Hey all,

I'm new to the forum, but I've had a 1983 F-150 with the 300 straight six for a couple months now. Me and a friend with some mechanical knowledge have spent a while caring for it because it has been neglected and/or left sitting for some time in its life.
When I got it, it ran but not well. We have replaced the non-feedback 1bbl carburetor, including the choke heater hose. We also did the valve cover gasket and the ported vacuum switch along with a few other small things while we were there. The spark plugs and wires appear to have been replaced shortly before I bought the truck. When I bought it, it wouldn't hold an idle at all -- so replacing the carburetor gave me huge improvements. At least it idles most of the time now.

My current problem is that the engine will sometimes try to idle very, very low if you don't give it some gas. The choke seems to work fine on startup; the truck will hold a nice high idle until it's been warmed up. Even when warm, it usually will keep itself running nicely. But once you give it gas and move it, it will tend to start dying when you have the clutch disengaged or the transmission in neutral. It's not as simple as an idle speed adjustment -- I have messed with that already. And the truck drives well when I'm in gear. It has only had problems with idling.

There is an exhaust leak from a couple of rusty, large holes in the muffler. The exhaust leaks bad enough that all the engine noise comes from the muffler itself, not from the end of the exhaust pipe. I'm planning to get this leak patched up ASAP. The cat is also probably original and thoroughly clogged up on the inside. I don't know if I'm going to have it removed or replaced yet.

My main question is: Is this engine known for being exceptionally picky with exhaust leaks? I can't find anything else that would be making it run badly. If anyone has a similar experience with idle issues or exhaust leaks on this engine, then that helps too.

Thanks and happy trucking!

 

Exhaust leaks way down the line on the muffler and the pipe are not going to affect it. If the cat is clogged, that can cause a problem. For test purposes, and since the exhaust is full of holes anyway, see if there is a two bolt flange under there that you can take the cat loose. Then run it some in the yard and see if the idle problem goes away.

The only other thing that comes to mind with the idle problem is carb freezing. I don't know where you live, but if it's around 40 degrees or below and damp or raining, you can have a problem like this. Another big clue is if after you get somewhere and park it for 10 minutes or so, then it clears up and runs fine after that. After you run it and get were you are going and turn it off, heat will make it's way up in the carb and melt the ice and keep it from icing after that.

300 sixes are not known for icing, but I bet it can still happen, since the factory had the heat tube on those engines also from the exhaust manifold. But I would check the cat first.

 

You say the truck has been sitting for a while, how long?
Bad fuel can and will cause idle issues. You can try running it off a can of good fuel just remember you have a carb bowl full of bad fuel it has to burn thru before the good stuff get in.

also besides adjusting the idle speed did you try adjusting the idle mixture?

Check for vacuum leaks using carb cleaner or the smoke test.
Dave - - - -

 

Franklin2:
Thanks, I will try loosening the cat and see if it helps at all. I am not with the truck at the moment but in about a week I'll be able to test that out. The temperature has ranged from about 30 to 60ish degrees since I replaced the carburetor, and it behaves about the same each time. I was actually pretty impressed with how well it started when it was cold.

FuzzFace2:
I don't know how long it's sat for. I bought the truck from someone who didn't own it for long, as he has a lot of projects to care for. The people he bought it from are the original owners, I think. But given the overall state of things, I thought either it sat or someone drove it without keeping up on maintenance. For example, the brake fluid is a little watery, the oil filter is old enough it's actually leaking some oil where it connects to the block, the tires are from 2012, and the old carubretor was in a state of running super rich and flooding, and couldn't be adjusted properly. As for the fuel, I have run it through a full tank and a half since I got it, and got rid of the old carburetor which was worn out and had the old fuel in it. So I'd guess I've gone through at least most of the bad fuel so far. I've driven the truck around 100 miles since I got it.
I did the idle mixture too. When it was too lean, the truck would barely idle at all. Maybe I should try turning it towards rich a little bit more. And my friend conducted a vacuum leak test and didn't see anything, but I will look into that more.

