'91 300CE compression/leakdown test results - help needed! - PeachParts Mercedes-Benz Forum

rgnprof · #1 06-19-2013, 05:57 PM

Just checked the compression on my '91 300CE with 80K miles - a little history. We overheated the car last year when the radiator neck broke and spewed coolant all over the place - didn't run it long after that but it seems like I have had numerous running problems since. I replaced the radiator, new water pump, belt and tensioner, etc. Car does not run hot now and doesn't smoke, but I've had a variety of starting issues and it won't run in closed loop.

Results were from a stone cold engine - car hasn't run for a while as I have pulled the fuel distributor etc.

Cylinder 1 - 160-170 psi
Cylinder 2 - 160-170 psi
Cylinder 3 - 125-135 psi (adding oil brought it up to 160 psi and after numerous test #3 steadied around 145-150 psi)
Cylinder 4 - 160-170 psi
Cylinder 5 - 150-160 psi
Cylinder 6 - 160-170 psi

Was going to just run a leakdown test on Cylinder 3 but I did it on all and results were "low" (20-30%) on cylinders 1,2,4,5, and 6 and "moderate" (45% - 60%) on #3 - adding some oil reduced the % to the upper 30% range. This was done using a rather cheap Milton tester and I couldn't really hear air escaping anywhere (coolant cap and oil filler cap removed, listening at adjacent cylinders, exhaust pipe...)

Clearly, #3 is low but should I be worried? All 6 spark plugs look great (I thought the car was running rich, but you wouldn't know it looking at the plugs).

Any advice or suggestions?

Thanks,
Ryan

rgnprof · #2 06-20-2013, 09:47 AM

Bump!

Anyone offer some advice here? Thanks!

ryan

mbonly · #3 06-24-2013, 12:33 AM

If that cylinder is low you should feel it when the car accelerates. If thats the case you probably smell unburned fuel out the tail pipe

Hirnbeiss · #4 06-24-2013, 07:40 AM

Worry that a 23 year old car might poop itself? Nah. Worry about how to replace the rings in one cylinder and redo the head while you're at it? Sure.

pmckechnie · #5 06-24-2013, 08:27 AM

That cyl is a little low, but not enough to cause starting problems or failure to go into closed loop mode. I've seen 300Es with NO compression on one cyl and they start and run. I would think something else is causing a starting problem and closed loop problem.
Could you give some more info on what is happening when you try to start it? Does it take a long time to start? Does it start but run very rough? After it starts, does it run fairly smooth? If warm, will it restart easier than when cold?

Paul

rgnprof · #6 06-24-2013, 09:05 AM

I agree Paul, I just got the car all back together and it is running much better actually...and starting fine. Idle still a little rough and I initially had a fast idle problem but I traced that to a broken throttle cable (actually, a plastic piece on the cable was broken which was preventing the cable from returning all the way to the stop/idle position causing the throttle microswitch to stay open at idle).

I have numerous posts on my duty cycle and open loop problems - just search by my user name. In doing this most recent work, I pulled all the plugs, cap and rotor to check out the ignition system - all of which are relatively new parts. All of these look fine... I pulled the fuel distributor and replaced the mixture tower, pulled the throttle valve switch and tested it. A slight adjustment to the throttle valve switch got me the factory manual readings (in ohms) and now the car is running in closed loop. I am really excited! I think I started messing with the mixture WAY too quickly in the process and got the car so far 'out of whack', that the computer couldn't compensate enough and get the car in closed loop. It's also possible that I messed up the mixture screw in the old tower. I THINK my previous starting problems - most noticeable over the cold, winter months - might have been due to the mixture being so far off. Right now the car is starting right off w/o any problems....it does have a slight hesitation off idle and I'm still - very carefully - tweaking the mixture.

I'm wondering if this broken throttle cable was part, or all of my problem all along? I have no idea how long this thing was busted...

My problem now is in trying to figure out which duty cylce numbers mean rich or lean...I understand that Mercedes uses the term 'duty cycle' opposite of the industry standard, but I'm still having trouble figuring out that if my car is in closed loop between, say 40%-46% - is that Rich or Lean??

Thanks.

Ryan

pmckechnie · #7 06-24-2013, 01:02 PM

Ryan, I can't answer your question as to rich/lean but can explain to some extent what the 40% to 46% means. The ECU reads the O2 sensor and determines if the mixture is rich or lean and changes the duty cycle to adjust the mixture. In theory, 50% +/- 10% is ideal. If you adjust the mixture screw to a rich or lean direction, the ECU will change the duty cycle to get back to where it was, mixture wise. In other words, if every thing is working as designed 40% to 46% is neither rich or lean, it is correct mixture. Did that make any sense? In other words, you could turn the mixture screw a little one way and see what the duty cycle does. If it goes the wrong way, turn the mixture screw the other way. I do know you turn the screw CW to richen the mixture and CCW to lean the mixture. I'm sorry, I don't know which way to turn it to get the duty cycle where you want it but that is easy to figure out once you start adjusting.
I'm sure others can tell you which way to turn the screw to get the duty cycle where you want it.
Paul

rgnprof · #8 06-24-2013, 02:06 PM

My confusion comes from the different readings I get depending on if the red lead goes in hole 2 or hole 3...

I've posted about this before - according to Landiss - BOSCH KE3-JETRONIC MIXTURE ADJUSTMENT - black probe from meter goes in #2 (ground) and red probe in #3 - static test on my '91 300CE indicates a mixture of 70% with KOEO...which is what Landiss claims it should be. On my '86 560SL - it's the other way around - I have to put the red probe in #2 to get a static reading of 70%. A little confusing...
I know that turning the mixture CW riches and CCw leans - and this appears to be happening when I turn the mixture screw on my 300CE.

I guess my question is - if I'm now running the 300CE at 40%-46% - am I running a little lean, or a little rich?

And, as you have probably figured out, if you switch the leads you get get the 'opposite' readings...does that make sense? It would read 30% static reading with KOEO...

pmckechnie · #9 06-25-2013, 10:27 AM

Ryan, I'm sorry but I will have to back away from your question. I understand what you are asking but I don't have a 300E or a 560SL, and my 500SEL doesn't have Lambda control (it's a EURO engine in a US car). The only thing I can suggest is to adjust the lambda to 50% +/- 10%. At 50% the meter can be hooked either way and you will still get 50%.

Paul