Chrysler Mitsubishi 3.0L V6 Engine Service (2023)

Introduced in 1987, the Chrysler/Mitsubishi 3.0L V6 engine provided a much-needed boost in power for Dodge, Plymouth and Chrysler minivans over the existing 2.2L and 2.6L four cylinder engines. The original engine was rated at 140 horsepower at 5,000 rpm with 170 ft.-lbs. of torque at 2,800 rpm.

Built by Mitsubishi, the MMC 3.0L V6 has single overhead cams (SOHC) and a large bedpan-like cast aluminum air intake plenum atop the motor. The block is cast iron with aluminum heads and two valves per cylinder.

Over the years, this engine has been used in almost every Chrysler front-wheel-drive platform except the L-bodies (Omni/Horizon and Charger). So if you are working on a LeBaron, Spirit, Acclaim, Shadow, Sundance, Stealth or Chrysler TC, you may find this engine when you open the hood. The last year it was offered in Chrysler vehicles was 2000.

You will also find the MMC 3.0L V6 in 1989 to 1998 Hyundai Sonota models as well as Mitsubishi models including the 1991 to 1994 Diamante, Montero, Mighty Max pickup and 3000GT. The Mitsubishi version is called a "6G7" engine, which also included a dual overhead cam (DOHC) four-valve per cylinder high-output version of the 3.0L V6 in both naturally aspirated and twin-turbo versions.

All versions of this engine share the same basic block, but the Mitsubishi engines are not directly interchangeable with the Chrysler engines because of differences in oil pump and water pipe locations, and intake manifold designs.

Evolving Design
Though the basic 3. 0L V6 engine design remained essentially the same over its 10-year production run, there were some changes from one model year to the next. The 1987-1988 model year engines had a 52 mm throttle body while later versions changed to a smaller 48 mm throttle body along with a redesigned intake plenum to reduce hood clearance. In 1989, the PCV system and valve covers were changed and the exhaust manifold outlets were increased from 2.25" to 2.50" for better breathing. In 1990, the flat tappet OHC cams and rockers were replaced with roller cams and rockers to reduce friction.

In 1990, Chrysler replaced the SMEC (Single Module Electronics) computer with a more powerful SBEC (Single Board Electronics) computer - then replaced it in 1992 with an even better SMEC II computer when the change was made from ordinary multiport fuel injection to sequential fuel injection. The new setup provided each injector with its own driver circuit in the computer so each injector could be fired individually in sync with the opening of each intake valve. Earlier computers had two injectors sharing each driver circuit and gang fired the injectors once every revolution of the crankshaft.

In 1993, a flash reprogrammable computer was introduced on California minivans with the 3.0L V6. In 1994, the reprogrammable computer was added to the federal minivans.

In 1996, power output was increased to 150 hp by increasing compression slightly. The exhaust manifolds were also revised slightly to fit the redesigned minivan engine compartment.

The Engine's Weaknesses
Over the years, the MMC 3.0L V6 has proven itself to be a real workhorse capable of racking up lots of miles. Some of these engines have gone 200,000 to 300,000 miles in aging minivans! But the years and miles also have revealed the engine's major weaknesses. The main ones are oil leaks, oil consumption and valve guide/seal problems. The bottom end has proven to be rock solid but the valve guides and seals (especially in the earlier motors) have been troublesome.

In the back of each of the OHC heads is a black round rubber cam plug. If this plug pops out, the engine will quickly dump its oil supply and self-destruct. The cam plugs tend to loosen up during extremely cold weather so it is important to make sure they are leak-free and tight. If you replace these plugs, make sure you use the later "improved" versions, which are wider than the original narrow plugs.

The Positive Crankcase Ventilation (PCV) system on the 3.0L V6 sucks blowby vapors from the crankcase through the baffles that are built into the valve covers. There is also a drain hole in the covers so excess oil can drain back into the engine. If the drain hole becomes plugged, oil can be drawn into the PCV system causing a huge increase in the engine's appetite for oil. Likewise, if the baffles are clogged with sludge, pressure can build up in the crankcase forcing oil to leak past other gaskets and seals in the engine. There is no way to clean the baffles because they are located between the inner and outer liners in the valve covers. So if the valve covers are dirty and caked with varnish and sludge, they should be replaced with new ones.

To prevent sludge from returning, change the oil and filter regularly. Short-trip, stop-and-go driving especially during cold weather accelerates the buildup of moisture in the crankcase making 3,000-mile or three-month oil changes a must.

The 3.0L V6 uses pressed-in valve guides, which may work loose as the engine ages. The problem occurs mostly with the exhaust guides and allows oil to leak past the guide seal into the exhaust port. Chrysler says oil consumption is excessive if the engine uses more than one quart in 1,000 miles on vehicles with less than 50,000 miles on the odometer, or more than one quart in 750 miles on higher mileage engines. On engines built since 11/15/91 (build code BDL7.067), snap rings have been installed on the exhaust guides at the factory to prevent them from slipping, and on engines built since 10/2/92 (build code BDL7.416) the intake and exhaust valve stem seals were revised to improve oil control. The build code label can be found on the intake plenum.

