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More Electrical/Idle Problems
#1
Hey guys,

I started up my 71 mach today (351C-4V) and it was idling rough, kind of a bumpy up and down idle.. I then decided to randomly press the brakes and the engine would struggle. Same with turning the headlights on. First I thought the brake problem was a vacuum leak but its actually just the brake lights causing the car to struggle when I press down.

A few months back I had installed a redone wiring harness, headlight harness, and fuse block from Midlife which originally made this issue seem to go away.
As of 4/10/2018 I also installed a new engine gauge harness and ignition switch. The car still stalls when I turn the headlights on or use the brake lights.
Thanks in advance
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#2
Pressing the brakes to make the engine struggle sure sounds like a vacuum leak to me; that and bumpy up and down idle (like surging?) indicates the same.

Let me check your shorts!
http://midlifeharness.com

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#3
(04-05-2018, 07:17 PM)midlife Wrote: Pressing the brakes to make the engine struggle sure sounds like a vacuum leak to me; that and bumpy up and down idle (like surging?) indicates the same.

Oh damn, time to spray carb spray all over the usual spots. I am guessing intake manifold leak because I pulled it off once and it had no gaskets.
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#4
(04-05-2018, 07:33 PM)icejawa Wrote:
(04-05-2018, 07:17 PM)midlife Wrote: Pressing the brakes to make the engine struggle sure sounds like a vacuum leak to me; that and bumpy up and down idle (like surging?) indicates the same.

Oh damn, time to spray carb spray all over the usual spots. I am guessing intake manifold leak because I pulled it off once and it had no gaskets.

It has a metal "valley pan" that also acts as a gasket for the intake ports, and rubber seals on the ends.
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#5
(04-05-2018, 07:38 PM)rvrtrash Wrote:
(04-05-2018, 07:33 PM)icejawa Wrote:
(04-05-2018, 07:17 PM)midlife Wrote: Pressing the brakes to make the engine struggle sure sounds like a vacuum leak to me; that and bumpy up and down idle (like surging?) indicates the same.

Oh damn, time to spray carb spray all over the usual spots. I am guessing intake manifold leak because I pulled it off once and it had no gaskets.

It has a metal "valley pan" that also acts as a gasket for the intake ports, and rubber seals on the ends.

Ah yes. I've also read that the manifold bolts can come loose and cause vacuum leaks. People also claim that a 4 piece gasket set works better than the valley pan gasket. Maybe I should get that setup. I should've probably mentioned I have an edelbrock performer manifold on there right now
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#6
If the idle is smooth without pressing the brake pedal and becomes rough when pressing the brake pedal, it could be a bad (leaking)power brake booster diaphragm. I had these symptoms once.

1973 H Code Convertible - Medium Copper Metallic - June 8, 1973, Built Ford Marketing Sales Vehicle
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#7
[ People also claim that a 4 piece gasket set works better than the valley pan gasket]
When you are in Europe and you need a new gasket, this means today at least 2 weeks waiting, and in the past 1 to 2 months waiting.
because of this detail, I've been forced to run without the metal gasket until the new one would arrive. Both on 2V and 4V config.
The only difference is that your carb/intake gets hotter faster, which for short distance trip or during winter improves a bit the fuel mix when air is cold.
Once at operating temp, things changes and the 351, become less responsive to the pedal. I'd say, use the metal gasket.

[ I've also read that the manifold bolts can come loose and cause vacuum leaks.]
I don't want to play the myth buster here, but you got to really do your best to screw up the assembly of your engine to get this kind of situation. Or use the wrong/damaged/corroded/dirty bolts and not torque them in right order/force etc..
In my experience, its pretty much the opposite when done by the book. Even if no bolts would hold the intake, it would not come off without a fight and some kind of lift. Bolts would certainly not come loose either. There has to be something not done right so this can occur. frankly I wouldn't know what.

As you have a vacuum leak, aside making sure you do not assemble anything before contact areas are mirror clean and dry (which can be tedious and hard on your back when engine is in the car), there is one thing I would advise to go outside the book for your install. In my eyes, the cleveland has a small design flaw for the front and rear "s" gaskets, especially if you use after market intakes or have resurfaced your heads. Install these 2 gaskets first on block, with a thin layer of sealant and let dry a few minutes. Then at the 4 corners where block/heads/intake meet, no matter the gaskets quality, always add an extra drop of sealant there and triple check these two gaskets do not move a bit during install. If you do this, I assure you, you'd need to drill your intake to get a leak again Smile

73 modified Grandé 351C. Almost done. 
71 429CJ. In progress
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#8
I never run the turkey pan. Never had an issue. I use the gaskets at the ports and a large bead of silicone on the ends. Ditch the end rubber gaskets.

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Kevin

1971 Mach 1
408C Stroker
C4 w/3,000 stall
8.8" Rear w/3.73's
Disc brakes all way around.

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#9
(04-06-2018, 06:07 AM)turtle5353 Wrote: Ditch the end rubber gaskets.
That's one way to fix the "flaw" Smile

73 modified Grandé 351C. Almost done. 
71 429CJ. In progress
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#10
I was unable to find a vacuum leak near the intake, so I am guessing either brake vacuum leak, or could it be the alternator? When I turn the headlights on the same thing happens as pressing the brakes does. The ALT gauge surges in a pattern. (assuming it works properly with the new wiring harness)
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