Updated date:

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


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



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.


perrya (author) on August 29, 2020:

well, if there is a loud clacking sound from the engine, it may indicate the valves need adjustment, pinging indicates some internal engine damage. as to sputtering, try changing the fuel filter which might be clogged, fuel lines can also be clogged

Joe on August 27, 2020:

I have a 67 Monza coupe with 110hp engine and PG trans. When it first starts up, it doesn't idle great but idles well and runs fine. After driving for about 15 minutes, the valves start pinging. It also has a problem where the whole car seems to shake when trying to hold the gas pedal at a constant speed. when I'm accelerating it runs fine, but when I try to hold it at a constant speed it shudders, spits, and sputters. Any thoughts on possible issues?

perrya (author) on January 01, 2020:

Maybe carb adjustment?

Lucas D on December 31, 2019:

I have a 64 Corvair with a manual trans' the car idles fine but stalls when I put it in gear and try to move.

I have check timing, put in new plugs, check fuel lines, and recently installed a new fuel filter.

Any ideas on what the issue is?..

perrya (author) on March 30, 2019:

Having never had that issue, the Governor or fluid might need changing.

Gary on March 30, 2019:

My 65 Corvair PG sometimes can’t decide to either up shift or downshift at low to mid throttle. It rapidly attempts to upshift then downshift and only decides after I let off the throttle some. I tried replacing the Modulator but no change. Might this indicate a Governor issue or internal issue?

perrya (author) on December 07, 2018:

I believe you should you should add ATF because checking it should be done when the fluid is hot.

Tony on November 27, 2018:

I have a transmission leak in my 64 corvair but am not sure where to start . It reads full when car is not running but while running fluid level reads add fluid . What is the problem

Marc Waters on October 28, 2018:

My 65 corvair engine runs good as soon as I put it in drive it dies even when I put foot on brake it dies no matter what I do can you please help me out

perrya (author) on March 18, 2018:

have no idea, sorry

Steve strawn on March 18, 2018:

My 63 corvair monza automatic powerglide stopped shifting into 2nd. I dropped trans pan and found a small set screw. Any idea on what it could be? This is actually steves son and i would really love to help him out he has recently restored the car and it means a great deal to him

perrya (author) on May 05, 2015:

Many shifting issues are tied to internal issues because AT trannys have bands that overtime may become damaged or worn.

disgrig on May 05, 2015:

My '62 Corvair Monza automatic shifts hard (jerks) when shifting from neutral into Drive and also when coming to stop/downshifting. Any ideas on what to start with?


Steve on August 02, 2014:

My PG in my 66 Vair does not shift int second. Have driven it up to 52 mph with no shift. Any Ideas?

perrya (author) on February 06, 2014:

Sorry, no, but I have the shop manuals

Wayne H on February 02, 2014:

how do you go about changing "O" seal in the Throttle valve on a 66 corvair monza

thomas on September 02, 2012:

Powerglide leaks oil sometimes all oil, sometime a little bit of oil and sometime no oil leak????

perrya (author) on August 30, 2012:

So u did all the usual tuneup things, new coil? fuel pump still good? A vacuum leak would cause a bad fuel-air ratio.

Richard R. Cook on August 30, 2012:

I've covered all those bases so wondering if vacuum leak in the intake portion of head could cause poor fuel supply to that bank, not allowing the 1,3,5 cylinders to fire fully. This problem is effecting the one bank only and uniformly. Can I get the good 150 reading on these cylinders and have a vacuum leak in the intake portion of the head/manifold? I know what a correctly firing plug looks like, grey or light brown. Engine misses and backfires under any load (acceleration). I've worked on my cars for years but have no experience with the Corvair engine.

perrya (author) on August 30, 2012:

Other that plugs being dry, black, are they burning fine? Maybe you need to reset timing? plug wires? I don't know what has already been done.

Richard Cook on August 30, 2012:

No oil. They are dry but obviously not firing as they do on the other head.

perrya (author) on August 29, 2012:

burning dark? oily dark or sooty dark?

