Ford Sierra RS Cosworth buyer's guide

Every RS500 was black.

Most were, but of the 500 cars 52 were Moonstone Blue and 56 were white.

A 4x4 converted to rear drive is worthless.

Specialists accept a conversion done properly with a T5 gearbox, though the car is no longer original and turning it back into a 4x4 is difficult and costly. Ask why it was done, as it can hide a failed centre differential.

Any modification ruins a Cosworth.

Sensible, documented upgrades by a known Cosworth specialist are widely accepted. The problems come from cheap chips, poor fuel supply and undocumented work. Original cars are still the most sought after.

A whale tail and a turbo badge mean a three-door Cosworth.

Ordinary three-door Sierras have been dressed up as Cosworths for decades. Only the VIN, the stamped numbers, the factory features and the history prove a car is genuine.

All YB engines are the same.

The YBB in early cars is generally regarded as the weakest of the road engines, and the 4x4's 200 block is the strongest. The code, block casting and cam cover colour should match the car.

Identity, genuine cars and replicas

Start here. A Cosworth that is not what it claims to be is a poor buy whatever its condition.

• Match the VIN on the V5C with the VIN plate in the engine bay and the number stamped in the floorpan. Ground off, re-stamped or welded over numbers are a serious warning sign
• Three-door and RS500 VINs begin WF0EXXGBBE and Sapphires begin WF0FXXGBBF. Check the full sequence against the V5C, not just the start
• Check the engine code on the block. Expect YBB in a three-door or rear drive Sapphire, YBD in an RS500 and YBJ or YBG in a 4x4. A different code means a replacement engine
• On a 4x4 look for the 200 casting on the block, not 205. Cam covers should be red before August 1991 and green afterwards
• On a claimed 4x4, confirm the louvred bonnet, clear front indicators, smoked rear lamps and 4x4 wing badges, then check under the car for the front differential and driveshafts
• An RS500 should have the second boot lid spoiler, revised front bumper, second injector rail and documented history. Ask the Ford RS Owners Club registrar to help confirm it
• Three-door UK cars came only in Diamond White, Black or Moonstone Blue. Another colour means a respray, which needs explaining
• Ask whether the car has ever been through a DVLA identity inspection, and why

Theft and accident history

Cosworths were among the most stolen cars in Britain in the 1990s and many were written off. Assume nothing.

• Run a full paid history check for unpaid finance, stolen markers, write off categories and plate changes
• A recorded insurance write off or recovered theft is not automatically a reason to walk away, but it needs repair evidence and it affects desirability
• Look for creased floorpans, rippled inner wings, a distorted boot floor and uneven panel gaps
• Check panels and glass for original date stamps and factory seam sealer, as many cars were crashed and repaired
• Ask how the car has been secured and whether the alarm or immobiliser has caused electrical faults

Bodywork and rust

The Sierra structure is conventional steel and rusts in predictable places. Accident damage often hides rust.

• Inner wings, especially under the coolant header tank and around the suspension turrets, which can fold inwards
• Bulkhead around the battery tray and under the fuse box
• Front crossmember and front chassis rails
• Rear chassis box sections beside the spring seats, worst on Sapphires
• Floorpan, inner and outer sills, boot floor around the bumper mounts and the rear arches
• Where the front wings meet the slam panel, checked with the bonnet open
• On three-doors the centre of the rear panel; on Sapphires the central support for the boot spoiler
• Sunroof surrounds for leaks and hidden rot in the floor
• Cosworth bumpers, side skirts and 4x4 vented bonnets are costly; check for sagging skirts and snapped grille lugs

YB engine and turbo

The YB is strong when maintained but expensive when neglected. Ask for a specialist's bills and see it started from cold.

• Listen on a cold start for knocking, tapping or piston slap; a compression test is worthwhile
• Look for coolant leaks on the exhaust side of the block and for any sign of overheating, as head gasket failure is common
• Smoke: blue under boost suggests a worn turbo or valve seals, white points to the head gasket, black suggests running rich
• Check the turbo compressor wheel for play with the engine off; there should be none
• Check that the cooling fans cut in when hot. Fan wiring is known to chafe and melt
• The fuel pump sits under the boot floor and should receive around 13 volts. Weak wiring or a cheap pump can make the engine run lean and destroy pistons
• Look for split or loose boost hoses and leaks around the fuel pressure regulator
• A misfire under load can be plugs, leads, coil or distributor sensor, but it can also signal serious damage
• Ask when the cambelt was last changed, with a dated invoice
• Engine bay wiring degrades with heat and age and can feed false signals to the ECU

Gearbox and drivetrain

Rear drive cars use the Borg Warner T5, the 4x4 uses the MT75. Both have known weak spots.

