What Should You Replace When Replacing an Engine or Transmission?

Your vehicle needs:

AN ENGINE.

Or:

A TRANSMISSION.

That's already one of the largest repair bills most vehicle owners will ever face.

Then your mechanic says:

“WHILE WE'RE IN THERE…”

Rear main seal.

Engine mounts.

Transmission mount.

Coolant hoses.

Thermostat.

Belts.

Spark plugs.

Transmission fluid.

Filters.

Maybe even:

A WATER PUMP.

And suddenly you're wondering:

“ARE THEY TRYING TO SELL ME MORE PARTS?”

Not necessarily.

Because engine and transmission replacement create something unusual:

ACCESS.

Components that normally require:

HOURS OF LABOR

may suddenly be:

RIGHT IN FRONT OF THE TECHNICIAN.

That can create one of the best opportunities you'll ever have to perform certain repairs:

BEFORE THE POWERTRAIN GOES BACK INTO THE VEHICLE.

But that does not mean:

REPLACE EVERYTHING.

The smart approach is:

INSPECT EVERYTHING THAT BECOMES ACCESSIBLE.

REPLACE KNOWN WEAR ITEMS WHEN JUSTIFIED.

ADDRESS LEAKS NOW.

CONSIDER LABOR OVERLAP.

and:

DON'T SPEND MONEY ON HEALTHY PARTS JUST BECAUSE THEY'RE EASY TO REACH.

Why “While You're In There” Matters So Much

Imagine a seal costs:

$40.

Replacing it normally might require:

8 HOURS OF LABOR.

But during an engine or transmission replacement:

The components are already separated.

The additional labor may be:

MINIMAL.

That's where:

A SMALL ADDITIONAL COST TODAY

could potentially prevent:

A LARGE LABOR BILL LATER.

This is called:

LABOR OVERLAP.

And few automotive repairs create more labor overlap than:

ENGINE AND TRANSMISSION REPLACEMENT.

But Don't Confuse Labor Overlap With Parts Cannon

There are two extremes.

BAD STRATEGY #1

“Only replace the engine. Don't touch anything else.”

That could mean reinstalling:

KNOWN LEAKING OR NEAR-END-OF-LIFE COMPONENTS.

BAD STRATEGY #2

“The engine is out, so replace everything.”

That can waste:

A LOT OF MONEY.

The better strategy is:

CONDITION + AGE + MILEAGE + ACCESS + FAILURE RISK + LABOR SAVINGS.

First: Understand What You're Actually Buying

Before discussing additional parts, determine:

WHAT ENGINE OR TRANSMISSION IS BEING INSTALLED?

You may be choosing between:

NEW.

REMANUFACTURED.

REBUILT.

or:

USED.

These are not:

THE SAME THING.

Used Engine or Transmission

A used assembly typically comes from:

ANOTHER VEHICLE.

Ask:

• What is the mileage?

• What vehicle did it come from?

• Was it tested?

• Is there documentation?

• What warranty is included?

• Does the warranty cover labor?

• What accessories are included?

• Which components must be transferred from the old unit?

A used engine with:

70,000 MILES

should not automatically be treated like:

A BRAND-NEW ENGINE.

Remanufactured Engine or Transmission

A remanufactured unit has generally undergone a more extensive rebuilding process according to the remanufacturer's procedures.

But:

WARRANTIES AND BUILD STANDARDS VARY.

Ask exactly:

WHAT IS INCLUDED?

Long Block vs. Complete Engine

This matters enormously.

A long block may include:

• Engine block

• Crankshaft

• Pistons

• Connecting rods

• Cylinder heads

• Valve train

But may not include:

• Intake manifold

• Exhaust manifold

• Alternator

• Starter

• A/C compressor

• Fuel system

• Sensors

• Turbocharger

• Water pump

• Engine mounts

• Other accessories

Those components may have to be:

TRANSFERRED FROM YOUR OLD ENGINE.

That Means the Condition of Transferred Parts Matters

If your replacement engine doesn't include the:

WATER PUMP

and your old water pump has:

180,000 MILES,

you should at least ask:

“DOES IT MAKE SENSE TO PUT THIS OLD PUMP ON THE REPLACEMENT ENGINE?”

