SOHC V6 - replace, rebuild or remanufactured?

[2000StreetRod]

In my opinion, except for three design flaws, the 4.0L SOHC V6 would be a well built, rugged and reliable power plant that could last 300K miles before requiring a rebuild. The block that is reinforced with a cradle (upper oil pan) is strong as is the crankshaft. The connecting rods, pistons, valves, rings and bearings should last a long time if the engine is maintained properly.

Unfortunately, the unreliability of the camshaft and balance shaft timing components give the engine an overall inferior reputation.

The earlier 4.0L OHV V6 used a reliable ratchet type timing chain tensioner.

The tensioner spring is strong enough to keep the chain taught on the slack side while the other side is in traction. When the engine is decelerating and the tensioner side goes into traction the ratchet prevents the tensioner from overcoming the spring pressure. As the chain stretches with age and its contact surfaces on the tensioner and guide wear the ratchet allows the spring to compensate. This superior design is implemented in the 4.6L SOHC and DOHC V8.

The inferior SOHC V6 camshaft timing design uses a spring/hydraulic piston camshaft chain tensioner.

The longer one is for the front (left) timing chain and the shorter one is for the rear (right) timing chain. A cassette consists of a guide assembly, sprockets and chain. The tensioner presses against the cassette guide on the slack side. The left cassette is shown below.

The right cassette is shown below.

Due to high failures the cassettes were improved during the 2nd generation models and all of the improvements were incorporated in the 2002 and later models. Some parts of the "plastic" guides were reinforced with metal. However, the "finger" on the left guide (shown below) still breaks off.

The traction side of the right cassette was never reinforced and has several failure points.

Unfortunately, the engine or transmission must be removed in order to replace the right cassette.

The primary (crankshaft to jackshaft) chain tensioner is a leaf spring type that breaks with age.

The base of the tensioner was enlarged and the number of spring leaves increased from 3 to 6.

The balance shaft chain tensioner is rather frail and fails with age.

The block cradle must be removed to replace the balance shaft chain tensioner because the base mounting bolts are vertical and not accessible with the block cradle in place.

 

[2000StreetRod]

Thermostat housing & cracked head

The thermostat housing is the second reliability issue with the SOHC V6. The housing is made of composite ("plastic" type) material that cracks with age.

Additionally, on the 2nd generation engines the two sensors (engine coolant temperature sensor for the PCM & temperature sender for the instrument cluster) screw into threaded bushings in the housing. Often when the sensor are replaced the bushing comes loose from the housing and leaks.

On the 3rd generation and later models the sensors are retained to the bushing inserts with clips reducing the chance of leakage when the sensors are replaced.

The susceptibility of the head coolant passages to crack when the engine overheats is the third reliability issue. Apparently, the casting thickness is marginal for coolant passages near the valves.

The SOHC V6 is a fairly expensive and somewhat complex engine to rebuild. A complete gasket set is not cheap, nor are rings and bearings. Also, there are numerous torque-to-yield bolts that can't be reused. Head resurfacing, crankshaft journal grinding, and valve seat grinding machine shop expenses rapidly add up to a significant cost. Valves, valve springs, hydraulics lash adjusters and cam followers are parts that may need replacement.

The cheapest and fastest method to correct a major engine deficiency with the SOHC V6 is to purchase an engine salvaged from another vehicle. The long blocks are essentially identical for Rangers, Explorers, Mountaineers, Mustangs and even the Land Rover Discovery III. However, intake manifolds, engine wiring harnesses, valve covers, sensors, throttle bodies, EGR and PCV configurations vary by year and model. The least reconfiguration work is achieved by purchasing an engine of the same year and model as the defective one. Most engines from 4WD vehicles have the balance shaft but a few may not. Most engines from 2WD vehicles don't have the balance shaft but some do. My 2005 Mustang 4.0L V6 engine assembly instructions include the balance shaft even though no production Mustangs that I'm aware of had 4WD. Unfortunately, a salvage engine may have more than 100K miles. The ones with less than 50K miles usually pretty expensive. The timing chain related components typically do not fail before 100K miles. Mine failed at 150K miles. Some members have experienced failures with less than 50K miles. Driving habits and periodic maintenance probably affect the service life of the components but I'm not sure there is a direct correlation. Using a salvaged engine as is assumes the risk of experiencing another timing chain related failure.

The decision to rebuild an engine should be based on the condition of the engine and the length of expected service to the owner. As I previously stated, the engine (with a few exceptions) incorporates quality, long lasting components and can take considerable abuse. Mine is boosted with about 6 psi and it frequently experiences 6250 or slightly more rpm. It still has the original head and block internals and they seem to be holding up although the compression test results are a little low. I plan to keep the vehicle probably for as long as I can drive (I'm 69 years old) and will rebuild the engine as soon as I think it is needed. With my pre-oiler that may not be until 300K miles. My suggestion is that if your engine has good compression, has never had lubrication problems (low or no oil pressure), no indication of coolant in the oil or the exhaust and less than 250K miles is just replace the timing chain related components provided you have the time, tools, knowledge and inclination. If you can't accurately follow and understand details engine assembly instructions due to impatience don't try it. The SOHC V6 is an interference engine meaning that when the camshafts are not correctly timed the pistons can strike the valves. Even some repair shop personnel with no first hand experience with the engine have destroyed the valves of a newly assembled engine at engine start due to incorrect camshaft timing.

