Another 2001 4.0 P0306 issue
12-06-2010, 06:39 AM
#61
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Year: 2001
Model: Cherokee
Engine: 4.0
To recap...
Things we have learned from this episode:
We have a 2001 4.0 liter with less than 89000 miles which had a persistent P0306 Cylinder 6 Misfire condition.
Vehicle safety/emissions test technician said we needed a "tuneup." So we used fuel injector cleaner, followed by Seafoming the intake, gas tank and engine oil. (Yes, we made a few oil changes...) We started trying to clear the code in September. Still, everything we applied "cleaner/solvent" to and allowed for it to work, did not clear the P0306 code.
1. Generally, if you have multiple P codes, address the highest numbered code first, the rest will most likely fall under one of the higher codes. We only had the P0306 code to deal with.
2. DIAGNOSIS!
a. Compression test - This was the major indicator of our particular problem. We initially chose to ignore it or explain it away until it was the sole remaining item needing to be addressed. Basic compression test showed us cylinder 6 was 10 psi less than the others, but still, 1-5 were 165 psi and #6 was at 150 psi. Leak down test told us it wasn't a blown head gasket, etc... we just thought that #6 was still fine at this point.
b. Fuel pressure test - A lot of needless time and effort saved by checking the fuel PSI with engine running as well as an engine off leak down test. Told us the fuel pump/filter and fuel injectors were running fine. We still swapped injectors 5 & 6 with no effect on the code and then just replaced all 6 injectors for improving fuel consumption and (mebbe?) some additional power. New injectors did not clear the P0306 code.
c. Noid light test - showed us the electrical connections were good to the injector electrical contacts.
d. With the engine running, unplugging/replugging the injector connectors one at a time and noticing the engine changes for 1 thru 5 and no changes with #6.
e. Vacuum test - Wildly fluctuating gauge told us there was a vacuum leak. We looked and found a cracked fitting on the brake booster line (see f. below.) This did not clear the P0306 code but did make the engine run smoother.
f. Visual (and audio) inspection - The cracked brake booster vacuum line fitting was sitting in plain sight. The crack was approximately 1 mm wide and went from the outside edge well down into the middle of the plastic fitting... so smooth and even that it appeared as if it was manufactured that way... if you didn't know there was not supposed to be a crack there you would think it was supposed to be there. Initially, moving your head around inside the engine compartment with the engine running (WARNING! Secure any long hair or loose clothing first!) and listening for vacuum leaks, air rushing, etc. Putting a finger over the brake booster fitting made an increased whistling noise, so we replaced that fitting... but it still didn't fix the P0306 code. Using a squirt water bottle or can of carb cleaner near the intake manifold can also confirm (or not) any intake manifold leaks.
g. Code Scanner Tool - other than always showing you the problem you need to address, some can also show you the "Freeze Parameters" snapshot of engine data to help you diagnose your problem. We initially ignored this too.
h. Ask for help - Every issue has most likely been experienced by someone somewhere. Sure, not everyone has the same exact resolution, but it cannot hurt to get an outside perspective or second opinion. There is a difference between a valid, pertinent outside comment and one just thrown out on the table that has no bearing at all. Take every outside opinion with a grain of salt, validate it as to whether or not it applies to your specific problem, but try not to call "BS" on irrelevant comments as it may cause the one comment/opinion you are looking for to not be made.
Be thankful people are trying to help you.
i. Finally, throw money at it - this is the most taken approach by the DIY'r (I'm guilty of this, too!)... based upon incorrectly assessing or ignoring the diagnostic conditions and reading other similar (but unrelated?) posts from other threads can have you religiously buying parts that don't need replacing:
O2 sensors - know how many are on your car. 4.0 Liter in-line 6 with California emissions has more than 2 O2 sensors. The exhaust manifold on ours does 3 into 2 into 1. With 2 O2 sensors right after the 3 into 2 join, any external comments that it was an O2 sensor did not make sense as if the aft O2 sensor was bad then cylinders 4 & 5 should also have thrown codes. O2 sensor(s) not the cause of our P0306 code. We did not replace O2 sensors.
Camshaft position sensor - $replaced$ with no effect
Crankshaft position sensor - $replaced$ with worsening effect; reinstalled original with no effect
Idle Air control Valve - $replaced$ with no effect
Mass Air Flow Sensor - $replaced$ with no effect
Fuel injector electrical connectors - $replaced$ as some of the originals would not remain locked down on the injector... fixed the connector/connection issue but not the P0303 code issue.
