misfire
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5th Wheeling
Joined: Oct 2022
Posts: 31
Likes: 1
misfire
I hate this truck, I now have a completely random misfire. I’ve replaced plugs, rotor, rebuilt distributer, wires, checked spark gap, compression, got a new coil, battery/alternator, hell I even replaced the starter just for the fun of it which is a problem in of itself.
no idea what the missfire is from, it’s usually after the engine warms up a tiny bit and there may be a vaccuum leak or two but that’s not going to cause that.
im a licensed mechanic so feel free to use big words
no idea what the missfire is from, it’s usually after the engine warms up a tiny bit and there may be a vaccuum leak or two but that’s not going to cause that.
im a licensed mechanic so feel free to use big words
Lead Driver
Joined: Nov 2013
Posts: 7,178
Likes: 1,174
Superlative, as I've always been inclined to eschew obfuscation!
The $64,000 question:
When did you first notice the misfire? Is this a new problem after changing all those parts for some other reason? Or were those parts changed trying to eliminate the misfire?
The $64,000 question:
When did you first notice the misfire? Is this a new problem after changing all those parts for some other reason? Or were those parts changed trying to eliminate the misfire?
Laughing Gas
Joined: Nov 2013
Posts: 1,022
Likes: 79
From: A dirty little town in OR
Can you be more specific than "Random misfire"?
Have you isolated a faulty cylinder, or does it skip around? What do the plugs look like (fouling)? Have you put a timing light to it? Maybe put the inductive lead on each wire one at a time to see if you can isolate a cylinder by watching the light?
You've gone through the usual electrical suspects, but if it's a dead cylinder it could also be somewhere in the valve train from a wiped cam lobe on up to a sticky valve.
Since it's an 85, there could be an issue with the precambrian EEC IV computer or a sensor (if all of that stuff is still present). In that case, you get to spend some quality time with a multimeter and Google checking for acceptable parameters for each sensor... l don't know too much about that stuff except it either works or it doesn't. Mine didn't, so it all went away.
Have you isolated a faulty cylinder, or does it skip around? What do the plugs look like (fouling)? Have you put a timing light to it? Maybe put the inductive lead on each wire one at a time to see if you can isolate a cylinder by watching the light?
You've gone through the usual electrical suspects, but if it's a dead cylinder it could also be somewhere in the valve train from a wiped cam lobe on up to a sticky valve.
Since it's an 85, there could be an issue with the precambrian EEC IV computer or a sensor (if all of that stuff is still present). In that case, you get to spend some quality time with a multimeter and Google checking for acceptable parameters for each sensor... l don't know too much about that stuff except it either works or it doesn't. Mine didn't, so it all went away.
Thread Starter
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5th Wheeling
Joined: Oct 2022
Posts: 31
Likes: 1
Truck was duraspark swapped when I got it. Misfire was originally caused by a bad carb, PO had drilled out the main jet and other wacky things so I replaced it with a YFA clone. Misfire was pretty much always present after I re-revived it after it sat for a year. Always annoying and it made it hard to get up to speed. Fast forward to when I change the intake and exhaust gasket and throw some EFI headers on it, misfire gets worse. No vaccum leaks and it’s very intermittent. Random cylinders and it runs fine when cold.
Hopefully I get the money around for an intake swap soon so it’ll be able to compliment its rear and and 5 speed swap I’ve already done but this random misfire is stumping me. Last thing to check is valvetrain area.
Hopefully I get the money around for an intake swap soon so it’ll be able to compliment its rear and and 5 speed swap I’ve already done but this random misfire is stumping me. Last thing to check is valvetrain area.
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5th Wheeling
Joined: Oct 2022
Posts: 31
Likes: 1
all the stuff I’ve done has somewhat alleviated the misfire but it’s still present at random. Between the new starter needing shimmed and me not having much money to begin with I really don’t want to send the head off to be fixed.
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Laughing Gas
Joined: Nov 2013
Posts: 1,022
Likes: 79
From: A dirty little town in OR
You may have a cracked distributor cap hucking sparks all over the place or a couple of plug wires cross-firing w each other... easy enough to spot unharnessed electrons in the dark.
Throw a timing light on it and see if the timing jumps around, an indicator of a bad timing gear. Having sheared-off teeth on the distributor drive gear can cause erratic timing too.
I'd also look deeper into fuel/air management. Pull and read the plugs, they'll tell you the engine's secrets. I'd also double-down on vacuum leaks.
Having the plugs out, it's a good time to run a compression test... very low could indicate worn rings and/or sticky halves, broken valve springs, etc. I doubt any of that is the cause of what your chasing, but it'll at least tell you if your cylinders are sealing up. Also a good time to check your firing order (1-5-3-6-2-4) and adjust if necessary.
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Thread Starter
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5th Wheeling
Joined: Oct 2022
Posts: 31
Likes: 1
i don’t have access to a timing light but I have the fuel air dialed in as good as it’ll get. Should I move some spark plug wires around and see if they’re messin with each other?
Timing was like 40 degrees Retarded before I fixed it to 10 BTDC and then advanced it a little further because it runs better. Does it warming up point to any root causes? Most other people say exhaust valves going bad.
compression and firing order are just fine since I did the tests last week.