If you don't have the time, skills, tools, and space to replace the timing chain related components or rebuild the engine then you should consider purchasing a remanufactured engine with a warranty. They are fairly expensive but may be a bargain for your circumstances. Make sure they use quality components (usually the result of offering a multi-year warranty) and have replaced or serviced all normal wear parts to specified tolerances. Just removing and installing an engine yourself instead of paying someone to do it will save a significant amount of money.

 

[bodkin1]

The thermostat housing is the second reliability issue with the SOHC V6. The housing is made of composite ("plastic" type) material that cracks with age.
View attachment 87985
Additionally, on the 2nd generation engines the two sensors (engine coolant temperature sensor for the PCM & temperature sender for the instrument cluster) screw into threaded bushings in the housing. Often when the sensor are replaced the bushing comes loose from the housing and leaks.

On the 3rd generation and later models the sensors are retained to the bushing inserts with clips reducing the chance of leakage when the sensors are replaced.
View attachment 87972

The susceptibility of the head coolant passages to crack when the engine overheats is the third reliability issue. Apparently, the casting thickness is marginal for coolant passages near the valves.

The SOHC V6 is a fairly expensive and somewhat complex engine to rebuild. A complete gasket set is not cheap, nor are rings and bearings. Also, there are numerous torque-to-yield bolts that can't be reused. Head resurfacing, crankshaft journal grinding, and valve seat grinding machine shop expenses rapidly add up to a significant cost. Valves, valve springs, hydraulics lash adjusters and cam followers are parts that may need replacement.

The cheapest and fastest method to correct a major engine deficiency with the SOHC V6 is to purchase an engine salvaged from another vehicle. The long blocks are essentially identical for Rangers, Explorers, Mountaineers, Mustangs and even the Land Rover Discovery III. However, intake manifolds, engine wiring harnesses, valve covers, sensors, throttle bodies, EGR and PCV configurations vary by year and model. The least reconfiguration work is achieved by purchasing an engine of the same year and model as the defective one. Most engines from 4WD vehicles have the balance shaft but a few may not. Most engines from 2WD vehicles don't have the balance shaft but some do. My 2005 Mustang 4.0L V6 engine assembly instructions include the balance shaft even though no production Mustangs that I'm aware of had 4WD. Unfortunately, a salvage engine may have more than 100K miles. The ones with less than 50K miles usually pretty expensive. The timing chain related components typically do not fail before 100K miles. Mine failed at 150K miles. Some members have experienced failures with less than 50K miles. Driving habits and periodic maintenance probably affect the service life of the components but I'm not sure there is a direct correlation. Using a salvaged engine as is assumes the risk of experiencing another timing chain related failure.

The decision to rebuild an engine should be based on the condition of the engine and the length of expected service to the owner. As I previously stated, the engine (with a few exceptions) incorporates quality, long lasting components and can take considerable abuse. Mine is boosted with about 6 psi and it frequently experiences 6250 or slightly more rpm. It still has the original head and block internals and they seem to be holding up although the compression test results are a little low. I plan to keep the vehicle probably for as long as I can drive (I'm 69 years old) and will rebuild the engine as soon as I think it is needed. With my pre-oiler that may not be until 300K miles. My suggestion is that if your engine has good compression, has never had lubrication problems (low or no oil pressure), no indication of coolant in the oil or the exhaust and less than 250K miles is just replace the timing chain related components provided you have the time, tools, knowledge and inclination. If you can't accurately follow and understand details engine assembly instructions due to impatience don't try it. The SOHC V6 is an interference engine meaning that when the camshafts are not correctly timed the pistons can strike the valves. Even some repair shop personnel with no first hand experience with the engine have destroyed the valves of a newly assembled engine at engine start due to incorrect camshaft timing.

If you don't have the time, skills, tools, and space to replace the timing chain related components or rebuild the engine then you should consider purchasing a remanufactured engine with a warranty. They are fairly expensive but may be a bargain for your circumstances. Make sure they use quality components (usually the result of offering a multi-year warranty) and have replaced or serviced all normal wear parts to specified tolerances. Just removing and installing an engine yourself instead of paying someone to do it will save a significant amount of money.

The timing chain guides/thermostat housing...

A Ford-Fisher Price joint collaboration.

Ditto on that last part. However, should my chain/guides finally go south, a shop is where it's going. I lack the time, tools, equipment, experience and patience to do such a project...

 

[4D X]

I never actually realized how many seals/gaskets a motor can have. My plan when buying a used motor was to just replace the seals and gaskets that were quick and necessary.
Facts turn out that when a butcher shop pulls a motor they cut all motor to engine wiring leaving you with one option on the 4L SOHC and that is to pull everything save the heads to retrieve the complete wire harness that runs underneath the lower intake manifold.
Building it back up from this point alone will cost you way more money than you could ever possibly conceive in gaskets/seals and special tools (if you dont already have them). The better route is to just get an overhaul kit right from the beginning. It pays for itself when you start adding up the cost of the used motor you just bought only to strip down to the same equivalent as your original motor to replace the timing sys.
Can't say I wasn't warned in so many words but didn't listen. So I extend my experience in this matter to anyone in similar situation.

NOTE: For a little bit more money if you can afford it, best yet to just get a re manufactured motor. If you dig your vehicle its worth it when you dont need to worry about shop costs .