That's the type of decision this article is about.

PART ONE: WHEN REPLACING AN ENGINE

1. Rear Main Seal

This is one of the classic:

WHILE-YOU'RE-IN-THERE ITEMS.

The rear main seal sits at the rear of the engine around the:

CRANKSHAFT.

Its job is to prevent:

ENGINE OIL LEAKAGE.

Why the Rear Main Seal Is Important

When the engine and transmission are installed:

The rear main seal may be buried between them.

Accessing it later can require:

REMOVING OR SEPARATING THE TRANSMISSION AGAIN.

But when the engine and transmission are already apart:

Access may be:

DRAMATICALLY EASIER.

Should Every Rear Main Seal Automatically Be Replaced?

Not necessarily.

Consider:

• Engine mileage

• Seal condition

• Evidence of leakage

• Replacement-engine history

• Seal design

• Manufacturer procedure

• Part quality

• Additional labor required

But if the seal is:

ALREADY LEAKING,

ignoring it while everything is apart can be:

VERY EXPENSIVE LATER.

2. Front Crankshaft Seal

The front crank seal should also be:

INSPECTED.

If it is:

• Leaking

• Hardened

• Damaged

• High mileage

• Easily accessible during the replacement

replacement may make sense.

Again:

ACCESS MATTERS.

3. Camshaft Seals

On applicable engines:

Camshaft seals can become difficult to access after:

FULL REASSEMBLY.

If they show leakage or are located behind a timing system already being serviced:

This can be an excellent time to:

ADDRESS THEM.

But don't automatically replace good seals unless:

THE REPAIR PROCEDURE OR CONDITION JUSTIFIES IT.

4. Timing Belt

If the replacement engine uses a:

TIMING BELT,

this deserves serious consideration.

Ask:

WHEN WAS IT LAST REPLACED?

If you don't know:

That's important.

A Used Engine With Unknown Timing-Belt History Is a Risk

Suppose the used engine has:

85,000 MILES.

But nobody knows when the timing belt was replaced.

If the belt is due at:

100,000 MILES,

installing the engine with the old belt may mean paying for:

ANOTHER MAJOR SERVICE SOON.

Consider the Complete Timing System

Depending on the engine:

A timing service may include:

TIMING BELT.

TENSIONER.

IDLER PULLEYS.

and possibly:

WATER PUMP.

Especially when the pump is driven by:

THE TIMING BELT.

5. Timing Chain Components

Does an engine replacement mean:

NEW TIMING CHAINS?

No.

But if the replacement engine has:

• Known chain stretch

• Worn guides

• Noisy tensioners

• High mileage

• Known design concerns

• Timing-related faults

then servicing those components before installation may make:

FINANCIAL SENSE.

6. Water Pump

This is another major:

ACCESS-DEPENDENT DECISION.

Some water pumps are:

VERY EASY TO REPLACE WITH THE ENGINE INSTALLED.

Others are:

BURIED DEEP INSIDE THE ENGINE.

Some are driven by:

THE TIMING BELT.

Others may be:

INTERNAL OR DIFFICULT TO ACCESS.

Ask One Important Question

“HOW MUCH LABOR WOULD THIS WATER PUMP COST TO REPLACE AFTER THE ENGINE IS INSTALLED?”

That changes the decision.

A healthy, easily accessible pump may not need replacement.

An old, difficult-to-access pump with major labor overlap may be:

WORTH DOING NOW.

7. Thermostat

Inspect or consider replacing the thermostat based on:

• Mileage

• Age

• Overheating history

• Engine failure cause

• Accessibility

• Manufacturer recommendation

If the original engine failed because of:

OVERHEATING,

the cooling system deserves:

EXTRA ATTENTION.

8. Difficult-to-Access Coolant Hoses

This is a huge one.

Some engines have:

SMALL COOLANT HOSES

located:

BEHIND OR UNDER THE ENGINE.

When the engine is installed:

They can be extremely difficult to access.

With the engine removed:

They may be:

RIGHT THERE.

Inspect Them Carefully

Look for:

• Cracking

• Swelling

• Hardening

• Oil contamination

• Heat damage

• Soft spots

• Coolant residue

• Brittle plastic fittings

If a $30 hose is buried behind the engine and already deteriorating:

Replacing it now may prevent:

A VERY EXPENSIVE FUTURE LABOR BILL.

