Transmission fluid plays a critical role in not just lubricating the moving parts, but it also helps the valve body transmit the pressure necessary to change gears, as well as helping to cool the transmission components. Anytime something affects the quality and consistency of the transmission fluid it will affect transmission performance on every level.
If you’ve noticed your transmission slipping out of gear, hesitating to make gear changes, or shifting into gears with a heavy “Clunk” the first thing to do is check the transmission fluid. Let’s say you do, and you notice bubbles in the transmission fluid or it looks frothy. What does this mean?
Usually bubbly or frothy-looking transmission fluid is a sign that there’s air getting into the system. This could be due to a leak, the transmission fluid being low, or even a clogged transmission fluid filter. Though foamy, frothy, or bubbly transmission fluid could also be a sign of too much transmission fluid in the system.
To understand how to get your transmission acting properly, and eliminate the bubbles, you’re going to have to troubleshoot what the transmission dipstick is trying to tell you.
The Startling Link Between Transmission Fluid Bubbles and Car Performance
Transmission fluid is integral to the functions of gear shifting, lubrication of moving parts, and temperature regulation within the transmission. The presence of bubbles in the fluid can significantly compromise its ability to effectively fulfill these critical roles.
So here’s how a simple air bubbles in your transmission fluid can cripple your car’s performance.
1. Transmission Fluid Lubricates
Air bubbles in the transmission fluid affect the oil’s ability to lubricate moving parts. This can lead to excess friction in the transmission’s many moving parts. It can also cause gears to grind on each other depositing little metal flakes in the fluid.
2. Transmission Fluid’s Role in Shifting Gears
When you have air bubbles in the transmission fluid it can affect the transmission’s ability to transfer pressure to and through the valve body, which initiates the gear-changing process. This can make it hard for the valve body and the TCM to activate the clutch packs and bands responsible for gear changes. You end up with the transmission getting stuck in one gear, or hesitating to change gears, finally shifting hard when the engine RPMs are too high.
3. Transmission Fluids Role in Heat Management
Air bubbles can prevent transmission fluid from efficiently flowing to the radiator and back, which can allow the transmission to grow gradually hotter. This can be a double-whammy effect as transmission temperature continues to rise it can cause the fluid to degrade. This can further affect lubrication leading to a runaway heating effect which can damage many transmission components.
6 Leading Causes of Bubbles in Your Transmission Fluid And How To address Them
Low transmission fluid, over-filled transmission fluid, and a clogged transmission filter are some of the top reasons why you might be seeing bubbles or frothiness on your transmission dipstick. Though the underlying concern here is how the fluid level was affected.
1. Low Transmission Fluid
Low transmission fluid can allow too much air in the system, which shows up as tiny bubbles. If you check your dipstick or your seal-transmission check valve and you find that the fluid level is low for the temperature of the transmission, your knee-jerk reaction is to simply add more.
The Quick Fix for Low Transmission Fluid
You might be able to add and refill your transmission fluid in small doses and let the car sit to allow the bubbles to settle out. Though this is just a quick fix that might resolve the problem for a day or two.
The problem is that transmissions don’t use or burn fluid in anyway. So, if you have low transmission fluid it means there is a leak somewhere. If you don’t find and fix that leak, you will continue to lose fluid and once again have bubbles or frothiness affecting transmission performance.
2. A Leak in a Transmission Seal or Pan Gasket
Underlying causes for these bubbles your transmission dipstick often trace back to leaks in the transmission seal or oil pan gasket, which allow small volumes of air into the system. This usually starts out as a miniscule transmission fluid leak, but the volume of air that comes in might not make the leak apparent on the dipstick.
Compounding this problem is that most minor leaks in seals or the pan gasket can start out so small in the early days that they only leak when the car is running. Shortly after putting the transmission in park or neutral, the pressure normalizes, and the leak stops. Yet the air remains in the system. Usually, it’s only when the leak in the seal or pan gasket gets severe that you notice drops on your garage floor or a major decline in the transmission fluid on the dipstick.
How to Find a Transmission Leak
If it’s a simple small leak in a seal or an O-ring the transmission fluid will likely continue to leak even when you park the car. If it’s a leak in an axle seal or a minor leak in the pan gasket, the transmission fluid might only have a noticeable leak when the car is running.
Your best bet for finding where the leak starts with giving the underside of the transmission a thorough cleaning. Leaking transmission fluid has a knack for moving around and following surfaces which can sometimes make it hard to spot.
Fixing a Transmission Oil Pan Gasket Leak
One of the most common transmission fluid leaks that can lead to bubbles and frothy fluid is a failure in the pan gasket. Pan gaskets have a limited lifespan. Some can start to fail around 60,000 miles. Even faster if the transmission has had overheating problems in the past.
