Just bought a Grand Cherokee with P0300 P0301 P0302...egads!

My next step would be to put a scan tool on it and watch the data lines to see if anything strange is happening. Also, a compression and leak down test would be a good idea.

Thanks for the reply! Yep, today we did compression/leakdown tests... normal. EGR is fine, O2 sensor is good- all checks out. So, since there's nothin' else left- we're thinking that maybe the brain has gone the way of HAL in 2001 A Space Oddysey... There seems to be tests for everything- except for the main computer itself! peace- g.

I know you said you swapped plugs and coil packs but did you check to see if you are getting spark? Is there a signal and battery power to the coils? Is there injector pulse to those two cylinders? I have seen multiple injectors fail at one time, it can happen.

Im going to assume that this is a 4.7 v8 and not a 4.0 ltr 6 because I've never seen a 4.0 ltr with an egr so either you checked something that wasn't really an egr or you have the 4.7. if so then it's very unlikely your pcm is the issue, more then likley you have the wrong plugs, dirty injectors or a valve/head gasket issue. Never seen a pcm fix a misfire code yet!

I'll keep ya posted on what this finally turns out being... the obd info said it COULD be an EGR (the 4.7 as you correctly stated- doesn't have one)... but good compression and leakdown tests should have eliminated the possibility of valves/head gasket issues (unless there's something you know that I don't), and again- it ain't the injectors (we swapped bad cylinders for good), a noid light confirmed injector firing, and we swapped coil packs and they all work well. Since there's nothing else we can think of, I just ordered a remanufactured brain for $199... you're not the first person to say it won't fix it, but since we've checked and double checked everything else- what's the alternative? Any other thoughts- I'd love to hear 'em!

