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390 suddenly stops running

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Old 07-15-2014, 07:15 PM   #1
jckinthebox
 
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Default 390 suddenly stops running

I have a 1968 coupe with a 390/C6 that I have been restoring for a couple years and I was a few hours away from road worthy when all of a sudden the 390 that had been gotten to running decent developed a severe chop to the idle and stalled. I tried restarting it while giving it some gas and stomping on the pedal made virtually no difference, it sound like all of a sudden its decided to run on a couple cylinders here and there..

We pulled the valve covers off and all rockers looked fine. Checked compression, all good. Replaced coil checked spark plugs and wires. Im about out of ideas, any help as to what in the world my issue could be would be helpful.

Thanks,
Jack
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Old 07-15-2014, 08:52 PM   #2
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I had a similar situation with my 68. Would run good for a while then die suddenly. I hooked it up to a battery charger and it stayed running turned out there was not enough voltage at idle and it would stall. The funny part was the battery would start the car fine but not keep it running. Hook up a charger if you have one and see if it helps.
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Old 07-15-2014, 09:34 PM   #3
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I'm fairly certain that the battery isn't an issue, like I said it's been running fine for a while and then decided to quit on me, I'm thinking more along the lines of a timing chain issue or maybe a distributor. Are either if those p
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Old 07-15-2014, 09:35 PM   #4
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*possibilities?
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Old 07-16-2014, 02:17 AM   #5
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Could be fuel related. How long did it sit and how old is the gas? Could have sucked something up and the carb is clogged. For it to just stop running all of a sudden that would be my guess. Try to manually choke it to see if that helps or use some gas poured down the carb to get it started first and see if it runs for a few seconds on all cylinders.. If it sounds normal, when you start up by pouring a littl fuel in, then its probably carb related. If it doesnt sound "normal" before it dies, you may have a slipping timing chain.. maybe..
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Old 07-16-2014, 02:53 AM   #6
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I agree with Justin on the fuel issue.

Also, does your 390 have a fuel rail installed? I don't know how the old FE motors ran with those fuel rails, if you have one, I would by-pass it. The fuel hits that tube and boils from the heat coming off of your intake. Other than that, its probably some sort of vapor lock, either in the fuel line or heat off of the intake hitting your carb bowls.

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Old 07-16-2014, 06:56 AM   #7
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Ill definitely give that a try but im hesitant to think that it will be the fix because when the car isn't running and you pump the gas you can clearly see gas being dumped into the carb. Although I suppose if it were clogged it could be one of the jets or something. Ill try it out and see what happens.

Thanks,
Jack
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Old 07-16-2014, 11:12 AM   #8
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spark seems more likely than fuel if the accelerator pumps are dumping fuel into the carb then it should run for a few seconds at least.

Take a volt meter to the battery whats the voltage?

Pull the boot off the coil and place one end of something like a pipe about 1" above the coil where the boot was and the other end across a good ground like the intake or bare metal and crank the motor over do you see spark jump out of the coil to the pipe?

Take your volt meter to the coil wires whats the voltage?

What ignition are you running...points or electronic?

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Old 07-16-2014, 11:33 AM   #9
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battery has 14+ volts on full charge which is when all these tests are done so that most likely isn't the issue and the coil is not even a day old at this point so I would hope that its fine but I can try to see how far the spark wants to jump. Its a points style ignition and which coil wires are you talking about?

Seeing that, as far as I can tell, i have both fuel and ignition, im just lost.

Im hopefully going to have some time to pull the timing chain cover after work today and check out the alignment there
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Old 07-16-2014, 11:58 AM   #10
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You cant just keep ignoring suggestions you've gotta try at least the simple ones or find a simple way to disprove them.

Before we spend hrs pulling a timing cover lets cover the basics!!

1) you pulled the valve cover with the valve cover off do you see the valves move when you crank over the motor...obviously yes because you claim compression was fine on all cylinders so right there we know with nearly a 100% confidence that the timing chain isnt broken...it could have skipped a tooth but you claim it ran at idle for a few seconds...it ran crappy but it ran then died never to restart....well that makes a skipped tooth seem less likely and maybe that whole timing cover adventure is misguided at this point...maybe

2) you threw a new coil at it but if the old coil wasn't getting power a new coil wont help

3) NEW is not a verification of "GOOD" new and good should never ever be assumed to mean the same thing...ever

4) you see fuel going into the carb ventures when you move the throttle even if nothing else on the carb was working with the exception of the throttle plates 3 good shots of accel pump squirt and a 10 second wait before crank would get the engine to fire and run for a few seconds....it does not so let us investigate something more likely than fuel at this time.

