outer ring has slipped bad it can move backwards and contact the timing chain cover.

 

lmao mine does the same exact thing. def from the compressor. idk if its bad or not...a'c is blowin fine....ive already spent liek 300 tryin to get a/c workin so its gonna stay the same damn way for awhile

 

Yeah, I will probably wire up a switch this summer, I attempted one last summer but I used too small of a gauge of wire and I didn't secure the wires so one of the accessories on the driver belt cut right through it.

So you guys think it's the A/C compressor? I'll shut off the A/C and listen whether it still happens and post back.

 

I'll take a picture of mine and post it up. I just came off the fuse block, 12v to switch, then to the fan relay on driver's side of firewall. I'm not 100% on which relay it is, or which contact on the relay off the top of my head.
This way you can run small guage wire, and not run anywhere near the accessories or radiator.

 

Alright man, that would help alot man.