Dealer suspects a cracked head. What do the experts think?

primetime4

primetime4

TJ Enthusiast
Supporting Member
Joined
Sep 13, 2018
Messages
457
Location
Northern Virginia
I have posted about this in a few other posts but hoping to get more responses here. I have been chasing a cylinder misfires for a few months now. All this appeared to start after I changed out a bad crankshaft position sensor. Since Mopar units are no longer an option, I installed an NTK sensor and the Jeep started and ran great.

Fast forward a few weeks while driving on the interstate over 70 mph at approximately 3100 rpm and I start getting a flashing check engine light. There are no noticeable changes to how the engine runs at high or low RPMs. It continues to run very smoothly and idles fine. I check the codes and get p0304 and p0306.

From here I changed out the spark plugs for a new set of NGK (old ones still looked fine but figured a refresh wouldn't hurt). Jeep still runs fine after this with no noticeable change but the issue still continued. Did not check the codes since I planned to take it to a local shop.

Took it to a shop to get diagnosed and shop said I need a new crank position sensor (which I just changed) and a coil rail. I asked them how they figured this and they just said the "codes told them". Regardless, because I could still buy a Mopar coil rail, I figured it would not hurt to freshen things up with a new one so I had one installed at a second shop. Jeep continues to run fine with no noticeable change but the issue persists. This time, I see codes p0300, p0406, p0306.

This time, I took it to the dealer hoping the dealer has a fancy scan tool that can view live data. Dealer performs a fuel induction cleaning service to verify that carbon buildup is not the issue with no noticeable change. Just to confirm it was not the NTK sensor at fault, I bought a NAPA Echelin sensor and asked the dealer to install. No noticeable change. Finally, the dealer runs a compression tests with the following results:

Cylinder 1: 175 psi
Cylinder 2: 175 psi
Cylinder 3: 150 psi
Cylinder 4: 160 psi
Cylinder 5: 180 psi
Cylinder 6: 160 psi

Seeing this, they performed a leakdown test and reported that cylinder 3 has a 45% leakdown through the exhaust valve!!! The dealer now suspects a faulty valve and/or cracked head for cylinder 3.

A few notes about my 2001 which most likely has the 0331 head. Through this entire saga, my Jeep has continued to run fine. If anything, performance has improved especially after installing the new plugs but in the 20+ years I have owned it, I would say it runs the same as the first day I drove it off the lot. This Jeep has also been garaged for the past 10 years and rarely sees any extended use and certainly no hard use. I did recently drive it for 8 hours under 3100 rpm with no check engine light and no issues so temperature does not seem to be a factor here. It is only above 3100 that the flashing CEL occurs.

A buddy of mine that builds a lot of Jeeps is calling BS on this and insists it is an electrical issue or issue with the cam crank sync. He says if it was cylinder 3 like the dealer indicates, the code should confirm this and so far I have not seen it. He also says that at 3100 rpm, the engine does not even have a chance to leakdown.

I am prepared to buy a new head with all new valves and valve train but hoping for some additional inputs here. Could this be the dreaded 0331 head cracking? I've only put about 60k miles on this over the past 10 years of very light weekend driving so I have my doubts about this too.

The final code check after picking up the Jeep from the dealer: p0304 and p1389??? Now I am wondering if there a speed sensor at play here that could be causing this?

Thanks.
 
X 2 on letting your buddy figure it out, instead of throwing money into a dealership's deep pockets.
p.s. My mechanic gives free estimates. If he checks things out and doesn't fix it, no charge until it is.
 
  • Like
Reactions: jeepguy03 and Irun
Unfortunately my buddy lives in another state and has been chatting with one of his buds that owns a shop to provide me with some guidance. I might make the drive to take my Jeep there but it's a bit of a haul.

I did come across a few posts that mention a "relearning" routine that is needed whenever a sensor is changed similar to this video here.


However, I have also heard others say that the crank sensor is a simple magnetic pickup with not much to it. In any case, I was hoping the dealer would at least do this when they swapped out the sensor but I honestly doubt it.
 
My 2002 OEM CPS gave up while on vacation a couple of years ago. In the days prior to it failing I could feel occasional momentary loss of power while cruising at higher rpm. Kind of like a head on wind gust. The sensor was replaced with an aftermarket sensor from Oreillys. That one worked but a few days later at high rpm cruise that one caused the flashing check engine light, multiple cylinder misfire, and codes for the same. It still ran well enough to get to an auto parts for yet another aftermarket sensor. That one lasted a few months and did the same thing at high rpm cruise. But this time all that cleared up after turning off the engine and restarting. I then installed a Mopar sensor and haven’t had any issue since.
I never had any codes unless I also had obvious misfire. I never had the problem begin at modest rpm, only 2800 rpm-ish plus cruise. Lowering rpm after the issue began did not clear the misfire.
Unless your losing coolant you might just try yet another CPS.
 
First thing I would do it put a mopar sensor in there. Rock auto shows them I stock for a 01’ 4.0 manual trans, not sure what trans you have.
 
  • Like
Reactions: Irun
I have the 3 speed auto and have tried ordering this part from the various Mopar sites with no luck. Also called the dealer asking if they could find one and everyone indicates it has been discontinued. What’s really bad is these don’t even come up on eBay but if anyone has a lead on one I would love to buy it.
 
  • Like
Reactions: J.Pierce
JKP said:
Pretty certain you don't have the good TUPY head.