• T5: notchy when cold is normal, but check for crunching into second and third, and listen for third gear problems
• T5: reverse has no synchromesh and can baulk
• MT75: crunching, especially on downshifts, points to worn synchromesh; a whine suggests a tired gearbox
• 4x4: make sure the front wheels are actually driven. A worn viscous coupling means no front drive
• 4x4: noise between first and second can indicate transfer box slip; clonks and snatch can point to the centre differential
• Listen for rear differential whine and check for one wheel spinning, a sign of a worn limited-slip differential
• Test for clutch slip by accelerating hard in a high gear; revs should not rise faster than speed
• A heavy clutch can be caused by exhaust heat damaging the clutch cable sheath
• Vibration can come from propshaft joints or worn engine and gearbox mounts; loud clicking on lock points to a CV joint

Suspension, steering and brakes

The layout is MacPherson struts at the front and semi-trailing arms at the rear. Worn bushes are almost universal.

• Front wander and vague steering point to worn track control arm bushes
• Rear sloppiness points to worn trailing arm bushes; polyurethane replacements are common
• Check for slack in the steering rack, split gaiters and a noisy power steering pump
• Heavily lowered cars or stiff coilovers can be harsh on the road and hard on CV joints
• Warped front discs and sticking rear calipers and handbrake mechanisms are common
• 4x4 front pads wear quickly
• Check the ABS warning light goes out; faults can be sensors, wiring, the pump or fuses

Interior and trim

Cosworth specific trim is scarce. A complete, original interior adds to a car's appeal.

• Dashboard tops crack and distort, particularly on three-doors
• Recaro driver's seat bolsters wear and the foam collapses. A recliner knob on the console side suggests a passenger seat has been swapped across
• Sagging headlinings and lifting door card vinyl are common and hard to put right
• Check the cloth suits the year: 4x4s from August 1991 have Space cloth
• Worn leather on the gearknob and steering wheel shows real use

Modifications and paperwork

Most Cosworths have been modified to some degree. The question is how well, and whether it is documented.

• Ask for a full list of modifications, with invoices from a recognised Cosworth specialist
• Cheap uprating chips can overfuel the engine and dilute the oil
• Ask whether the original parts removed during modification come with the car
• Check the insurer has been told about every modification
• Look for a stamped service book, specialist invoices and old MOT certificates that support the mileage

How can I tell a genuine Sierra Cosworth from a replica?

Check the VIN on the V5C against the VIN plate and floorpan stamp, look at the engine code, and confirm the factory features for that version. Ask the Ford RS Owners Club registrar if anything is unclear.

Which engine code should my car have?

YBB for the three-door and rear drive Sapphire, YBD for the RS500, YBJ for early 4x4s and YBG for 4x4s from August 1991 with a catalyst. The 4x4 block should carry a 200 casting.

Is a Sierra Cosworth reliable?

A well maintained YB engine is durable, but neglect is punished. Head gaskets, turbos, wiring and the fuel pump supply are the main risks. Specialist service history matters more than mileage.

Which gearbox is stronger, the T5 or the MT75?

Both have weak points. The T5 can suffer third gear and synchro wear and has no reverse synchro. The MT75 on the 4x4 is known for worn synchromesh. Test both thoroughly.

Should I avoid a car that was stolen or written off?

Not always, but a recorded write off or theft recovery needs evidence of a proper repair and confirmed identity. It also affects how other buyers will see the car when you sell.

Is a converted rear drive 4x4 a problem?

A conversion done properly with a T5 gearbox is accepted by specialists, but the car is no longer original and returning it to four-wheel drive is costly. Find out why it was done.

Which version should I buy?

The three-door and RS500 are the most collectable. The Sapphire 4x4 is the most usable, with four doors and four-wheel drive. The rear drive Sapphire offers the original engine and gearbox in a quieter four-door body.