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Discussion Starter #1
Hey guys,

I've been fighting a misfire since last year (I know I'm lazy).

I ruled out all the plug/wire/distributor/injectors, good fuel pressure. Does not burn oil.

Engine has 192k (previous owner said rebuilt once but had no proof/receipts)

Compression test results:

1- 120-125
2- 150
3- 145
4- 145
5- 150
6- 145

Leak down test results:
1- 21%
2- 19%
3 - 28%
4- 15%
5- 13%
6- 14%

1-4 slight hiss noise from valve cover and exhaust. 5-6 silent.

So, do I need to rebuild the head? That is what I'm thinking based on the results.

Just want someone to confirm my suspicion.

Thanks

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Premium Member
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2,289 Posts
With those numbers and 192k, your looking at a total rebuild, top and bottom end. Anything greater than 7% is bad news.
 

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Possibly, were you able to hear where the air was escaping to? Usually you can hear it in the exhaust pipe, carb/throttle body, oil filler, PCV hole, or maybe all the above.
 

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Might be worth putting a rebuilt head and do another test. Won't be to much trouble or or extra cost.
 

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Commercial Member
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Discussion Starter #7
Well, no going back now;)

Local shop gets high praise so gave them a call, seemed knowledgeable, $200 +parts for a full rebuild.

Will see how it goes.

Nothing looking bad so far, push rods all straight and rockers look ok.


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Discussion Starter #8
It's out. Rigged up a chain hoist to the garage beam and lifted the intake and exhaust manifold along with the head all at once. Worked out well.

Nothing glaring so far, all gaskets look ok. Cylinder walls look good (no scrapes or anything). I think the exhaust manifold is cracked so will grab a new one. Will see if the machine shop can test the springs (just my own curiosity).


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$200 for a Cylinder Head rebuild? That's the deal of the century. I recently put a TUPY head on a 00' Cherokee. Cost me $450 for Cleaning, Magnaflux, Valve Job, Resurfacing & Assembly. The new parts the machine shop installed were just springs & seals. They cleaned & machined the original valves. What does that $200 job entail? You're not trying to reuse valve springs are you?
 

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Discussion Starter #12 (Edited)
Success! If anyone needs machine shop work near Spokane, definitely go to Cylinder Head Service on the north side. Great guys and fair price.

Cleaned, tested for cracks, replaced valve seats and guides, valves, springs, pressure tested, milled and painted for $322 out the door.


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120-150 is normal to good compression for 4.0l. I figure after the heads are rebuild you should have 150 for all 6 cylinder unless rings are worn. Good luck on putting here back together.
Do you any photos of the cylinder walls? Just wondering.

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Discussion Starter #14
120-150 is normal to good compression for 4.0l. I figure after the heads are rebuild you should have 150 for all 6 cylinder unless rings are worn. Good luck on putting here back together.
Do you any photos of the cylinder walls? Just wondering.

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I had a misfire I couldn't get rid of since last year, so I finally did the leak down test which pushed me to get further in.

Burns no oil, cylinder walls all look good, no unusual marks etc.

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Discussion Starter #15
Starting reassembly, got the head, exhaust, and intake all port matched (why not it was pretty easy and should flow a bit better).
Head is on, new bolts, torqued down, and manifolds are on. (Getting the bottom bolts tight is going to be annoying!).

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manifolds are on. (Getting the bottom bolts tight is going to be annoying!)
Yup. Pretty straight forward job. That's why that motor is such a legend. Next time, maybe invest some manifold studs. They make locating and fastening the manifolds a little less annoying. I've been using allen drive set screws with prevailing torque lock nuts and hardened wide diameter washers; like these...



or M10 Washers on the 3/8 studs work fine as well
 

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2003 Rubicon, manual trans, Atlas transfer case, Nth Degree Mobility kit and more...
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120-150 is normal to good compression for 4.0l. I figure after the heads are rebuild you should have 150 for all 6 cylinder unless rings are worn. Good luck on putting here back together.
Do you any photos of the cylinder walls? Just wondering.
Just did my 03 cold.

1 185
2 190
3 188
4 190
5 194
6 188

Cold engine because I am looking for a misfire when cold. Normally this should be done engine at operating temp.

Al in Tucson
 

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Just did my 03 cold.

1 185
2 190
3 188
4 190
5 194
6 188

Cold engine because I am looking for a misfire when cold. Normally this should be done engine at operating temp.
Was that test done dry or wet? That looks almost too good to be true. If accurate, you now know to look elsewhere for your misfire concerns. Typically the compression of a 4.0L engine with over 100k Mi will look like this:
Cyl 1-135
Cyl 2-155
Cyl 3-155
Cyl 4-155
Cyl 5-155
Cyl 6-135
 

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Discussion Starter #20
Woohoo, it's alive!

Took a bit (air in the fuel rail, purged with the valve and let out some bubbles), then had to hold pedal to the floor. Eventually it roared to life.

Idling perfect! (For as long as it's misfired and check engine light, it had a rough idle and a bad sound near the manifold).

Pretty sure my old exhaust manifold was also cracked.

I just have to finish flushing the cooling system, and then give it an oil change. Go grab fresh gas and see how the milage is.

So happy!


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