can a leak on the intake manifold cause problem when my transmission shifts - Hot Rod Forum : Hotrodders Bulletin Board
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Old 07-28-2004, 04:46 PM
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can a leak on the intake manifold cause problem when my transmission shifts

ive ordered gaskets from my intake but it takes a long time for me to get them in germany i was driving the autobahn out here going about 100mph and my automatic transmission wouldnt go into 4th gear could this be from a vacuum loss due to the intake leaking

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Old 07-28-2004, 05:12 PM
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I don't know what kind of car you drive, but some automatic transmissions have a modulator valve that is vacuum controlled. If you have a manifold leak then that would definitely cause low vacuum and shifting problems. You would know your car best, if there is vacuum tubing going to your transmission than that could be the problem.
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Old 07-29-2004, 04:23 PM
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In a word, YES.

But only if your tranny has a vacuum-operated modulator. And without knowing what car & what tranny, this is only conjecture.

If it does have a vacuum modulator, then it typically requires 12 inches of vacuum at idle to operate properly. If you were running 100mph, then your vacuum signal at the carburetor would be lower than at idle, and that, combined with a leaking intake, could easily prevent shifting.

So, my suggestion would be to check the hoses going to the modulator, then fix the intake & see if the shifting problem fixes itself. If not then start looking at the modulator.
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Old 07-29-2004, 05:08 PM
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my corvette has a 700r4 transmission and a 5.7 engine with crossfire injection
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Old 07-29-2004, 06:00 PM
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O.K. I haven't seen a 700 R4 with a vacuum actuated modulator valve. Sometimes the 4th gear won't engage because of the TV cable that runs from the throttle body to the trans. it's incorrectly set and won't allow the trans to shift to overdrive. Most of the time though it will still shift when you let off the gas (very hard to tell if it does though). Make sure the TV linkage is actually hooked up in the trans. When you remove them or let them go slack they sometimes slip out of the lever in the trans. you can feel a springyness in the cable at the throttle body if it's all cool.

The 700R4 also has a locking torque converter so if you listen carefully you can hear the engine drop RPM 5 times 1st gear, 2nd gear, 3rd gear, 4th gear, and (not as obvious) lock. Sometimes the solenoid that engages the fourth gear burns out and (sucks to be you), the OD won't kick in. I'm not a trans. guy, but the only way I know to test this is to remove the pan and manually energize the solenoids. There's two of them and they're pretty obvious when you remove the pan and filter.

The torque converter solenoid is locked out by a vacuum switch on some vehicles. vacuum must be high in order to engage the lock. It looks like a device with two wires in one side and a vacuum tube in the other. (You can trace the wires back from the top of the passenger side of the trans). Check it with a vacuum gauge and continuity tester before taking the pan off. Vacuum high, on. Vacuum low (or non existant), off.

You may want to get a shop manual if you plan on repairing this corvette yourself. They're only about $150 and they have vehicle and model year specific information that will prove to be invaluable.
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Old 07-29-2004, 09:19 PM
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Good info, flyfisher.

On that one, I'm bowing out. I know a fair amount about TH-350/400's, less about PG's, and almost nothing about 200-4R's & 700R4's. If the tranny's got wires hanging out, I avoid it like the plague.

Good luck, dopey.
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Old 07-30-2004, 11:57 AM
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Exclamation 700R4

My understanding is you have a 1984'85 Corvette. With everything normal ,Cable adjusted ,TCC lock up working , no intake leak, normal fuel pressure. It may be normal for no 3/4 shift at above 80% throttle at that speed. GM made changes in the 3/4 shift valve train to correct this on Police cars and certain performance cars. in 1989 you could install this valve and spring to keep it in fourth under full throttle above a certain vehicle speed. I installed one in my 1991 700R4 (4L60) .B&M also makes a valve I believe for the same papas. Paul
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Old 08-01-2004, 10:43 AM
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hey guys thanks for the help turns out it was mainly just the tv cable was able to run 13.5s at the drag race in bitberg this weekend
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