Timing was like 40 degrees Retarded before I fixed it to 10 BTDC and then advanced it a little further because it runs better. Does it warming up point to any root causes? Most other people say exhaust valves going bad.
compression and firing order are just fine since I did the tests last week.
Moderator
Joined: Jan 2001
Posts: 56,992
Likes: 2,741
From: Virginia
i don’t have access to a timing light but I have the fuel air dialed in as good as it’ll get. Should I move some spark plug wires around and see if they’re messin with each other?
Timing was like 40 degrees Retarded before I fixed it to 10 BTDC and then advanced it a little further because it runs better. Does it warming up point to any root causes? Most other people say exhaust valves going bad.
compression and firing order are just fine since I did the tests last week.
Timing was like 40 degrees Retarded before I fixed it to 10 BTDC and then advanced it a little further because it runs better. Does it warming up point to any root causes? Most other people say exhaust valves going bad.
compression and firing order are just fine since I did the tests last week.
Lead Driver
Joined: Jun 2013
Posts: 5,668
Likes: 1,160
From: Tolland, CT
What brand of plug wires ?
Does the rotor look like it's making a clean contact with the cap ?
You need to make sure those simple things are working well, but I guess you know that.
Can you put a volt meter on each plug wire to test it for resistance ?
I see you have new plugs installed, do you remember what the old plugs looked like ?
Can you do a vacuum system pressure test ? Not sure, if what us turbo guys call a "boost leak test" will work for you. I've done that on newer cars, but not on these old trucks. The idea is to pressurize the vacuum system and listen for leaks, you may have to plug a large opening, such as the air filter can, if a hose goes there. You only need a few psi to hear a leak. I would just give the hose I'm feeding pressure into a short quick blast of air from my compressor, you don't want to blow any hoses off.
A smoke test is another way to find a leak.
Does the rotor look like it's making a clean contact with the cap ?
You need to make sure those simple things are working well, but I guess you know that.
Can you put a volt meter on each plug wire to test it for resistance ?
I see you have new plugs installed, do you remember what the old plugs looked like ?
Can you do a vacuum system pressure test ? Not sure, if what us turbo guys call a "boost leak test" will work for you. I've done that on newer cars, but not on these old trucks. The idea is to pressurize the vacuum system and listen for leaks, you may have to plug a large opening, such as the air filter can, if a hose goes there. You only need a few psi to hear a leak. I would just give the hose I'm feeding pressure into a short quick blast of air from my compressor, you don't want to blow any hoses off.
A smoke test is another way to find a leak.
More Turbo
Joined: Oct 2020
Posts: 529
Likes: 54
From: Houston
[QUOTE=bullnosebuford;20914525]i don’t have access to a timing light but I have the fuel air dialed in as good as it’ll get. Should I move some spark plug wires around and see if they’re messin with each other [QUOTE]
A zero cost way to check if the distributor cap/rotor are causing problems is to get the engine running nice and warm. Then remove one plug wire at a time. If the spark inside the cap is jumping randomly, the engine may settle down to a nice, smooth, steady miss. It won't be fighting itself. Try that for all the plugs.
This test can also spot one or more bad wires. You might also try giving the wires and dizzy a spritz of water while running. If the wires are leaky, the missing will likely get worse.
These are Not guaranteed tests, but if they show a problem, the odds of real problems (valves, cam, timing gears, etc.) goes way down.
A zero cost way to check if the distributor cap/rotor are causing problems is to get the engine running nice and warm. Then remove one plug wire at a time. If the spark inside the cap is jumping randomly, the engine may settle down to a nice, smooth, steady miss. It won't be fighting itself. Try that for all the plugs.
This test can also spot one or more bad wires. You might also try giving the wires and dizzy a spritz of water while running. If the wires are leaky, the missing will likely get worse.
These are Not guaranteed tests, but if they show a problem, the odds of real problems (valves, cam, timing gears, etc.) goes way down.
Lead Driver
Joined: Nov 2013
Posts: 7,178
Likes: 1,174
Not trying to bust your behind, but I'm seeing two wildly contradictory statements. It's really hard to say what is going on with your poor truck, but checking the timing is one of the basics that needs to be considered. More than just checking the base timing, look to see if the timing fluctuates at various RPMs. Not saying your distributor is definitely worn out, but it should be checked. I had a troubleshooting nightmare about a year ago with loss of power above a certain RPM, a brand new problem on a trip several hundred miles from home. I convinced myself of several possibilities before finally nursing the truck home and eventually working through some basics. Only then did I discover the timing was WAY off, possibly due to a slipping hold-down clamp on the distributor. My stubbornness set me back a couple of days.
Then you also run the possibility one or more of your new parts is bad. Did you see any worsening after changing all those parts? I'd also be leery of your new Chinese carb. Some forum members have reported good results, while others said they didn't even make good paperweights. QC seems to be all over the place (or non-existant) at the Fling Dung factory.
Thread Starter
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5th Wheeling
Joined: Oct 2022
Posts: 31
Likes: 1
Going to run a vaccuum and timing test when my gauge and light get here from the other house, going to run a smoke test tomorow to see if a vac leak is what’s killing the engine.
the YFA carb also made it misfire so it really hasn’t changed the problem, just moved the truck to the driveway and the misfire is barley noticeable. Turn it off and back on again and the misfire is back big time. This 300 makes no sense.
