The DOHC engine

8v DOHC Engine : Saw service in Sierra and Sierra Sapphire 2.0 models from 1989 onwards as a replacement for the Pinto. Initially available in both carburettored and injection format, the DOHC is often overlooked by tuners - yet it is a capable engine.

They are economical around town but still flexible enough to provide the pull when necessary - the carburettor'd version produces 109 PS / 174Nm while the EFi version produces 125 PS / 174Nm.

Quoted economy figures were pretty good too - even by the current standards - the carburettor'd version could return up to 50.4mpg while the EFi version was slightly behind at 48.7mpg (on a run at a constant 56mph)

By late 1991, all 2.0 DOHC engines were EFi variants. After August 1992, all DOHC cars sold in the UK were equipped with a catalytic convertor however prior to this time, the fitment had been optional.

 

Technical overview :

Both carburettored and EFI versions are 8-valve engines (they are sometimes incorrectly listed on eBay as being 16-valve engines) featuring a double-overhead camshaft arrangement powered by a single roller chain driven directly from the crankshaft. A smaller ancilliary chain driven directly from the same point on the crankshaft powers the oil pump. The timing chain itself is tensioned hydraulically.

Cylinder heads are of cast aluminium. The cylinder blocks themselves are cast iron. Sumps are aluminium. Camshaft and timing covers are made from a plastic compound.

The cylinder heads - although similar - are not the same between carburettor and EFi versions. The EFi versions feature revised inlet camshaft timing, and holes machined into the casting to accomodate the fuel injectors which are a direct fit into the side of the cylinder head. Tappets are hydraulic and therefore self-adjusting but do rely on the oil being changed regularly to prevent them sticking and becomming noisy.

Recommended grades of oil: 5w30, 10w30, 10w40

 

DOHC EFi engine:

DOHC engine with EFi

 

DOHC Carburettor engine:

DOHC engine with carburettor

 

What goes wrong? :

Timing chains can fail - although Ford state that the chain "is designed to last the service life of the engine" this can be variable depending on circumstance and a failure can mean the destruction of the engine.

The exact cause of timing chain failure is open to conjecture - although in the past it had been believed that the grade of oil used in the engine could contibute to the hydraulic chain tensioner overtensioning the chain leading to premature breakage, it is currently believed that the nylon chain guides that run vertically down the exhaust side of the timing case, and the nylon components of the tensioner assembly, break up over time and get lodged in the chain or in the chain path causing the chain to de-rail, jam and break.

Replacement chain kits are available from several aftermarket sources although the nylon guides are dealer-only parts. It is the recommendation of the website author that the chain and guide assemblies are replaced every 100,000 - 130,000 miles.

 

Cylinder heads are a known weak point - they are prone to cracking if allowed to overheat seriously so the cooling system should be maintained in A1 condition. Cylinder heads can also warp which results in an appetite for cylinder head gaskets. Replacement of a 'head gasket can cost upto £600 to replace depending on circumstance and owner ability with the socket set.

The original fibre-based headgasket was particularly problematic early on - a fault traced down to the original Ford Superplus antifreeze being too corrosive for the gasket which dramatically shortened its life. Ford replaced antifreeze which improved the situation but the fibre gasket wasn't particularly tolerant to the warping of the cylinder head when the engine got hot and was eventually superceeded in the mid-1990's by a multi-layer laminate steel item which is both stronger and more tolerant to head movement than the original item.

This modified gasket should be fitted as a matter of course when an engine is stripped and resolves most issues. It is stocked by Ford dealers nationwide under finish number 1101405 and costs (at time of writing) around £35 including VAT.

Symptoms of cylinder head or gasket problems usually include hearing gurgling or washing-machine type noises behind the dashboard on first startup - trapped combustion gases moving around the cooling system. However noise on its own should not be taken as proof of a problem - the cooling system can be difficult to bleed and air could easily accumulate through other means, such as a leaky hose clip.

 

Engine electrics can suffer with age - especially on the EFi version. Part of the EFi wiring loom runs across the right-hand engine mounting arm (when viewed from within the car) and the two wiring plugs there are prone to picking up dirt and moisture from the road surface over time leading to poor contacts and intermittant running problems.

On the whole, the EEC-IV engine management system itself is reliable. Sensors can fail with age but are cheap and easy to replace. The idle speed control valve (ISCV) is prone to getting sticky with age leading to a poor or erratic idle speed - basic cleaning of the valve can remedy it if caught early enough.

 

Engine covers and seals can leak oil - especially if they have been disturbed in the past. The plastic material naturally goes slightly porus to oil over time anyway but if tightened down incorrectly can leak significantly.This is usually more an annoyance than a serious problem - although it has the potential to get messy!

Like any other, the engines are prone to burning some oil over time - especially as mileage racks up and valve stem seals wear. Replacing the valve stem seals is a relatively straightforward job although necessitates the removal of the cylinder head.

 

Despite these characteristic areas for potential problems, there are plenty of units on the road and running perfectly reliably. It is a very responsive unit with a satisfylingly flat torque curve and dozens of positive tesimonies to bolster the feeling that it is one of the better engines ever made by Ford for the European market.

The 8v DOHC engine fitted in the Sierra and Granada models was the basis of the 16v variant used in the Mk.5 Escort RS2000, and the 2.0 and 2.3 16v DOHC engines used in the Ford Scorpio and Ford Galaxy models - many of the internal bottom-end components are interchangable and as a result, the availability of parts for these engines - even from main dealers - is still good.

 

Owner testimony :

"Personally I'm very fond of it - it's perhaps a little harsh and makes that sort of slight pinking sound that all Sierras seem to make when hot and on load sometimes, but it's very reliable, satisfyingly responsive and respectably economical. I get around 36mpg round town and recently managed 42.3mpg on a run. My Dad has a 2 litre Mondeo and I definitely wouldn't swap! The 16 valves may give it 11bhp more than mine and I'm sure it could just about outperform it in 0-60 trials - but it just won't pull from low revs where you actually need it. The Sierra responds immediately, the Mondeo doesn't really wake up until you've passed 4000rpm, by which time you don't need it anymore. It's thirstier too - I can't do much better than 35mpg with the Mondeo.

You mentioned oil consumption - I change it every 6000 miles and there's no noticeable consumption during that period. Some of the seals do weep a bit, especially around the cam cover and crankshaft pulley, but it doesn't drip. I think the crankshaft isn't as well balanced on my specimen as it could be though - there are lots of drillings in the pulley for this purpose but there's a slight resonance at 4000rpm which I've not noticed on other 2.0i DOHC engines I've driven. Perhaps this has some role in the oil leak at the pulley.

My mechanic says they're pretty solid engines - one guy who drives a Granada with the same engine has reportedly covered 250000 miles without any rebuilding! The tappets rattle a bit on a cold start, so I think low mileage cars suffer with this more than motorway high- milers because of the more frequent cold starts. Having said that, I don't see this as being any worse than any other engine I've had anything to do with.

The most common fault I'm aware of with the DOHC is the same as the 2.9 - erratic idling caused by the idle-speed control valve gunking up, although I've not had this yet. The only non-routine thing I've had to do is rewire the plug to the temperature gauge sender as it had an dodgy connection.
"

Carl Davis, describing his experiences with his 1992-model Sierra GLX.



 

 

 

This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. RIPA Notice: No consent is given for interception of page transmision.