Chrysler Technical Service Bulletin 09-18-92 describes the oil consumption problem and offers the following repair advice. If a 3.0L V6 is burning too much oil and/or blue smoke is visible in the exhaust, check compression and verify the correct operation of the PCV system to rule out these as possible causes of the oil usage. If both are okay, remove the intake plenum and manifold, then the valve cover, rocker shaft and cam bearing cap assembly on the front head. Remove one of the cam journal caps from the assembly and reinstall it on the head to hold the cam in place so the cam does not come out of the journals when the engine is rotated later.

Examine the valve guides. If any can be moved with finger pressure or appear to be higher or lower than the others, the head will have to be removed so the guides can be replaced. Chrysler says the guides must be replaced if the distance from the top of the exhaust guide to the valve guide boss on the head is 0.335" (8.5 mm) or less.

If the guides are not loose and have not slipped, a special tool (MD998790) can be used to cut grooves in the exhaust guides so clips can be installed to prevent them from coming loose. Use a magnet and a shop towel to remove the cuttings. Then install new positive valve seals on the guides, reassemble the valvetrain and valve cover and repeat the same procedure on the rear head.

Engine Replacement
If you are rebuilding or replacing a high-mileage 3. 0L V6 in a Chrysler FWD car or minivan, one thing you have to watch out for is to make sure the engine and transaxle are properly centered when the engine is reinstalled in the vehicle. This is necessary to prevent the FWD halfshafts from pulling out of the transaxle when the front suspension is extended to maximum travel.

To check centering, refer to a manual for the specified halfshaft dimensions (they vary from one application to another). Then use a tape measure to check the distance between the inner edges of the inboard and outboard CV joints on both shafts. If adjustment is needed, support the engine, loosen the side motor mounts and slide the engine left or right as needed to achieve the proper dimensions. Retighten the right side motor mount first, then the left mount to hold the engine in position.

Something else to pay close attention to is the radiator and cooling system. If the original engine failed due to overheating, the radiator may be clogged and may have to be cleaned or replaced. Also, when refilling the cooling system, make sure all of the air is out of the system before the vehicle is driven. Check the operation of the cooling fan, too, to make sure it comes on when the engine is hot and when the A/C is turned on.

Maintenance
The 3. 0L SOHC and DOHC V6 both use a rubber timing belt to drive the overhead cams. The recommended replacement interval is 60,000 miles with labor times ranging from 2.0 to 4.4 hours, depending on the vehicle model and power accessories. The easiest application to replace the timing belt on is the 3000 GT (SOHC version). All of the other applications will take 3.5 to 4.4 hours to change.

The SOHC version of the 3. 0L V6 has enough valve-to-piston clearance so a belt failure should not cause any internal engine damage. But on the DOHC versions in the Dodge Stealth, Mitsubishi 3000GT and Diamante, it is an interference application. A timing belt failure on one of these engines will usually bend one or more valves. The same goes for the 3.0L V6 engines in the Hyundai applications.

Replacing a timing belt on a 3.0L V6 is pretty much the same as replacing a timing belt on any other FWD engine. You have to remove everything that is in the way, disconnect the passenger side motor mount, and raise the engine slightly to remove the belt covers. Rotate the engine to align the timing marks (refer to a manual for their exact locations), then loosen the belt tensioner bolt, push the tensioner back and replace the belt.

Pay close attention to the condition of the belt tensioner. On high-mileage engines, the tensioner also should be replaced.

The water pump pulley runs off the timing belt, and considering how much effort is involved to get at these parts it is probably a good idea to replace the water pump, too. Also, check the tube that runs from the back of the pump to the back of the engine. Replace it if it is corroded or leaking. The service life of the OEM water pumps is about the same as the timing belt, so it makes sense to replace both at the same time.

Driveability Diagnosis
There is nothing different about the 3. 0L V6 that you have not seen on other engines of this vintage. Chrysler and Mitsubishi applications use a speed-density fuel injection system that relies on the throttle position, MAP sensor and temperature to estimate airflow at various engine speeds. The Chrysler multipoint fuel injection system runs at 48 psi. On Hyundai, a mass airflow fuel injection system is used.

The ignition system uses an optical distributor with a coil mounted on the intake manifold. Early engines were equipped with regular plugs while later engines had 100,000-mile platinum plugs.

