Tested compression on my 300SD

[ykobayashi]

I’ve been having some rough idle on my 82 300SD. Over the last few months I’ve really been enjoying my 85 300D and I’ve noticed it doesn’t shake a bit at idle. It is so smooth, I noticed the engine shock on he driver’s side has broken out of its rubber isolator and it really doesn’t need the shock. It’s that smooth. Very different from my 300SD that shakes at idle and stammers to a start in the morning.

So that put a spotlight on the SD. The rough running is a rocking like rack damper pin rocking. I changed the bolt back in 2010 and I started by buying another this month thinking the old one was worn out. No luck. Still rocks at idle. I changed filters. Changed o-ring in filter. New cigar hose. Adjusted valves twice (actually once but checked it after a few hundred miles). I put in a new set of monark injectors that I pop tested at 1950 psi. This actually made the shake worse. My old injectors were squirting, leaking and popping at low pressures.

New engine mounts. New engine shocks. Checked for air leaks. Sprayed the goo out of turbo. Adjusted idle. Rebuilt the lift pump. Twiddled with the rack damper pin dozens of times. I got little improvements on the way but the engine still rocks. Especially when hot.

Oddly the car is smooth over 1000 rpm and after all my work this month it makes amazing power coming up the 10% grade to my home. I mean it has really improved and it makes good smooth power. No smoke. But it rocks at idle. Almost makes me seasick at red lights. Hit the pedal and at 1000rpm it mellows out.

I have not adjusted timing ever on this car. The reason is when I got it in 2010 I tried and rounded off the bolt on the back of the pump. Since it ran well I never touched it. At some point I’d planned to take off the oil filter and properly extract the bolt. I even have the filter and pump gaskets sitting on the shelf...for a few years. Just never felt I needed it. Till now.

So that is the background. I started getting a little flustered and realized I may have a tired pump. As I read more posts here I realized that maybe one of my cylinders may be weak too and that no tweaking would bring back compression. Before I attacked the pump I thought I’d check compression.

So I decided to bite the bullet and get out my compression tester. I bought the cheap one at Harbor Freight in 2010 and I never was too happy with the way it would leak off pressure if I wiggled the quick connect. I’m still on the fence whether I should trust this thing. I took multiple measurements to get to these numbers. Some of the low pistons started out at 150 psi and came up to 200s after cracking a lot. Not sure if this is the gauge or car but I think it’s the car.

Anyway here are the results. Warm engine.
#1 390 psi
#2 390 psi
#3 280 psi
#4 220 psi
#5 230 psi

I don’t think I’m ever going to get a glassy smooth idle like my 300d with these numbers. I’m really bummed out. I guess the next step is so a leakdown and try to see where I’m losing pressure. And tackle the injection timing.

My teenage son shook some sense into me and said “is it really broken? It drives great.” He has a point and maybe I’ll just let him drive it.

Opinions? Start looking for a new engine or car? Keep driving and forget about it? I have really put a lot of work into this car and it is really nice save the idle.