Vacuum problems?? - Vintage Mustang Forums
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post #1 of 46 (permalink) Old 08-24-2019, 07:15 AM Thread Starter
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Vacuum problems??

I've been diagnosing various problems that seem vacuum related with my 68 factory A/C system. Yesterday it acted really strange. The automatic trans kept slipping out of gear, forward or reverse. I could put it in neutral, rev it a bit, drop back into gear and it would usually catch. Then I also noticed the power brakes were super sensitive. I only had it running for a minute while I moved it from one garage bay to another. Sound like a vacuum issue? A few days earlier I noticed a back leak from the A/C reservoir canister (i.e it wouldn't hold it's vacuum when the engine is revved or after it is shut off) so I installed an inline check valve in the hose supplying vacuum to the canister. That seemed to fix the back leak. Could it be a culprit?

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post #2 of 46 (permalink) Old 08-24-2019, 07:53 AM
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Your trans issue sounds like it's low on fluid. The vacuum modulator has no effect on shifting in or out of a gear, it only affects part-throttle shift points.

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post #3 of 46 (permalink) Old 08-24-2019, 09:18 AM Thread Starter
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Your trans issue sounds like it's low on fluid. The vacuum modulator has no effect on shifting in or out of a gear, it only affects part-throttle shift points.
What is the proper way to check trans fluid level? The Shop Manual doesn't go into that, at least not in the auto trans section. I kinda remember to check some old trans you had to have it in drive gear at idle.

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post #4 of 46 (permalink) Old 08-24-2019, 10:19 AM
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What is the proper way to check trans fluid level? The Shop Manual doesn't go into that, at least not in the auto trans section. I kinda remember to check some old trans you had to have it in drive gear at idle.
Start the engine and let it run to operating temperature, park on level ground and with engine still running, pull the dipstick, wipe, replace and pull again to read.

It is likely you're low on fluid, look for a puddle when you move it, and when looking for a vacuum leak there is a short section of hose from the engine to the modulator line and even a tiny section of hose from the modulator line to the valve.
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post #5 of 46 (permalink) Old 08-24-2019, 10:59 AM Thread Starter
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Doh, Checked it like you said and nothing shows up on the stick! I also found I have an exhaust leak at the rear most port. I got my finger close enough to feel the hot exhaust pulsating out from there. I guess I have more work to do on this old thing.

Forgot to ask: is there a modern equivalent to the type ATF listed in the old Shop Manual? Seems like I can never find the type listed for these old cars.

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Last edited by Winch; 08-24-2019 at 11:03 AM.
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post #6 of 46 (permalink) Old 08-24-2019, 11:21 AM
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C4s are kinda leak-prone, especially if they sit too much.
I would STAY AWAY from stop-leaks.

The exhaust leak may be "cured" with getting the bolts to the right torque, but don't overdo it, you may hafta RnR it. Be sure you aren't feeling exhaust coming from a heat riser tube port that has no tube in it.

Type F is still available, I have gotten it at Wallet-Mart, AutoZane, O'Really's, Adverse Auto (haha) and is what yours was designed to use. You can use the Mercon Dexron type in a new build but I wouldn't mix the two for a top-off.

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post #7 of 46 (permalink) Old 08-24-2019, 11:23 AM
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Doh, Checked it like you said and nothing shows up on the stick! I also found I have an exhaust leak at the rear most port. I got my finger close enough to feel the hot exhaust pulsating out from there. I guess I have more work to do on this old thing.

Forgot to ask: is there a modern equivalent to the type ATF listed in the old Shop Manual? Seems like I can never find the type listed for these old cars.

Use Type F. You can get it at pretty much any decent parts store. Even the Ford dealers carry it again.

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post #8 of 46 (permalink) Old 08-24-2019, 12:34 PM Thread Starter
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C4s are kinda leak-prone, especially if they sit too much.
I would STAY AWAY from stop-leaks.

The exhaust leak may be "cured" with getting the bolts to the right torque, but don't overdo it, you may hafta RnR it. Be sure you aren't feeling exhaust coming from a heat riser tube port that has no tube in it.

