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Hey everyone, i have a 1987 Ford F-250 with a 460 Big Block W/ a 4 barrel carburetor. I notice that the truck has been having idle issues, it misses ONLY at idle, and its not a repetitive miss either, its random. I can floor it and the truck blazes tires but at idle it misses, i can put my hand over the exhaust and feel strong pulses of air when the engine misses. I’ve replaced the intake manifold gasket, changed the timing chain, got a new cap, rotor, plugs, plug wires, I’ve turned the idle mixture screws in until the engine started breaking up and i turned the needles back 1/4 turn, I’ve even rebuilt the carburetor to no avail. Lately i checked the voltage going to the ignition coil while the key is on and i only get about 6.5 volts. Is that my problem or is it something else? I was thinking of just running a jumper wire from the battery to the coil and run it on full 12 volts for a minute to see if its weak spark or something. What do you guys think? Thanks.
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The fact that it runs perfect when you open it up, makes me still think vacuum leak (Lean at idle) somewhere. I would think intermittent spark issue would continue on acceleration. Just for note on charging system though, you should check battery voltage with engine off, idling and raising RPMs. The HEI spark testers are great for testing spark at end of plug wire to determine if spark issue exist and then you could move up to distributer, etc but sounds as if you have already swapped alot of that.
Thanks for replying so fast. I took a smoke machine and tested for vacuum leaks and found a little one, got it plugged but not difference. As for the charging system, i have noticed that at night with the lights on they are really dim at idle, then i raise the rpm to about 1200 and the lights get way brighter. Bad alternator? would that cause the misfire at idle you think? Again thanks for replying.
Got the alternator tested by my good friends at Orileys. It tested out perfect, so now I’m not really sure what it is. I haven’t replaced the carburetor base gasket yet, probably do that tomorrow. But I’m out of ideas, I could go through the whole ignition system, replacing the ignition coil, the pickup coil, the ICM, but I get the same question, will it stop my misfire? Hope someone has an answer for me, I’m kinda stuck. Thanks again.
Was going to suggest having charging system tested. The only concern is the auto parts store don’t typically test with lights and accessories on. If you have a basic mutimeter. You can test battery voltage with car off. Should be near 12.6, then test at idle with headlights, blower and wipers on. Should be near 14-14.5. you can raise rpm to 2 , 000 and see if change. Also for vacuum. Any changes if you plug brake booster. Any exhaust leak. Do you have strong consistent spark at idle
Have you checked timing, maybe able to rent timing light. After that, compression, fuel. I wouldn’t throw more parts untill you know what your looking at. If going to replace everything in guess. It is cheaper to pay an hour diagnosis fee at trusted local shop
Okay, I’ll test the voltage with all the accessories on tomorrow. I tried blocking the brake booster with a plug at the manifold, but no difference. The truck does have that famous exhaust leak on the passenger side manifold though. Not sure how to test for strong spark at idle, guess I’ll have to get one of those spark testers.
I’ve also checked timing, it’s set how the emissions label says, 8* btdc with vacuum advanced unplugged. I’ll have to rent one of those compression testers also. I think I have a fuel pressure guage lying around somewhere. Thanks again.
Okay, I tested the charging system. With the truck off the battery is at 12.6 volts, with the truck idling I get 14.4 or so, and when I turned on the radio the blower the wipers, and the lights, it dropped to about 13.2 then I reved it to 2000 and it got worse (around 12.7). Is that supposed to happen? I’m not sure that will cause my misfire though.
Sounds like your alternator is bad or there is voltage drop in between alternator and battery. The fact that your voltage get worse with load and acceleration and your misfire improves, that is probably not the culprit all though poor electrical can cause all kinds of weird things.
As far as the exhaust leak, do the misfires appear to be coming from that bank alone (Power Balance Test). Exhaust leaks can cause lean conditions.
Any word on consistent strong spark with HEI tester? Any word on fuel? Did you spray carb cleaner, water around base of carb to check for leaking gasket? Sorry those odd misfires are really hard to help without hands on truck.Everytime it misses, it sounds like it comes from the passenger side (where the exhaust leak is) but it’s really hard to tell. I got a new carburetor base gasket, but it didn’t help, but I did need to replace it because the old one was really bad. I’m trying to find my fuel pressure guage but I just can’t, and I’m gonna have to wait till I get paid Thursday to rent a compression tester and spark tester. I was told about the pcv valve? It rattles when I shake it, so it’s good right? I looked a little closer and found that the exhaust leak is coming from the collector, maybe just tighten it? It’s just so rusty, not sure that bolt will move. Thanks.
Not sure PCV valve would cause but there usually only a few bucks and maintenance item. Try a little penetrating oil before snugging exhaust bolts, probably should put in new gasket if you can get it apart. Dont get carried away with over tightening and brake bolts. Sorry with all those other things checked, unless you find something out of the norm, it is hard to know where to point you. Best of luck and let us know what you find out.
I was told to test spark by taking off the main wire coming off the ignition coil, from the distributor and lift it up so the spark jumps to the distributer and see if the spark stops when the misfire does? They said I might have to use my slow motion camera to see if the spark stops? I just can’t get those exhaust bolts off, I’m thinking I might have to break them. I’ll see if I can take my torch to it and see if I can get it off that way. Thanks, you’ve been a great help.
Are the throttle shafts supposed to have play, I can shake them up and down with quite a bit of play, would that cause a vacuum leak? The primary throttle blades don’t have play in them. Just curious.
I am not a very smart carb guy. I know that the throttle shafts should have minimal to no play and it can cause vacuum leaks. They sell bush kits to repair this but I do not know if it would help you or not. Sorry.
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