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Runs great until I hit the highway? #2677205
07/13/19 09:40 PM
07/13/19 09:40 PM
Joined: Aug 2006
Posts: 567
Michigan
Whitey Nocar Offline OP
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Whitey Nocar  Offline OP
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Any thoughts on why my ‘68 Coronet wants to not run at highway speeds?
Only driven it a handful of times this year, it runs great on “city” streets. Today I jumped on the highway and it didn’t want to run over 55mph felt like it wanted to stall when I would accelerate. Jumped off the highway about 3 miles down the road and ran great again. I’m a little lost here. Would love it if some had a simple “try this” idea before I take it in.
Thanks in advance

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2677207
07/13/19 09:50 PM
07/13/19 09:50 PM
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Nampa, ID
None2Slow Offline
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Is your fuel tank vented? I had this problem on my 63 Valiant. It had a non-vented fuel cap and when I would drive it at speeds, it would fall on its face. 1 day I pulled into the fuel island after spirited driving and when I took the cap off, you could hear the air rush in and the tank make a "donk" sound. When I started it back up, ran great for a little while, then would act up again. Popped the cap and the air would rush in and it was fine again. Car was a 6cyl originally and had a V8 swap. Its been off the road for a while now going to a better V8 and this time I will get a new vented tank and larger fuel lines.

If you tank is vented, the vent may be plugged, or your fuel pump coukd be bad or your fuel lines clogged.

Re: Runs great until I hit the highway? [Re: None2Slow] #2677216
07/13/19 10:15 PM
07/13/19 10:15 PM
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Michigan
Whitey Nocar Offline OP
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Interesting. So last summer I changed the gasket on the sending unit because I had a small leak when tank was over 1/2 full. I camera’d the inside of the tank and was sure the vent tubes were hooked up backwards. I must have saw a diagram somewhere which made me think the gas was leaking from the short tube because it was hooked up backwards. Maybe I just switch them back and see what happens.

Anyone have a diagram on what vents get hooked to which tubes inside the tank. Just looked online and can’t find one... makes me wonder what I saw that made me think it was backwards to begin with

Last edited by Whitey Nocar; 07/13/19 10:17 PM.
Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2677315
07/14/19 11:48 AM
07/14/19 11:48 AM
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Southeast Pa.
SALEM1912 Offline
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X2 on a tank issue,sock could be getting clogged. More info on the engine set up. Weak fuel pump?

Re: Runs great until I hit the highway? [Re: SALEM1912] #2677317
07/14/19 11:57 AM
07/14/19 11:57 AM
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Omaha Ne
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TJP Offline
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Fuel filter ?? loose or deteriorated hose on suction side of system shruggy

Last edited by TJP; 07/14/19 11:59 AM.
Re: Runs great until I hit the highway? [Re: TJP] #2677318
07/14/19 12:01 PM
07/14/19 12:01 PM
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nowhere
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Sniper Offline
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Not enough info to guess.

Is it a big block? there was a rash of soft fuel pump pushrods awhile back that had similar symptoms.

Re: Runs great until I hit the highway? [Re: Sniper] #2677355
07/14/19 03:50 PM
07/14/19 03:50 PM
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so see if you run in town, down shift to 2 and see if you see the same problems.
RPM being the same as highway speeds, but at lower actual speeds might tell you something. IF it doesn't do it, then it could eliminate fuel issues and point to other things.

Re: Runs great until I hit the highway? [Re: Andrewh] #2677380
07/14/19 05:36 PM
07/14/19 05:36 PM
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Lincoln Nebraska
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hookup a can of gas in the eng compartment to the pump inlet & retry it. this'll elim everything from that point rearward.


live every 24 hour block of time like it's your last day on earth
Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2677381
07/14/19 05:40 PM
07/14/19 05:40 PM
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Florida
cbusters Offline
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"T" in a temp fuel pressure gauge. Take a highway drive. One test eliminates the fuel supply.

Re: Runs great until I hit the highway? [Re: cbusters] #2677382
07/14/19 05:47 PM
07/14/19 05:47 PM
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Omaha Ne
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TJP Offline
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may also be ignition related, Points or electronic ?

Re: Runs great until I hit the highway? [Re: TJP] #2677397
07/14/19 06:56 PM
07/14/19 06:56 PM
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Michigan
Whitey Nocar Offline OP
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Swapped the vent tubes. Same issue. Saw posts about the vent tubes inside tank saying the long tube in tank should be hooked to short tube outside the tank, and the short tube in the tank to the long vent that goes up in the trunk. Anyone one know if that is indeed accurate?

This is just the original 318, automatic trans, does not have electronic ignition, but points were replaced about a year ago with new plugs and wires, does have intake and Holly 650 with headers, other than that it’s stock.

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2677411
07/14/19 07:35 PM
07/14/19 07:35 PM
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Dacula, GA
I_bleed_MOPAR Offline
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Lots of variables. As others have said, I'm thinking fuel pressure or ignition problem without actually being able to put my hands on it. Does it act up under hard acceleration on city streets? Are you getting on it hard on the freeway when it acts up or just cruising at highway speeds? Holley carb- have you checked the float levels?
Many years ago I had a 340 that wouldn't go over a certain illegal triple digit speed. It would start breaking up badly. Under that number it ran fine. My Dad drove it and without me telling him anything, discovered it had a bad alternator (shame on you, Dad wink biggrin ).