Richard Cook on August 29, 2012:

1965 Corvair Monza vacuum leak. Is it possible to have an intake manifold leak with the manifold/head being common and comparession reading of 150# on all cylinders? I was wrong on my first set of readings. All plugs burning clean on one head/manifold all plugs burning dark on other head/manifold. Is there a separat seal between the head surface and the common intake surface?

perrya (author) on August 27, 2012:

High compression is usually caused by either excessive carbon build up, excessive fuel being injected or oil\coolant getting in. The corvair should have a reading of 130 psi

Richard Cook on August 26, 2012:

Thanks for the input but I still don't understand the readings. The only spec I have found says compression should be between 120 and 130. If this is true how can my reading even be achieved??

perrya (author) on August 26, 2012:

Low compression either from valves or rings. You can do the test wet and dry. Check the lbs. dry and then give a little shot of oil in the plug hole and check the lbs. If the pressure goes up wet, then the rings are losing the pressure. If the pressure is the same wet or dry, then it's a good chance that it needs a valve job.

Richard Cook on August 25, 2012:

Just started working on 65 Corvair Monza. Compression is #1 146,#2 157,#3 170,#4 160,#5 165, #6 170. Book says 120-130 is normal what do my reading tell you??

rare69 on August 22, 2012:

'66 PG increases RPM when Low is selected . The throttle lever rotates clockwise to push the accelerator lever up to a high idle speed.

linkage adjusted by manual from gas pedal to carbs with no difference. cannot downshift in drive at WOT.

Had this car 30+ years always same symptoms.

donnie on July 30, 2012:

65 corvair that has high oil pressure and blows filter apart

perrya (author) on July 12, 2012:

Good luck!

cowgirl79 on July 12, 2012:

Multiple issues? I was afraid of that! Guess I will start replacing parts until it runs right, hopefully I can figure it out soon. Thanks so much!

perrya (author) on July 12, 2012:

You may have multiple issues with the knocking. This is one of those things where you need to fix one thing, and see if it fixes the issue. It may need a new distributor as you said.

cowgirl79 on July 12, 2012:

I have a 61 Monza 900 (auto)- replaced the points 2 days after I bought it, ran fine for a few weeks, fuel pump killed over, replaced it. NOW, it runs fine for about 15-20 minutes and chokes out. Idles fine in neutral, but as soon as I put it in gear, it starts knocking really bad and has no power. If I leave it in neutral, and rev the engine a few times, then put it back in gear, it will creep along then suddenly it runs ok for a few minutes longer. Thinking distributor...??? It almost seems like it kicks itself out of timing, but I am not sure. Any ideas??

perrya (author) on June 17, 2012:

My 67 has a different linkage, so i am not sure.

Mark on June 17, 2012:

Hello! I have a '63 Monza 900. It's been a great car. My question is about the throttle linkage that runs underneath the car from the accelerator back. About halfway back, there is a small linkaga that hold together two parts of the shaft. After 48 years, that finally decided to pop apart on my car. Are there any cheap fixes that I can do to avoid a towing bill up to my corvair guy? He's about a half-hour away from where I live. p.s. Where is Santa Rosa?

DonV on April 14, 2012:

I have just checked the vacume advance on my 64 corvair. It takes aalot of ahnd pressure to move the old one. The engine has two vacume tubes on each carb. with the vacume tube going to only one(other one has been capped off). I checked for amount of vacume from just the one carb and even when the engine is reved up there is very little vacume. Do I need to run more tubing to both carbs for more vacume then to the vacume advance?

ri on December 13, 2011:

My 400 trans is only working in reverse do I need to replace trans mission or vacuum modulator..... It also goes like 5 mph in drive

perrya (author) on July 28, 2011:

well, in a 65+, they are in each corner of the fender, behind tail lights and headlights. They DO make a big difference in stability on roads at higher speeds.

Steve on July 28, 2011:

I have a 63 Monza 900 Convertible. What and where are these oil ballasts I've heard of? I'd like to check if I have them, and if they have fluid in them...

terry on July 22, 2011:

i have a 64 corvair monza 900 coupe with the automatic sound like my starter is not engauging half the time or teeth are broke...does this model have a regular flywheel or is it a ring gear weled on the torque converter?pics of the torque converter would help if you could find some...thank

frankh on July 12, 2011:

I will keep looking into the problem, thank you very much for your ideas.

frankh on July 12, 2011:

She drove it approx. 20 miles with the squeeling, said that it drove fine and shifted like normal, just the noise. I haven't driven the car since. Have been a little worried that it may cause expensive damage.

