300 Idle Issues
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Junior User
Joined: Aug 2007
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From: NEPA
300 Idle Issues
Hey again fellas, my 82 F100's 300 six (3spd manual) is having some idle problems I can't seem to figure out.
I start it, let it high-idle, kick it down, and let it warm up. Everything goes normal. When I stop at red lights or stop signs, however, the idle gets really low and sputters/chugs for a little while (up to 20-30 seconds at times). It then comes back to normal, as if nothing were wrong. Runs fine under throttle and response seems okay as well. This is annoying because trying to take off under the low idle make the truck buck and kick like rodeo bull. This sucks.
I've recently replaced the fuel filter as well as the distributor cap/rotor/wires, but this problem just started.
I'm thinking vacuum issue, perhaps? Any and all help is appreciated. Thanks in advance.
I start it, let it high-idle, kick it down, and let it warm up. Everything goes normal. When I stop at red lights or stop signs, however, the idle gets really low and sputters/chugs for a little while (up to 20-30 seconds at times). It then comes back to normal, as if nothing were wrong. Runs fine under throttle and response seems okay as well. This is annoying because trying to take off under the low idle make the truck buck and kick like rodeo bull. This sucks.
I've recently replaced the fuel filter as well as the distributor cap/rotor/wires, but this problem just started.
I'm thinking vacuum issue, perhaps? Any and all help is appreciated. Thanks in advance.
Cargo Master
Joined: Aug 2007
Posts: 2,558
Likes: 69
From: New Zealand
Hey again fellas, my 82 F100's 300 six (3spd manual) is having some idle problems I can't seem to figure out.
I start it, let it high-idle, kick it down, and let it warm up. Everything goes normal. When I stop at red lights or stop signs, however, the idle gets really low and sputters/chugs for a little while (up to 20-30 seconds at times). It then comes back to normal, as if nothing were wrong. Runs fine under throttle and response seems okay as well. This is annoying because trying to take off under the low idle make the truck buck and kick like rodeo bull. This sucks.
I've recently replaced the fuel filter as well as the distributor cap/rotor/wires, but this problem just started.
I'm thinking vacuum issue, perhaps? Any and all help is appreciated. Thanks in advance.
I start it, let it high-idle, kick it down, and let it warm up. Everything goes normal. When I stop at red lights or stop signs, however, the idle gets really low and sputters/chugs for a little while (up to 20-30 seconds at times). It then comes back to normal, as if nothing were wrong. Runs fine under throttle and response seems okay as well. This is annoying because trying to take off under the low idle make the truck buck and kick like rodeo bull. This sucks.
I've recently replaced the fuel filter as well as the distributor cap/rotor/wires, but this problem just started.
I'm thinking vacuum issue, perhaps? Any and all help is appreciated. Thanks in advance.
If not, the carb can ice up, partly blocking the throat & affecting the mixture, the idle drops like you describe; then it sucks the lump of ice out of the venturi & runs normally again.........then the cycle repeats.
In my experience this is worst when the weather is cold & wet (vs cold & dry); sometimes very humid weather can be bad, too.
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Junior User
Joined: Aug 2007
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From: NEPA
This is possible. The breaks do seem to require more effort than they used to, but it's not like the pedal goes to the floor like it would when pads are gone or there is a fluid leak. I'll look into it. Thanks bud.
I haven't had that tube hooked up for quite some time (the flex pipe was in horrible shape) and I've never had this problem until just now. I'll get a new pipe and rig it back up, see how it goes. Thanks for this, too. I wouldn't have considered this one on my own.
Do you have your hot air tubes etc hooked up (& working) to preheat the intake air?
If not, the carb can ice up, partly blocking the throat & affecting the mixture, the idle drops like you describe; then it sucks the lump of ice out of the venturi & runs normally again.........then the cycle repeats.
In my experience this is worst when the weather is cold & wet (vs cold & dry); sometimes very humid weather can be bad, too.
If not, the carb can ice up, partly blocking the throat & affecting the mixture, the idle drops like you describe; then it sucks the lump of ice out of the venturi & runs normally again.........then the cycle repeats.
In my experience this is worst when the weather is cold & wet (vs cold & dry); sometimes very humid weather can be bad, too.
Fleet Mechanic
Joined: Jan 2011
Posts: 1,479
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From: West Valley City
Could you possibly have a vacuum leak? These carbs are notorious for having the base plate loosen up from vibration. If you put your hand over the top of the carb, see if it wiggles. If it does, then remove the carb and tighten the screws underneath the carb and put it back on.
Good luck.
Good luck.
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Junior User
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From: NEPA
That did happen to me once before, but the carb is secure now. When I was having bad vacuum leaks in the past, the truck would pretty much refuse to idle at all. This is a bit different and I can't seem to pin down the problem.
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Junior User
Joined: Aug 2007
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From: NEPA
I recorded a short clip of how it's behaving. Turns out hitting the brakes has nothing to do with making it chug like this, but the brakes do feel weird (they're normal at first and will stop the truck, but then the pedal gets dead heavy if I hit it hard/go deep on it).