Type F is still available, I have gotten it at Wallet-Mart, AutoZane, O'Really's, Adverse Auto (haha) and is what yours was designed to use. You can use the Mercon Dexron type in a new build but I wouldn't mix the two for a top-off.
here's where I feel the exhaust leak. Looks like the gasket has burned through. Looking at this again I remember what a bear that A/C compressor was and it appears it has to be moved to get at the front bolts. I might put this one off for a bit.
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File Type: jpg Exhaust Port Leak.jpg (386.6 KB, 8 views)

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post #9 of 46 (permalink) Old 08-24-2019, 06:29 PM
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Also, after adding ATF, cycle through all the gears to fill the hydraulic circuits and get an accurate reading. Check level in neutral.
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post #10 of 46 (permalink) Old 08-26-2019, 02:55 PM Thread Starter
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Put a quart and a half of type F in it. Seems to have fixed that problem. I got it a little over the full mark. So far I haven't seem any foaming. Do I need to worry about that?

Now back to that exhaust manifold gasket. It definitely needs replaced. I'm having a problem getting a socket on any of the bolts. I'm sure they are 1/2 inch but I can't get either 6 point or 12 point on any of them. I've wire brushed one and used PB Blaster with no luck yet. I tried a 14 mm on one but it started to round off the edges and of course 13 mm won't go on either. Any tips?

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post #11 of 46 (permalink) Old 08-26-2019, 03:16 PM
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Put a quart and a half of type F in it. Seems to have fixed that problem. I got it a little over the full mark. So far I haven't seem any foaming. Do I need to worry about that?

Now back to that exhaust manifold gasket. It definitely needs replaced. I'm having a problem getting a socket on any of the bolts. I'm sure they are 1/2 inch but I can't get either 6 point or 12 point on any of them. I've wire brushed one and used PB Blaster with no luck yet. I tried a 14 mm on one but it started to round off the edges and of course 13 mm won't go on either. Any tips?
The bolt heads are probably rusted to the point that they are "in between" sizes. IIRC, all the Ford exhaust manifold bolts are 3/8-16 and should have a 9/16" head. 14mm is only .012" smaller than 9/16. It might grab it if you use a good 6 point socket.

Be very careful when removing the bolts. Crack it loose, then tighten again, loosen 1/8 turn, tighten. You want to go maybe 1/8 turn more at a time. Keep hosing it with Blaster until it starts to come out easily. If you feel resistance, stop, spray it, and turn it back in a couple turns.

Nothing worthwhile is ever quick, cheap or easy, those that can't do, complain

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Last edited by Hemikiller; 08-26-2019 at 03:19 PM.
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post #12 of 46 (permalink) Old 08-26-2019, 05:48 PM Thread Starter
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I just ordered a 17/32 socket from Home Depot. It's 12 point and for a 3/8 drive so it might not work. I did see a 1/2 drive 6 point 17/32 on Amazon which I might have to try if the 3/8 drive doesn't work.

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post #13 of 46 (permalink) Old 08-26-2019, 06:06 PM
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Take Hemikillers advice..all of it!
Especially..the use a 6 point socket. TOO easy to round a rusted bolt with 12pt..
Good luck.. and take your time..allow the penetrant time to work.. you won't want to spend your time removing broken manifold bolts from your engine.

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post #14 of 46 (permalink) Old 08-28-2019, 08:49 PM Thread Starter
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Looks like there are 2 brands of exhaust manifold gaskets available, Fel-Pro and Mahle. Is either better? It's interesting that Fel-Pro costs more at O'Reilly's but half the cost of Mahle at Rock Auto.

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post #15 of 46 (permalink) Old 08-29-2019, 01:16 PM
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Keep in mind that sometimes the easiest way to gain access to a nut or bolt is from a different vantage point. For example, on a big block motor some of the lower exhaust manifold bolts are best reached from below the car using extensions, and universal joints to get around obstacles. Also start several days early with PB Blaster (penetrating oil) on all the nuts and bolts reapplying it several times. Rusted nuts and bolts are prone to breaking off if forced. Too much force can lead to broken bolt extractions and they are usually time consuming and a PITA. Be patient. If nothing works, look into heat (a torch) to take advantage of expansion and contraction of the rusted metal. Lastly, once the exhaust manifold is off the engine, have it checked for warpage of the mating surface. A machine shop can true up that surface by re-surfacing the mating surface if needed.

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