Rough Idle until warmed up
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Rough Idle until warmed up
Recently purchased '85 E350 w/300 V6. Starts like a champ but idle rough upon start. Sometimes dies if I don't keep the revs up. However, when she is warmed up she idles just fine. Faulty electric choke?
Senior User
Joined: May 2010
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From: Spartanburg, SC
How would you replace that metal vacuum line if it was rusted off? Mine was broken off at the exhaust manifold and so I removed it. Can you buy a replacement for it and how does it attach to the exhaust manifold?
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Joined: Dec 2007
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From: Boise, Idaho
It doesn't screw into the manifold or anything. It's just a piece of tubing that exits the carb, and sits in an "indent" in the manifold. The carb being bolted down basically holds it in place.
Other things that may be causing the rough idle when cold though.
- Warped intake manifold. The warping causes a slight vacuum leak that seals itself up as the metal expands from the heat.
- Cracked vacuum lines (especially if you have those brittle color coded plastic ones). The engine is more sensitive to vacuum leaks when cold.
- Choke isn't open enough. Easy to tell, take the air filter off when the engine is cold, fully open and close the throttle and look at the choke plate. It should be about 1/16" - 1/8" open. Any less, and it'll be difficult to start. Any more, and it'll be getting too much cold air. Adjust it by loosening the choke housing and twisting. Once done, fully warm up the engine and be sure the choke is 100% open. Being 100% open after fully warmed up is more important in the long run.
Other things that may be causing the rough idle when cold though.
- Warped intake manifold. The warping causes a slight vacuum leak that seals itself up as the metal expands from the heat.
- Cracked vacuum lines (especially if you have those brittle color coded plastic ones). The engine is more sensitive to vacuum leaks when cold.
- Choke isn't open enough. Easy to tell, take the air filter off when the engine is cold, fully open and close the throttle and look at the choke plate. It should be about 1/16" - 1/8" open. Any less, and it'll be difficult to start. Any more, and it'll be getting too much cold air. Adjust it by loosening the choke housing and twisting. Once done, fully warm up the engine and be sure the choke is 100% open. Being 100% open after fully warmed up is more important in the long run.
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Joined: Dec 2007
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From: Boise, Idaho
The tube that runs down is to aid in warming the choke up. When I got mine, it had both that and an electric choke that worked together. A small vacuum pulls air through the tube and pulls warm air up from the manifold.
Mine was rusted out when I got mine and I have 15° winters here and I never missed it. I did eventually fix it, but didn't notice much difference.
Mine was rusted out when I got mine and I have 15° winters here and I never missed it. I did eventually fix it, but didn't notice much difference.
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From: Sheffield, AL
With it plugged up [most people simply crimp the tube where it enters the choke housing], the choke takes longer to fully open up. This occurs every time you start your "not warm" engine. This wastes fuel and theoretically allows more oil to be washed off the cylinder walls, which will shorten engine life.
If you do this, be sure to put a rubber cap on the open port at the top of the carburetor, where the clean air is drawn from to provide air which was to be pulled through the tube which is heated by the exhaust manifold prior to entering the choke housing.
By the way, there is a small vacuum port inside the choke housing, so if the line is not closed, one will have a vacuum leak.
If you do this, be sure to put a rubber cap on the open port at the top of the carburetor, where the clean air is drawn from to provide air which was to be pulled through the tube which is heated by the exhaust manifold prior to entering the choke housing.
By the way, there is a small vacuum port inside the choke housing, so if the line is not closed, one will have a vacuum leak.
Senior User
Joined: May 2010
Posts: 350
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From: Spartanburg, SC
So today, I decided to do the deed. I took some brake line and customed fitted a line from the carb to the manifold. I drilled it so the fit is snug and it worked out well. Do you think it will need any heat tape on it?
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