Remove the oil fill cap, look straight down. No TUPY, you have the problematic head.

Look for antifreeze, oil sludge in the area immediately under the fill cap. If you have it up to operating temp when you look down, check for bubbles.

Also, see this TSB. https://www.wranglerforum.com/threads/service-bulletin-for-multi-cyl-misfire.109037/post-1501443
Click to expand...
This was a very interesting read. I definitely drive it on the conservative side so expected carbon build up may be possible. But the dealer did a cleaning and the issue appears to still be happening. I might give it a couple of hard revs on occasion and see if it helps. I’ve never had it above 4500 rpm that the TSB calls for and maybe I need to.
 
I wonder if anyone has had any luck with installing a troublesome aftermarket CPS either a little closer or a little farther out than the sacrificial spacer on the nose provides for.
 
primetime4 said:
I have the 3 speed auto and have tried ordering this part from the various Mopar sites with no luck. Also called the dealer asking if they could find one and everyone indicates it has been discontinued. What’s really bad is these don’t even come up on eBay but if anyone has a lead on one I would love to buy it.
Click to expand...
I just spoke with the dealer near me and they say the mopar version crosses over to the Spectra Premium S10087. I just bought this one on Amazon and will be the guinea pig to try it.
 
The problem with going to the dealer is these Jeeps are now minimum 16 years old. Most dealerships have younger techs straight out of school that have probably never even seen a 4.0L, nevermind work on one. The dealer training programs usually focus on vehicles that are currently on the lot or still within warranty. Your best bet would be finding someone who knows older vehicles/Jeeps well.

I've replaced a few cracked 4.0L heads, and none of them had a misfire or even compression loss. The tell-tale sign was a leakdown test, testing the coolant for exhaust gases, or engine oil for coolant contamination. If the problem went on long enough you would have low hot idle oil pressure (dropping to 0 psi) which at that point means the cam bearings are scorched in it.
 
  • Like
Reactions: freedom_in_4low
Tj Jay said:
I just spoke with the dealer near me and they say the mopar version crosses over to the Spectra Premium S10087. I just bought this one on Amazon and will be the guinea pig to try it.
Click to expand...
I had the dealer swap my NTK sensor out for the NAPA part and they reported that the live data was identical between the two but I am definitely interested to hear how the Spectra works out.

Other than both of us having 2001 autos, I see that you are also on 33s. Did you regear for 33s? The only other change to my Jeep right before the misfires was a regear to 4.10 when I moved up to 33s. I started doing some research on speed sensor but not sure if that even applies to my Jeep and if it could be a factor. But that is the only thing to change other than the CPS. Someone mentioned the tone wheel impacting the CPS so maybe the gear change caused this relationship to change enough that the computer doesn't like it. Grasping here but there seems to be a trend here with 2001 - 2002 models possibly due to age but maybe something else.

Regarding oil and coolant issues. My Jeep has no issues with mixing or consumption. It's only above 31100 - 3200 rpm when I get the CEL.
 
primetime4 said:
I had the dealer swap my NTK sensor out for the NAPA part and they reported that the live data was identical between the two but I am definitely interested to hear how the Spectra works out.

Other than both of us having 2001 autos, I see that you are also on 33s. Did you regear for 33s? The only other change to my Jeep right before the misfires was a regear to 4.10 when I moved up to 33s. I started doing some research on speed sensor but not sure if that even applies to my Jeep and if it could be a factor. But that is the only thing to change other than the CPS. Someone mentioned the tone wheel impacting the CPS so maybe the gear change caused this relationship to change enough that the computer doesn't like it. Grasping here but there seems to be a trend here with 2001 - 2002 models possibly due to age but maybe something else.

Regarding oil and coolant issues. My Jeep has no issues with mixing or consumption. It's only above 31100 - 3200 rpm when I get the CEL.
Click to expand...
My bet is the cylinder head is fine, and you should find a shop that won't just load up the parts cannon.
 
  • Like
Reactions: TheBoogieman
primetime4 said:
I had the dealer swap my NTK sensor out for the NAPA part and they reported that the live data was identical between the two but I am definitely interested to hear how the Spectra works out.

Other than both of us having 2001 autos, I see that you are also on 33s. Did you regear for 33s? The only other change to my Jeep right before the misfires was a regear to 4.10 when I moved up to 33s. I started doing some research on speed sensor but not sure if that even applies to my Jeep and if it could be a factor. But that is the only thing to change other than the CPS. Someone mentioned the tone wheel impacting the CPS so maybe the gear change caused this relationship to change enough that the computer doesn't like it. Grasping here but there seems to be a trend here with 2001 - 2002 models possibly due to age but maybe something else.

Regarding oil and coolant issues. My Jeep has no issues with mixing or consumption. It's only above 31100 - 3200 rpm when I get the CEL.
Click to expand...
I'll report back once I install the new CPS sensor and test drive. Yes, I had to regear due to having 33s. I probably drove 6 mos with 33s and 3.07 gears before regearing with no issues, but I probably never got over 3k rpm. I also added the correct speedo gears if that's what you are referring to by speed sensor.
 
If your head is cracked, you'll usually notice coolant loss. Another way is to turn off the hot engine, wait a few minutes, then peer down through the oil filler cap and see if you can see coolant bubbling up. It's usually pretty obvious.
 
Top Bottom
Back