this may or may not help you, however looks like it may apply, TSB from mopar. NUMBER: 09-003-03 GROUP: Engine DATE: Aug. 01, 2003 SUBJECT: 4.0L Multiple Cylinder Misfire OVERVIEW: This bulletin involves inspection of all engine exhaust valves and a decarbonizing procedure if necessary. MODELS: 1999 - 2004 (WJ) Grand Cherokee (All Markets) 2001 - 2004 (WG) Grand Cherokee (International Markets) 1999 - 2004 (TJ) Wrangler 1999 - 2001 (XJ) Cherokee NOTE : THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L ENGINE (SALES CODE = ERH). SYMPTOM/CONDITION: The customer may experience an incident of engine misfire during certain vehicle operating conditions. The misfire may occur when the vehicle is operated between 80 - 112 KPH (50 - 70 MPH) and under light loading conditions, e.g. slight uphill road grades. This condition may occur at all ambient conditions, but is more noticeable when ambient conditions are less than 0°C (32°F). If the vehicle is equipped with On-Board Diagnostic (OBD), a MIL illumination may also have occurred due to Diagnostic Trouble Code (DTC) P0300 - Multiple Cylinder Misfire. Various single cylinder misfire DTC's may also be present. If the frequency of misfire is high the Powertrain Control Module (PCM) may place the engine in "Limp-In" mode. The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem. DIAGNOSIS: 1. This condition may occur when the engine is not allowed to run at engine RPM's that are greater than 3,200 RPM. At 3,200 RPM or higher the engine exhaust valves will rotate if not impeded by high carbon deposits. Low engine RPM's and high carbon deposits are associated with short trip driving where the vehicle engine is not allowed to fully warm to normal engine operating temperatures. Cold ambient temperatures will increase engine warm-up time and add to the opportunity of carbon deposit build-up on the stem of the engine exhaust valve. 2. Verify that an engine misfire condition is present. Use of the DRBIII(R) during a road test, or a Co-Pilot data recording, may help to determine engine misfire and misfire counts. If carbon deposit accumulation is severe, then a cylinder leak down test may detect one or more cylinders leaking greater than 15%. Save any misfire DTC Freeze Frame Data that was stored for later misfire correction verification. 3. Verify that the engine misfire condition is not caused by faulty engine mechanical or electrical components. 4. If the engine mechanical and electrical systems are operating properly perform the Repair Procedure. POLICY: Reimbursable within the provisions of the warranty. Now the fun part, The Repair Procedure..... 1. Raise vehicle hood. 2. Remove the engine valve cover and all six exhaust valve rocker arms (the intake rocker arms are also removed during this step). Refer to the appropriate vehicle Service Manual for detailed removal instructions. The valve cover gasket is reuseable. Keep each pair of rocker arms matched to their respective valve and cylinder. 3. Inspect the end, or tip, of each exhaust valve stem where it makes contact with the respective rocker arm. 4. Determine if each exhaust valve is rotating within its respective valve guide. An exhaust valve that is rotating will have a "bulls eye" or circular wear pattern on the face of the valve stem tip. If the exhaust valve is not rotating a straight mark-like pattern will be present across the face of the valve stem tip. 5. If there are exhaust valves which are not rotating then proceed to the VALVE ROTATION section of this Repair Procedure. 6. If all exhaust valves are rotating, then this bulletin does not apply and further diagnosis is required. Install the engine rocker arms and valve cover. Refer to the appropriate vehicle Service Manual for detailed installation instructions. VALVE ROTATION: 1. If one or more engine exhaust valves are not rotating, perform the valve rotation procedure to all six (6) exhaust valves. NOTE :IT IS IMPORTANT THAT THE VALVE ROTATION SECTION OF THIS REPAIR PROCEDURE BE PERFORMED. 2. Clean and mark the tip of each exhaust valve stem with a paint marker. The paint mark will be used later to assist with determining if the exhaust valve has been rotated 90°. 3. Bring number one ( # 1) cylinder piston to top dead center using the mark on the crankshaft front dampener/pulley. This step is important to prevent the possibility of the exhaust valve from falling completely into the cylinder. 4. Install the essential service tool valve spring compressor, MD-998772A, to the # 1 cylinder exhaust valve spring. NOTE : THE FOLLOWING VALVE ROTATION PROCEDURE WAS DEVELOPED WITH THE USE OF VALVE SPRING COMPRESSOR TOOL MD-998772A. THE MD-998772A IS AN ESSENTIAL SERVICE TOOL AND PART OF THE DEALERSHIP REQUIRED TOOLS. THE MD-998772A COMPRESSOR ENGAGES THE ENTIRE PERIMETER OF THE VALVE SPRING RETAINER, UNLIKE OTHER MAKE VALVE SPRING COMPRESSORS. 5. Compress the # 1 cylinder exhaust valve spring enough to gain access so that the exhaust valve can be rotated 90°. Rotate the exhaust valve 90°. Slowly remove the compression on the exhaust valve spring. Verify that the valve keeper is properly seated to the valve stem and valve spring retainer. 6. The 4.0L firing order is 1 - 5 - 3 - 6 - 2 - 4. Without rotating the engine crankshaft repeat steps 3 through 5 to cylinder # 6. 7. Rotate the engine crankshaft 1200 and repeat steps 3 through 5 to cylinders # 5 and # 2. 8. Rotate the engine crankshaft another 1200 and repeat steps 3 through 5 to cylinders # 3 and # 4. 9. Install all cylinder rocker arms (intake and exhausts) and retaining bridge. Make certain that the push rods are properly seated to their respective rocker arm and lifter. Tighten the respective cylinder bridge/rocker arm cap screws to 30 Nm (21 ft. lbs.) when each cylinder piston is at top dead center (cylinder intake and exhaust valves are closed). 10. Install the engine cylinder head valve cover. Tighten the valve cover bolts to 10 Nm (85 in. lbs.). Refer to the appropriate vehicle Service Manual for detailed assembly instructions. DECARBONING COMBUSTION CHAMBER AND VALVES: 1. Start the vehicle engine and allow the engine to reach normal operating temperature. 2. Remove the air tube from the engine throttle body. 3. With the engine at idle, spray the entire contents of Mopar Combustion Chamber Cleaner, p/n 04318001AB, directly into the throttle body. As the cleaner is being ingested, allow the vehicle to "load up" with the cleaner to the point that the engine is almost stalling out. Maintain this condition until all of the cleaner is used/ingested. 4. Stop the engine once the entire can of cleaner has been ingested. 5. Install the air tube to the throttle body. 6. With the hood closed and the vehicle parked inside the garage, allow the vehicle engine to heat soak for two to three hours. This will ensure that the engine will maintain its temperature and will allow proper solvent penetration. 7. After engine soak, start the engine and drive the vehicle until the engine is has reached normal engine operating temperatures. 8. If the vehicle is equipped with an automatic transmission, place the gear selector into "L" (low). If the vehicle is equipped with a manual transmission, place the transmission into first gear. 9. In a safe vehicle operating location that will allow the vehicle to be driven safely and at the posted speed limit, accelerate the vehicle until the engine reaches 4500 RPM. 10. Hold the engine speed at this RPM for 15 seconds. 11. Slow down and in a safe location pull to the side of the road. Allow the engine to idle for five seconds. 12. Repeat steps 9 through 11 five more times. 13. With the vehicle at operating temperature and using any available Freeze Frame data recorded when the misfire DTC occurred, verify that the misfire condition has been corrected. 14. Erase any engine DTC's once the misfire condition has been corrected.

Here is another tsb as well.. NUMBER: 18-010-01 GROUP: Vehicle Performance DATE: April 20, 2001 SUBJECT: Rough Idle at Cold Temperatures OVERVIEW: This bulletin involves selectively erasing and reprogramming the Powertrain Control Module (PCM) with new software (WJ/TJ - calibration level 01Cal14C, XJ - calibration level 15B.) NOTE : THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L (ERH) ENGINE BUILT PRIOR TO APRIL 30, 2001 AND CALIBRATION LEVEL PRIOR TO (LESS OR LOWER THAN) THOSE LISTED IN THE OVERVIEW ABOVE. SYMPTOM/CONDITION: Some vehicle engines may exhibit a rough idle and/or MIL light "on" condition upon initial start-up after a cold soak when temperatures are below 50°F (10°C).

