350 gets close to stalling when stopping

[Nilocsemliw]

It doesn’t happen every time I stop, but occasionally the rpms drop <500. It’s on my 1994 k1500 with a 5.7. I don’t have any issues with my idle after startup or once up to temp. This occurs when coming to a stop sign at ~30 mph. I push the clutch all the way in and shift to neutral when coming to a stop to try and avoid the issue and it still happens. The only thing I have done with the ignition system is replace the idle air control valve since I got it. Any idea on where to start the parts cannon? Or is this an easy fix?

I scrolled through the stupid engine questions thread and didn’t see anything like my issue.

 

[thegawd]

check for a leaky vacume brake booster.... it leaks more when the brakes are pressed.... the computer tries to compensate and then the truck falls on its face or stalls when you let off and give it the gas. when its leaking really bad you will hear a hissing noise and you will basically have unassisted brakes. I think all these old brake boosters are at the end of there life.

Al

 

[alpinecrick]

It doesn’t happen every time I stop, but occasionally the rpms drop <500. It’s on my 1994 k1500 with a 5.7. I don’t have any issues with my idle after startup or once up to temp. This occurs when coming to a stop sign at ~30 mph. I push the clutch all the way in and shift to neutral when coming to a stop to try and avoid the issue and it still happens. The only thing I have done with the ignition system is replace the idle air control valve since I got it. Any idea on where to start the parts cannon? Or is this an easy fix?

I scrolled through the stupid engine questions thread and didn’t see anything like my issue.

Can you put a scanner on it and see what your idle rpms actually are? Does this happen when coming to stop. Does it do it going down the road and putting the clutch in and letting it idle?
What brand of IAC?

 

[Nilocsemliw]

Can you put a scanner on it and see what your idle rpms actually are? Does this happen when coming to stop. Does it do it going down the road and putting the clutch in and letting it idle?
What brand of IAC?

It’s a 1994 so my obd2 scanner won’t work. It only happens when using both brake and clutch at same time. It’s a AC Delco IAC. I will check for vacuum leaks tomorrow it does have a bad hiss that sounds almost like a beep.

 

[97C1500TJ]

I second the brake booster. This gave out on me and made a loud long whoosh sound before the engine stalled. It’s a pretty easy replacement.

 

[96TahoeDoug]

No messing around change the fuel filter I did mine at 96k with onset of idle symptoms it was butt plugged black slip pouring out, more idle misses at 98k change again last night, purrssss real nice. $7.99 AutoZone cheapest. Statistics wise I can't believe people change them at all let alone 30k and it's rare I see it as a first case must do but it is a must do.

 

[Schurkey]

Yes to pretty-much all of the above; but as usual I favor connecting a scan tool and looking at the data stream.

Find out if the IAC is actually being commanded properly; can the scan tool raise and lower the idle speed? What is commanded idle speed vs. actual idle speed? Does the O2 sensor reading make sense? MAP sensor reading? RPM signal?

Wouldn't hurt to verify fuel pressure, look at the spray of fuel coming out of the injectors. Connect a timing light, verify initial timing and that it advances when you plug the connector back in.

 

[Schurkey]

It only happens when using both brake and clutch at same time... ...it does have a bad hiss that sounds almost like a beep.

Does it only hiss/beep when you push the brake pedal? Is it coming from the power booster or the booster plumbing? Or do you hear it in the cab?

 

[Nilocsemliw]

No messing around change the fuel filter I did mine at 96k with onset of idle symptoms it was butt plugged black slip pouring out, more idle misses at 98k change again last night, purrssss real nice. $7.99 AutoZone cheapest. Statistics wise I can't believe people change them at all let alone 30k and it's rare I see it as a first case must do but it is a must do.

I already have a AC delco fuel filter and will swap it in the morning. Thank you for your insight.

 

[Nilocsemliw]

Does it only hiss/beep when you push the brake pedal? Is it coming from the power booster or the booster plumbing? Or do you hear it in the cab?

It always has the hiss. I hear it loudest in the cab.

I have seen that your go to is always a scan tool. Which one do you recommend?