Compression test? W124/M104

englishdas

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Just done a plug change, and I have to say, the ones I pulled out don't look great.

Although, I'm wondering if the oily gluey buildup on the threads is just the stuff that stops em jamming. (Antilock grease? The name of the stuff alludes me right now.)

I've never done a compression test before, even though I've swapped the odd cylinder head in my time, So what do I need to know? First, will just a boggo £25 kit from halfords do the trick? I hate using halfords, it just seems wrong, but I'm no expert.

secondly, what compression figures am I specifically looking for on the M104 engine block?

third, I've read the blurb on compression kits, saying do one plug at a time (I do know to remove all HT leads even if following this method) but I was watching my new favourite programme (wheeler dealers, AMAZING) and Edd China yanked all the plugs when testing a motor the other day.

I suppose the next question then would be, is reseating valve seals a massive headache best left to somebody I can send the head off to if the compression test reveals leaks, or is it something anyone who can do a head gasket swap can do in an afternoon.? (not including head stripping time of course.)

The reason I'm doing this by the way is my MPG (not the M104's strongest point anyhow) has dropped to amazingly bad levels, and so bad I doubt it's just a dodgy air filter, and the new plugs havent made a difference, really, but do seem to have took care of a bit of a flat spot. However, overtaking a tractor, the engine revved like a Yamaha R1 the other day and went like a rocket, so I'm not really lacking in performance I don't think. I'm sure this is all unrelated...
 

hotrodder

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The gunge on the plug threads is probably the result of whoever last changed the plugs going nuts with copper grease/anti seize. Can always post a good close up pic of the old plugs if you've not got rid of 'em yet. Any idea how old they are? What state were the working ends (electrodes/insulator) in?

The motorised furniture maker pulled all the plugs to give the starter motor/battery an easier time. Cranking speed among others effects the readings... compression checks aren't the most useful thing imo, they're a quick and easy of checking all cylinders are pulling their weight i.e. all are within 10% of each other but a 'leak down test' is far more useful imo. A leak down test involves pressurising a cylinder with compressed air and measuring how much leakage their is. If loads is escaping you can narrow down where it's coming from as with a bad inlet valve air will escape through the inlet etc. I don't know off the top of my head but i suspect compression test numbers would be along the lines of 10 bar min upto maybe 14 bar. Leaving the throttle closed will effect those numbers along with engine temp etc

Having said all that if it's pulling and running well i wouldn't worry too much about something 'big' being wrong inside... M104 engines are pretty bullet proof, their biggest achilles' heel is the origional comedy wiring loom. They like a headgasket too but 1) the design was changed and most have eaten at least one gasket by now and 2) they tend to fail in a slow, non catastrophic way- external oil leaks rather than mixing/eating their fluids. The M103 is known for valve guide seals at high mileages but i don't think the m104 suffers, at least not to the same extent

What sort of mpg are you getting and what sort of driving conditions? The way i usually drive mine (off peak, mix of m/way and rural roads... not gently outside of built up areas, sitting at ahem mph on the m/way when conditions allow) i typically average low mid twenties. A trip across sarf London at the wrong time of day can make a noticable difference as they're pretty brutal with fuel in stop/start traffic. Take it easy on the m/way and i can see v high twenties - just breaking into the thirties

Getting any fault codes read would be a good starting place if/once you're sure something is wrong... these store quite a bit of data considering their age/the primative interface. They'll report mixture adaptions, implausable numbers from sensors etc. Oh, unless you know the engine loom has been replaced the first thing to do is CHECK IT. There's a 'sticky' thread on the subject at the top of the engine forum
 

kth286

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Have you ever gotten the fault codes read outs from the diagnostic plug ?

That may tell you a lot.

You need to get recommended to a techie that understands your generation merc. and with access to a simple code reader.

Secondly you could connect a vacuum gauge to the engine inlet as a prior
step to doing a compression test.

It can tell you a great deal about the state of your engine, and seems a lost art in today's climate of electronic diagnostics.

Halfords will probably sell you a gauge for around £20, and it will come with lots of information regarding interpreting the readings.

Let us know how you are getting on.
 

jaymanek

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Compression test is the way forward... You should get readings of 12bar+
Remove all the plugs and dont forget to remove the king lead!

If thats all fine, id be looking at the intake system for your fuel consumption issue.

p.s. a head gasket job should not be done in an afternoon, you need to take your head to be machined flat..
 
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englishdas

englishdas

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Good point about the head job Jay, I've been known to risk it with success, but that was on piddly engines that I didn't care too much for, whereas I think this one would deserve the correct treatment.

I think all of everyone's advice is most useful. May just go get it plugged in as y'all suggest. Power is definitely not an issue, but the plugs don't look rich, will post a pic of em (i numbered them all for cylinder number too) so I think you may be onto something with intake issues.

Will report back...
 

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