1983 911SC making intermittent sucking sound at idle

[Miniwerks]

For lack of a better descriptor perhaps, my engine was making a new sound when I started it yesterday. On cold start it had a brief surging idle and quickly smoothed out. As the car was idling, I heard a new sound that was like a sucking or squeeeeshing sound every 10-15 seconds or so and would only last for a few seconds each time. At first I thought it was a belt noise but I don't believe so. Took the car for a 15 minute drive, parked it in front of the garage, did a few things in the garage, went back out to pull it in and would not start, turns over fine but will not fire. Waited overnight to try cold start rather than warm start and same result. If my fuel accumulator was about to fail, would it make noise? It was definitely coming from the engine bay. In hindsight, wish I had taken a video to capture the sound, would have been more helpful than my "descriptors".

[Miniwerks]

Read in one of the threads with similar starting issue that one check is to remove the airbox lid and air filter and with key in power on position, push the silver lever beneath the fuel distributor. I was not able to find that thread again. Wondering how long to hold the lever upwards before cranking over?

[john walker's workshop]

Try turning the idle speed air bylass screw a tad in ot out until the noise stops. Probably just an air whistle. The big slot screw with the spring under it on the throttle housing.

[Miniwerks]

Originally Posted by john walker's workshop

Try turning the idle speed air bylass screw a tad in ot out until the noise stops. Probably just an air whistle. The big slot screw with the spring under it on the throttle housing?

Thank you John, will give that a try once I get her going again. Despite the odd noise, it did run fine and has for months but failed to start after the last short drive. Need to check fuel pressure, make sure I am getting spark.

[timmy2]

Do you have a pop off valve installed in the air box?
If yes, check the o-ring is sealing. Or the unit hasn’t worked itself loose.

[Miniwerks]

Originally Posted by timmy2

Do you have a pop off valve installed in the air box?
If yes, check the o-ring is sealing. Or the unit hasn’t worked itself loose.

I do have a pop-off valve, will check it today. Thanks for the tip.

[Miniwerks]

Tried something different this morning, left the key in the power position for about a minute then cranked. The car tried to start very briefly. Tried cranking over again and no start. Did again with key in power position and waited, tried to start for a second or two. Fuel pump/pressure? I guess best way to rule in or out is to disconnect the fuel line at top of the fuel accumulator and crank over? Best to let car sit overnight before doing so?

[911obgyn]

Check pop off first

[boyt911sc]

Originally Posted by Miniwerks

Tried something different this morning, left the key in the power position for about a minute then cranked. The car tried to start very briefly. Tried cranking over again and no start. Did again with key in power position and waited, tried to start for a second or two. Fuel pump/pressure? I guess best way to rule in or out is to disconnect the fuel line at top of the fuel accumulator and crank over? Best to let car sit overnight before doing so?

What are you trying to achieve or test? Does the FP run when you turn the ignition switch to ON/RUN position (not START)? There are several things you need test or check to identify the culprit for your no-start problem:

• FP relay.
• Fuel pump.
• Ignition.
• Unmetered air.
• Cold start valve.
• Fuel Pressures.
• Etc.

Tony

[proporsche]

hopefully you did not hear any sound of mini bang..-in bad case air box split....

Ivan

[Miniwerks]

Originally Posted by proporsche

hopefully you did not hear any sound of mini bang..-in bad case air box split....

Ivan

No noises, just the low whine with key in power on position.

[Miniwerks]

Originally Posted by boyt911sc

What are you trying to achieve or test? Does the FP run when you turn the ignition switch to ON/RUN position (not START)? There are several things you need test or check to identify the culprit for your no-start problem:

• FP relay.
• Fuel pump.
• Ignition.
• Unmetered air.
• Cold start valve.
• Fuel Pressures.
• Etc.

Tony

Thanks Tony, maybe dumb question but to check my fuel pump relay, can I swap for one of the other relays?

[Miniwerks]

Got it running. Simply swapped the relay for the ac and fired up on first turn of the key. Will order new FP relay. Feeling pretty dumb now but if it ever happens on the road, I will know what to do.

[Schulisco]

Congrats!

The red relays contain a suppressor diode, the black ones not. You have to use the red relays for the fuel pump, you *can* use them as well on other circuits as the black ones, but not vice versa.

https://forums.pelicanparts.com/porsche-911-technical-forum/305738-930-fuel-pump-relays-red-black.html

Thomas

[Miniwerks]

Originally Posted by Schulisco

Congrats!

The red relays contain a suppressor diode, the black ones not. You have to use the red relays for the fuel pump, you *can* use them as well on other circuits as the black ones, but not vice versa.

https://forums.pelicanparts.com/porsche-911-technical-forum/305738-930-fuel-pump-relays-red-black.html

Thomas

Thanks Thomas, I will definitely replace with the correct red relay. I saw this part description on another site for the black relay and assumed it was ok for the fuel pump.

[Schulisco]

Originally Posted by Miniwerks

Thanks Thomas, I will definitely replace with the correct red relay. I saw this part description on another site for the black relay and assumed it was ok for the fuel pump.

The suppressor diode of the red relays protects the electronics in your car. Every time the coil is switched off a transient will be generated. This principle is desired and used on the ignition, but only there. Not for the rest of the car. For the function of the fuel pump it's not necessary, a black relay will do the job as well.

https://components101.com/articles/how-to-use-tvs-diodes-for-transient-voltage-suppression

Thomas