Possible motor problem help?
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From: East Texas
Possible motor problem help?
I have a 77 F150 with a 390 and C6 transmission. The motor and transmission are not original to the truck, but they are original in that neither has been rebuilt. Now, on to the problem.
Just this past week, my truck was running fine. All of a sudden, it would not go faster than 40-45 while on a straight stretch, 35-40 going uphill, and it tops out at 50 going down a hill. It will also make the RPM's rise high (idk to what extent because the tach is missing, but I can hear it). I have to let off the gas for it to shift.
Everything I have replaced so far is as follows:
-Fuel pump
-Fuel filters
-Vacuum hoses
-Spark plugs/Plug wires
-Distributor
-Starter
-Starter Solenoid
I have not checked out the exhaust yet. There is no cat, and the p/o welded stacks on in place of original exhaust. I have never ran stacks so idk if that would be a problem.
It is an old truck, mostly all original, so it is due its problems. I am fixing it up little by little, but it is my only vehicle and I need to get this problem fixed. Any help would definitely be appreciated.
Just this past week, my truck was running fine. All of a sudden, it would not go faster than 40-45 while on a straight stretch, 35-40 going uphill, and it tops out at 50 going down a hill. It will also make the RPM's rise high (idk to what extent because the tach is missing, but I can hear it). I have to let off the gas for it to shift.
Everything I have replaced so far is as follows:
-Fuel pump
-Fuel filters
-Vacuum hoses
-Spark plugs/Plug wires
-Distributor
-Starter
-Starter Solenoid
I have not checked out the exhaust yet. There is no cat, and the p/o welded stacks on in place of original exhaust. I have never ran stacks so idk if that would be a problem.
It is an old truck, mostly all original, so it is due its problems. I am fixing it up little by little, but it is my only vehicle and I need to get this problem fixed. Any help would definitely be appreciated.
Posting Guru
Joined: Dec 2012
Posts: 1,722
Likes: 7
From: Shelton, Wa
The more detail on the problem the easier it is for people to try and help.
Does it Idle without anything unusual happening?
Have you checked the vacuum line for the Vacuum Modulator on the transmission? There should be a rubber line from back of intake manifold to a hard metal line that goes down to rear of transmission on passenger side, then another rubber line to connect that to the Vacuum Modulator itself.
Did this problem happen "all of a sudden" or did it start very mild and slowly get worse?
What were you doing, driving wise, when the problem started? Off road, puttering down main street, drag racing?
Could you post a short video of the problem?
Does it Idle without anything unusual happening?
Have you checked the vacuum line for the Vacuum Modulator on the transmission? There should be a rubber line from back of intake manifold to a hard metal line that goes down to rear of transmission on passenger side, then another rubber line to connect that to the Vacuum Modulator itself.
Did this problem happen "all of a sudden" or did it start very mild and slowly get worse?
What were you doing, driving wise, when the problem started? Off road, puttering down main street, drag racing?
Could you post a short video of the problem?
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New User
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From: East Texas
It idles fine, maybe a little rough sometimes. The only unusual thing that happens during idle is that it starts to overheat. No overheating while driving though.
I haven't checked the vacuum modulator yet
The problem did happen all at once. I drove it home from work one morning, and when i went to work that night, thats when the problem started. I was just driving down the highway like usual. I do know the p/o took it mudding quite a bit, and before i got it it had been sitting for a few months.
I haven't checked the vacuum modulator yet
The problem did happen all at once. I drove it home from work one morning, and when i went to work that night, thats when the problem started. I was just driving down the highway like usual. I do know the p/o took it mudding quite a bit, and before i got it it had been sitting for a few months.
Thread Starter
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New User
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From: East Texas
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Joined: Apr 2009
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From: Stanley, VA
From the vague description I would agree with the others, check the vacuum modulator on the transmission. Pull the vacuum line off and see if transmission fluid leaks out. If it does, the modulator is bad. If it doesn't, it could still be bad.
Also check the vacuum line that goes to it. Look for any problems with the line or the rubber hoses on either end. Make sure they're connected and not loose, cracked or otherwise deteriorated.
The modulators are easy to change, one bolt holds a clamp on that retains the modulator. It has an o-ring seal and just slips in. There is also a pin, sometimes it stays in the trans, sometimes it comes out with the modulator. If it comes out, don't lose it.
Modulators are identified by the color of the stripe on the exterior. The most common is black, if that doesn't show up in the catalog, use the green one. If you have a pink stripe modulator, it will have two vacuum lines instead of one. Replace that modulator with one of the other ones and use the vacuum line that goes to manifold vacuum and block off the other line.
Also check the vacuum line that goes to it. Look for any problems with the line or the rubber hoses on either end. Make sure they're connected and not loose, cracked or otherwise deteriorated.
The modulators are easy to change, one bolt holds a clamp on that retains the modulator. It has an o-ring seal and just slips in. There is also a pin, sometimes it stays in the trans, sometimes it comes out with the modulator. If it comes out, don't lose it.
Modulators are identified by the color of the stripe on the exterior. The most common is black, if that doesn't show up in the catalog, use the green one. If you have a pink stripe modulator, it will have two vacuum lines instead of one. Replace that modulator with one of the other ones and use the vacuum line that goes to manifold vacuum and block off the other line.
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Thread Starter
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New User
Joined: Aug 2014
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From: East Texas
Changed out the modulator and got new universal joints because i found they needed replacing. Still having the same problem though. I even manually shifted, and it shifted better, but it would still not drive over 45 mph.
Posting Guru
Joined: Dec 2012
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From: Shelton, Wa
When you try to go over 45 does it just stop revving high/going faster or does it continue to rev higher but not go faster?
If it revs but doesn't increase speed then it's slipping bad and would suggest trans fluid flush with filter replacement and adjust the band after.
If it revs but doesn't increase speed then it's slipping bad and would suggest trans fluid flush with filter replacement and adjust the band after.
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