The 3.6 Petrol V6 engine is featured in the WK2 Grand Cherokee and its in the JK wranglers. The KL Cherokee also features a variant of this engine which is a 3.2 and mounted east to west however many of the faults we are about to discuss apply to that engine too.
Recently we have seen a huge run on these engines coming to our stores with misfire codes and running rough, or in some cases running fine and still misfiring. In majority of these cases the vehicles either come to us straight from another workshop by the mechanics at that shop or the shop has got to the end of their diagnostic skill and have told the owner to seek our help.
Unfortunately many shops see a misfire issue and sell spark plug and or ignition coil replacement with no actual diagnosis carried out. This is called using a parts cannon and is poor form on the mechanics part. This is simply parts fitting and not running the diagnostic process, effectively this costs the customer a lot of money as the manifold requires removal to access the plugs for a complete guess as to what is wrong.
This engine very rarely has ignition coil failure. It does happen from time to time, but not often. Similarly, plugs are rarely the issue. Here is a step by step guide for how we follow the diagnostic process for these engines. This is not the process we use for all vehicles. We have chosen the path of least resistance taking into consideration the time required to remove manifolds and wiper cowlings.
STEP1: Code scan: the number of people asking what could be wrong when the check engine light comes on, baffles me. This light (the orange engine light and the red lightning bolt) apply to literally thousands of possible faults. You must get the vehicle scanned before anyone can help you. Yes a code scan costs money the computer some shops uses costs upwards of $20,000 but that’s another blog.
Are there codes for injectors ? (these engines are known for internal injector shorts) if you have a specific injector code remove the injectors and get them tested. If one has failed, replace all injectors. If one has died the others are not far behind, trust me you will have to replace another one in a week.
Do you have cylinder specific codes for misfire? if yes we know where to look. If generic “multiple misfire detected we are likely facing a mechanical fault not electrical.
Step 2: Smoke test it. These engines sucks air from failed upper and lower manifold gaskets breaks the plastic emissions pipe the runs across the top of the motor, have two sources of vacume leak on the back of the manifold and in some cases have melted the inlet manifold.
If you have a vacume leak, fix it, clear codes and you should be sorted.
Step 3: Carry out cooling system checks. These engines are very common for blowing head gaskets. Carry out TK tests and cooling system pressure tests. If any coolant is getting into the combustion chamber it will stop the spark plug from firing and cause a misfire.
other tell tail signs this is your issue are
– gurgling or fish tank noises behind the dash usually on start up
– starter motor failure including going through several starter motors in a short period of time.
If you have head gasket issues, the heads have to come off. (tow it to GDL)
Step 4: Remove the manifold (upper half only) Remove coils and plugs and check for spark. If no spark you need to be looking at coil resistance. If you don’t have the equipment or electrical know how you can move suspected plug and coil to a different cylinder. try moving all suspect coils and plugs to the drivers side bank, this removes the need to remove the manifold over and over again. (you can access cylinder 2,4 and 6 without removing the manifold.)
Step 5: Got spark, got fuel. Now do a compression test. These engines are known for burning exhaust valves and breaking valve springs. If you are low or absent on compression in one or more cylinders do a leak down test. This will tell you if its top (valves or springs) or bottom end (piston rings or pistons) Of note its highly unlikely for these engines to have bottom end issues.
NOTE: a good compression on these engines is not a guarantee that the top end is good. If you have good compression and no other issues move to the next step.
Step 5a) before moving to the next step do a compression test the following way:
make sure all spark is disconnected but keep the bottom half of the inlet manifold and the injector rail connected. Carry out the compression test with the injectors connected. This will result in live fuel in the chambers so follow appropriate safety precautions. This is effetely carrying out a wet compression test. If your numbers are extremely high (200PSI in a cylinder) you have a dribbling or leaking injector. This is a very common issue with these cars, Don’t skip this step.
Step 6. Remove the valve covers and rotate the motor by hand. Look for worn cam lobes and valve springs that are not opening or not opening enough. Worn cam lobes can cause misfire codes as the mixtures in the camber are not at 14.7:1. this fault will cause lifter ticking noise and or misfire codes without a rough idle. If cam lobes are worn replace camshafts, rockers and lifters.
Note: If a cam lobe is worn the metal from the lobe will have gone though the whole engine and an assessment of the bottom end of the motor should be carried out and likely a full rebuild advised.