5) based on the above lets do something to confirm that we have spark at the coil. This coil is the main coil that plugs into the distributor....I am unaware of the existence of another coil that may come to mind. Do the pipe test and see if the spark will jump out of the coil to the grounded pipe...this is super simple and takes almost no effort....If you see a nice fat spark then go ahead and check out that timing chain. If you see no spark then its something else like a broken wire or bad points

In place of the pipe you can use a wrench or bare wire anything thats metal that can touch a ground like a bolt or unpainted frame, motor piece and the other end hovers 1" above the coil where the boot fit onto.
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Old 07-16-2014, 12:06 PM   #11
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6) pull the cap off the distributor and crank the engine over does the rotor spin if no then its a stripped dizzy gear...also a very very easy test and a hypothesis that 100% fits the symptoms this is a very likely culprit...and takes about 3 min to check.
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Old 07-16-2014, 12:10 PM   #12
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I apologize if Im making it seem like im disregarding your suggsetions.

1. yes pulled the covers and all valves seemed to be in order.

2 the reasoning behind the new coil was that the secondary coil was reading over 14000 ohms of resistance which by my research is too high.

3. I agree completely however I test the resistance of the new one before it went in and checked spark at the plugs after installation which was why i believed it to be functional.

5. I will give it a try tonight

And I was asking which wire haha im assuming you mean the high tension lead going to the distributor but wasnt sure

Once again really appreciate the input,
Jack
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Old 07-16-2014, 01:56 PM   #13
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still running points/condenser system or electronic? If Old school, could be the condenser is going out - heats up then fails, cools down and is fine. Electronic could be the control module is getting overly hot and failing until it cools down.

Fuel Vapor Lock, again, heats up and boils/vaporizes the fuel in the fuel line until it cools and recondenses. If it just started heating up in your area and this problem started about the same time, those are the things I'd look at first.
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Old 07-16-2014, 05:28 PM   #14
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I am unfamiliar with whatever system uses a secondary coil. By coil voltage I meant what normally would be power to the coil typically 6 ,9 or 12v. NOT coil output which is typically 30k , 35k or 45k.

I was also unaware that you had checked and verified spark at the plugs.

SO that means

1) we have spark at the plugs and you can see it right? if yes this changes everything

2) lets make one last fuel test because now we have spark. Take some clean fresh fuel in a jar and pour a bit less than a 1/2 oz or so (a shot glass is perfect) pour a portion of the fuel down each carb barrel until there is none left in the shot glass. Get in the car and hold 1/4 throttle count to 15 then crank it over. It should run and stay alive for at least 5 seconds.

3) if after doing 3 you get nothing then its almost certainly a timing issue, timing chain skipped, dizzy skipped a tooth, dizzy is loose and rotated far enough retard the engine to the point where it wont start.... if you are sure the spark is hot at each plug then and 2 did nothing then look to here to 3
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Old 07-16-2014, 07:58 PM   #15
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hello;

umm, check the timing. the pin on the distributor gear might have sheared or the gears might have broken.

remove number 1 plug.

hold a ling thin screw driver in the hole while someone competent rotates the engine until the screw driver goes up then just starts to go down.

back the engine up around 5 degrees.

if the timing mark is off by 40 degrees or more your chain jumped.
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Old 07-16-2014, 08:31 PM   #16
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Have you checked all of the vacuum lines? Run a Vacuum test? Could be a simple as a broken rubber hose. Another thing to check on these engines is the wire from the distributor to the coil. This flexes every time the distributor rotates due to vacuum or centrifical advance. They can break inside and look good. If you grab wire in the middle and give it a good pull it will stretch if the inside wire is broken. Sometimes the insulation will hold the conductor close enough to check OK, but when the engine starts the vibration will cause it to become intermittent.
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Old 07-22-2014, 12:23 PM   #17
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For anyone who is looking for an answer we had checked just about everything and drained to oil to find antifreeze coming out. As far as im concerned this is almost definitely a manifold or head gasket so we are currently tearing down the top end for a clean and rebuild. Will post after updating whether or not this has fixed the issue.
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Old 07-22-2014, 04:14 PM   #18
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hello;

if your intake is chinese they do not fit well.
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Old 07-22-2014, 11:42 PM   #19
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Does it not fire at all?

If you have okay compression in all cylinder it should run probably even without a head gasket for a few seconds...it should do something.

If the intake gasket were missing it should also run for a few seconds when you pour fuel down the carb it would run ****ty in both cases but it would run.

You say you have:

spark

Fuel

Air

Compression

all thats left is timing

If its not doing anything then I am of the opinion that either you are mistaken about one of the above or it really is timing.

If it was just running and died typically its spark, fuel, or timing.

Intake gaskets and head gaskets are usually a slow death where you run around chasing your tail as they get worse until you have an Ah-ha! moment.

I bet if it pulled the intake off and dumped fuel down the intake runners on the head and cranked it over it would run for an instant or at least break its self.

If it does nothing...I would re-investigate the former go back to basics...


Just my thoughts

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Old 07-22-2014, 11:42 PM
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