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Common 1960-69 Corvair Issues: Transmission Modulator and Engine Overheating

Author:

Perrya knows a fair amount about the Corvair, America's aluminum-engine classic car.

The Corvair Powerglide automatic transmission remains one of the most stable and resilient transmissions Chevrolet made. Amazingly, the Corvair's air-cooled engine seldom overheats or goes above 400ºF.

Nevertheless, after a car has been idle for many years, issues commonly develop with the automatic transmission and the engine overheating.

The following troubleshooting techniques apply to all models from 1960-1969.

1. Powerglide Transmission: Modulator and Other Issues

Modulator

Modulator

The modulator, should it fail, can cause problems with shifting, and if the small rubber connector tube is clogged it can contribute to idle problems. The device contains a diaphragm and valve. If your engine is hot and you cannot determine checking the Powerglide (PG) fluid level when the engine is running ( it reads "empty" on stick), the modulator may be the issue as it is allowing the fluid to leak out of where it is supposed to be. Loss of tranny fluid where you cannot locate a leak may indicate a ruptured diaphragm. A ruptured diaphragm allows the oil to be drawn into the intake manifold causing excessive white smoke. If there is no smoke, check the rear axle lubricant as tranny oil can enter if the pinion shaft seal is worn or bad.

To Test the Modulator Before Removing It:

Attach a vacuum gauge to the lower end of the modulator hose and start the engine. At idle speed, the gauge should read within 1 inch. If it doesn't, something is blocking the tube. Open the throttle and there should be a quick drop in vacuum. If there is none, or the drop is delayed, the restriction is internal within the vacuum line somewhere.

If both readings are OK, the oil pressure at idle with fluid level full and engine hot and modulator hose connected and the transmission in Neutral/Drive should be 47-57 psi.

If there is no oil pressure, there is a broken front pump drive shaft, or shaft is disengaged from the front pump drive hub. If there is low oil pressure, it may be caused by a dislodged plug from the front of the hollow front pump drive shaft.

Removing the Modulator

The modulator is a 2-3” diameter mushroom-shaped vacuum thing that can be installed in less than 30 min (with the right tools) in the side of the automatic transmission. Access is difficult so jack up the car and remove the rear right tire. It is located in a recessed area and using the special tool from Clark’s Corvair makes it easy.

  • Odds are nothing has ever been done to this part, so spray some lubricant to loosen it before starting to remove it.
  • Remove the short rubber tube and verify it is not clogged.
  • Verify that the tubing attached to the rubber tube remains solid with no holes.
  • Torque it a couple times and remove it.
  • There is a small valve in the hole, if it sticks or binds while in the bore hole (it does not turn freely), remove it and inspect for roughness.
  • Use a fine slipstone to polish roughness out by rubbing the valve on the stone. Rotate the valve as it is moved across the stone surface.
  • Insert it and make sure it turns freely. Part of the valve is actually sharp and square, leave these areas alone.
  • If this valve does not need smoothing, leave it alone and install a new modulator with the gasket centered on the threads.
  • Carefully thread the unit back on with the special tool or water pump pliers.

Normal Powerglide Shifting Points

Normal shifting during driving (assuming no issues) should occur at 11 mph and at 44 mph, ± 3 mph. Going 45 mph and more and a downshift to L (low) will not cause the PG to shift. When the speed drops below 45 mph while still in L, the downshift will occur.

NEVER shift from D to R while moving; doing so will damage the PG.

You can push start your Powerglide at 18 mph.

If you are going 41 mph or less and you push the accelerator pedal through the detent point with the accelerator, the PG will shift into Low, and up shift at 44 mph or so. "Detent" means the point at which the pedal is pressing or pulling through a definite resistance.

If Your Car Starts in Gear

If your engine starts while the car is in any of the gears (the car should start only in N):

  1. Loosen the mounting screws, and move the switch housing forward or backwards until the starter operates only in Neutral. Tighten screws and retest. Verify the backups also work.
  2. Make sure the selector lever is D and check. Rotate the throttle lever clockwise. The hole in the large arm must be below the oil pan rail and measure 3\8”. If the hole is above the pan rail, the cable ball end did not enter the slotted lever and must be reinstalled.

If Shifting Is Rough

If your shifting is rough, or too early or late, your governor may be defective, or improper Throttle Valve (TV) pressure may be the cause.

To check the TV:

  1. Attach a 0-100 psi gauge to the TV test port on tranny (this will hard to locate). It is at the 8 o’clock position.
  2. Disconnect the accel rod from the tranny throttle lever.
  3. Raise the rear wheels 5” off the ground.
  4. Start engine and idle.
  5. Disconnect the throttle rod from the cross-shaft, and the modulator hose from the Balance Tube.
  6. Slowly pull the rod toward the rear of car. Pressure should be 40 psi to the detent position, and 52-54 psi through Detent position. If lower than 52 psi, low pressure is causing it to shift early, if more than 54 psi, excessive pressure causes it to shift later.
  7. Pressure in Low should be 66-77 psi. Pressure in N or R is 0.
  8. To adjust the TV pressure, remove tranny oil pan. Using an Allen wrench, hold the Throttle Valve plunger as you turn the jam nut CCW 1 full turn to raise TV pressure 3 psi. One full turn CW will lower it 3 psi. To change only 1 psi, move to the flat area of jam.

If the TV seems okay, you may have a defective governor.

Checking Tranny Oil

Check tranny oil after engine has run 15 min and is running. If the oil is more than 1\2” above the FULL mark, drain excessive amount to avoid oil foaming and other problems.

Engine Overheating

Corvair engine with no top shroud: Note the finned cylinders and spark plugs. Debris stuck between fins causes overheating.

Corvair engine with no top shroud: Note the finned cylinders and spark plugs. Debris stuck between fins causes overheating.

The main cause for overheating is debris clogging up the cylinder fins which are vital for proper cooling. If your engine top shroud or the oil is hot to the touch after running for 5-10 minutes, you might want to “de-flash” your engine. Corvair engines do not normally run so hot that the top engine shroud is too hot to touch—warm, yes, but not hot. The oil from the stick should not be hot either. If it pings or smokes, the passages are blocked.

  1. Clean the oil cooler and fan.
  2. Remove the top engine shroud if you need to by removing the carbs and alternator.
  3. Spray compressed air in from the bottom and see what comes flying out of the fan area.
  4. Once you do, you'll probably see that you need to pull the lower shrouds to clean it properly. This is especially the case if the vehicle has been parked for a period of time. Cleaning the fins out is VERY important and easy to do. Slow timing may also cause engine overheating as does running too lean in the carbs.
  5. Shine a light under the car so that you can see what needs to be de-flashed.
  6. Use a cheap keyhole saw from the local home depot or tool store to insert between the fins to punch out debris or excess metal. If the exhaust manifolds are still on, they block the light from below and limit how far the blade can go through the head, but you can still do the job with them on.

More on the Chevrolet Corvair

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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