Help identify this farty noise right after engine starts

[francesconyc]

I posted this video in a longer thread but I guess it's worth keeping it separate for future readers and to isolate the problem

I made a video loop to show these strange "farty" sounds (grinding??)

https://youtu.be/c9z1WxBvrrA

it seems coming from the back/below but hard to say


anyone has an idea or ever noticed before?

[walt]

Can barely hear it, bad audio file.

[ErrorMargin]

Originally Posted by francesconyc

I made a video loop to show these strange "farty" sounds…

The video shows a big drop in idle speed concurrent with the farty noise. Down to about 250 rpm for a moment and then back up.

Try to confirm that the idle speed drop is happening at the same time and then try to figure out why it is momentarily dropping like that.

[francesconyc]

Yes exactly. That why im on this forum. I have no idea what is it.
Yes it happens at the exact time of the rom drop.

I didnt notice it again afterwards. But in hindsight it was not the first time either. (Even before replacing fuel pump filters and accumulator)

Mayeb the pop-of valve? Air pressure in the tank?
I found something on youtube about the tank farting but those were modern veicles idk

[icarp]

Sounds like the starter motor

[francesconyc]

Originally Posted by icarp

Sounds like the starter motor

Mm interesting.

[ErrorMargin]

Originally Posted by francesconyc

I found something on youtube about the tank farting but those were modern veicles idk

I don’t know either, but I’d start by focusing on the RPM drop as the problem instead of the noise.

[ben parrish]

The RPM drop most likely IS the noise. How is your idle after that drop? ICV valve been cleaned and tested?

[francesconyc]

Originally Posted by ben parrish

The RPM drop most likely IS the noise. How is your idle after that drop? ICV valve been cleaned and tested?

good point.

the idle is rough for 30 seconds (could be less but feels infinite and ONLY at startup.


when the car gets back to normal, then I get a sharp 1,000 rpm always, no matter if I am at intersections, at stops, in traffic, etc. cars run flawlessly with no sputter or any lack of responsiveness


Literally, my issue (described in other threads) is at startup.

[boyt911sc]

Originally Posted by ben parrish

The RPM drop most likely IS the noise. How is your idle after that drop? ICV valve been cleaned and tested?

The car is ‘82 SC. Therefore, no ICV. That’s for Motronic.

Tony

[ben parrish]

Ahhh…it’s been a while since I messed with cis but don’t I recall a cold start valve?
Can you get a fuel pressure gauge and see if you drop when the car idle dips?

[Rsnellie]

Fart noise sounds like a belt slip when the idle is trying to catch back up. I used to hear this when turning on the AC compressor. My thought is it’s gone as soon as you get your cis pressures and idle working within factory specs. Get your set of fuel pressure test gauges and save yourself 1,000s of dollars because the real pros on this forum, who do this for a living, can tell you EXACTLY where to look… or keep guessing and eventually get there $$$$. If the car has years of deferred maintenance this isn’t the worst thing for it, but you’ll need the set later down the road to fine tune it again.

[EC900]

Working alone is always a pain. Extra set of eyes and ears or a hand isn’t always available.
I found plenty of use for my 3 obsolete iphone cameras and cheap $10 tripods when trying to find mystery sounds and functions. Even lowered one down a 4” drain tile to find that a section of clay pipe had shifted caused a bit of washout and undermined a foundation.

[francesconyc]

Originally Posted by Rsnellie

Fart noise sounds like a belt slip when the idle is trying to catch back up.

this is interesting. it does feel like a sound coming from the engine but is not the engine. it must be the belt. I hope the skip doesn't damage in the long run.

Im overall leaning towards the idea that after changing new fuel pump, new filter, new accumulator, the problem may be that now the engine would need some adjustment (warm valve? etc) because who knows what the PO did given that he had a noisy pump.

this is the problem with buying cars from people who don't drive much: sure ehtye keep them in garage and maintain them well, but if they drive only 2 days a month they won't perceive all these noises.

[porsche930dude]

I vote starter. Pull it out and lube up the bendix with atf.

[francesconyc]

a new hi-torque starter is in my wishlist. after if ix other stuff I definitely want to change that as well