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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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I just did another vacuum reading and thought I would tell you guys what I get for readings. At idle I get about 15 inches and it occasionally drops to 14 inches (most likely when the engine has a big misfire). Just wondering if 15 inches is not enough. I took brake clean and sprayed all over. I sprayed the bushings for the secondaries and nothing, sprayed the intake manifold gaskets and the idle gets a little lower, but I have to spray a ton, to the point that there are puddles of brake clean on the manifold, I’m thinking the change in idle might be from the fumes being sucked into the carb, from spraying so much. What do you guys think?
That’s low vacuum. See if this helps:
http://www.gregsengine.com/using-a-vacuum-gauge.htmlThanks, I’ve replaced the timing chain already. And I’ve set the timing how it says too on the emissions label. 8* btdc with vacuum advance unplugged and blocked. I’m leaning more twords worn rings, cause it has low steady vacuum and I’m not sure, I need to test this again but pretty sure when doing the throttle snap test the vacuum goes to zero and only goes to about 20 inches. Again not sure, will test again and post my findings. Thanks again.
I sprayed brake clean around the shafts and nothing happened. I tried to wiggle them while the engine was running but they don’t wiggle at all. I think while runing they make a seal. They don’t wiggle much at all, but they do wiggle. I just didn’t see any difference when I sprayed around the shafts.
I want to thank everyone for helping me, I will definately use all this knowledge in the future. I figured it out (I think). The 460 has very low compression. Here is the picture of my test. I only did a wet test to one cylinder to make sure it was the rings. I don’t think I put enough oil in, but it when the up, that proves to me that the engine has worn rings and most likely worn cylinder walls. Thanks everyone for your help. I will definately post back when I’m done rebuilding the engine.
This is low compression right? I Googled this and there are people that say it’s bad and people who say its not bad. Just to save you guys the math, the difference in cylinders from the lowest to the highest is 12.2%.
It is low compression, and the wet test does suggest rings, but just in case have you adjusted the valves (if they’re adjustable)?
EDIT: Nevermind, they’re not adjustable. Should have checked before posting.
One more thing: do the wet test on all cylinders.
Alright, will do the wet test on all cylinders, when I get time. Is this the source of the misfire you think?
OK, so those numbers suggest that some cylinders have problems with rings and/or cylinder walls, and that some cylinders have valve or head gasket problems. It does look like it’s time for an engine replace/rebuild.
I want to thank everyone who helped me. I am planning on taking the engine out tomorrow and possibly tear it down. I will update you guys periodically during the rebuild. Thanks again.
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