9. Heater Hoses

Heater hoses should also be:

INSPECTED.

Especially connections located near:

THE FIREWALL OR REAR OF THE ENGINE.

Again:

Don't replace good hoses automatically.

But don't reinstall:

A CRACKED 15-YEAR-OLD HOSE

simply because:

“IT WASN'T PART OF THE ENGINE.”

10. Plastic Coolant Pipes and Fittings

Modern engines often use:

PLASTIC COOLING-SYSTEM COMPONENTS.

Over time:

Heat cycling can make them:

BRITTLE.

If an old plastic fitting is extremely difficult to access after engine installation:

Inspect it carefully while:

ACCESS IS EASY.

11. Engine Mounts

Engine mounts support the engine while isolating:

VIBRATION.

They should be inspected whenever an engine is being:

REMOVED OR REPLACED.

Replace Mounts When They Are Actually Bad

Look for:

• Torn rubber

• Separation

• Collapsed mount

• Fluid leakage from hydraulic mounts

• Excessive movement

• Broken brackets

If a mount is:

FAILED,

this is usually an excellent time to replace it.

Don't Automatically Replace Every Mount

If the mounts are:

HEALTHY,

you may not need them.

But ask:

HOW DIFFICULT WILL THIS MOUNT BE TO REPLACE LATER?

12. Transmission Mount

Even during an engine replacement:

Inspect the:

TRANSMISSION MOUNT.

A collapsed mount can affect:

• Drivetrain movement

• Vibration

• Shift feel

• Exhaust stress

• Axle angles

Replace based on:

CONDITION.

13. Serpentine Belt

If the old belt is:

• Cracked

• Frayed

• Contaminated

• Glazed

• Near end of life

this may be a sensible time to install:

A NEW BELT.

14. Belt Tensioner and Idler Pulleys

Don't automatically replace these because:

THE BELT IS OFF.

Inspect:

• Bearing noise

• Pulley play

• Weak tension

• Misalignment

• Damage

Replace:

WHAT IS ACTUALLY WORN.

15. Spark Plugs

If spark plugs are:

DUE SOON

and they're difficult to access with the engine installed:

Replacing them before installation may save:

SIGNIFICANT LABOR.

This is especially useful on engines where:

REAR BANK ACCESS IS DIFFICULT.

16. Ignition Coils

Do not automatically replace healthy coils.

We've covered this before.

Inspect:

COILS + BOOTS.

Replace:

FAILED OR DAMAGED COMPONENTS.

A new engine does not automatically need:

A FULL SET OF COILS.

17. Valve-Cover Gaskets

If the replacement engine has visible:

VALVE-COVER LEAKAGE,

addressing it before installation may be much easier.

But again:

NO LEAK + GOOD SEALS ≠ AUTOMATIC REPLACEMENT.

18. Oil Pan Gasket / Sealant

Inspect the:

OIL PAN.

Look for:

• Leaks

• Damage

• Rust

• Dents

• Cracks

• Failed sealant

Depending on vehicle design:

The oil pan may be dramatically easier to service while:

THE ENGINE IS OUT.

19. Oil Cooler and Oil Cooler Seals

Some engines have oil coolers or housings located:

DEEP IN THE ENGINE VALLEY

or other difficult locations.

If the replacement engine has known leakage there:

This may be the time to:

FIX IT.

20. Intake-Manifold Gaskets

If the intake manifold must be transferred:

Use new gaskets when:

THE SERVICE PROCEDURE REQUIRES THEM.

Don't reuse a sealing component that isn't designed for:

REUSE.

21. Exhaust-Manifold Gaskets

Same principle.

If exhaust manifolds are removed or transferred:

Replace required:

GASKETS AND HARDWARE

according to the service procedure and condition.

22. Turbocharger — If Equipped

A turbocharged engine deserves:

ADDITIONAL INSPECTION.

Check:

• Shaft play where appropriate

• Oil leakage

• Coolant leakage

• Oil-feed lines

• Oil-return lines

• Coolant lines

• Cracks

• Actuator operation

• Related hoses

A Failed Old Engine May Have Damaged the Turbo

If the old engine suffered:

OIL STARVATION

or:

METAL CONTAMINATION,

the turbocharger may have been exposed to:

THE SAME CONTAMINATED OIL.

