what is the general consensus on cylinder pressures?


swynx

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confirmed spark and fuel. did compression test with fuel relay unplugged, all spark plugs removed, and throttle all the way open. reason for the compression test is a no start condition.

i believe i remember that a low cylinder will cause a no start.

i got on the driver side

105/120/140/55 (80 wet)

passenger was

165/110/90(only when wet)/100

this one passenger cylinder that read 90, would bump up to 30 psi, and fall to zero.

obviously it needs a ring job.

i know that ive read you dont want a variance of more than 10%, but could not remember about a no start with compression results. i thought tho that anything under 90 and youll have issues.
 


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RonD

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Compression ratio will tell you expected compression test results

General rule of thumb is Compression ratio X 18

So 9.0:1 ratio would be 9 x 18 = 162psi
9.3 x 18 = 167psi

So what engine are you testing?

My 400k 4.0l OHV(9.0:1) runs 150-160 in all cylinders, 155 average

Compression heats up the gasoline to vaporize it enough so it can be ignited by a spark
Liquid gasoline won't ignite by spark, yes the "movies guys" take liberties, lol.

Vaporization much reach 30% for start up, and this is very dependent on outside temp, temp of the gasoline
120psi might be too low compression on a cold day, 90psi might be enough on a hot day
Ether(starting fluid) has a low vapor point which is why its used as a starting fluid for gas and diesel engine
Try Ether

If not firing with Ether then its a spark issue as well as compression, which is VERY low and all over the map, 165 to 55psi, thats not rings it valves
 

swynx

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its a chrysler 4.7 v8. stupid thing dropped a valve seat at 147k. i was in a hurry to fix it before i left for work so i neglected to check all the cylinders before i tore it apart, then when i got back i was in even more of a hurry, because its the wifes winter rig and winter is here. so i just checked the one cylinder i figured had dropped a valve seat. tore the head off put a new one on, now it wont start. ran fine before the valve seat dropped.

i quadruple checked the timing was right before i buttoned it back up. the passenger side head is the one that i replaced with a new unit. now it just wont fire up.
 

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Must be an epidemic on those things dropping valve seats.

Second one on here this week...
 

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4.7l spec is 9.5:1 CR, but also saw 9.8:1, so expected is 170psi at sea level to 2,000ft

Only one cylinder is spec, 165psi, it should fire if spark occurs at the right time, this also means cam timing, at least on that bank is OK
But the others are leaking badly

You can do WET test on all and if they just go up a bit then rings are fine, heads will be the issue, if one goes up above 140 then its got bad rings

Also look at the PCV Valve and hose, if its full of oil then rings are on there way out as well
Oil vapor inside the crank case/valve covers is caused by Blow-by, the hot gases that "blow-by" the rings when a cylinder fires vaporizes the oil coating on cylinder walls and piston sides.
PCV system is there to suck this oil vapor out, and burn it in the engine
Normally its not all that much, cheap oil will vaporize more but still shouldn't be too bad
As rings wear out blow-by increases and so does the oil vapor it causes
 
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rusty ol ranger

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Are you *sure* you got the timing 100% correct?

Having that many cylinders that low on different banks really reads toward timing. The chances of it having that many valves screwed at once are pretty low.

Unless they got nailed by a piston.
 

swynx

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Are you *sure* you got the timing 100% correct?

Having that many cylinders that low on different banks really reads toward timing. The chances of it having that many valves screwed at once are pretty low.

Unless they got nailed by a piston.
all 4 timing marks were lined up when i put it together. spun it over a few times, and the crank and idler lined up after like 4 spins. other than that they say that both cam gears "v8" symbol should point to 12 at the same time, and they did. there isnt much room in there for the timing to be off. they almost always smash the valve seat against the piston with the valve.

Must be an epidemic on those things dropping valve seats.

Second one on here this week...
its super common, just takes over heating them once. which she overheated this one. thats when i found out this thing had been abused. instead of replacing the o ring on the thermostat they just put gasket maker. then when i tore it apart i found more gasket maker on outside surface of the oil pan/timing cover.


i guess ill pull off the valve covers and see what i can confirm for timing. maybe try starting fluid. possibly the jeep forums. for what its worth, the 4.7 sucks ass to work on. after i had pulled the whole thing apart, it definitely would have been easier to pull the engine out. there is a lot of dumb designs where you cant get 2 hands.
 

swynx

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got the valve covers off and first thing i noticed was that when comparing the new/old sprocket for the camshaft position sensors. they are different. none of the holes line up, and when i line up the new/old sprockets together, the mark i made on the oem sprocket doesnt line up with the mark i wouldve made on the oem cam. its about 90* off. i always mark my old stuff in random spots for a reference point

i knew that there was a jtec kit and a ngc kit. checked my order and i ordered a jtec kit.

im hoping that i can just swap my oem target wheel/sprocket on, and that everything else is the same.
 


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