Fuel injector(s) - $replaced$ with no effect
So... now what? Sticky valve? Burnt valve? Bent valve? Cracked head?
We tried a "valve rotation" on #6 but this too did not clear the P0306 code.
Cylinder head - $replaced$ and waiting to see if code will come back... we don't think it will. (also needed new gaskets, head bolts, rocker arm assemblies, oil and filter)
Additional thoughts on cylinder head work:
Removing the bottom intake/exhaust manifold bolts is a PITA.
Reinstalling the bottom intake/exhaust manifold bolts was easier as we completely removed the intake to clean it. We aligned the exhaust manifold on the new head and screwed in all of the bolts 1 to 2 threads and then slid/rotated the intake manifold into place. TIGHTENING those bottom manifold bolts is a PITA.
The intake ports were really gummed/cruddy. A large can of carb cleaner with a toothbrush resulted in a clean and smooth intake manifold.
Original rocker arm assemblies did not align with the valves on the new head. Sure, we could have tweaked the bridges for alignment, but new assemblies had new bridges and rocker arms... so we obtained new rocker arm assemblies. Same old push rods though.
Getting to the radiator drain petcock from underneath is doable, the manual refers to removing the front grill for access. We opened the drain from underneath but somehow cross-threaded the (plastic!) valve when trying to screw it back in. We did not know this until refilling the radiator and having it all leak back out. So, we removed the front grill, unscrewed the valve and then threaded it back in straight. No problem now.
Tools for the job... lol, we have them all now... and I have a 14 y.o and a 16 y.o. that want Jeeps, too...
Bottom line: we learned a lot about our particular vehicle problem. We are now letting the new head break in and will enjoy our Cherokee for another 100k miles.
Questions?
Things we have learned from this episode:
We have a 2001 4.0 liter with less than 89000 miles which had a persistent P0306 Cylinder 6 Misfire condition.
Vehicle safety/emissions test technician said we needed a "tuneup." So we used fuel injector cleaner, followed by Seafoming the intake, gas tank and engine oil. (Yes, we made a few oil changes...) We started trying to clear the code in September. Still, everything we applied "cleaner/solvent" to and allowed for it to work, did not clear the P0306 code.
1. Generally, if you have multiple P codes, address the highest numbered code first, the rest will most likely fall under one of the higher codes. We only had the P0306 code to deal with.
2. DIAGNOSIS!
a. Compression test - This was the major indicator of our particular problem. We initially chose to ignore it or explain it away until it was the sole remaining item needing to be addressed. Basic compression test showed us cylinder 6 was 10 psi less than the others, but still, 1-5 were 165 psi and #6 was at 150 psi. Leak down test told us it wasn't a blown head gasket, etc... we just thought that #6 was still fine at this point.
b. Fuel pressure test - A lot of needless time and effort saved by checking the fuel PSI with engine running as well as an engine off leak down test. Told us the fuel pump/filter and fuel injectors were running fine. We still swapped injectors 5 & 6 with no effect on the code and then just replaced all 6 injectors for improving fuel consumption and (mebbe?) some additional power. New injectors did not clear the P0306 code.
c. Noid light test - showed us the electrical connections were good to the injector electrical contacts.
d. With the engine running, unplugging/replugging the injector connectors one at a time and noticing the engine changes for 1 thru 5 and no changes with #6.
e. Vacuum test - Wildly fluctuating gauge told us there was a vacuum leak. We looked and found a cracked fitting on the brake booster line (see f. below.) This did not clear the P0306 code but did make the engine run smoother.
f. Visual (and audio) inspection - The cracked brake booster vacuum line fitting was sitting in plain sight. The crack was approximately 1 mm wide and went from the outside edge well down into the middle of the plastic fitting... so smooth and even that it appeared as if it was manufactured that way... if you didn't know there was not supposed to be a crack there you would think it was supposed to be there. Initially, moving your head around inside the engine compartment with the engine running (WARNING! Secure any long hair or loose clothing first!) and listening for vacuum leaks, air rushing, etc. Putting a finger over the brake booster fitting made an increased whistling noise, so we replaced that fitting... but it still didn't fix the P0306 code. Using a squirt water bottle or can of carb cleaner near the intake manifold can also confirm (or not) any intake manifold leaks.
g. Code Scanner Tool - other than always showing you the problem you need to address, some can also show you the "Freeze Parameters" snapshot of engine data to help you diagnose your problem. We initially ignored this too.
h. Ask for help - Every issue has most likely been experienced by someone somewhere. Sure, not everyone has the same exact resolution, but it cannot hurt to get an outside perspective or second opinion. There is a difference between a valid, pertinent outside comment and one just thrown out on the table that has no bearing at all. Take every outside opinion with a grain of salt, validate it as to whether or not it applies to your specific problem, but try not to call "BS" on irrelevant comments as it may cause the one comment/opinion you are looking for to not be made.
