Rough Idle
Thread Starter
Instructor
Joined: Mar 2008
Posts: 235
Likes: 1
From: Waynesburg PA
Rough Idle
Started to idle rough and hesitate/miss on acceleration. Smooths out after about 1500 rpm. Idles better in park/neutral but still alittle rough. Idling at about 700 in park and 550 in gear. Could it be vacuum leak? Was idling nice then got rough about 10 days ago. Does the idle need turned up. 77 L-48 with newer Q-jet. I changed fuel filter and plugs but didn't help
Instructor
Joined: Sep 2007
Posts: 218
Likes: 7
From: Tonbridge, Kent
If you get a small length of hose and put it up to your ear you can move the other end around the intake seals and other areas of the vacuum system and find where the leak is, as it eliminates the background noise of the engine.
Instructor
Joined: Jun 2008
Posts: 249
Likes: 0
From: Avon Connecticut
i have a '78 L48 and it starting doing exactly as you stated just 2 days ago.
park/neutral is around 700
drive and reverse drops, sometimes too low to keep running and stalls
but once i get it up to speed and above 1500 rpm, it runs very smooth.
thanks for any info!
park/neutral is around 700
drive and reverse drops, sometimes too low to keep running and stalls
but once i get it up to speed and above 1500 rpm, it runs very smooth.
thanks for any info!
Safety Car
Joined: Oct 2007
Posts: 4,360
Likes: 1,724
From: Seattle Area Washington
If you think you may have a vacuum leak try these three steps.
There is a metal vacuum fitting at the rear of the intake manifold. If you have cruise control it will have three ports on it.
1. The port pointing towards the passenger fender is for the cruise control. Pull the vacuum hose off that line and plug port on the metal fitting on the intake, start the car see how it runs.
2. If that didn’t help then re-connect that vacuum line and pull the top vacuum hose line of the same fitting and plug it. That port is for the vacuum modulator on your transmission. Start car see how it runs.
3. If that didn’t help re-connect that line and pull the rubber vacuum hose pointing towards the drivers fender, plug metal fitting on intake manifold and start the car see how it runs.
These three steps have now removed most of the major vacuum lines from troubleshooting process.
If your car ran better during any of the three then we can get more specific in a troubleshooting way to isolate the problem in a single area.
If you have no cruise control disregard step 1
There is a metal vacuum fitting at the rear of the intake manifold. If you have cruise control it will have three ports on it.
1. The port pointing towards the passenger fender is for the cruise control. Pull the vacuum hose off that line and plug port on the metal fitting on the intake, start the car see how it runs.
2. If that didn’t help then re-connect that vacuum line and pull the top vacuum hose line of the same fitting and plug it. That port is for the vacuum modulator on your transmission. Start car see how it runs.
3. If that didn’t help re-connect that line and pull the rubber vacuum hose pointing towards the drivers fender, plug metal fitting on intake manifold and start the car see how it runs.
These three steps have now removed most of the major vacuum lines from troubleshooting process.
If your car ran better during any of the three then we can get more specific in a troubleshooting way to isolate the problem in a single area.
If you have no cruise control disregard step 1
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Thread Starter
Instructor
Joined: Mar 2008
Posts: 235
Likes: 1
From: Waynesburg PA
Thanks Joe, I will try tomorrow, was just out messing with it and it was cold and idling OK. Seem to only happen at full temp, could this mean something? Carb is new, less than 2 years old about 1500 miles on it. It has never stalled but sort of pulses between 500 & 550 in gear hot.
OHH, dumb question, with what/how do you plug the ports (again I am a newbie).
OHH, dumb question, with what/how do you plug the ports (again I am a newbie).
Safety Car
Joined: Oct 2007
Posts: 4,360
Likes: 1,724
From: Seattle Area Washington
Go to your local auto parts house and ask at the counter if the have a HELP section. These rubber plugs come in a kit and they are variable sizes. These are only made for vacuum and in no way should they be used to cap off any fuel or corrosive fluids.
Here is what I use.
Is the choke on your carburetor opening up all the way when you’re at full operating temp?
Here is what I use.
Is the choke on your carburetor opening up all the way when you’re at full operating temp?
Thread Starter
Instructor
Joined: Mar 2008
Posts: 235
Likes: 1
From: Waynesburg PA
I got those plugs and tried the 3 ports, no real difference. Maybe alittle better with the straight up one plugged. That one has a little rubber boot then to a solid steel line. Where would this go, transmission? What to check here?
Choke is full open (straight up and down) when hot.
What else could it be, timming. still rough in gear 500 to 550 rpm and kinda surging. It doesn't sound like a miss, but I could be wrong.
Choke is full open (straight up and down) when hot.
What else could it be, timming. still rough in gear 500 to 550 rpm and kinda surging. It doesn't sound like a miss, but I could be wrong.
Thread Starter
Instructor
Joined: Mar 2008
Posts: 235
Likes: 1
From: Waynesburg PA
Found my vacuum leak. It was on a tee where the headlight vac line and a line that is a small wire-like vac line split right after the metal check valve and white plastic filter. The wire line had about an inch of old rubber vac hose that was hard and brittle and not sealing to the barbs on the tee. I did the classice bubba, BLACK ELECTRICIAL TAPE, but it sealed it until I find out how to splice a new piece of rubber hose onto this wire-like line. Anybody know how to splice this?
