What Is Over Boost & What Causes It? (Explained)
Updated: Oct 16, 2021
Over boost is what happens when the forced induction system is forcing more air into the engine than it's supposed to, it can be caused by a multitude of reasons.
This can cause some major issues with the engine if it goes on for too long, such as overheating, head gasket failure, misfiring, knock, etc.
Over boost can be caused by a wastegate stuck in the closed position, a stuck wastegate control solenoid or actuator, blocked wastegate hose, faulty boost sensors or ECU, or possibly even faulty knock sensors.
Below, I'll explain the ways to diagnose and fix over boosting and what the risks and symptoms of over boost are.
Table of Contents:
Risks of Over Boost
There are multiple risks of a car over boosting, mostly to do with the engine and drivetrain, though the risks depend on how much the car over boosts by.
There are other possible risks others than those listed above, these are just the most common.
Blown Head Gasket
A blown head gasket can be caused by over boosting, usually over a prolonged period of time if the engine overheats.
This could cause a loss of compression, coolant or oil leaks and other issues.
You can quickly diagnose a blown head gasket by using a compression test kit, checking the oil cap for a milky white substance, checking the engine bay for leaks, and checking the exhaust for a blue or white coloured smoke.
Overheating can be a problem if a vehicle over boosts for a prolonged period of time.
Usually the car is set up specifically for the amount of power and torque the engine produces, when a car over boosts it produces more power.
This extra power can cause too much to build up in the coolant and oil, increasing engine temperature if the radiators can't deal with that heat.
Overheating can result in various and possibly severe engine issues.
Warped & Cracked Components
Sometimes, if the over boost is high enough it can result in cracking, warping or snapping engine components.
This can be seen on the pistons, cylinder liners, connecting rods, valves, crankshaft, and possibly other components.
Usually other issues will arise before these do, though.
With the extra power and torque produced by an over boost situation it can cause the connecting rods to bend and warp.
This is more common on engines with cast aluminium or cast iron rods, though it can still happen with forged rods or even steel and titanium connecting rods, but it's less likely.
Sometimes, if a strong enough engine knock occurs it can cause bent rods.
As mentioned above, engine knock can cause many issues, especially severe engine damage, such as bent rods, cracked cylinder liners and pistons.
Engine knock occurs when the combustion process is uncontrolled and happens at the incorrect time, there are multiple types of knock but they're all unwanted.
Knock is more likely to occur when turbo boost pressures are higher, such as in the case of over boost.
Gearbox, Clutch & Driveshaft Failure
Due to extra torque and power seen in an over boost situation, the clutch gearbox and drivetrain components such as driveshafts, CV joints, etc, can be put under extra stress.
This extra stress can cause damage and failures, possibly snapping, cracking, overheating, and failing altogether.
Some early signs of clutch failure is slipping and difficulty accelerating even though rpms are increasing, signs of gearbox failure are gears popping into neutral and oil overheating, sometimes driveshafts and CV joints can begin to make a clicking sound.
Sometimes when too much boost occurs, the spark from the spark plugs can be blown out, causing an incomplete combustion, resulting in a misfire.
Other times, the incorrect fuel mixture is injected, resulting in a misfire. Early signs of a misfire include difficulty accelerating, and a hesitation or judder when accelerating.
Symptoms of Over Boost
Some of the indications and symptoms of overboost have already been addressed, however there are some more symptoms of overboost that I'll describe below.
There are some variations of over boost which have different symptoms, sometimes the ECU and failsafe mechanisms will stop over boost from occurring and cut power, attempting to avoid damage to the engine, sometimes there are no mechanisms in place.
If the ECU and failsafe mechanisms can take control of the over boost, then power will be cut resulting in loss of power and "limp mode" occurring, where the vehicle will not boost.
This can result in the following symptoms:
Loss of power and not boosting when accelerating.
Hesitation and difficulty accelerating.
Usually illumination of the "check engine" light.
Providing the over boost was only for a brief moment, the engine and car will likely be fine and not incur any damage. However, ifs the over boost was for a prolonged period and time and knock occurred, there could potentially be some damage.
Another type of over boost is where the ECU or engine has no mechanism or failsafe to avoid it, resulting in aggressive over boost, hitting the limit of the fuelling system.
This will result in the following symptoms:
The car will feel like it's "hitting a wall", suddenly cutting out on power after an aggressive spike of power.
Loss of traction, wheel spin and possibly swerving.
If the over boost was severe enough, the engine and drivetrain may incur some damage, possibly resulting in other issues and symptoms of those.
The "check engine" light may illuminate.
Coolant or oil temperature may spike.
The engine may emit a knocking or pinging sound, causing by engine knock.
The engine may misfire, causing hesitation or juddering.
There are various other symptoms of over boost, especially if the engine or drivetrain experience other issues as a result.
One other type of over boost is where the engine maintains its normal functioning but hits a higher boost pressure than it should, usually this is caused by a sticking wastegate actuator, blocked hose, faulty sensor, etc.
One of the other symptoms of this type of over boost is actually under boost, sometimes it may reach too low boost pressures, followed by too high boost pressures.
You may also experience turbo lag, a hesitation when accelerating caused by the engine not reaching target turbo boost pressures quickly enough.
Usually this type of over boost doesn't cause any specific symptoms other than usually more mild versions of those already discussed.
It'll mostly just result in excessive power output, causing wheel spin, clutch slippage, and possibly leading to engine or drivetrain damage in the long-term.
Many times if boost pressure exceeds 4 psi for longer than 5 seconds then an OBD fault code P0234 may appear when faults are scanned for.
Causes of Over Boost
Over boost has a variety of causes, mostly related to the wastegate. The most common causes of over boost are the following.
Damaged or sticking wastegate actuator
Sticking wastegate control valve solenoid
Blocked or partially blocked wastegate control hose
Faulty boost pressure sensor
Faulty or damaged wiring or connectors
Faulty or damaged knock sensors
Defective PCM/ECU programming
Sometimes it could be caused by multiple of these factors.
How to Fix Over Boost
Fixing over boost depends largely upon its cause.
The best way of checking for a cause of over boost is by scanning the ECU for fault codes, sometimes another fault would have flagged up.
Taking the car to a mechanic is the best way of getting over boost properly diagnosed and promptly fixed.
The most frequent solution to over boost is to replace the various air sensors (BPS, MAP, MAF, etc). If is defective it will not provide the correct pressure measurement to the PCM.
The wastegate may also need to be changed if it is stuck. Replacing a clogged or stuck wastegate valve or damaged wastegate lines may be needed too.
If the fault code is generated again, indicating an over boost situation, the most frequent cause is intermittent wastegate binding or sticking.
It is always a good idea to properly diagnose and fix an over boost situation if one arises, leaving it unfixed can result in a massively reduced engine lifespan and engine failure.