Author Topic: Compression test on European M116.985 in 1977 350SE  (Read 446 times)

Berggreen

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #15 on: 11 August 2020, 04:14 AM »
Plugs show that the engine is running way too lean, that is why it is overheating. Fix that first. Those compression figures are very good for a cold engine.

Great, I am happy to hear the compression readings are at least ok.. And do you agree to my procedure with 5 strokes of the needle? I was worried that I maybe had generated false high readings with 5 strokes, because if I did more, I did not see that the pressure level was increasing more.

Agree...the spark plugs, apart from the oily one, are very white. However, initially I was associating this to the fact that I have done less than 500 km with these plugs. Maybe they need some more km, before I can determine if the mixture is too lean. What do you think?

Sorry, I am a newbie with these M116 engines and K-jetronic injection systems. How do I regulate the full range mixture? Doing so, I guess I have to adjust the idle mixture afterwards to compensate. Or?

I have two 280SL w113s as well, and on these cars I know all the tricks to adjust the mixture on the old BOSCH injection pumps. But I am still a novice on the K-jettronics, but eager to learn! ;)

Thanks for your input and help - much appreciate it. :D

Christian

ptashek

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #16 on: 11 August 2020, 08:45 AM »
Agree...the spark plugs, apart from the oily one, are very white. However, initially I was associating this to the fact that I have done less than 500 km with these plugs. Maybe they need some more km, before I can determine if the mixture is too lean. What do you think?


The only way to check properly is hooking up an exhaust gas analyser, and checking for CO% while mixture is adjusted at the distributor.
CO should be ~1.5% to 2.5%
1993 "Pearl Blue" W124 280TE
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Randys01

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #17 on: 13 August 2020, 10:45 PM »
In summary we have a hi mileage engine returning to service after a long sleep.  It was blowing oil like crazy and now seems to be settling in as it's getting some use. Of course the valve stem seals are going to be dry and cranky: they might come good. The compressions are curiously impressive: espec for a cold engine . Industry practice is of course on a warm motor...be interesting to see the diff.  Theoretically the comp would be a bit higher when warm. One wonders how much a carbon build up is contributing to these impressive figures.

I don't feel the mixture is too lean looking at the new plugs.

What does intrigue me tho is no 5...the odd one. This appears to be a plug not firing.........I would expect a near regular miss on this cylinder . ..but this has not been reported. If this plug is only 500 kms old something weird is going on. It appears to me to be more of an electrical gremlin than a plug so fouled it can't fire. OR you have a leaking injector.


On the matter of mixture.......in K jetronic this is determined by fuel control pressure.  We're only interested in hot values at road speeds   ..3.4 to 3.8 bar.   BUT you need a test kit to determine this.!
Adjusting the fuel flap richness screw is only really relevant to idle/fast idle values and is really emission control centric.

As the story is evolving the cracked ring theory is losing ground ...
btw hot idle oil press at 750 rpm in the wet sump V8's on fresh-ish oil should be 1.5 bar minimum at 80/85. deg C.
I think on a hot day your engine was running a bit hot might be more to do with the cooling system's age than  a lean mixture scenario.

Sort out no 5...keep driving! See what develops. ::)




Berggreen

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #18 on: 16 August 2020, 06:22 AM »
In summary we have a hi mileage engine returning to service after a long sleep.  It was blowing oil like crazy and now seems to be settling in as it's getting some use. Of course the valve stem seals are going to be dry and cranky: they might come good. The compressions are curiously impressive: espec for a cold engine . Industry practice is of course on a warm motor...be interesting to see the diff.  Theoretically the comp would be a bit higher when warm. One wonders how much a carbon build up is contributing to these impressive figures.

I don't feel the mixture is too lean looking at the new plugs.

What does intrigue me tho is no 5...the odd one. This appears to be a plug not firing.........I would expect a near regular miss on this cylinder . ..but this has not been reported. If this plug is only 500 kms old something weird is going on. It appears to me to be more of an electrical gremlin than a plug so fouled it can't fire. OR you have a leaking injector.