Don't automatically bolt it onto:

THE REPLACEMENT ENGINE

without evaluation.

23. Engine Oil and Filter

A replacement engine should receive:

THE CORRECT ENGINE OIL

and:

A PROPER FILTER

according to the engine supplier and manufacturer requirements.

This isn't the place to guess.

24. Coolant

The cooling system will generally need:

FRESH CORRECT-SPECIFICATION COOLANT.

Use the coolant required by:

THE VEHICLE / ENGINE APPLICATION.

Not simply:

WHATEVER COLOR LOOKS SIMILAR.

PART TWO: WHEN REPLACING A TRANSMISSION

25. Transmission Fluid

A replacement transmission should be filled with:

THE EXACT REQUIRED FLUID SPECIFICATION.

Modern transmissions can be extremely sensitive to:

INCORRECT FLUID.

“ATF” is not:

ONE UNIVERSAL PRODUCT.

26. Transmission Filter

Depending on transmission design:

The filter may be:

• External

• Internal

• Pan-mounted

• Integrated

• Considered non-serviceable under normal maintenance

If the replacement transmission allows filter service and the procedure calls for it:

This may be a sensible time.

27. Transmission Pan Gasket

If the transmission pan is removed:

Use the appropriate:

GASKET OR SEALANT

during reassembly.

If the pan is:

DAMAGED OR RUSTED,

address that before installation.

28. Front Pump / Input Seal

The transmission's front seal can become:

VERY EXPENSIVE TO ACCESS LATER.

Why?

Because it sits near the:

TORQUE CONVERTER / INPUT AREA.

If it leaks after installation:

The transmission may have to:

COME BACK OUT.

Should It Always Be Replaced?

Not automatically.

But on a used transmission:

Ask about:

• Mileage

• Leakage

• Seal condition

• Supplier recommendation

• Warranty requirements

• Accessibility

This is a classic:

LABOR-OVERLAP DECISION.

29. Output / Axle Seals

Inspect:

AXLE OR OUTPUT SHAFT SEALS.

If they show:

LEAKAGE OR DAMAGE,

replacing them before installation can be:

MUCH EASIER.

30. Torque Converter

If replacing an automatic transmission:

Ask:

IS THE TORQUE CONVERTER INCLUDED?

This is extremely important.

A remanufactured transmission may include:

A REMANUFACTURED TORQUE CONVERTER.

A used transmission may come with:

ITS ORIGINAL CONVERTER.

A Failed Transmission Can Contaminate the Torque Converter

If the old transmission suffered:

INTERNAL CATASTROPHIC FAILURE,

metal and clutch material may circulate through:

THE TRANSMISSION FLUID.

Reusing a contaminated converter can jeopardize:

THE REPLACEMENT TRANSMISSION.

31. Transmission Cooler

This is one of the most important parts of a transmission replacement.

Automatic transmissions often circulate fluid through:

A TRANSMISSION COOLER.

That cooler may be:

• Inside the radiator

• External

• Separate heat exchanger

• Combination system

A Failed Transmission Can Contaminate the Cooler

If the old transmission sent:

METAL OR CLUTCH MATERIAL

through the cooler:

Installing a replacement transmission without addressing that contamination can send debris:

RIGHT BACK INTO THE NEW TRANSMISSION.

Cooler Flushing or Replacement May Be Required

Depending on:

FAILURE TYPE + COOLER DESIGN + TRANSMISSION SUPPLIER REQUIREMENTS,

the cooler may require:

FLUSHING

or:

REPLACEMENT.

Some coolers cannot be reliably cleaned after severe contamination.

Follow:

THE WARRANTY AND MANUFACTURER REQUIREMENTS.

32. Transmission Cooler Lines

Inspect cooler lines for:

• Leaks

• Corrosion

• Damage

• Kinks

• Restricted flow

• Damaged fittings

Don't put a replacement transmission behind:

A RESTRICTED OR LEAKING COOLER CIRCUIT.

33. Transmission Mount

This is another obvious inspection point.

