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leak down vs compresion test

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SilentUnicorn

SilentUnicorn

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folks i am working on a ford v6, yea i know, but any way...engines has a miss at all speeds, lock of power ruff idle. traced the problem to #2 cyl. i have replaced all six injectors, compresion is good nice and even on both banks, around 175 or so, got my hands on a leakdown tester and did it up this evening, the numbers are all 6-8% on all but #2 which is 26% and i hear air leaking into the valve cover. Is this a no brainer or what? i cant seem to get my head around the compresion being so good and this cyl obviously being bad, am i to assume that there is a crack thru the head into the rocker area? all thoughts on this are welcome.

mark
 
It may be bad rings on that piston with air leaking past them into the crankcase then up through to the valve cover since there's no way out down below....

Another possibility might be a bad head gasket venting into those passages. But I'd also expect oil in the coolant or vice versa- if you have neither try testing and look for bubbles in the coolant.
 
Broken valve spring comes to mind, as does a burnt valve...

SteveL
 
I'm with James,
Hearing air in the valve cover is blowby from the rings. The air is leaking into the crankcase, part of which is the valve covers.
Do you have spark at that cyl. If the compression test was good, it should still be firing at least at higher speeds.
 
thats what gets me. i did forget to mention that the coolant overflow is full of oil, and i mean full of oil. the level is good but it is all oil. there is occasional white smoke but not much. the underside of the v/c was pretty milky with signs of water. if it was rings, or a head gasket, wouldn't the compression be somewhat lower? when i goose WOT i can get an intermenint backfire in the intake. what is happening to the fuel charge? and another thing is that the engine doesnt seem to want o get to temp very well it comes up and drops off, almost like it is cooling to much.

cap plug and wires are all new, and the fire is in the hole, i have visually checked that the plug is firing. i have even swapped plugs around, makes no difference. i checked the inj when i had the rail off and all are firing. compression is the same with plugs out or in.

theory: a crack in the head into the oil return passage including part of the water jacket.

theory:
headgasket blown between oil return and water jacket and cyl? would this allow compression to build but also quench the spark? (its kinda tuff to read these plugs but they are all somwhat black and sooty, but they havent really been driven on much)


more thought from my rambling mind...

mark
 
You're right on. 20% is maximum allowed on leak down, best I recall. You certainly have either a failed head gasket or cracked head, given all the other indications.
 
Has any one ever thought of a bent valve? That would do it in a leak down, I have seen this at many a drag race. Someone may have floated the valves and one got bent. Just a slight bend would do it for ya, and the motor would still operate.
 
The good feature of a leak down test is that it can locate problems & also eliminate problems. He hears leakage into the crankcase/valve cover. A bent valve would cause leakage into the intake or exhaust systems, not the crankcase. Especially with oil in the coolant, the cracked head or leaking headgasket are the most likely sources of the problem.
D
 
is this a 4.0L?? There is an oil passage that goes
through the head gasket to the head, to lube the rocker
shaft... or am I thinking about the 2.8L?? You may have
a blown head gasket between a coolant hole and the oil
passage, a crack in the block between the oil gallerey
and coolant side, or a cracked head, which the 2.9L and
the 4.0L are somewhat famous for...

SteveL
 
yea its a 2.9, i have been haunting the ranger forum a little and have learned that these motors can crack a head if ya look at them wrong. looks like i will be pulling the head in the near future, of course it couldn't be the ez side. its the passenger side and the vehicle has ac, which will make it a lil bit of a challenge. but i think it can be done in place.

mark
 
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