Car compression test

Quick question - I need to do some more digging when I get home to find out some actual values, but for now....

If a compression test showed that all 6 cylinders were showing a pressure reading of ~40% of what they should be... Would the engline likely to run in any shape or form?

[Sid Squid]

Yes, it will almost certainly run - but if the compressions are very low then perhaps not well. It's a suck-it-and-see situation and you might be pleasantly surprised, as long as the compressions are all within a reasonable range, that is the highest and lowest don't differ by too much, then it will often run quite happily, although obviously the power and economy will be compromised.

Originally Posted by Sid Squid

Yes, it will almost certainly run - but if the compressions are very low then perhaps not well. It's a suck-it-and-see situation and you might be pleasantly surprised, as long as the compressions are all within a reasonable range, that is the highest and lowest don't differ by too much, then it will often run quite happily, although obviously the power and economy will be compromised.

Cool! Thanks

[embee]

How is the compression test being done? Normally the figures are quoted for a hot engine, all plugs out, cranking with wide open throttle and a decent battery charge (maybe 200rpm+).

If you do it with a closed throttle you'll get very low figures, when you say "40% of what they should be" it makes me suspicious.

Originally Posted by embee

How is the compression test being done? Normally the figures are quoted for a hot engine, all plugs out, cranking with wide open throttle and a decent battery charge (maybe 200rpm+).

If you do it with a closed throttle you'll get very low figures, when you say "40% of what they should be" it makes me suspicious.

When I said "40% of what they should" take that with a pinch of salt. I need to find the exact figures, but I left my Haynes in car, which is at the mechanics doing the test. I'm waiting on a merc forum for the correct values and tolerances. I was just trying to ascertain whether the values I have been given are reasonable given that the car runs, or whether the mechanic is trying to pull a fast one! I have a feeling he is hamming things up more than is actually the case to try and convince my girlfriend get the engine rebuilt.

As for how it is being done, I'm afraid I don't know because I'm not doing it!

FWIW Test results are

CYL# 1 2 3 4 5 6
PSI 135 90 130 130 90 110

Just trying to figure out whether it is just 2 & 5 which are low or the whole lot is and 2 & 5 just happen to be the worst. Searching a Merc forum came up with a thread that quoted a Max of 350 PSI and Min of 220 PSI, but since there were 3 engines (with different no. of cyl and petrol/diesel) available for this model, I can't tell from the posts which one they are talking about which is why you should take a big pinch of salt with the 40% thing!

EDIT: They are talking about a diesel - must be which would explain the high compression. Doh!

Originally Posted by lukemillar

When I said "40% of what they should" take that with a pinch of salt. I need to find the exact figures, but I left my Haynes in car, which is at the mechanics doing the test. I'm waiting on a merc forum for the correct values and tolerances. I was just trying to ascertain whether the values I have been given are reasonable given that the car runs, or whether the mechanic is trying to pull a fast one! I have a feeling he is hamming things up more than is actually the case to try and convince my girlfriend get the engine rebuilt.

As for how it is being done, I'm afraid I don't know because I'm not doing it!

FWIW Test results are

CYL# 1 2 3 4 5 6
PSI 135 90 130 130 90 110

Just trying to figure out whether it is just 2 & 5 which are low or the whole lot is and 2 & 5 just happen to be the worst. Searching a Merc forum came up with a thread that quoted a Max of 350 PSI and Min of 220 PSI, but since there were 3 engines (with different no. of cyl and petrol/diesel) available for this model, I can't tell from the posts which one they are talking about which is why you should take a big pinch of salt with the 40% thing!

EDIT: They are talking about a diesel - must be which would explain the high compression. Doh!

When I had a compression test done on an old car (2.8 z3 - petrol) the pressures were about right at 120psi. So those don't sound too far off, although the 90s sound a bit low?

90 on 2 and 5 though - could this be deliberate, in order to "preserve" the crank?