perrya (author) on July 12, 2011:

No, maybe some of the teeth in the converter are missing. Maybe the bands in the AT need repaired or adjusted. Does it shift fine?

frankh on July 12, 2011:

I took the fan belt off of the car to rule out the cooling fan,idler pulley and the generator. Started the car and narrowed down the noise to somewhere near the torque converter/flywheel area. Is there some sort of cooling fan inside the bell housing?

perrya (author) on July 10, 2011:

Squeeling? sounds like the fan belt. What are the other symptoms when driving and shifting?

frankh on July 10, 2011:

My girlfriend has a 64 convertible that has a horrible squeeling noise coming from the torque converter area. I am not familiar with the corvair automatic transmission and was looking for some ideas.

Chris Watkins on March 29, 2011:

great blog thanks!!!!

perrya (author) on March 27, 2011:

I agree with Danny, except minor things. my 67 Corvair coupe, 4 spd runs like new, it only has 56K on it.

Transolution on March 26, 2011:

Great detailed HUB. It's obvious you know what your talking about when it comes to the Corvair. Might have to come back with some questions for you when I have them.

claysclan1 on December 19, 2010:

In September of 2010 I had a family emergency, since I do not like to fly, I drove my 1968 Corvair Monza two door from Huntington Beach to Columbus Ohio in three days with know problems. I did have Greg Peterson give it the once over before my trip out of Long Beach California. The car now has 112,000 miles and I will be driving it back to California soon, The car is not restored but solid being from California, This should answer all question about reliabilty. I did finally replace the harmonic balance in Nov of 2010 in Ohio

Nissan Frontier on November 30, 2010:

This is such a great help buddy. Thank you so much for reminding everyone here that it is not always about changing units every time but exerting effort to maintain the cars they own.

There are plenty of ways to save and this one will serve that purpose big time.


transmission slippage on November 25, 2010:

This is so real! The details are awesome. it is so rare to see writers putting his head on his piece in an internet page accessible to people without charge.

autopart1 on November 04, 2010:

Hi friend i see your hub this is nice. If you want a best Chevy Used Parts please visit our site...........

usedmotor on October 13, 2010:

I see your blog this is nice. you want a best Japanese engines please visit our site.

perrya (author) on October 05, 2010:

well, many things may cause some of the issues: dirty carbs and ventura jets inside them, does not take much.The carb airflow is not synched, carbs are not adjusted to spec, vacuum leaks. Automatics are much more difficult than a stick.

Danny on October 05, 2010:

My corvair starts right up with no problems stays running in nuetral fine. when i put into reverse or drive though it bogs out. I try for about 15 minutes and then it works but it seems to hesitate and even die sometimes at stop signs right when i start to hit the gas? After its been driven for 15 minutes or so it runs great with no problems. Any ideas?

perrya (author) on September 29, 2010:

Check for a broken E clip, which is inside the tranny. Pieces of it may be jamming things up. Remove the oil pan and then the valve body to find the pieces. A broken E clip (cheap part)usually keeps the tranny in low.

hank on September 28, 2010:

65 powerglide stuck in 1st and rev. Cannot upshift while driving. Any thoughts?


perrya (author) on May 28, 2010:

Hmmm, because the engine is air cooled, de-flashing the fins usually solves the issue. How hot is your engine running?

car scratch repair on May 28, 2010:

I have a problem with my engine overheating. I try to keep the fan as cool as possible but still having problems. Any other ideas?

perrya (author) on March 22, 2010:

have no idea what you are referring to.

kyle on March 21, 2010:

why dont u just heliarc the case did that on oweer boger worked fine

perrya (author) on March 15, 2010:

Very cool

onicroster from New York on March 15, 2010:

hey, thanks perrya..it really helped me out on way to home. just saw an old lady messing up with her car. I helped her out...thanks to you :-)

perrya (author) on March 03, 2010:

Sorry, no. Never had that issue!

bubba on March 03, 2010:

hi perrya. do you have any lit. with photos on how to remove the insides of a good trans. (broken case), and replace in a good case? for a 65 PG. thanks

Sexy jonty from India on August 15, 2009:

Very well written hub .....

very much informative ......

Thank you very much for your great hub, for good advice, good wishes and support. Thanks for sharing your experience with all of us.

Related Articles