While searching technical service information database systems for driveability bulletins on this engine, I found several of interest:

TSB 18-01-93 describes a hard/no-start condition that may occur during cold weather. Models affected include 1993 Sundance, Shadow, Caravan, Grand Caravan, Voyager and Grand Voyager with the 3.0L V6. The problem may be caused by the engine coolant temperature (ECT) wiring connector. To diagnose the fault, connect a scan tool to the vehicles diagnostic connector and look for a Code 22 (ECT Sensor Voltage Too High). If you find a Code 22, check the coolant sensor to see if it is defective (the sensors resistance should change in proportion to temperature and be within the range of specifications listed in the manual). If the sensor checks out okay, the fix is to replace the coolant sensor wiring connector. Chrysler recommends soldering the new connector wires with rosin core solder and then sealing the connection with heat-shrink tubing, followed by electrical tape.

Another problem you may encounter is a shudder that occurs when decelerating. TSB 18-17-93 describes a shudder condition that occurs when slowing from 40 to 25 mph on 1992-1993 FWD cars and minivans. If no diagnostic trouble codes are found, the cure is to reprogram the PCM with a DRB II scan tool. This requires flash cartridge CH3501, a flash programming adapter CH1500, an authorized software update label 4669020 and an authorized modification label 425086.

On 1992 federal emissions vehicles, the PCM cannot be flashed so it must be replaced with an updated PCM: 4778002 for Acclaim, Spirit, LeBaron Sedan and Convertible, Sundance and Shadow with automatic transmission; 4778012 for Dynasty and New Yorker; 4778008 for Daytona, LeBaron, Sundance and Shadow with manual transmission; and 4778018 for the minivans.

TSB 18-05-97 deals with engine performance problems caused by a buildup of carbon deposits on the intake valves on any 3.0L V6. Symptoms that may indicate this condition include engine stalling when pulling out of a driveway, rough idle, lack of power and hesitation. The cause, says Chrysler, is cheap gas that does not contain enough detergent additives to keep the intake valves clean. A contributing factor also can be oil leaking past work intake valve guides and seals.

To diagnose the condition, verify there are no diagnostic trouble codes in the PCM and the PCM has the latest software updates (where available).

To remove light deposits from the intake valves, try adding a can of concentrated injector cleaner to a full tank of gasoline, or cleaning the injectors on the vehicle. The solvent should loosen and wash away most of the deposits.

If injector cleaner does not work because the deposits are too thick, Chrysler says the heads may have to be removed and disassembled so the intake valves and combustion chambers can be cleaned with a wire brush. We have also heard of people blast cleaning the valves through the intake ports with pulverized walnut shells or a similar soft media.

Air Bubbles In Cooling System
Many 3. 0L alleged head gasket failures appear as air bubbles in the cooling system. The cause of these failures on the 3.0L Mitsubishi engine have typically been attributed to a cracked cylinder head or a leaking head gasket. However, this is not always the case. This engine has two inlet coolant pipes that connect from the heater core and the rear of the engine to the water pump. The pipes are located in the valley of the cylinder block. These pipes are sealed with O-rings on the ends and become easily distorted when the engine is disassembled. If the pipes become distorted or the O-rings are not replaced air can enter the cooling system immediately after start-up.

To check for this after air is detected in a system, check for hydrocarbons in the coolant. If no hydrocarbons are present, look for these pipes to be leaking.

Excessive Oil Consumption
Excessive oil consumption on the 3. 0L Mitsubishi engine has been an issue. There are numerous reasons for this problem on the Mitsubishi engine. From a PCV system not operating correctly, to not maintaining correct valve guide clearances, to not using high quality valve stem seals. This engine design allows for large amounts of oil to stay present in the valve spring and camshaft area of the cylinder head. Using a high quality "positive" type valve stem seal is extremely important in preventing excessive oil consumption on these engines.

Preventing Head Gasket Failures
Head gasket failures on the 3. 0L Mitsubishi have always been an issue. Most head gasket failures typically happen on engines that have had recent cylinder head service or have been remanufactured. Causes for these failures can be related to a number of things.

Always clean or replace the EGR valve using a new gasket. EGR valves that have bad carbon buildup can cause a restriction of the flow of exhaust gases or, if it is not working properly or is leaking, it can change the combustion process. The center two cylinders are usually the ones affected with burning or cracking of the combustion grommet, which then would cause exhaust gases to enter the cooling system, potentially causing overheating.

Always be sure to replace or check the cooling system on the vehicle. Older cooling systems on these vehicles are another cause of possible head gasket failure. These older systems do not flow well and do not allow for the adequate heat transfer to cool the engine. This inability to transfer heat from the coolant can cause increased operating temperatures and potential head gasket failures.

Always use the correct torque specs. Some early service manuals show head bolt torque specs to be 65-72 ft. lbs. Later on the engine manufacturer changed the spec to 80 ft.-lbs. Since the specs differ based on the VIN code and the year, be sure to reference an up-to-date torque spec guide.

Use a quality head gasket. Because of the cast iron block and aluminum cylinder heads the amount of thermal motion on this engine is high. This motion can destroy some head gaskets of a lesser design.

Information courtesy of Corteco.

Adapted from an article written by Larry Carley for Underhood Service magazine

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