The DIY temptation here is to simply crank down hard on some of the transmission oil pan bolts. This might even work for a little while. Unfortunately, the flange on the oil pan can deform easily, making the gasket fit improperly.
It’s also possible, if you or a previous owner replaced the last pan gasket as part of a transmission fluid change, that there were bits of the old gasket still in place. This can cause a gap that worsens over time, letting air into the system and allowing fluid to slowly seep out.
In a scenario like this, you’ll need to replace the entire oil pan and use a new gasket.
The parts cost for a new transmission oil pan can range from $35 to $100. It often includes a new gasket.
If you’re a modestly capable DIY mechanic, you should be able to handle this yourself in the better part of a Saturday afternoon. Though it will require draining most of the transmission fluid. So, while you’re at it, you might as well do a transmission fluid replacement.
This will add another $100 or so to the repair cost for fluid and a new filter.
If you have a mechanic perform the transmission oil pan replacement and transmission fluid change, you can expect it to add another $120 to $175 to the average repair bill of $400.
3. Degraded Transmission Fluid
Transmission fluid in a degraded state can also retain bubbles and other particulate matter, leading to compromised transmission performance. Generally, most transmission fluids have a lifespan of 60,000 to 100,000 miles. However, factors like overheating, heavy towing, or lack of proper maintenance can hasten this degradation, causing the fluid to lose its vital lubricating properties. As a consequence, it can trap any air present in the system, leading to the formation of bubbles or a frothy appearance in the transmission fluid.
You can identify degraded transmission fluid by its dark color. Often, it will turn brown and start to froth. In contrast, good transmission fluid should appear translucent and have a light pink hue.
How to Fix Degraded Transmission Fluid
Fixing degraded transmission fluid starts with a complete flush and fill, as well as replacing the transmission fluid filter. This will help remove the air bubbles as well as purge the transmission of sludge or other particulate matter.
If you’re a modestly capable DIY mechanic you should be able to replace your transmission fluid yourself in just a few hours.
The cost of the fluid and new filter, with a fresh pan gasket, will only cost you around $100 to $125.
Having a mechanic change your transmission fluid will add another $100 to $125 to an average repair bill of $200 to $250.
The more important thing you have to consider is what caused your transmission fluid to degrade in the first place. If it’s been less than 50,000 miles since your last transmission fluid change, you might have another fault that caused the fluid to go bad and develop bubbles.
Overheating is the most common cause of rapidly degraded transmission fluid. It’s often due to heavy towing or having to drive in a lot of stop-and-go traffic. If this sounds like your driving style, you might want to consider installing a transmission cooler/radiator or upgrading the size of your transmission oil pan to reduce the risk of overheating.
4. A Clogged Transmission Fluid Filter
A clogged transmission fluid filter can also contribute to the formation of bubbles or frothy fluid, due to changes in fluid dynamics within the transmission. In such a situation, air present in the transmission oil pan gets drawn up while the fluid struggles to pass through the obstructed filter. This air then circulates through the transmission, causing cavitation with gears and other moving parts, ultimately saturating the transmission fluid with tiny bubbles.
Other signs that a clogged transmission filter is causing the bubbles in your transmission fluid include strange burning smells, trouble shifting gears, and odd rattling noises. A lot of times, the transmission fluid on the dipstick will also look dark or have particles trapped in it that can also affect transmission performance.
Sometimes the stress on the system caused by a clogged transmission filter will lead to fluid leaks. These leaks are usually around the pan gasket, O-rings, or the transmission’s breather valve.
How to Fix a Clogged Transmission Filter
If it’s been more than 50,000 miles, or whatever your car’s manufacturer recommends since your last transmission fluid change, the wise move is to have a complete flush and fill. This will involve replacing the transmission filter along with the fluid, and the pan gasket seal.
You can usually do this yourself for around $100 to $120.
If you want a mechanic to replace your transmission filter and perform a complete flush and fill, you can expect it to add another $100 to $150 to the final repair bill.
5. Overfilled Transmission Fluid
Ironically, overfilled transmission fluid can also cause bubbles or frothiness just like low transmission fluid can. When there’s too much transmission fluid, it creates high pressure within the transmission housing. This pressure triggers a cavitating effect that allows air bubbles to blend with the fluid.
A lot of times overfilled transmission fluid looks “Frothy” rather than “Bubbly.” You’ll also get the same hesitating shifting and hard gear changes that you do with low transmission fluid. Though one of the distinctive signs of too much transmission fluid is that you’ll see it leaking out of the transmission’s breather valve.
