Compression

1968 - 1976: 220, 220D, 230, 230.4, 230.6, 240D, 250, 250C, 250CE, 280, 280E, 280CE, 300D
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Raijin
C Class
Posts: 31
Joined: Wed 24 May, 2017 8:24 am
Model you own: w114
Location: Tweed heads NSW/ Gold coast QLD

Compression

Post by Raijin » Sat 29 Jul, 2017 7:22 pm

I have done my test and have 80 psi on 1 and 6, 95 on all the rest.
I have only read that generally they should be 100 but I have found nothing specific for the 68 250.
Even my Haynes book does not have figures on it.

CraigB
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Re: Compression

Post by CraigB » Sat 29 Jul, 2017 9:05 pm

Generally I would have thought higher, but the main thing, using the same gauge, is the difference between cylinders. It could be rings or valves. A bit of oil down the bore will settle around the rings and give a temporary seal. If the compression stays the same it is the valves and if it goes up, the rings. Another way is leak down test, but that's another story.

And make sure your tester is sealing properly. That does sound low on all to me (160/170 on my gauge?) but i am not an expert.
Craig Baulderstone
Lurch
280s's
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Raijin
C Class
Posts: 31
Joined: Wed 24 May, 2017 8:24 am
Model you own: w114
Location: Tweed heads NSW/ Gold coast QLD

Re: Compression

Post by Raijin » Sat 29 Jul, 2017 10:04 pm

Thanks Craig I will give the oil trick a go. Could be a head gasket I have a lot of old oil I have been cleaning off the engine. Damn I don't want to rebuild this engine.

CraigB
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Re: Compression

Post by CraigB » Sat 29 Jul, 2017 10:20 pm

that's a fairly minor drop i would suggest. In the old days i remember having a car low on compression and put a product called redex directly in the bore and then ran it in the fuel. Apparently it was supposed to loosen gummed up rings. Whatever it did it did pick up my compression. Have you checked valve clearances?

What engine is it? A lot easier to do head/valves on SOHC than M110 Dohc.
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

Raijin
C Class
Posts: 31
Joined: Wed 24 May, 2017 8:24 am
Model you own: w114
Location: Tweed heads NSW/ Gold coast QLD

Re: Compression

Post by Raijin » Sun 30 Jul, 2017 12:20 pm

Not sure if it's single or double my guess would be single as it's the standard 250 twin carb I assume they would have put the dohc on the S class cars. But you are right again could be as simple as valve clearance not having any idea what ppl have done to the car in the past.

CraigB
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Re: Compression

Post by CraigB » Sun 30 Jul, 2017 4:13 pm

Yep i missed it was a 250 - definitely SOHC. I'm no expert but even if it is 15 down I think you will still get it to run alright. Also has the car been sitting around? Give it an oil change and i nice country run and check again. Maybe a bit of corrosion under valve seats that have been sitting open and rings gummed up a bit - nice good high speed run could settle all that down and then test again. Reminds me of a 'to remain nameless' exotic car expert who would chuck the keys to a young mate of mine (a mechanic there) and get him to take it for a thrash before he would attempt to tune it. Was always jealous of his job!
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

Raijin
C Class
Posts: 31
Joined: Wed 24 May, 2017 8:24 am
Model you own: w114
Location: Tweed heads NSW/ Gold coast QLD

Re: Compression

Post by Raijin » Sun 30 Jul, 2017 4:58 pm

LOL I have re done the test I warmed the car up first had it running for about 5 to 10 min.
The figures I got were
1: 95 and 95 with oil
2: 100 and 105 oil
3: 100 and 105 oil
4: 105 and 110 oil
5: 100 and 105 oil
6: 95 and 107 oil
So I got much better readings with the car warmed up a bit it has been sitting around for I have no idea how long.
I cant take it for a drive as yet as it does not have breaks and its not registered.
Giving the engine a general check was step one getting the brakes going is step 2.
going to get a new master cylinder as the one on the car must be original its so rusted and I can see any of the levels of the fluid.
hopefully a new Master cylinder and some pads and a good bleed hopefully the brakes will be going.

But I think you could be right it probably just needs the cobwebs cleared.

CraigB
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Re: Compression

Post by CraigB » Sun 30 Jul, 2017 9:14 pm

Yes, given that i wouldn't worry at the motor at this stage.

Just some recommendations on the brakes. I hate bleeding brakes and having hassles and rebleeding brakes etc etc. I've been on this forum for many years and so many times people are chasing problems, replacing stuff bit by bit and so many times it is the flexible brake lines. There are just 4 and not expensive. Often they collapse inside, trap air or act like a valve and jam a wheel on. They are supposed to be replaced every 10 years. If the car has been sitting there is every chance at least one of your caliper pistons is seized or slow. Have a look online how much the rubber repair kits are and it is a very cheap fix - just a bit of time but not a hard job. If you are getting a new master and replace the flex lines and rubbers in calipers and give them a clean up, there really isn't anything in the system to go wrong and you will have many years of trouble free brakes.
Craig Baulderstone
Lurch
280s's
280SE3.5
280SL
350SL
500SEC's
560SEL's

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Mercmad
Mercedes Demigod
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Re: Compression

Post by Mercmad » Mon 11 Sep, 2017 1:26 pm

It's a bit low on compression but that will come right with constant running and clean oil . Normally around 120 PSi is OK for these engines . As long as it doesn't knock or smoke and there are no major bubbles coming out of the coolant ,it's fine.

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