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Originally Posted by lilblkbnz
Does anyone know what an acceptable range is on an 85 380se. I've searched but can't find any hard numbers. I'm trying to find why It's fouling plug #2. That cylinder shows 145 psi cold. The others are above 150. The car has 205k miles. Acceptable?? Car starts easily hot or cold and runs good anywhere above idle. It has new plugs, wires, cap, rotor, injector seals and fuel filter. But still has rough idle. I've tried swapping an injector from another cylinder but the problem remains. Any ideas appreciated.
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Compression pressure is a quick way to check.
It tells you nothing about how the compression is achieved.
You need a leak down tester to quantify the pressure or lack there of.
A L/D will tell you if piston rings or valves or gaskets are leaking and how much.
The motors I test need to have below15 % leakage.
More than this means unless you repair the 'leak',
your wasting time patching or hoping it will get better.
My rebuilt blocks are often1% or less ....I test without the head installed.
Then after the head is on, I test again to see if the whole motor is okay.
Total leakage I'm looking for is 2 to 5 % maximum.
Used motors shipped here to install are checked and the owner told about the results..
I will not install a used motor with more than 20% leak down after I have seated the valves and wet tested.
This removes my time from hunting a misfire when it's running due to poor compression or wondering if the 'nailing' is due to wear.
Most 350 and 450's I have tested for compressions have been around 160 to 170 psi.
Full charged battery and throttle propped open, and usually allow 8 cycles to get a reading.
this is at 1100 feet above sea level...Your location may be different.
On diesels like the 240 and 300d's they will be 360 to 430 psi.
Anything below 350 psi is cause for concern.
That is, after cam timing and valve adjustments have been set.
The US cammed gasoline models usually show a little less, about a 30 psi reduction in compressions
although this is not always.
So....your pressures are about right as long as the conditions where the same as I use..
Your plug fouling is most likely a displaced or bad intake valve stem seal.
PS... You know Ironzy ? ATC Guy ?
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