Is 300 psi lousy compression? 85 300D - PeachParts Mercedes-Benz Forum

Carrameow · #1 04-12-2004, 08:38 AM

I took readings and got 360,340,300,340,300. I was told 370 is more the norm and a good engine but 300 was not a good reading...well you know I love rebuilding engines but there are so many cars around I just may buy the motor...or the car!!

TonyFromWestOz · #2 04-12-2004, 09:54 AM

Usually, a spread of +/- 10% (or is that 10% total?) is acceptable, so with a mean of around 330psi, +10% would be 363psi and - 10% would be 297psi.
If the spread is 10% total, then it could be better.
The more telling results are:
how the engine starts cold and
how much blow-by is happening.
If the engine starts and runs well and doesn't smoke out the oil filler too badly then it should be OK, regardless of the pressures.
There is a recent thread on compression testing, look it up.

240Joe · #3 04-12-2004, 07:29 PM

I can't help it....but did I predict this or what?

Forget about the compression tests...they mean nothing.

For the love of God man, please do what was suggested at least twice now!

Do you want to repair this car or play?

240Joe

compress ignite · #4 04-13-2004, 01:06 AM

What are the Factory's original Upper (and Lower) limits ?

Your decision is already made for you!

"Happy Motoring"

notlostmaybe · #5 04-13-2004, 01:48 AM

normal compression 24-30bar; 348-435psi. min. 20bar; 290psi

w123 cd manual

don

william rogers · #6 04-13-2004, 03:24 AM

I trust compression test about as much as I do campain promises.................

William Rogers.........

Carrameow · #7 04-14-2004, 11:40 AM

Dear Mr.240 Joe:
I found a 300D motor core in a yard. I usually go to a Mercedes pro for consultation because no one can equal day to day experience for 20 years and a second head is better than one.
My plan is to rebuild the second core, but to keep the car on the road until the second engine is finished.
Meanwhile I am starting to buy parts and amass resouces. I found five injectors on Ebay.
I really thank you for your belief in me and while I hope I dont have to go to a Shop for a while there are always two or three pros I rely on for advice.. you know what--even they disagree once in a while.

Again thanks for your advice...

Carrameow · #8 04-14-2004, 11:47 AM

I am starting to get my garage ready for a 300D rebuild. I am pricing it as:

300.00 Head Job
200.00 injectors
300.00 pistons/sleeves (oversize)
300.00 bearings
200.00 Seals, gaskets, etc.
0.00 Machine shop for redrilling
300.00 miscellaneous

$1400 Total

Unfortunately the used car market is flooded with 100K mile
Volvos, Toyotas, Corollas, Hondas and yes MERCEDES !!
QUITE A FEW PEOPLE HAVE TOLD ME THROWAWAY the
car, they may be right.

I am thinking the W123 cars were the last cars worth rebuilding....

samiam4 · #9 04-14-2004, 10:54 PM

You know $300 sounds like the price for *a* piston and *a* sleeve for 617.95X engine!

My time has a price... as much as I like W123's for ease of service, I cannot justify the money to rebuild a motor in a car worth $2k tops.

If it starts well cold and idles smooth hot- I'd just drive it! Have you checked the valve adjustment on the low cylinder? Did you do the compression test with the engine warm per the manual? If not, you'll get more variation and a lower compression reading.

Michael

kerry · #10 04-14-2004, 11:16 PM

Am I missing something here? Are you saying that you are going to rebuild the engine because of those compression ##'s? Why?

Carrameow · #11 04-15-2004, 10:44 AM

The engine lost oil because of clogged breather tube but i didnt notice it.
Because I was using ROTELLA oil, (heavy weight), I literally made it home 30 miles on maybe 1 -2 qts of oil.
By the time I got home oil pressure was 0 Bars!!
Engine didnt sieze but suspect bearings damaged.

My Mercedes POC suggested that 370 psi is more the norm for a strong engine.

Finally its summer and I am bored. My teenager is grown and engines are "my thing"

samiam4 · #12 04-15-2004, 12:55 PM

I think the recyclers use 345 psi as a min to sell a reliable motor.

As far as rebuilding... if you've scorn pistons/walls, damaged the crankshaft.. it would be a bunch cheaper to find a used engine. I knew a guy in colorado which got a factory rebuilt in a pickyour own place for $75!

I've done work to stay busy too... understand.

I think a 617 engine can run without damage on 2-3 quarts. Might want to change the oil a few times and have an oil analysis done. If you've got lots of copper... the bearings are past the babbit and might decide from there.

Michael

WANT '71 280SEL · #13 04-15-2004, 09:05 PM

psst. I've seen rebuilt Bosch injectors for $17 plus core. I think it was at bmaautoparts.com. I will double check to make sure.
Thanks
David

WANT '71 280SEL · #14 04-15-2004, 09:09 PM

okay, okay. I'm an idiot. Just looked and that was the CORE charge. They are like $53 rebuilt which is WAY more than fastlane. Sorry if I got your hopes up.
Thanks
David

lietuviai · #15 04-15-2004, 10:08 PM

I had compression readings on my 300D that were 380, 380, 380, 340, 295 and they were done about 7 years ago. The car still seems to run OK and idles fine. I think the 295 reading that I have might be from a leaking pre-chamber. I was working on the injectors not too long ago, some fuel had gotten down in the keyhole of the pre-chamber. While the car was running I noticed I had some slight bubbling coming from that keyhole. I have no idea whether that bubbling was caused from actual leakage from the cylinder's pre-chamber or it was from the fuel reaching the boiling point. Either way I don't feel like paying someone to do a leakdown test on it. If I was to do anything I'd rather just spend that money that would be spent on the leakdown test and put new seals in the prechambers.