1985 F-150; Idle Problems

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1985 F-150; Idle Problems

Postby 300 4x4 » Tue Aug 07, 2007 9:13 am

Idles with a few misses when cold. Once it has warmed up, it hits maybe one cylinder, shakes bad, and coughs black gas out the tail pipe.

There is a known vacuum leak at the PCV valve, but when I seal the leak by hand, it runs worse. If I take that hose off, it idles very nice, but high idle.

It seems to run very rich, but is a carb adjustment needed, or is there something else that may be a common problem I'm unaware of. New cap and rotor are installed as of yesterday.
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Postby Ferris Bueller » Wed Aug 08, 2007 1:34 am

A 300 with 2 dead cylinders barely runs. I highly doubt a 300 would run on only 1 cylinder. Carbs after 83 have been problematic as were the chokes also. Be patient as there are many more knowledgeable members than I on here that can help.
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Postby The Plankster Prankster » Wed Aug 08, 2007 3:07 am

if you have a factory setup, then you have the miserable EFC (electronic feedback carb, better known as "evil F***ed up carb")

the root of your symptoms appears to be a choke that is not properly connected, thus causing air starvation. as a matter of fact, i got your exact same symptoms one day when i forgot to turn off my hand choke :oops:
after the vehicle has been running for 5 minutes or better, pull off the air cleaner, and a properly functioning choke would be facing straight up, not restricting airflow at all. yours is probably staying mostly closed off.
your choke should be controlled by a combination of a wire and a tube coming from the exhaust manifold. if either item is damaged, it wont work well, and the pipe is known for rotting out.

if thats not your problem, its somewhere in the electronics :roll:

if you plan to keep the truck for awhile and you dont have to pass emissions, i would strongly advise getting rid of the electronic carb for a normal carb. but this also requires you to replace your ignition system
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Postby 300 4x4 » Wed Aug 08, 2007 9:22 am

Thank you both for the replies. I will try this out tonight. Thanks again.
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Postby 300 4x4 » Wed Aug 08, 2007 8:17 pm

Well, the choke plate may be the problem. The thing is, the choke plate is always open. I can close it by hand, it is very loose, but as soon as the throttle is opened, the choke opens 100%.
There is a metal line going to the back of the carb that is broken, and I think that may be what the problem is.
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Is this a coolant line running into the choke, where there might be a thermister or something that holds the choke closed?
Maybe this carb was tuned so that it would run without a choke?
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Postby Harte3 » Wed Aug 08, 2007 11:41 pm

That looks like the heat tube that runs to the choke. Had the same problem on mine and replaced the tube...after digging the rusted part out of the manifold. In the mean time I blocked the choke valve open which made start up and cold running a real joy.

I believe there is a small amount of vacuum that is used to draw the heat to the choke. With the tube broken you probably have a vac leak which someone has tried to compensate for by screwing with the carb. Get all your leaks fixed then go after the carb adjustment.
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Postby The Plankster Prankster » Thu Aug 09, 2007 2:29 am

normally, when that hot air pipe is broken, the choke will not open up like its supposed to. therefore, someone has done quite the bit of screwing around with the intent of making it start well when cold.
IF you plan on keeping that carb setup (which i wouldnt), your first step will be to replace that tube - it draws hot air from th ex manifold to tell the choke when to open, then get it adjusted properly (which it isnt now). once the choke is right, then screw with the rest of it....oh, i forgot, you have that electronic mess, so it may well be a sensor or something causing the computer to control it wrong, its hard to say and harder to fix
if possible, convert to an earlier carb and ignition setup and youll fave a lot less trouble
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Postby 300 4x4 » Thu Aug 09, 2007 12:22 pm