 

What shape are the plugs, wires, cap & rotor in?
If you don't know I would replace them then check the timing.


Once that is all set get the motor out to normal temp then adjust the idle mix again.
Turn it rich till the motor runs ruff then turn it in till it runs nice at highest RPM.
Some use a vacuum gauge to adjust the mixture, adjust to highest reading.
Dave ----

 

The plugs, wires, cap and rotor are all recent. However, it's entirely possible for me to have fouled a few of the plugs because the engine ran extremely rich with the old carburetor, before we were able to get it replaced. I was worried about timing, but I don't think it should be far off, because the engine runs pretty great when I'm driving around. But when I see the truck in a weekish, I will look into that and get it tuned right. Thank you!

 

To answer your questions about leaks, the carbed 300s are extremely infamous for vacuum leaks, intake/exhaust leaks, valve cover leaks (I see you've taken care of that already), etc, even though they are a very tough motor. My son's Bronco has the same setup (Duraspark II) and the same idle problems (idle too low/close to stalling randomly), which has been fixed for the moment by playing with the idle speed screw.

The short answer is to replace the EGR spacer>intake gasket, carb>EGR spacer gasket, carb mounting hardware if threads are worn, and intake/exhaust manifold gasket, then reevaluate. It's a lot of work, but after doing so with my '86 pickup a couple years ago, it's like a different truck with a smooth, steady idle around 600 RPM

 

Thanks for the useful information. I'm glad to see someone else has the same problem I do although I hope you can get that worked out too! I did a similar thing temporarily. I turned up the idle speed so that if it decides to run low, it probably won't actually turn off or flood out.

I remember changing out the "EGR gasket" while the old carb was off, but I suppose it might be worth a shot changing out the rest of the stuff. Before I tear it too much apart though, I might fix that muffler exhaust leak and see if it makes it better. And maybe I'll check the ported vacuum switch for leaks around its base or something just in case.

 

You know all this talk of EGR made me think of something.
Next time it is running / idling good remove the vacuum hose from the EGR and plug it.
Now go for a long test ride and see if it happens again. If it does not then it was the EGR hanging open at times and you now know where to look to fix this.
Dave ----

 

Okay, will do! Do you know which colored line is the EGR hose?

 

I don't think it has a color

It is easy to find the EGR as is sits right under the carb. The EGR plate is between the carb & intake and has a flatish can with a vacuum line to it.
The picture below is the EGR with the flatish can is facing you.


Again when it is running well pull & plug the vacuum line and drive it to see if it happens.

If all is good after the test drive then the EGR is hanging open at times. You can try and remove it, be carful of the gasket, and see if carbon is built up and the cause, a cleaning may fix it.

Or if you don't have inspections you can leave the hose off.

Or make a block off plate to fit between the valve & plate so it cant work and hook the hose back up so it looks like it works.
Dave ----

 

Okay. I don't think it will need cleaning; my friend and I did the EGR gasket while we had the carb off and it was not too gunked up in there. But it might still be worth a shot to plug the vacuum line anyways.
An EGR delete might be beneficial in the end. I've got a similar situation with the catalytic converter right now. Pretty sure it's factory original and super clogged up on the inside, and because of that the cat itself might be causing this whole issue. I don't know whether I'm going to replace or just remove the cat but I'm going to have the exhaust fixed soon.

 

As mentioned, the intake manifolds are connected and attached with the same a hardware as the exhaust manifold. Any exhaust leak can also mean a vacuum leak. Tighten all that up, see what happens. My '85 (the the fleet of '85s where I work, back in '86-'87) were in need of nearly constant attention in this area. A giant pita in an otherwise solid and reliable setup.

 

Thanks, I will check that as well. Was your '85 fuel injected or carbureted?

 

'85 carb. Computer controlled with miles of delicate plastic vacuum lines