Idle Issue - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Classics Fan · 11-13-2018, 01:49 PM

My 1971 Jimmy with the original 350 has recently developed a problem at idle. It runs fine for a minute or so then slowly begins to decrease to the point of running rough and threatens to die without feathering the throttle. When I do that it has some initial hesitation as well. The problem seems to have developed suddenly, not progressively. It runs fine down the road, no lack of power or ignition miss. Any thoughts or ideas on what could be causing the idle issue? TIA!
Posted via Mobile Device

SeventyOne · 11-13-2018, 01:58 PM

Whats your idle RPM?

Could be a vacuum leak.

**lks dcvn** · 11-13-2018, 02:44 PM

geezer#99 · 11-13-2018, 03:07 PM

It’s the weather!
Cooler moist air is the problem.
It creates a colder intake.
Cold fuel vapour inside the intake reverts back to a liquid. The liquid gas puddles in the intake. Some vapour gets to the chamber but is too lean to support god combustion. It runs rough.
Cure is a heat soak. Usually when it starts to run rough if you shut it off for 5 minutes and wait for the hot air in the lifter valley to heat the intake then you’ll be good to go.

Classics Fan · 11-13-2018, 04:32 PM

Quote:

Originally Posted by SeventyOne

Whats your idle RPM?

Could be a vacuum leak.

No tach. My best guess is around 800 RPM in park and 500-600 in drive. Vacuum hoses are in good condition but I will double-check the connections. Thanks for the suggestion.
Posted via Mobile Device

Classics Fan · 11-13-2018, 04:35 PM

Quote:

Originally Posted by geezer#99

It’s the weather!
Cooler moist air is the problem.
It creates a colder intake.
Cold fuel vapour inside the intake reverts back to a liquid. The liquid gas puddles in the intake. Some vapour gets to the chamber but is too lean to support god combustion. It runs rough.
Cure is a heat soak. Usually when it starts to run rough if you shut it off for 5 minutes and wait for the hot air in the lifter valley to heat the intake then you’ll be good to go.

The problem persists regardless of ambient and/or engine temps.
Posted via Mobile Device

SeventyOne · 11-13-2018, 05:07 PM

Quote:

Originally Posted by Classics Fan

No tach. My best guess is around 800 RPM in park and 500-600 in drive. Vacuum hoses are in good condition but I will double-check the connections. Thanks for the suggestion.
Posted via Mobile Device

Next i'd inspect/replace the plugs.

geezer#99 · 11-13-2018, 06:39 PM

Quote:

Originally Posted by Classics Fan

The problem persists regardless of ambient and/or engine temps.
Posted via Mobile Device

Ah!
So it happens all the time, not just when you start it cold.
That makes it easier!
I think I know what it is!

Classics Fan · 11-13-2018, 07:15 PM

Quote:

Originally Posted by geezer#99

Ah!
So it happens all the time, not just when you start it cold.
That makes it easier!
I think I know what it is!

I think the problem is solved, pending a test drive in a few minutes. I’m still interested in hearing your thoughts.
Posted via Mobile Device

Classics Fan · 11-13-2018, 08:37 PM

Quote:

Originally Posted by SeventyOne

Whats your idle RPM?

Could be a vacuum leak.

Mad remote troubleshooting and diagnosis, SeventyOne! I found the vacuum line to the PVC valve disconnected at a tee. Reconnected and it runs like new again! Thanks, and I appreciate everyone’s input! Hope to repay with some good advice of my own to someone in need in the future.
Posted via Mobile Device

SeventyOne · 11-13-2018, 08:41 PM

Awesome glad it was a cheap and easy fix!

**lks dcvn** · 11-14-2018, 02:54 PM

11-13-2018, 01:49 PM	#1
Classics Fan Registered User Join Date: Jan 2018 Location: Anacortes. WA Posts: 132	Idle Issue My 1971 Jimmy with the original 350 has recently developed a problem at idle. It runs fine for a minute or so then slowly begins to decrease to the point of running rough and threatens to die without feathering the throttle. When I do that it has some initial hesitation as well. The problem seems to have developed suddenly, not progressively. It runs fine down the road, no lack of power or ignition miss. Any thoughts or ideas on what could be causing the idle issue? TIA! Posted via Mobile Device

11-13-2018, 01:58 PM	#2
SeventyOne Registered User Join Date: Mar 2007 Location: Oakland, Ca Posts: 1,382	Re: Idle Issue Whats your idle RPM? Could be a vacuum leak.

11-13-2018, 02:44 PM	#3
lks dcvn Registered User Join Date: Jan 2007 Location: Columbus, Ohio Posts: 4,348	Re: Idle Issue

11-13-2018, 03:07 PM	#4
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,732	Re: Idle Issue It’s the weather! Cooler moist air is the problem. It creates a colder intake. Cold fuel vapour inside the intake reverts back to a liquid. The liquid gas puddles in the intake. Some vapour gets to the chamber but is too lean to support god combustion. It runs rough. Cure is a heat soak. Usually when it starts to run rough if you shut it off for 5 minutes and wait for the hot air in the lifter valley to heat the intake then you’ll be good to go.

11-13-2018, 08:41 PM	#11
SeventyOne Registered User Join Date: Mar 2007 Location: Oakland, Ca Posts: 1,382	Re: Idle Issue Awesome glad it was a cheap and easy fix!

11-14-2018, 02:54 PM	#12
lks dcvn Registered User Join Date: Jan 2007 Location: Columbus, Ohio Posts: 4,348	Re: Idle Issue



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.