If the transmission is already out:

Inspect the:

TRANSMISSION MOUNT.

Replace if:

• Torn

• Collapsed

• Separated

• Fluid leaking

• Structurally damaged

PART THREE: IF THE ENGINE AND TRANSMISSION ARE SEPARATED

This is where some of the largest:

LABOR SAVINGS

can happen.

34. Flexplate — Automatic Transmission

Automatic-transmission vehicles typically use a:

FLEXPLATE

between the engine and torque converter.

Inspect for:

• Cracks

• Damaged teeth

• Distortion

• Loose or damaged mounting areas

• Evidence of previous failure

If it's damaged:

REPLACE IT NOW.

35. Flywheel — Manual Transmission

Manual-transmission vehicles typically use a:

FLYWHEEL.

Inspect it carefully.

Depending on design and condition:

It may need:

RESURFACING OR REPLACEMENT.

36. Clutch — Manual Transmission

If the transmission is already removed from a manual-transmission vehicle:

This may be one of the biggest:

WHILE-YOU'RE-IN-THERE DECISIONS.

If the clutch has significant mileage:

Ask:

HOW MUCH LIFE IS LEFT?

Imagine Reinstalling a Nearly Worn-Out Clutch

Transmission goes back in.

Three months later:

CLUTCH STARTS SLIPPING.

Now:

The transmission has to:

COME BACK OUT.

That's exactly the kind of duplicate labor you're trying to avoid.

Consider the Complete Clutch System

Depending on the vehicle:

This may include:

CLUTCH DISC.

PRESSURE PLATE.

RELEASE BEARING.

PILOT BEARING / BUSHING.

FLYWHEEL SERVICE.

37. Rear Main Seal During Transmission Replacement

Even if you're replacing:

ONLY THE TRANSMISSION,

the engine rear main seal may now be:

ACCESSIBLE.

If it is visibly leaking:

This is one of the best opportunities to:

ADDRESS IT.

Because once the transmission is reinstalled:

Access disappears.

38. Pilot Bearing / Bushing

On applicable manual-transmission vehicles:

Inspect or replace the:

PILOT BEARING / BUSHING

according to condition and clutch-service procedure.

The part itself may be inexpensive.

Access later:

IS NOT.

39. Release Bearing

If you're performing clutch work:

A release bearing is commonly included as part of:

A COMPLETE CLUTCH SERVICE.

Again:

The labor required to reach it later is:

SUBSTANTIAL.

40. Hydraulic Slave Cylinder

Some manual transmissions use a:

CONCENTRIC SLAVE CYLINDER

inside the bellhousing.

This is a major:

LABOR-OVERLAP ITEM.

Why?

Because if it fails later:

The transmission may need to be:

REMOVED AGAIN.

If the slave cylinder has high mileage or is being disturbed during clutch service:

Replacement may make sense depending on:

CONDITION AND MANUFACTURER GUIDANCE.

41. Axles / CV Boots

Transmission removal often requires:

REMOVING OR DISCONNECTING CV AXLES.

Inspect:

• CV boots

• Joints

• Splines

• Axle seals

• Threads

• Retaining clips

Don't automatically replace good axles.

But don't reinstall:

A TORN CV BOOT

without discussing it.

42. Driveshaft Components

On rear-wheel-drive and four-wheel-drive vehicles:

Inspect the driveshaft and related components.

Depending on design:

Check:

• U-joints

• Center support bearing

• Flex disc

• Slip joint

• CV joints

• Seals

Replace based on:

CONDITION.

43. Transfer Case — AWD / 4WD

If transmission replacement involves:

TRANSFER-CASE REMOVAL,

inspect:

• Input/output seals

• Fluid

• Mounting surfaces

• Splines

• Couplers

• Leaks

Again:

ACCESS CAN CREATE OPPORTUNITY.

44. Wiring and Electrical Connectors

Engine and transmission replacement involves disconnecting:

A LOT OF WIRING.

Inspect connectors for:

• Broken locks

• Heat damage

• Corrosion

• Oil intrusion

• Damaged pins

• Chafed wiring

• Brittle insulation

A replacement powertrain won't fix:

BAD WIRING.

45. Grounds

Engine and transmission grounds are extremely important.

