Low idle, missing
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Low idle, missing
Hello, I have a 81 f150 with a 351m engine and while on my way home after stopping at a stop sign my truck started misfiring while accelerating. When I reached the next stop sign the idle was very low and "lopey". It will still start and run, but misses quite a bit. There have been no back fires at all. A little history on it- I bought it about a year ago and immediately replaced tune-up parts, cap-rotor-plugs-wires and rebuilt it's eldlebroc performer carb. It has always ran great with zero problems. In a few days I will test fuel pressure and check for vacuum leaks, test coil, ect... I already checked battery voltage (14v). If anyone has any advice on other areas to check, that would be great. This is my first ford and I know every rig has its different quirks and common problems.
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From: Northeast, OK
Welcome to FTE!
I have encountered this a couple of times on Edelbrock carbs, and both times it was stuff in the gas getting past the fuel filter and clogging something in the idle circuit. I found that pulling the idle air screws out and blowing compressed air though the holes fixed the problem.
But, if you are getting junk in the carb you may want to replace the fuel filter or, as I did, add a second one. In my case the truck had two tanks so I was able to add a filter in the short hose from the switching valve to the steel line running to the pump. And, I have one just ahead of the carb.
Also, I've found that the hoses used in 1981 (I have two 1981's) were not made with ethanol in mind. And all of the hoses were in bad shape. In fact, the one atop the rear tank was so gooey that I wadded it up in a ball and it stayed there. So I recommend you replace all the hoses with new fuel line. You can see the hoses here: http://www.garysgaragemahal.com/1980...s--bronco.html. But, if you have two tanks there is one at each tank, one from each hard line to the valve, one from the valve to the hard line, and one from the hard line to the pump.
I have encountered this a couple of times on Edelbrock carbs, and both times it was stuff in the gas getting past the fuel filter and clogging something in the idle circuit. I found that pulling the idle air screws out and blowing compressed air though the holes fixed the problem.
But, if you are getting junk in the carb you may want to replace the fuel filter or, as I did, add a second one. In my case the truck had two tanks so I was able to add a filter in the short hose from the switching valve to the steel line running to the pump. And, I have one just ahead of the carb.
Also, I've found that the hoses used in 1981 (I have two 1981's) were not made with ethanol in mind. And all of the hoses were in bad shape. In fact, the one atop the rear tank was so gooey that I wadded it up in a ball and it stayed there. So I recommend you replace all the hoses with new fuel line. You can see the hoses here: http://www.garysgaragemahal.com/1980...s--bronco.html. But, if you have two tanks there is one at each tank, one from each hard line to the valve, one from the valve to the hard line, and one from the hard line to the pump.
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I will definitely check that out. The previous owed installed a electric fuel pump on the frame rail near the switching valve with a filter before it and another inline filter near the carb.... however that does not mean that one of the hoses didn't break down, a filter clog, or the carb got crud in it. When I rebuilt the carb last may I cleaned out all the passage ways with carb clean, then compressed air, then ultrasonic cleaned it and finally another compressed air blast out. That would be a real bummer if crud made it past the filters into my clean carb! I work 12 hr days, so it will be a few days until I can do some trouble shooting. I will post the results of what I find. In the meantime, if anyone has more suggestions of what to check out- let me know. What are the chances this could be ignition related?
FTE Legend
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From: Northeast, OK
It could be an ignition problem, as a loose wire or bad plug could certainly cause the symptoms. So, as Dave said, check that first.
However, an electric pump on an Eddy carb can be an issue. Edelbrocks don't like more than 6 psi of fuel pressure or the fuel can overpower the needle and the bowl can overfill. So an inexpensive fuel pressure regulator set to 5 PSI would be a good addition. Make sure it is a non-return style, like this one I got from Summit: https://www.summitracing.com/oh/part...3033/overview/. And I added this gauge, which screws into a port on the regulator: https://www.summitracing.com/oh/part...3122/overview/.
However, an electric pump on an Eddy carb can be an issue. Edelbrocks don't like more than 6 psi of fuel pressure or the fuel can overpower the needle and the bowl can overfill. So an inexpensive fuel pressure regulator set to 5 PSI would be a good addition. Make sure it is a non-return style, like this one I got from Summit: https://www.summitracing.com/oh/part...3033/overview/. And I added this gauge, which screws into a port on the regulator: https://www.summitracing.com/oh/part...3122/overview/.
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From: Northeast, OK
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So.... I had a little time to check out the truck today. I installed my fuel pressure gauge with some clear tygon tubing so I could check for pressure or bubbles- 5.75 psi and fuel flowing good. I pulled out the idle screws and blew out the passage with compressed air. Runs the same. I swapped out all the plug wires with my old (known good) set. Runs the same. Checked vacuum hoses, all appear fine. I tested the vold age across the coil while running and it was only 4.3v. Now, correct me if I'm wrong, but isn't the dura spark system supposed to have 7v at the coil?
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FTE Legend
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From: Northeast, OK
Yes, it should have about 7 volts at the coil. However, your voltmeter cannot measure the waveform when the engine is running, so will give a false reading. The only way to measure it is with the engine off.
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After looking at it today, it doesn't seem to be running rich or lean- no overwhelming fuel smell and i can Rev it up with no backfiring (like a bad power valve). I was poking around with some starting fluid looking for leaks and I sprayed some right down the carb and there was zero difference at all.
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A miss under load may be ignition. What happens is that the more load you have on the engine the higher the pressure in the cylinders goes. More pressure means it takes more voltage to get the spark to jump across the gap in the spark plug. And as the voltage goes up the more chance there is of it jumping to another cylinder or to ground due to a bad wire, cap, or rotor. Or, the more chance there is of the voltage not getting high enough to fire the plug due to a faulty ignition.
You've replaced most of the usual suspects, so that may not be it. But, a bad wire or plug would probably cause the symptoms you are seeing.
You've replaced most of the usual suspects, so that may not be it. But, a bad wire or plug would probably cause the symptoms you are seeing.
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I will get on it tomorrow. I work 12 hrs a day graveyard and have a cattle ranch that requires daily chores. So, I only have a small amount of time for trouble shooting this! I will test ignition components, clean ground wires, check out plugs, and check for bad connections.