Compression tester for Diesel

[ejparrott]

I'm thinking about buying a compression tester. I'm not sure yet that there's anything wrong with the TDi in my 88, but she's definitely poorly, going to start with an injector swap tomorrow and sort new air filter pluming and a new filter over the weekend I hope.

I haven't got lots of money to spend so I need to make sure I buy what I need first time. Has anyone any recommendations for a set?

What is the maximum pressure a TDi should generate on test? Have seen a couple of kits that go to 70bar/1000psi which are stated as for Diesel, is that enough?

Thanks guys

[vulcan bomber]

A decent TDi will read about 480PSi on each cylinder.

[muddy]

Drilled and tapped glowplug with any old gauge?

[ejparrott]

Drilled and tapped glowplug with any old gauge?

Is it really that simple?

[vulcan bomber]

Is it really that simple?

Why wouldnt it be...... Trouble is, do you use a glow plug that the end snapped off still in the head.

Threads m10x1 iirc

[muddy]

You could have some kind of valve so the gauge holds its pressure to make reading easier but it would still work without.

Will.

[vulcan bomber]

You could have some kind of valve so the gauge holds its pressure to make reading easier but it would still work without.

Will.

probably wise, when i've compression testedstuff in the past its taken a few rotations of the engine before you get the proper reading.

[ejparrott]

you're making me think now... I suppose thinking about it yes, all it is is a pressure gauge connected to the bore. It either needs an assistant to keep the engine turning over, or a check valve to maintain the pressure so you can read it before bleeding it off again.

[Retroanaconda]

Do the gauges not have a max reading needle?

[vulcan bomber]

Could lend you a poorly injector Ed and go in that way if you wanted.

[David Sparkes]

I appreciate a compression test seems the most straightforward way, but in the diesel it seems to mean either expensive kit, or messing around plus moderately expensive kit (home made adaptor plus a gauge that both reads high enough and has a resettable maximum pressure pointer).

Doesn't this 'complication + expense' justify going for the 'better' option of a blow-by test facility.

I am assuming a suitable compressor exists, but such a facility works equally as well on both spark and compression engines, with different adaptors, and works at a lower pressure.

A compression test will only show unbalanced pressures across the cylinders, but (I understand) a flow test will allow more detailed analysis: is the leakage past the piston, or inlet, or exhaust valves, or a head gasket to water jacket leak.

I confess, so far I have survived without needing to do any of the above, so it's really a 'vapour' suggestion :-)

HTH

[GW8IZR]

That sounds like an interesting project

I assume the pressure test would have to beat or near compression stroke peak so apart from the engine itself how do we generate the pressure?

An electrical transducer to read the values into a scope with persistence would seem obvious for reading and probably interpretation of the results. Does picoscope do this already with an appropriate probe.

[David Sparkes]

... I assume the pressure test would have to beat or near compression stroke peak so apart from the engine itself how do we generate the pressure?

An external supply of compressed air is used, which is why I wrote "I am assuming a suitable compressor exists".

The engine is stationary, which saves on battery stress.

I cannot recall if the gauge needs to register flow, or whether it's the difference in pressure at source, and pressure close to the point of entry into the cylinder.

Picoscope probably can do the same as a compression test, probably by monitoring the change in electrical power flowing to the starter motor (less current flows if the cylinder is weak on compression).

I don't recall seeing anything about Pico doing a leakage test.

Mind you, if starting from scratch, Picoscope is a whole different ball game, both in initial expense, and learning how to use it.

I'd suggest these factors mean it's not relevant to the original enquiry here.

HTH

[SOA 93]

Pressurizing the cylinder with compressed air is called 'Leak-down test', I've just Bing'd 'leak down tester' and there is a good page on Wikipedia.

The professional kits are usually far more expensive than compression testers.

Having seen the sort of things you can make I don't think making one of these is going to cause you too much trouble.

[GW8IZR]

Far too many variables using starter current for this.

Any pic has the required precision given an appropriate transducer- the reason for suggesting picoscope is its always worth looking how someone else has tackled a problem for ideas.

The use of an external compressor makes sense as you could make measurements with the piston in various places on the stroke after disabling the valve gear.

[pete3000]

I've got one of these, does the job nicely hardly going to break the bank at £35. Multitude of adaptors for glowplug threads

http://www.rdgtools.co.uk/acatalog/DIESEL-ENGINE-COMPRESSION-TEST-KIT-6848.html

Pete

[Ally V8]

My diesel compression tester has sat in my toolbox unused since having a Picoscope.Putting a current clamp around the battery lead and watching current draw is VERY accurate.I've used it many times on TD5 engines which suffer badly from bent con rods.

I also have their pressure transducer which can measure up to 500psi from memory if you really want an actual figure.This can also be used with the engine running to check valve timing,not done it myself,but have seen it done - fascinating stuff.

The thing to remember is that it doesn't matter which cylinder is down on compression,or why - it still needs to come apart.

[GW8IZR]

I don't understand how the current probe is an accurate indication of compression- I can see it being an indicator of a delta between cylinders

On my picoscope the delta on a petrol engine in good shape with one plug out is fairly small - I haven't really played much with it as I use the scope for a very different use.

Your last point though is enough to have reminded me this is a bit futile anyway, my 30 quid gauge has served me well enough to know when I need to break out the spanners - EndEx :-)

[ejparrott]

I've got one of these, does the job nicely hardly going to break the bank at £35. Multitude of adaptors for glowplug threads

http://www.rdgtools.co.uk/acatalog/DIESEL-ENGINE-COMPRESSION-TEST-KIT-6848.html

Pete

That's helpful, thanks, I use RDG for my model engineering occasionally. I'm looking for something budget but good that will tell me things are not dreadful. I'm certainly not in the market for tuning efforts. The 200 in my 88 seems poorly, and I can't quite put my finger on why. I suspecting injectors are part of it, being long overdue changing, and I have a set to go in tomorrow that are supposed to be good. If she's still not happy I'll be looking for something else.

[Ally V8]

This is a current trace from a very poorly TD5 where a local garage did a head gasket but didn't check piston protusions . It was then sold "as is" and I think it went bang shortly after.

Sorry about the poor quality photo,it was just from my phone to show the garage what I had found.You can see there is 72A difference between the highest,(Best cyl ) and the worst one.Seeing that screen is an immediate "Yes - its got to come apart"

The good thing for me is that very little time is needed to do this and its very accurate,I don't have to take anything apart.This is important as garages often just want to know whats wrong so they can either pass it on or fix it themselves.They are happy to pay a diagnostic fee,but not hours of labour.

[GW8IZR]

That's excellent- thanks for sharing.

I had less delta plug in plug out on a petrol and dismissed it as a useful test, I'm going to try a couple of tdi,s I have here and I have a very recalcitrant Allis Chalmers to measure as well - be interesting to see this - to me anyway :-)

[Ally V8]

I hope it has helped,I've used the Pico for all sorts of old stuff too. The oldest was a 1917 Wade dragsaw with a single cylinder 2 stroke water cooled engine.It has trembler coil ignition and a strange carb which has a single jet in the valve seat of a tapered drop valve.Hard to explain really, but it had a running fault which made it backfire and stop after about 30 secs of running.We thought it was an extra ignition event as the operation of the trembler coil is a bit iffy,with very worn bush type main bearings with no seals etc.The scope clearly showed that the spark operation was quite accurate despite the wear, the backfire was caused by poor transfer to the combustion chamber because the carb valve was failing to seat and seal.A temporary cure was affected by using elastic bands to make the carb valve rise then seat.

It is an excellent tool,taking the guesswork out of many things,being able to go back in time to see what drops out or stays when something is failing is invaluable.

[GW8IZR]

Yes it has helped. I last used my picoscope to debug an ALC circuit which is a simple Cartesian loop controlling the gain of a pair of amplifiers in a digital radio transmitter but under software control. Being able to time stamp changes and measure multiple sensors made a fairly tedious job into a pleasure. Very different use from automotive applications but how many tools can go from a TDi to a DMUX :-)

[ejparrott]

Well I've changed the injectors this morning, the old ones were quite sooty, not sure how they normally are. She's not smoking at idle once the carp had cleared, i'll take her out later and she how she behaves.

[ejparrott]

...must be good injectors these....she's only gone and dropped a core plug now!

It never rains but it pours....