[embee]

As a very rough guide, simply the comp ratio times 20psi will give you a guide, but with words of caution.

There's a good guide to comp tests on the vauxhall owners site .

It seems a bit suspect that 2&5 are low (I assume straight six?), often those cyls have some sort of heat issue if the design is made up of two sets of 3 cyls in principle. Sometimes valve seat distortion can show up in "middle" cylinders.
Usually those sort of issues are well documented in the owners' forums if they are common.

What are the problems with the engine?

Originally Posted by embee

As a very rough guide, simply the comp ratio times 20psi will give you a guide, but with words of caution.

There's a good guide to comp tests on the vauxhall owners site .

It seems a bit suspect that 2&5 are low (I assume straight six?), often those cyls have some sort of heat issue if the design is made up of two sets of 3 cyls in principle. Sometimes valve seat distortion can show up in "middle" cylinders.
Usually those sort of issues are well documented in the owners' forums if they are common.

What are the problems with the engine?

Yep - straight 6, with twin Zenith carbs feeding 3 pot eachs. Owners forum is a little hazy as the car in question is A 76 Mercedes 230 (w114)

Story to date:

Car is bought unseen in Auckland. Previous owner is completely clueless. He knows that cars need oil so randomly empties a litre of oil into the engine. Never checks the dipstick either! Consequently the excess oil is blown through the airbox, coating the carbs and air filter. This is how we get the car. Car runs fine though idles very high and uses very little oil.

Swap out the air fliter, plugs + leads, distributor cap, rotor arm + points as these are incredibly worn. Set timing and points gap. Car now hesitates when cold when going off gas -> on gas i.e moving off on junctions or roundabouts etc. I know the air filter is letting a lot more air through, so I assume the carbs could just do with a setup and balance. With the help of the merc forum, I become a Zenith carb expert overnight ( read into that what you will ) as these things are quite complex.

I manage to get a uniSyn and adaptor from the US (thanks BannanaMan ) and manage to balance the carbs and get the idle within spec (was running very rich and had a too high idle). Car runs really well on test drive, but the next morning, it stalls at most junctions/roundabouts (when cold). Also sounds like it's not firing on 1 cyl when cold either. Both choke plates are fucntioning correctly BTW.

There is additional adjustment on the carbs that maintains the idle speed when going from P to D (auto. box) which I haven't adjusted yet. Also fast idle settings which I also haven't touched. However, given I am getting around 1/2 -> 1 day off a week at the moment, my girlfriend decides to take it to the mechanics to probe further. Also saves me from buying things like a compression tester etc.

Compression test comes back as posted above - Mechanic says car is backfiring through one carb, has low compression and could probably do with a rebuild (new rings, valves etc. The cost of a rebuild isn't worth it given the value of the car, so we agree that he is going to do a combustion chamber clean, possibly re-grind the valve seats, do the tappets and re-set up the carbs.

He claims to have 25 years experience working with carbs yet what he tells me contradicts what the guy on the Merc forum says. He says that the front carb is the main carb and the rear carb is secondary so they shouldn't be balanced together. This goes against the Merc forum guy, the Haynes manual and even the orginal manual for the carbs! Which is here is anyone is interested (carb manual link on left) http://www.jaimekop.com/

So that's pretty much where we are at. We'll see what happens when we get the car back in a week and how it runs.

[embee]

If it's been breathing oil carryover due to random overfilling it could just be loads of deposit build-up on the backs of the inlet valves. Carbs also tend to exacerbate deposit build-up (compared to FI). They can easily get to the point where the deposit more or less fills the back of the inlet valve/port.


It might be a big job but I'd be considering having the head off for a general clean up. You'll probably find the components themselves are OK underneath all the crud.

It'd be nice to get access to a bore-scope (endoscope) and have a look down the holes, but that might not be easy to come by. I wouldn't be surprised if you can't get some USB plug-in mini camera these days for a few bucks from China.............