Just to possibly rule out some other ideas, this truck has:
New EGR (stated in video)
New distributor cap, rotor, and wires
Rebuilt carb, properly bolted down
New fuel filter
It also does this no matter where I set the timing, idle speed, and idle mix. It will bog down after letting off, then come back to normal after a few minutes. This makes crawling in first gear with a 2.38 rear end nearly impossible. It bucks and snorts and hates my guts.
<iframe src="//www.youtube.com/embed/nhSDDolcKAE" allowfullscreen="" frameborder="0" height="315" width="560"></iframe>
Just to possibly rule out some other ideas, this truck has:
New EGR (stated in video)
New distributor cap, rotor, and wires
Rebuilt carb, properly bolted down
New fuel filter
It also does this no matter where I set the timing, idle speed, and idle mix. It will bog down after letting off, then come back to normal after a few minutes. This makes crawling in first gear with a 2.38 rear end nearly impossible. It bucks and snorts and hates my guts.
<iframe src="//www.youtube.com/embed/nhSDDolcKAE" allowfullscreen="" frameborder="0" height="315" width="560"></iframe>
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Moderator
Joined: Nov 2007
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From: Denver Metro Area, CO
If you ask me, it's idling WAY too fast, it looks like what you're doing with your hand is effectively "kicking the throttle down" or, put another way, releasing the choke.
No saying that's what's happening, just that's what it looks like.
When you pull the vacuum line off the EGR, plug the end with your thumb else you have a vacuum leak which can cause/contribute to bad things.
Do you have a hand-held vacuum pump?
No saying that's what's happening, just that's what it looks like.
When you pull the vacuum line off the EGR, plug the end with your thumb else you have a vacuum leak which can cause/contribute to bad things.
Do you have a hand-held vacuum pump?
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Junior User
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From: NEPA
Can it re-choke itself? I mean, I give it two pumps and it starts on high idle, I kick it down, it rattle and sucks, and then in time gets out of that funk. I figured it was idling too high as well, but without a tach I can't really tell.
I don't have a vacuum pump, either.
I don't have a vacuum pump, either.
Moderator
Joined: Nov 2007
Posts: 22,415
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From: Denver Metro Area, CO
It *looks to me* like the idle speed is adjusted way too low (causing the low-rpm stumbling) and that the choke hasn't decided if the engine should be at high-idle or not.
You never answered - pull the vacuum from the EGR and PLUG IT WITH YOUR THUMB, what happens then?
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Junior User
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From: NEPA
It doesn't really matter how long I let it high idle, I've done it for as little as 30 seconds and as long as 3 full minutes. Kicking it down makes it stutter. Giving it gas after it's been warmed up for 20 minutes or longer does the same. It doesn't matter if I drive it for 10 miles or just bump it by hand in my driveway, there is always stutter after throttle.
I adjusted the idle speed to the extreme high side and it still happens.
Plugging the line to the EGR brings the idle speed up from the stuttering. Giving it throttle while plugged still resulted in dropping idle speed afterward.
If I let it go or reattach the line to the EGR, the idle does not drop back down. It drops back down if I then give it throttle, still.
I adjusted the idle speed to the extreme high side and it still happens.
Plugging the line to the EGR brings the idle speed up from the stuttering. Giving it throttle while plugged still resulted in dropping idle speed afterward.
If I let it go or reattach the line to the EGR, the idle does not drop back down. It drops back down if I then give it throttle, still.
Moderator
Joined: Nov 2007
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From: Denver Metro Area, CO
The EGR's vacuum hose shouldn't have vacuum unless 1) the engine is warmed up, and 2) RPM is 2,000+.
It's hard to tell in the video but your engine very well could be idling above that threshold given its current situation; it's also possible your vacuum hoses are installed incorrectly.
Disconnect the EGR's tube and plug the hose; actually, look on the back side of the valve and see if you can see the vacuum diaphragm moving in and out as vacuum is applied to it.
But this also has me wondering about the vacuum supply to the distributor, are you using ported or manifold vacuum. Then again, cripe, I'm not even sure if you're running DS2 or TFI-IV ignition and if you even *have* a vacuum line to your distributor....?
It's hard to tell in the video but your engine very well could be idling above that threshold given its current situation; it's also possible your vacuum hoses are installed incorrectly.
Disconnect the EGR's tube and plug the hose; actually, look on the back side of the valve and see if you can see the vacuum diaphragm moving in and out as vacuum is applied to it.
But this also has me wondering about the vacuum supply to the distributor, are you using ported or manifold vacuum. Then again, cripe, I'm not even sure if you're running DS2 or TFI-IV ignition and if you even *have* a vacuum line to your distributor....?
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Junior User
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From: NEPA
I'll check the EGR diaphragm in the morning. I did read that it's only supposed to move between idle and WOT. I'll report back with what I see.
I actually have a Duraspark I and a vacuum line coming off the dizzy. Here are pics of both:
Hope this helps, and thanks for taking the time help with this. It's nice to feel bit less hopeless about it.
I actually have a Duraspark I and a vacuum line coming off the dizzy. Here are pics of both:
Hope this helps, and thanks for taking the time help with this. It's nice to feel bit less hopeless about it.