One other thing. It only started to run really rough and rich after my girlfriend had to drive it for a week. One day she accidentally drove for 10 miles in 2nd gear.
After that happened she said it smelled like gas really bad and it stalled on her when she put it in reverse.
I popped the hood and while running, the fuel pump was spitting out gas from the back side, out of something that appeared to need a plug of some sort. So I plugged it back up with a makeshift plug, and there was no performance difference from the engine.
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Postby The Plankster Prankster » Fri Aug 10, 2007 2:35 am

replace the fuel pump ASAP - then change the oil imediately! a fuel pump that blew out that bad may well be blowing fuel into your oil as well :!:
83 F250 flatbed 300-6, NP435, 4X4, 5800lbs empty weight
87 E-350 6.9 diesel, c6, 3.55s, powertrax no-slip locker, onboard 120v power and compressed air, built out with toolboxes and toys
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Postby 300 4x4 » Fri Aug 10, 2007 11:21 am

How/where would the gas get into the oil? It didn't seem like anything actually broke, just as if a plug fell out. I'll post the picture asap, but I'll heed your advise on the replacement as well.
Thanks again for the reply.
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Postby Harte3 » Fri Aug 10, 2007 12:26 pm

It will get in to the oil! Sometimes the fuel pump goes bad and it all goes to the inside with no evidence on the outside it is happening.
'83 F150 300, 0.030 over, Offy DP, Holley 4160/1848-1 465 cfm, Comp Cam 260H. P/P head, EFI exhaust manifolds, Walker Y Pipe, Super Cat, Turbo muffler, Recurved DSII, Mallory HyFire 6a, ACCEL Super Stock Coil, Taylor 8mm Wires, EFI plugs.
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Postby 300 4x4 » Fri Aug 10, 2007 3:18 pm

Yea, I found out when I bought the new pump, that the hole that gas was leaking from was an indicator for a torn diaphragm. If the diaphragm is torn, gas will leak out the hole, and travel down to the cam.
So my new theory based on the previous posts and other outside resources is that the oil is flooded with gasoline. The vacuum manifold is inhaling gasoline from the PCV valve. Which explains running rich, and why the PCV hole in the valve cover shoots out smoke.

So hopefully the fuel pump replacement and oil change will remedy most of the problems. Then I'll have to start working on that choke.

The Plankster Prankster -- You said that you would remove the EFC for a "normal carb". Do you have any inexpensive suggestions? Thanks again.
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Postby crigby801 » Fri Aug 10, 2007 4:13 pm

Plankster, when you say to get a carb without the electronics crap, what carb are you talking about? I have one with the exact problems.
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Postby The Plankster Prankster » Sat Aug 11, 2007 2:53 am

the carb most people want is a carter YF found on our engines from 83-earlier.
some of the electronic carbs have their own weird bolt spacing, with the intent of preventing a swap, so if yours does you will need to replace the manifold with the one from the wrecking yard. my 86 did, but other people here were on the standard bolt spacing
please note that this conversion requires you to change out your ignition system for a duraspark (or points if you like old school like me :) ), which is also available from your donor at the wrecking yard.
failing to change your ignition will result in a lack of spark advance, which sucks!! i mean youll have no power and twice the fuel consumption :roll:
once that is done, you're free to remove the computer from under the dash, above the gas pedal, with its mess of wiring.
none of it is too expensive if you buy used parts
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Postby chico » Mon Aug 13, 2007 12:11 pm

one quick, easy way to tell if the fuel has ALREADY gotten in your oil is to check the dipstick. first, see HOW gassy it smells. fumey is one thing, flammable is quite another.

second, the level itself. if it has gone up, STOP RUNNING IT RIGHT NOW.

in fact, you should change the pump, oil and filter right now as it is, since you know you have had a pump failure. the fuel gets in the crankcase via the diaphragm, which is ruptured.
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Postby 300 4x4 » Mon Aug 13, 2007 1:31 pm

Problem fixed!

-Fuel Pump $30
-Oil & Filter Change $20

When I was letting the oil out and it was low enough where it was dripping out, gas vapor was pouring out the oil pan. So I could have probably fixed it all with only $50. During the investigation however, I replaced:

-cap & rotor $13
-spark plugs $10
-spark wires $i forget

Runs like a dream when It's warm. Now I remember how I have to keep on the gas when It's cold. It makes sense now. Thanks for all the help, and for the old carb suggestion.
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