Be thankful people are trying to help you.
i. Finally, throw money at it - this is the most taken approach by the DIY'r (I'm guilty of this, too!)... based upon incorrectly assessing or ignoring the diagnostic conditions and reading other similar (but unrelated?) posts from other threads can have you religiously buying parts that don't need replacing:
O2 sensors - know how many are on your car. 4.0 Liter in-line 6 with California emissions has more than 2 O2 sensors. The exhaust manifold on ours does 3 into 2 into 1. With 2 O2 sensors right after the 3 into 2 join, any external comments that it was an O2 sensor did not make sense as if the aft O2 sensor was bad then cylinders 4 & 5 should also have thrown codes. O2 sensor(s) not the cause of our P0306 code. We did not replace O2 sensors.
Camshaft position sensor - $replaced$ with no effect
Crankshaft position sensor - $replaced$ with worsening effect; reinstalled original with no effect
Idle Air control Valve - $replaced$ with no effect
Mass Air Flow Sensor - $replaced$ with no effect
Fuel injector electrical connectors - $replaced$ as some of the originals would not remain locked down on the injector... fixed the connector/connection issue but not the P0303 code issue.
Fuel injector(s) - $replaced$ with no effect
So... now what? Sticky valve? Burnt valve? Bent valve? Cracked head?
We tried a "valve rotation" on #6 but this too did not clear the P0306 code.
Cylinder head - $replaced$ and waiting to see if code will come back... we don't think it will. (also needed new gaskets, head bolts, rocker arm assemblies, oil and filter)
Additional thoughts on cylinder head work:
Removing the bottom intake/exhaust manifold bolts is a PITA.
Reinstalling the bottom intake/exhaust manifold bolts was easier as we completely removed the intake to clean it. We aligned the exhaust manifold on the new head and screwed in all of the bolts 1 to 2 threads and then slid/rotated the intake manifold into place. TIGHTENING those bottom manifold bolts is a PITA.
The intake ports were really gummed/cruddy. A large can of carb cleaner with a toothbrush resulted in a clean and smooth intake manifold.
Original rocker arm assemblies did not align with the valves on the new head. Sure, we could have tweaked the bridges for alignment, but new assemblies had new bridges and rocker arms... so we obtained new rocker arm assemblies. Same old push rods though.
Getting to the radiator drain petcock from underneath is doable, the manual refers to removing the front grill for access. We opened the drain from underneath but somehow cross-threaded the (plastic!) valve when trying to screw it back in. We did not know this until refilling the radiator and having it all leak back out. So, we removed the front grill, unscrewed the valve and then threaded it back in straight. No problem now.
Tools for the job... lol, we have them all now... and I have a 14 y.o and a 16 y.o. that want Jeeps, too...
Bottom line: we learned a lot about our particular vehicle problem. We are now letting the new head break in and will enjoy our Cherokee for another 100k miles.
Questions?
Last edited by membrain; 12-06-2010 at 07:00 AM.
06-26-2014, 07:13 AM
#63
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Model: Cherokee
Been having similar problems with 2000 Jeep Cherokee SE 4.0L. P0300, P0301, and P0306 codes popped up. Replaced fuel injectors, seafoamed, changed spark plugs, checked coil, fresh gasoline, and new battery. Ends up to be number 1 and 6 injector connector ground/short. Intermittent faults are the toughest to diagnose. Don't go replacing parts until the wiring harness (especially the FI Connectors) can be ruled out. Many replace parts, move wires, and think they've fixed the problem with the new part; but actually corrected the wiring problem inadvertently. Be careful whenever disconnecting as the insulation becomes very brittle with age and heat under the hood. The 4.0L engine is damn near indestructible, but the crappy wiring harness appears to have come from a low cost bidder.
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