Date registered: Aug 2009
Vehicle: '72 250C, '74 280C, '85 500SEL, '81 300CD.
Location: Tucson, Az.
Mentioned: 2 Post(s)
Quoted: 49 Post(s)
The general rule of thumb is the lowest compression should be within 80% of the highest reading. Your readings are right there on the low end. 130 psi X .8 = 104 psi. I always toss the low and high readings out as a control so, 120 X .8 = 96 so you're still on the limit.
What concerns me more is the fact of having adjacent low compression readings such as you have between #4 & #5. Ordinarily I would suspect a blown head gasket but in your case I'd bet it's "normal" wear.
Your engine is tired but not worn out. Baby it until you can get the noise taken care of and you should be fine. Go easy on the throttle and try to keep the top speed down.
Last edited by Mike D; 08-22-2013 at 12:08 PM.