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How Long For Mechanic To Change An Engine? (Hidden delays)

Mechanics usually work pretty quickly, we’re always under pressure, in fact like many professions, we’re always under time deadlines. Nobody wants to be without their car or have the expense of a hired car.

A skilled mechanic using professional tools and equipment will change a car engine in one to two days. The time required is affected by 3 main factors:

  • Engine layout
  • Engine size
  • Replacement motor condition
  • Vehicle type

In this post, you’ll learn what it takes to replace a car engine. I’ll list the type of components that need to be removed and unforeseen issues that can slow down the process.

Engine Layout

Engine layouts come in a few different flavors and with the exception of some high-end sports cars, most are either Longitudinal or Transverse.

Longitudinal is the traditional engine layout, engine and transmission fitted length-ways with the engine out front.

This layout is still favored by trucks and higher-end Sedans. The longitudinal is perfect for rear-drive and four-wheel drive cars and trucks. This type of setup also happens to be the easiest and fastest to turn around.

Engine layout

The reasons are quite simple, there is a ton more space on longitudinal cars to swing wrenches, access shortens the on-the-job time. In addition, the transmission can remain in place.

How do you know if you have, longitudinal engine cover that runs parallel to the length of the car, the big giveaway is however the driveshaft tunnel down the center of the car that encroaches into the cabin.

Transverse is where the engine and transmission are mounted sideways in the engine bay. In 1954 Alfa-Romeo had plans to fit a transverse engine to a small Alfa, post-war financial difficulties killed it. DKW of Germany and Suzuki motor cars of Japan had similar products.

But it was the little British mini of 1959 that became the true pioneer and is still the blueprint for transversely fitted engines today.

The advantages of the transverse-mounted engine are space. The transverse layout typically drives the front wheels and so a tunnel down the length of the vehicle wasn’t needed. Neither did the engine encroach into the cabin.

All this compact comes at a price, while cabin space became plentiful, real estate under the hood got tighter. Changing an engine on a front-drive transverse vehicle can be a little more work.

First off the transmission must come out with the engine. Wiggle space under the hood is a lot tighter than the rear-drive.


More modern front-drive vehicles are designed around a sub-frame. On the assembly line engines, transmissions and suspension, and steering are fitted to the sub-frame and the car is lower onto the sub-frame.

This is the most common setup today and when working on these vehicles the mechanic must reverse the process.

Engine Size


The bigger the engine the more complex the operation. Complexity obviously takes more time to manage. A large engine in a truck isn’t an issue but a large engine fitted to a sedan will take the mechanic a little more time than a family saloon.

Bigger engines are more sophisticated and that means time. A family saloon likely won’t need cooler lines to engine components, but higher-end cars with large motors get hot and so it’s not unusual to have a water-cooled air conditioning pump, alternator, throttle body, fuel coolers, air-cooled power steering, transmission oil cooler, power steering cooler.

You get the idea, more is more work.

Replacement Engine Condition

Aux belt on an engine

The condition of the replacement motor is important. A new or used engine may come fully build or bare. If it’s arriving at the mechanics bay bare, it will need to be built. Components such as:

  • Intake manifolds
  • Throttle body
  • Fuel injectors and rail
  • Exhaust manifolds
  • EGR valve
  • Turbo charger (if applic)
  • Flywheel or Flexplate
  • Alternator

Many of these components will require new gaskets which can only be sourced from the main dealer. Fitting old gaskets runs a real risk of developing a problem such as a vacuum leak, exhaust gas leak, oil or coolant leak.

How Does A Mechanic Change An Engine?

Transverse engine

I always begin with a full inspection of the vehicle, I scan the onboard control module for trouble codes. I keep a record of them in case diagnostic is required with the new motor.

It is helpful to understand why the previous engine died, if for example, it was an external component such as a blocked rad or turbo oil seal fault, well the same faith awaits the replacement motor.

After initial analysis, the process begins with battery disconnection and air-con extraction. KAM (Keep Alive Memory) tool may be needed if the vehicle is voltage loss-sensitive.

Here’s a typical removal process for a regular family front-wheel-drive car. A car hoist is essential for this type of work.

Components that need to be removed include:

  • Battery
  • Air-box
  • Chassis to engine wiring loom
  • Ground straps
  • A/c lines to pump
  • Starter power supply
  • Fuel lines
  • EVAP line
  • Exhaust
  • WSS sensors front
  • Brake calipers front
  • Brake pad sensors front
  • Power steering cooler lines
  • Transmission cooler lines
  • Transmission selector
  • Steering gear bolt

And now here’s the process for removing:

  • Support sub-frame on axle stands and remove bolts.
  • Raise the hoist, lifting the vehicle up leaves engine, transmission, steering rack, suspension, driveshafts and knuckles sitting in one lump on the ground.

Removing the engine from the sub-frame:

  • Remove starter motor and un-bolt torque converter.
  • Unbolt the transmission from the engine and using a shop crane support. the engine wile removing the motor mounts.

The engine is now free.

When fitting a new engine, the following items will be a ton easier to fit while on a bench:

In addition, for manual transmission cars, check the clutch, pressure plate, flywheel, release bearing, and clutch slave cylinder. Replacing any of these components will mean a major strip down again.

After fitting the new motor, the engine will need the following:

  • Oil and filter
  • A/C recharge
  • New A/C O-ring seals
  • Rad checked for blockage
  • Inter-cooler checked for blockage and debris
  • Air filter

The engine may need a throttle body to relearn procedure, steering wheel calibration, window rest, etc.

Things That Can Go Wrong


Things can and will go wrong from time to time, however knowing a few of the variables will increase the chances of the job moving like butter.

Here are a few of the common ones, most could be foreseen.

  • Engine faulty – Replacement motor is faulty.
  • Broken bolts – Older cars corrode, if a mechanic sees and older corroded motor he knows what’s ahead.
  • Leaks – Replacement engine develops an oil or coolant leak while on test.
  • Damaged oil pan – Used engine arrives with damaged oil pan.
  • Parts on order – Unforeseen parts needed and not in stock.
  • Wrong engine code – Engine looks the same but doesn’t suit the car’s PCM mapping (or turbo charger size, if applic).
Engine code

Related Question

How many miles can an engine last? Most modern family-sized car engines can last for 200,000 miles or more without any major mechanical repairs, but only if well maintained and cared for.