How to Check for an Overfilled Transmission
Checking the transmission fluid is the easiest way to know if it’s overfilled. This means reading the dipstick and accounting for the current temperature of the transmission. If it’s cold, an overfilled transmission will have fluid far above the COLD or MIN line.
If you have a sealed transmission without a dipstick, you’ll have to check the fluid level via the inspection plug on the side of the housing. You should wait until the transmission is at the correct temperature range for your make and model. Different automakers have different specifications for when you should do this. Though you might need a scan tool to make sure you’re in the correct temperature range.
How to Fix Overfilled Transmission Fluid
You can use a simple siphon tool to extract a small volume of excess transmission fluid. You’ll also need a drain pan and a bunch of clean shop rags. Then make sure the transmission fluid is completely cool before siphoning. You can then put the parking brake on and extract the excess transmission fluid using the following steps.
If the cavitation effect in the transmission fluid is severe, and the frothiness is still a problem even after letting the car sit for a day or two, then you’ll need a complete transmission fluid and filter change.
6. A Failing Transmission Pump
A failing transmission pump can also result in the creation of fluid boiling or bubbles in the transmission fluid due to diminished circulation flow. As the condition worsens, you might notice your transmission struggling to shift up, shifting late, or even slipping out of gear. As the motor inside the pump starts to die you’ll likely also hear a whining noise.
If your transmission has over 100,000 miles on it, and you do a lot of stop-and-go driving or you frequently tow heavy loads it might simply be coming to the end of its natural life. Though many times overheating incidents are the root cause of a transmission pump failing. If your transmission fluid has bubbles and it’s also dark due to degradation from overheating, it might be time to have the transmission pump replaced.
A lot of newer cars will turn on a check engine or check transmission warning light when the transmission oil pump starts to fail. It might then throw one of the following codes.
How to Fix a Bad Transmission Pump
The only way to deal with a bad transmission pump is to completely replace it. This calls for opening the bell housing which can add a fair amount of labor cost to the final repair bill. It also tends to be just outside the range of what a typical DIY mechanic can handle.
Usually, you have to take it to a transmission specialist, which further ramps up the labor cost. Depending on your make and model, the transmission specialist might also recommend a transmission tune-up or even a partial rebuild.
The knee-jerk reaction is to think they’re trying to capitalize on your misfortune, but most of the time when the transmission pump goes out, it causes problems with other components as well. Especially if the transmission pump died prematurely due to overheating problems or just general wear and tear.
The part cost for a transmission pump can vary from as little as $210 to as much as $450.
Then you can expect another $450 to $700 in labor costs to have the transmission specialist replace it.
Though these figures are just for the pump. If the transmission needs a partial rebuild, which it probably will, you can expect another $450 to $700 added to the final repair bill.
Frequently Asked Questions
Is it bad to drive with bubbly or frothy transmission fluid?
Yes, it’s unfavorable to drive with bubbly or frothy transmission fluid. The bubbles in the transmission fluid can lead to hesitation in shifting and improper lubrication of moving components, as well as contribute to heat buildup. Therefore, it’s not advisable to drive more than a few miles under these conditions, either to reach home or to make it to a mechanic’s workshop.
Do I need to completely replace bubbly or frothy transmission fluid?
Most of the time the bubbles in transmission fluid won’t settle out on their own. There’s a chance that large bubbles from low transmission fluid might be okay if you let the car sit, but most of the time the wise move is to perform a complete flush and fill. Especially if the transmission fluid looks frothy.
What causes an overfilled transmission?
Too much transmission fluid is one of the more common causes of bubbles or frothy-looking transmission oil. This is usually an owner error, from checking the fluid at the wrong temperature. You might check the fluid when your transmission is cold, the fluid looks low, and you accidentally jump to the conclusion that it’s low and you add some. When really, the manufacturer usually recommends checking the fluid when the transmission is warm or near service temperature.
Bubbles in transmission fluid are often caused by cavitation due to either low or overfilled transmission fluid. You’ll likely also notice signs like hesitating gear changes, hard shifts, and symptoms of an overheating transmission.
If your fluid is low, you likely have a leak somewhere in the transmission housing, seals, O-rings lines, or pan gasket. Even if you refill the transmission fluid the leak will still allow more air into the system. So, the top priority is finding the leak.
It’s also possible for degraded transmission fluid or a failing transmission pump to cause bubbles or frothiness. Especially if you tow frequently, or your daily commute has a lot of stop-and-go traffic leading to overheating Most of the time when you have bubbles in the transmission fluid, you need to do a complete transmission flush and fill. The bubbles rarely settle out on their own. Then you’ll have to take care of any other repairs, like fixing a leak or replacing the transmission filter or pump in the process.
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.