5.7L 1984, Rough idle at start up, then ok after a minute or so

[tfret]

Extremely rough idle upon start up, as if two plug wires are off. Tachometer goes ape s*&t. Then suddenly smooths out after about 1 minute. It's not the choke. It seems ignition related. Could I have a couple plugs failing to fire but then start working? That's exactly how it acts.

 

[Bt Doctur]

if it happens at the initial start up ,it could be water in a cylinder or two

 

[Rick Stephens]

Pull plugs and take a look.

 

[tfret]

I understand. If a plug is wet it won’t fire at first. That makes sense. What would that be a symptom of? Bad head gasket?

 

[crazy charlie]

I understand. If a plug is wet it won’t fire at first. That makes sense. What would that be a symptom of? Bad head gasket?

Rather than speculating or asking others to speculate,check the plugs.You have been given reliable solid advice.Take it and go from there.Charlie

 

[nola mike]

I understand. If a plug is wet it won’t fire at first. That makes sense. What would that be a symptom of? Bad head gasket?

Typically a bad exhaust manifold leaking into the cylinders. Might try a cylinder drop test, though might be hard to figure out if it's running that rough. Tach issues make it sound electrical though. Try disconnecting gray tach wire from coil.

 

[tfret]

Ok finally got all the plugs pulled. All dry as a bone and look real good. No water evidence. I’ll try disconnecting the gray tach wire. I’ve never had an ignition coil go bad. Could that act intermittently when cold? I still believe it’s an electrical issue. I’ve checked every inch of wiring and it all looks good with solid connections.

 

[nola mike]

Ok finally got all the plugs pulled. All dry as a bone and look real good. No water evidence. I’ll try disconnecting the gray tach wire. I’ve never had an ignition coil go bad. Could that act intermittently when cold? I still believe it’s an electrical issue. I’ve checked every inch of wiring and it all looks good with solid connections.

You're not looking for a bad coil, you're looking to see if the wiring to the tach/tach itself is intermittently killing the spark. The coil if bad would be worse warm.

 

[Rick Stephens]

Glad no water and all clean plugs.

 

[tank1949]

Ok finally got all the plugs pulled. All dry as a bone and look real good. No water evidence. I’ll try disconnecting the gray tach wire. I’ve never had an ignition coil go bad. Could that act intermittently when cold? I still believe it’s an electrical issue. I’ve checked every inch of wiring and it all looks good with solid connections.

5.7 motor mounts are more solid than an auto's. Get a good rubber glove and pull wires and notice if rps fall off. At least you will be able to identify which cylinder/plugs/wires are going bad. It could be valve needs adjustment or bad riser dripping.... I seen new plugs fail. They get mishandled, dropped, etc and ceramic cracks. Not good!

 

[poconojoe]

Hope you figure it out.
I'm gonna follow your dilemma.

 

[tfret]

I can see where a stuck valve might behave this way. However I’m not sure that would cause the tach to go buzzerk like it does. It will actually read zero for several seconds at a time. I ran it yesterday after pulling all the plugs and reseating all the wires. No change. Took the gray tach wire off and today started it and it ran perfectly. So hopefully it’s just a bad wire. That’s easy enough to fix. Will be a week or two before I can test it again to be sure it’s not a valve sticking. But I’m feeling pretty optimistic about the tach wire being the culprit. I’ll update when I know more.

 

[tfret]

Update today. Boat has been sitting for a week. Rough as can be when i cranked her up today. Same as before. Extremely rough for several minutes. Even smoked some leaving soot on the water line. Last week after sitting just over night it fired up and ran smooth. This sort of makes me think it’s a lifter or something. I also noted today no throttle response. The more throttle I gave it the loader the air being sucked in became, but not more rpm. I hadn’t noticed that before. I also have some erratic voltmeter activity. I actually replaced the alternator earlier this year thinking the original might be getting flakey. So it still seems ignition electrical in a way. I’ll try to get some insulated gloves and pull some wires off to see if I can isolate it to a particular cylinder. Again after warming up good she runs alright. Thunderbolt IV module getting flakey? Right now I’m baffled.

 

[tfret]

Update 10/2. I tried misting fuel into the carb during rough idle to see if it’s fuel related. Like maybe water in the fuel or fuel pump problem. No change. It seems to be taking longer for the issue to clear up after starting. Today I attempted 3 times to get on plane before actually being able to. As before, once it begins running right it runs fine. So I’m back to thinking something mechanical is sticking. The heads were rebuilt a few years back so they aren’t original. The lifters are original to my knowledge. Could I have a bad lifter that starts working after several minutes of running?

 

[flashback]

Think you would hear it

 

[tank1949]

Update 10/2. I tried misting fuel into the carb during rough idle to see if it’s fuel related. Like maybe water in the fuel or fuel pump problem. No change. It seems to be taking longer for the issue to clear up after starting. Today I attempted 3 times to get on plane before actually being able to. As before, once it begins running right it runs fine. So I’m back to thinking something mechanical is sticking. The heads were rebuilt a few years back so they aren’t original. The lifters are original to my knowledge. Could I have a bad lifter that starts working after several minutes of running?

If you can't wiggle wires with glove on, run it at night with muffs on. Perhaps you can see wires arcing? Just a thought...

 

[tfret]

Think you would hear it

It has through hull exhaust that is pretty loud so I can’t really hear the engine that well.

 

[tfret]

If you can't wiggle wires with glove on, run it at night with muffs on. Perhaps you can see wires arcing? Just a thought...

I tried this and couldn’t find anything. After it’s hot it runs fine. So I think I’ve ruled out ignition. Wire aren’t that old either.

 

[tfret]

Anyone ever had a lifter failure?

 

[tank1949]

Anyone ever had a lifter failure?

yep... I assume that you know how to adjust valves. Pull valve covers and watch rocker arms at cold start up and compare them as motor warms. It will be messy. But, at least this procedure may give you peace of mind.