On the matter of mixture.......in K jetronic this is determined by fuel control pressure.  We're only interested in hot values at road speeds   ..3.4 to 3.8 bar.   BUT you need a test kit to determine this.!
Adjusting the fuel flap richness screw is only really relevant to idle/fast idle values and is really emission control centric.

As the story is evolving the cracked ring theory is losing ground ...
btw hot idle oil press at 750 rpm in the wet sump V8's on fresh-ish oil should be 1.5 bar minimum at 80/85. deg C.
I think on a hot day your engine was running a bit hot might be more to do with the cooling system's age than  a lean mixture scenario.

Sort out no 5...keep driving! See what develops. ::)

Thanks Randy, you hit it spot on and thanks for all your good advice!! :)

I have arranged with my mechanic to do the following, as a start:

1) Change the valve stem seals - I have them and they did not cost more than 70-80 EUR for a full set.

2) Change all injectors in the same process. Also cheap parts - around 120 EUR for a full set.

3) Change the thermostat - maybe also the radiator as it looks old and worn. Which thermostat do you advice to use?

4) Change spark plugs to NGK BP6ES plugs. Mostly because the current plugs are BP6E plugs, and I can’t get a replacement 6E plug for nr. 5!  :(

5) Maybe change the engine oil to a 15W50 semi-synthetic instead of the 10W40 currently in the engine.

Then let’s see where that brings me!

What do you think?

Thanx,

Christian 😎

Ps: I have been offered a replacement M116.985 engine from a 350SLC, which has done only 160.000 km. I have agreed to buy it as a spare to either use as spare part donor or to install, if things go sour with the original high mileage engine. The replacement is furthermore from 1981, but still with K-jetronic, thus it is of the second generation 985s with 205 HP against the early 985s with only 195 HP currently installed in the car. Replacement even has the aut. gearbox attached to it as well as the K-jetronic system. So it seems like a good deal for 1000 EUR delivered from Germany to Denmark - where shipping alone is around 250-300 EUR. :)

Randys01

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #19 on: 18 August 2020, 01:46 AM »
Nice to have a spare complete engine on hand but of course we need to get a definitive position on the existing.
1. proceed with the valve stem seals.
2. try a new thermo
3. just get a new plug for no 5. Even another brand for the purposes of the exercise.
4.Replace all injectors
5. don't worry about different oil...so long as what's in there is reasonably new.

I suspect:
- the valve stem seals will curb a lot of the smoking problem
 -the thermo  will make little diff but is good insurance to swap in a new one.
-new injectors are probably well overdue and can be reused in a better motor if this one doesn't improve
 : but in conjunction with  a new plug in 5 should present as a clean plug in a few 100kms of running.

Make sure your electricals are all good for all 8 cylinders..No 5 odd appearance could be due to no spark  as much as leaking injector.
 

ptashek

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #20 on: 18 August 2020, 02:44 AM »
hot idle oil press at 750 rpm in the wet sump V8's on fresh-ish oil should be 1.5 bar minimum at 80/85. deg C.

The manual says anything down to 0.5bar on hot idle is within acceptable limits, as long as it jumps to max as soon as RPM is increased.
1993 "Pearl Blue" W124 280TE
1988 "Arctic White" W124 200T
1979 "Icon Gold" W116 450SE (history, resto)

Randys01

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Re: Compression test on European M116.985 in 1977 350SE
« Reply #21 on: 19 August 2020, 09:22 PM »
Interesting..I've scanned my ref material many times and never seen it articulated in these terms. I've seen it written whereby they allude to low oil pressure at idle but no value ever stated.
Perhaps I should have said. "You want to see 1.5 at 750 at 85."  Anything less- whilst within limits- would concern me...you will note the 750 rpm caveat.  Yes sometimes hot idle will be less..hot day..in Drive at the lights..rpm sags to 500. Hot oil press will fall to  1 bar.
I've never seen less and don't want to.!