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compression test results???

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I'm struggling to diagnose a problem with my 300tdi. I'ts done 160,000 miles and this year I've been having a right old carry on with cooling problems. I've gone through two water pumps and three thermostats and I finally thought I'd fixed it after taking the whole cooling system apart, back flushing radiator etc, no blockages in heater matrix, all appeared ok. Drove to work next day (22miles) and lost most of the coolant on the way there throught the vent on the reservoir cap. Same thing on way home.

During this period of problems, the engine did get very hot on at least two occasions and I am now thinking that I have either a head gasket problem or a cracked head as the cooling system has far too much pressure and is just ejecting the fluid.

So, did a compression test today to see if I could diagnose the problem myself and am struggling to interpret the results, because, according to the Haynes manual, a difference of up to 70psi between cylinders is acceptable.

starting at the front of the block, cylinder 1 gave a reading of 25 bar/386psi

number 2 gave the same reading.

number 3 read 28 bar/432psinumber 4 read 24bar/371psi

Now, I've never been great at maths but according to my calculations, the biggest difference is 61psi between 3 and 4 and the difference between 3 and 2 is 57psi.

The specs in the manual state up to 70 psi difference is acceptable, so this is where I am struggling because there is definitely a problem somewhere??

I would very much appreciate any feedback from anyone who has either experienced this or who may be able to help me pinpoint the problem.

thanks for reading...........................................

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Problem is that it does not take much of a leak to pressurise the cooling system. Effectively you have either a head gasket problem or a head/block problem (split in the block) the latter is fairly unlikely on a Tdi.

I presume you've swapped the header tank cap just to rule that out? Assuming so, I'd take the head off, clean it up and have a good look for any cracks, peer down the bores as well and if you can't see anything, clap a new head gasket on and see what happens. You might see something obvious. Look also for any cylinder where the head/piston looks washed clean, this being a sign of water ingress into that cylinder.

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Hi Bogmonster, I did change the cap on the header tank, forgot to mention that.

I was thinking about just taking the head off as per your suggestion because it is the only logical thing to do next.

thanks for the reply and the information. Once I've done this, I will post the outcome on here.

thanks again.

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Took the head off today (finally), head gasket showed no sign of breaching to any of the cooling ports and appeared intact. cleaned the surface of the head and studied it carefully under a strong light and could not see any visible signs of a crack. No sign of water ingress to any of the bores.

I can only assume that the head is indeed cracked in a place, and, probably so small that it is not visible to the naked eye.

Does this seem to be a reasonable assumption that anyone would agree with?

thanks in advance for any help.

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try bearing blue or a good crack detection kit, paint it on the head, wipe off residue and if there is a crack the dye will have seeped into the crack and it will show up, some test kits supply a UV torch and the dye shows up in a darkened room.

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The Tdi head has been known to crack in No1 exhaust port but this can be fixed. I think you need to get the head tested especially as it is already off.

Your compression test readings show that your gauge needs calibrating. The manual says 24 bar max on a new engine. I have never measured a cylinder at over 20.

My engine (after K&N 'filter') was down to 12 bar on one cylinder, 16 or so on the rest. It would still start from warm, but smoked a lot. That was 140 000 kms ago - I didn't rebore but honed the bores and changed the rings. (and fitted a paper air filter).

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Well, in the end it turned out that the head was indeed cracked, a fair bit of rust on two of the head bolts makes me think that was the case, and no sign of any obvious damage to the head gasket. I had already bought a new head from Turner's, so I cleaned everything up and popped the new head on, one nice sunny day a couple of weeks ago. Trouble was that by the time I had finished it was dark and freezing cold. Charged the battery overnight, stuck it back in, bled the diesel through and turned the key, bingo, problem solved.

In response to your comment Jim, the compression tester was a brand new one with a calibration certificate. The manual I have also states that there is no need to provide a measurement but instead, it is the difference between the readings that is important. I would be interested to know if you measured the compression again after you had done all the work to your engine and what those readings were?

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I was going to check the actual compressions but was too scared! Well, I did try but the no 1 glow plug broke and is still broken. Tip: only use BERU glows as the one that seized was a cheap 'County' type. The engine started and ran well and that was good enough for me. That job was done at 166000 kms and the engine is still a good starter when cold at 310000. Engine overhaul next year though as the power is not too good. But it still starts and goes and doesn't use much oil.

This is possibly because I change the oil every 5000kms and the filter every second oil change. I use the best mineral oil I can get, nowadays a CI grade.

You are correct in that it is the difference in the readings that matter. I have my doubts about your 'calibration certificate' as the readings you get are impossible with a 300tdi.

I quote from the manual "Expected readings of a crank test, with the vehicle's battery fully charged, compression ratio 19.5:1, should be 24 bar."

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You are correct in that it is the difference in the readings that matter. I have my doubts about your 'calibration certificate' as the readings you get are impossible with a 300tdi.

I quote from the manual "Expected readings of a crank test, with the vehicle's battery fully charged, compression ratio 19.5:1, should be 24 bar."

My 200 TDi gave me 450 Psi on all the cylinders when I checked its compression a couple of years ago so it should be possible.

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My 200 TDi gave me 450 Psi on all the cylinders when I checked its compression a couple of years ago so it should be possible.

450 psi = 31.03 bar.

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Remember though you get compressive heating so while the theoretical comp ratio might be 20:1 or whatever it is, if you are squeezing that it will get awful hot, as per the requirement for making stinky old diesel go bang, and that will cause it to expand. Having said that I think the highest I have seen is about 28.

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