Thanks for the info! Unfortunately (or maybe fortunately) my motor if the 4.7 V-8, not the 4.0 6 cylinder... also, it's missing all the time, and in a consistent way... not just when cold, hot, no load, too much load, etc... it just always misses on cyl. 1 & 2- no matter what we swap out. Thanks 'tho!

So what's the next step?

Partially restricted/blocked Fuel rail ? Hope you don't have a hairline crack in the heads.

Hey guys! Thanks again for takin' an interest... well, as some predicted- a new brain did not fix the problem. It is now at the third (yes third!) shop to let them take a stab at it. Whatever the problem, it definately ain't run o' the mill... The idea of clogged fuel rails is a good one- 'tho I keep going back to the fact that cyl. 1 & 2 (and none others)both failed at the exact same time... not that that can't happen, but it's a little suspicious!

Some 4.7's do have egr valves, no 4.0 ltrs do! I think at this point you have a timing chain that has slipped or some bad valves!

pull your driver side valve cover (it is easyer to get to) The 4.7 is bad about rocker arms falling out. I just found one today that droped # 3 4 7 exhaust rockers. plan is to replace all lifters and the 3 bad rockers. soposedly carbon holds the valves open enought to let the rocker come out. so you will want to try the 4.0 decarbonizing method outlinde in this post when you are done they make a tool to compress the valve on the engine so you dont need to pull heads

Just wondering if you ever fixed that Cherokee. I have a 05 dakota with a p0300 problem. Dealer checked plugs, wiring (i think) and compression without solving the problem. Next step is checking injectors. Cylinder six has a carboned (black) plug burns different than all the others based on the color I pointed out. Freeze frame shows P0300 setting the service engine light, no specific cylinder. Taking to another dealer for more help in the future. May try some decarbonizing. Thanks.

How long does it take to foul the plug? which engine? the 4.7? before decarbonizing I'd remove the valve cover and check the rocker and valve springs! I've seen broken springs and rockers get missed on compression tests! What brand of plug? what's the gap? Try moving that coil to another cylinder and see if if the misfire moves cylinders.

Thanks for your response. At this point the plug doesn't foul out just gets carboned compared to the others. I brought a new coil for cylinder six and moved some around, that didn't help. I did a wiggle test on the wires, truck was stored outside for the fall and ended up with a mouse nest, so wiring is still suspect. I believe the rough idle started before that though and got worse to the point of setting the P0300. Plugs used are platnium AutotLite gapped at .40. I guess inspecting the rocker is a good step to take. Thought decarbonizing would be easiest and may help with a valve carbon problem. I can't believe a dealer service shop can't figure this out.

First of all lets not go down the dealer service dept rant again! Im a dealer service tech and anything can be fixed if the customer wants to pay for sufficent and thorough diagnosis! Most customers don't! I can't accurately and wouldn't diagnose the cause of a misfire for the price of scan test so because they didn't fix it for you doesn't mean they can't. Secondly that's the wrong plug for that engine, Autolites work Ok in fords and terrible in everything else so get the correct Champion plug for it, regap it, make sure the porcelain on the one you're removing isn't cracked and install it and see if it continues to miss on that cylinder. If it does then check the compression and or switch that cylinders injector with another to see if the injector is at fault. If you have access to one use a block tester to check for combustion gasses in the cooling system. if so then you're likely looking at a headgasket issue!

Just so you know, I don't expect to pay for the fix since it was a rough idle problem I reported while it was under warranty. The service dealer flashed the computer and sent me on my way. Thanks for paying attention to this, you're probably a service tech I would want to go. Anyway, the champion plugs I would get if I knew the exact type. I had Bosch plugs in there for about 30K miles (with same rough idle). Currently, I am trying on that cylinder 6 the old Bosch plug gapped at .20 for a hotter spark with 93 octane fuel to burn some carbon. So far, no p0300 code. I may try the Mopar combustion cleaner next. Being a mechanic from the 70's (not much a of one now). I do hear a sound like a worn tappit (old days) or hydralic lifter, that makes me curious mostly at cold start. This is a 4.7 V8.

Again if it you don't expect to pay for the fix how's it going to get fixed? Alot of 4.7's have a rough idle and never throw a misfire dtc! As for the correct champion plugs it would be the same number as what was installed originally at the assy line so cross it with the ones you removed or call the dealer! The parts dept there can get you the correct plug! I've had several 4.7's with rough idle that were fixed with using the correct plugs and veirfying they weren't cracked or had carbon tracks from incorrect torque, poor coil boot fit etc! I've also fixed several with the software update they performed. Would you rather they sell you a tune up and engine repair first before attempting to remedy it with a software flash? Carbon buildup happens alot on those as does broken or weak coil springs, bad coils, slipped rockers from sludge in the upper end, head gasket leaks, incorrect cam timing etc etc, etc