Hey guys… a new issue has just arose with my 69 Xr7. The car has a c4 automatic trans, with a 302 crate motor… mild performance goodies nothing outrageous. When going wide open throttle out of the hole, the car normally will shift into 2nd gear around 4800-5000 rpms at 55ish mph. Just recently the car has begun to surpass this rpm mark and not shift into 2nd gear until 5600-5800 rpms. I have not ever had this issue in the past until after I recently installed a set of ceramic coated long tube heades. I went out for my first test drive with the headers went WOT and nearly passed out when I saw the tachometer! I know for a fact my motor is not making any amount of useful power at that rpm range, and frankly I do not think I want to be a regular visitor to that RPM range ever again as it sounded like the motor was not enjoying it either. The weird thing is that it does not happen every time… when I tested it today going downhill it shifted normally at 5000ish rpms… but it seems to happen the majority of the time.
My first question is could the headers actually have anything to do with this? Next could a vacuum leak make this happen? Thanks for the help. The c4 otherwise is functions without any other issue.
I doubt anything has changed in the internals of the transmission like the governor or valve body that would change the shift points. The only other thing external would be the modulator. Trace the steel tubing and hose from where it connects to the carb or intake all the way to the modulator valve on the rear passenger side of the transmission. Exhaust piping runs through this area as well and may have melted the vacuum hose, or it good pulled loose while the exhaust work was being done.
If there was a problem with the modulator, like a leak from a melted or accidentally disconnected hose, wouldn’t that be a vacuum leak that the manifold was always experiencing? I would think that would result in a very rough running engine, noticeably bad, like if you pull a cap off an unused manifold vacuum port, no?
It’s quite possible you were driving towards a full moon. Think about it, if the moon can move an entire ocean, just imagine what it could do to a car! An engine that revs faster will reach a little bit higher rpm before the shift is completed. Although 800 rpms higher seems a bit much for that theory. Is it possible with making an upgrade to the performance that you are stomping on the pedal just a bit more to obtain a real WOT versus with other times you may have been about 90-95%? A little bit more pull on the kick-down might make that difference for when the trans shifts. If this were the case then the root cause of the problem is a loose connection between the steering wheel and driver’s seat. Another possibility is the trans fluid moving away from the filter pick-up.
From what I have seen, even a very small leak in the modulator line will cause more of a transmission problem before the engine starts to react.
A pinched hose will cause no engine issues, but screw up shift points as well.
That’s also the easiest place to look when starting to troubleshoot since it is all pretty accessible and easy to trace out.
I speak from experience on small screw ups on transmission work causing huge issues. When I put my modulator piston back in backwards. It wouldn’t shift unless I did it manually, and even then, 3rd gear was like being in neutral.
The engine and vacuum line to the trans should be at a zero vacuum state during WOT. If there is a collapsed or kinked hose to retain a small amount of vacuum at the modulator during WOT, then that would create an earlier shift. This would mean the problem’s symptom was occurring before the headers and now it’s not. I still have the feeling it might be something to do with pedal travel. Is there a floor mat that gets compressed during WOT? Regardless, of how the car is set up, I think the most plausible theory is a full moon—especially a harvest moon.