Tim

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2677652
07/15/19 12:43 PM
07/15/19 12:43 PM
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Omaha Ne
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TJP Offline
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Quote
but points were replaced about a year ago


Have you checked your dwell recently ??

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2677742
07/15/19 05:38 PM
07/15/19 05:38 PM
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Posts: 1,136
Columbus, GA
Michael Ecks Offline
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Originally Posted by Whitey Nocar
Swapped the vent tubes. Same issue. Saw posts about the vent tubes inside tank saying the long tube in tank should be hooked to short tube outside the tank, and the short tube in the tank to the long vent that goes up in the trunk. Anyone one know if that is indeed accurate?

This is just the original 318, automatic trans, does not have electronic ignition, but points were replaced about a year ago with new plugs and wires, does have intake and Holly 650 with headers, other than that it’s stock.


Might take a look into the rear bowl on the holley if it has sat for long periods of time. My car sat for a good while with me only starting it up to idle every three or four months. With no opening of the secondaries, the fuel in the rear bowl went bad, gummed up the needle and seat (and everything else) and eventually dried out completely. Started, Idled, and ran at low RPM fine. Everything was fine until the secondaries opened and then it was all air and no fuel back there.


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Re: Runs great until I hit the highway? [Re: Michael Ecks] #2679445
07/20/19 12:58 PM
07/20/19 12:58 PM
Joined: Aug 2006
Posts: 567
Michigan
Whitey Nocar Offline OP
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Tried the paper clip on diaphragm shaft to see if secondaries are opening up. Paper clip didn’t move down the shaft more than 1/8 of an inch, maybe a little cocked from where I put it but probably vibrated there. So would that be a bad diaphragm or bad vacuum pressure? Should the diaphragm shaft be able to move up and down by hand when car isn’t running? (Remember I spelled Holley above without the “e” 😬)

Also running in 2nd gear had no effect on the bogging down. Still bogged at a slightly lower speed

Any more suggestions?

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2679448
07/20/19 01:12 PM
07/20/19 01:12 PM
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Hamtramck, PA
Alaskan_TA Online content
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What gear ratio in the differential?

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2679531
07/20/19 06:33 PM
07/20/19 06:33 PM
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Omaha Ne
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TJP Offline
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Originally Posted by Whitey Nocar
Tried the paper clip on diaphragm shaft to see if secondaries are opening up. Paper clip didn’t move down the shaft more than 1/8 of an inch, maybe a little cocked from where I put it but probably vibrated there. So would that be a bad diaphragm or bad vacuum pressure? Should the diaphragm shaft be able to move up and down by hand when car isn’t running? (Remember I spelled Holley above without the “e” 😬)

Also running in 2nd gear had no effect on the bogging down. Still bogged at a slightly lower speed

Any more suggestions?


Have you checked your dwell recently ?? Just had a 66 vette that would shut off at about 4K, dwell was set at 19. Reset to 30 and all is well to 5500,

beer

Re: Runs great until I hit the highway? [Re: Whitey Nocar] #2679735
07/21/19 11:37 AM
07/21/19 11:37 AM
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Phila. Pa.
Mattax Offline
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When did you last put fuel in it?
'Winter fuel' in hot weather can cause that.
been there, done that.
its very similar to running out of fuel in the bowls.
Sometimes that's actually what's happening (if there's vapor lock)
But many times too much is evaporating in the bowl and intake so less fuel gets into the combustion chamber.
Drain off 5 gallons and put fresh summer fuel in.

Re: Solved. Runs great until I hit the highway? [Re: Mattax] #2685873
08/08/19 10:48 PM
08/08/19 10:48 PM
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Posts: 567
Michigan
Whitey Nocar Offline OP
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Thanks for the suggestions. TJP’s 1st guess was right. Turns out the fuel filter needed to be changed. Also, big thanks to Bill Meyer of Meyer’s Automotive in Howell. I described the symptoms over the phone and he said “ change your fuel filter, you’ll be all set” $5 later she’s running just fine.

Last edited by Whitey Nocar; 08/08/19 10:51 PM.
Re: Solved. Runs great until I hit the highway? [Re: Whitey Nocar] #2685958
08/09/19 09:39 AM
08/09/19 09:39 AM
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Omaha Ne
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TJP Offline
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Originally Posted by Whitey Nocar
Thanks for the suggestions. TJP’s 1st guess was right. Turns out the fuel filter needed to be changed. Also, big thanks to Bill Meyer of Meyer’s Automotive in Howell. I described the symptoms over the phone and he said “ change your fuel filter, you’ll be all set” $5 later she’s running just fine.


thanks for the report back, many times people just drop off once the problerm is solved. up beer

I also wanted to say that a plugged filter is usually an indication of a contaminated or rusty system. If it is rusty we do the entire system, tank, sending unit, lines, pump, and carb. IF you still have the old filter, cutting it open ( be careful of the fumes) or flushing through it backwards will normally tell one what the debris is that plugged it. beer


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