Poor grounds can cause:

STRANGE ELECTRICAL PROBLEMS.

Make sure:

• Ground straps are present

• Connections are clean

• Cables aren't broken

• Mounting points are correct

46. Sensors

Do not automatically replace:

EVERY SENSOR.

Some may transfer from the old powertrain.

Inspect them.

Replace if:

• Failed

• Damaged

• Contaminated

• Manufacturer-required

• Known to be faulty

A new engine doesn't automatically need:

NEW SENSORS EVERYWHERE.

47. Starter

The starter may become:

VERY EASY TO ACCESS

during engine or transmission replacement.

Does that mean replace it?

No.

But if it already has:

KNOWN PROBLEMS

or extremely high mileage:

This may be worth discussing.

48. Alternator

Same principle.

If healthy:

KEEP IT.

If failing:

THIS MAY BE A GOOD TIME.

49. A/C Compressor

Again:

Don't replace a healthy compressor simply because:

THE ENGINE IS OUT.

But inspect:

• Leakage

• Pulley/bearing condition

• Clutch condition where applicable

• Electrical connection

If the compressor is transferred:

Make sure it is:

SERVICEABLE.

50. Fuel-System Components

Inspect components being transferred such as:

• Fuel injectors

• Fuel rails

• Fuel lines

• High-pressure pump where applicable

• Seals

• Connectors

Use new sealing components when:

REQUIRED BY PROCEDURE.

51. Vacuum and PCV Hoses

Small hoses can create:

BIG PROBLEMS.

Inspect:

• PCV hoses

• Vacuum lines

• Brake-booster hose

• Emissions hoses

Especially those that become:

DIFFICULT TO ACCESS AFTER INSTALLATION.

52. Exhaust Connections

Inspect:

• Gaskets

• Studs

• Nuts

• Flex sections

• Flanges

• Broken bolts

Replace sealing components when:

DISTURBED AND REQUIRED.

53. Radiator

An engine replacement does not automatically require:

A NEW RADIATOR.

But if the original engine failed from:

OVERHEATING,

the radiator should be:

EVALUATED.

54. Cooling Fans

Same principle.

If overheating destroyed the old engine:

Verify:

THE COOLING FANS ACTUALLY WORK.

Installing a replacement engine without fixing:

THE ORIGINAL COOLING FAILURE

could destroy:

ENGINE NUMBER TWO.

Why Did the Original Engine or Transmission Fail?

This may be:

THE MOST IMPORTANT QUESTION IN THE ENTIRE ARTICLE.

Before installing a replacement:

Ask:

“WHAT KILLED THE ORIGINAL UNIT?”

If an Engine Failed From Oil Starvation...

Investigate:

WHY.

Was it:

• Oil leak?

• Failed oil pump?

• Low oil level?

• Blocked pickup?

• Turbo failure?

• Internal contamination?

Don't install a replacement engine while leaving:

THE ORIGINAL CAUSE UNCORRECTED.

If the Engine Failed From Overheating...

Check:

RADIATOR.

THERMOSTAT.

WATER PUMP.

FANS.

HOSES.

COOLANT LEAKS.

PRESSURE CAP.

OTHER COOLING-SYSTEM COMPONENTS.

If the Transmission Failed From Overheating...

Check:

TRANSMISSION COOLER.

COOLER LINES.

FLUID CONDITION.

COOLING SYSTEM WHERE APPLICABLE.

TOWING / LOAD HISTORY.

If the Transmission Failed Catastrophically...

Address:

CONTAMINATION.

That can include:

TORQUE CONVERTER.

COOLER.

COOLER LINES.

and possibly:

OTHER CONNECTED COMPONENTS.

A replacement unit deserves:

A CLEAN SYSTEM.

Ask About Warranty Requirements Before Making Decisions

This is extremely important.

Replacement engine and transmission suppliers may require specific procedures for:

WARRANTY VALIDITY.

Examples can include:

• New fluids

• New filters

• Cooler flushing

• Cooler replacement

• Thermostat replacement

• Certain seals

• Documentation

• Installation procedures

Follow the actual warranty requirements for:

YOUR UNIT.

What Should Usually Be Replaced or Serviced?

There is no universal list.

But generally: