P0304 P0300 Leak-down and compression test results

1999 Jeep Cherokee sport 4.0 AW4

So I have been getting p0304 and p0300 CELs every once in a while. It seems to go along with rough idle at times. When its under load it feels fine. I'm riding on 37" tires so its pretty slow to begin with, never noticed a significant decrease in power. I decided that after doing some research on the forums I would try a leak down test and compression test. Here are the results:

LD.............C-D.C-W
#1 - 5%....148 160
#2 - 5%... 159 170
#3 - 29%..145 156
#4 - 41%..135 151
#5 - 7%....165 170
#6 - 8%....151 170

So as we can see from the leak-down, there are problems in cylinder #3 and #4. When doing the leak down, both cylinders had leaking out of the exhaust pipe. All cylinders showed minimal leaking out of the crankcase, it was barely noticeable though. I was planning on just buying a reman head from www.headsonly.com . Comes with valves and springs preinstalled for $345 shipped. I thought that until I did the wet compression test on cylinder 3 and 4. By the way, the wet test was done warm and with a capful of oil. Now I'm not sure what to think.

What would you guys do with this engine?

your compression is in great shape man. I am at 120 at #1 and 150 at #6 and it ranges from 120-150 so you should be in good shape. Consider doing the head and call it done.

Contrary to what blondejon indicates, my assessment is just the opposite.

A wet test increase shows you that the rings are leaking by.

In my experience, putting a tight head on an engine with weak rings is going to accelerate the ring problem and it's likely that it will start using a good amount of oil as well.

Putting a head on that engine is a waste of money.

I have to wonder though if the valves where fixed on #3and 4 the numbers would look more like the rest of the cylinders. 140-150 dry and 160-170 wet.

You don't say how many miles you have. Being a cheapskate, I'd think about pulling the head and reseating the valves with valve grinding compound.

135k. I looked for a casting number and found 0630.

Are these also known to crack in between #3 and #4? or is it just the early 0331 heads?

I don't burn oil currently. No white or blue smoke. There's no signs of coolant in the oil. There were no bubbles in the radiator when I ran the leak-down test. There was air leaking out of the exhaust on #3 and #4.

I guess there is no way to be sure until I pull the head.

If you have a buddy with a borescope you could always check the exhaust valves for visible burning before pulling the head...

0630 heads are good; take it to a local shop to have the valves re done. Usually way cheaper than buying a reman.

the reading on #4 is what raises the red flag. 41% leak down, with air out the exhaust, yet a big jump on the wet test.

Wet/Dry readings should not change that significantly if the problem is (only) a valve.

I really don't trust the cap full of oil method when doing a wet test. The object is to seal the rings to the cylinder walls or in other words use the oil to fill the gaps. I used to use an old school, hand oil pump gun, with the piston at the bottom and try to squirt the the cylinder walls so the oil flows down onto the rings.

You don't want to overdo it with the oil. Oil doesn't compress, something is going to break if you overdo it.

Now I use a spray can of Permatex super slick and a nozzle extension. And spray, using kind of a twisting motion, so I get a spray pattern most of the way around the cylinder wall.

In the past, I have found that when adjoining cylinders exhibit compression loss, the head basket is suspect.

Tossing in my two cents on compression tests...

Oiling the cylinder will promote ring sealing, not the valves. Usually, if the compression dramatically rises when wet, then it is a ring issue, if it does not, then the valves are suspect.

OK then, why do I highlight the word dramatic? The compression will always "improve" with a wet cylinder. The question to be asked is how much did it improve?

There is another test I like. Using a modified spark plug, I introduce shop air into the cylinder and then check both the exhaust and the TB for pressure leaks. Provided your hearing is still good, you will hear the leaks... Obviously, the cylinder being tested needs be on the ignition stroke so that the valves are, nominally at any rate, closed.

Yup, it is Old School but... It does work.

To modify the spark plug, blow out the ceramic and thread the body to accept an air fitting. Attach you hose to the fitting and apply pressure. Easy as pie.

O-Gauge Steamer said:

In the past, I have found that when adjoining cylinders exhibit compression loss, the head basket is suspect.

Tossing in my two cents on compression tests...

Oiling the cylinder will promote ring sealing, not the valves. Usually, if the compression dramatically rises when wet, then it is a ring issue, if it does not, then the valves are suspect.

OK then, why do I highlight the word dramatic? The compression will always "improve" with a wet cylinder. The question to be asked is how much did it improve?

There is another test I like. Using a modified spark plug, I introduce shop air into the cylinder and then check both the exhaust and the TB for pressure leaks. Provided your hearing is still good, you will hear the leaks... Obviously, the cylinder being tested needs be on the ignition stroke so that the valves are, nominally at any rate, closed.

Yup, it is Old School but... It does work.

To modify the spark plug, blow out the ceramic and thread the body to accept an air fitting. Attach you hose to the fitting and apply pressure. Easy as pie.

Click to expand...

Pretty much the same thing I do, to add a note, keep the air pressure down, below 15 PSI, or you will force the piston down and turn the motor over.

I actually cut a couple of small wooden wedges I force between the belt on both sides of the harmonic balancer. I used hard wood, they have lasted for many years. Not guaranteed to stop rotation, but it helps if you over pressure a little. Also it really doesn't have to be shop air, I often use a bicycle tire foot pump, which works well enough.

I hollowed out a spark plug, cut the rubber off of an old tire valve, removed the inside part of the valve (valve stem, just screws out) and silver soldered the valve in there. It's lasted around twenty years, so far.

If it is the valves and you do a lot of short trip driving, try a high RPM run up the interstate in second gear over 3000 RPM. The valves seldom rotate unless you get the motor up into the higher RPM ranges. A tiny piece of trash stuck on the seat can cause issues, one reason I'm against pouring cleaners down the TB or the vacuum booster vacuum opening.

I went to Dodge truck school (back in the 60's ), their philosophy was, fix or replace only those parts that are flawed. In other words only the bad valves or even rings. The reason was, the part you have in the engine is tried and proven, anything you swap out might not be as good as what you have, new isn't necessarily better. We would troubleshoot and try to identify exactly what was causing the issue, order the needed parts, before we ever tore into the motor.

I've used a little fuel and even colored chalk dust to test valve seating, If you take the intake off you can cover the back of the valve in fuel and see if it leaks through into the cylinder. Or cover the back of the valve in carpenters string chalk and put some air into the spark plug hole (top of the compression stroke), if a cloud of colored dust flies out you know you have an issue..

It may be burnt valves, the reason vacuum leaks are so important to fix. A lean running motor can overheat the metal and cause issues. A good reason to check intake/exhaust manifold bolt torque occasionally. 3 and 4 are right in the middle, a good spot for vacuum issues to show up if the intake/exhaust manifold bolts are loose.