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Compression Testing - Diesel


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#1 JohnT

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Posted 04 November 2007 - 09:36 PM

I want to check out the condition of my engine (BMC Commander2.52ltr) does anyone know the PSI minimum limits and the norm. I have been told its prob going to be 250psi+. Nothing in the manual but it does indicate a compression ratio of 19.5 - 1.
Many thanks
JohnT
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#2 DaveR

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Posted 05 November 2007 - 07:41 AM

OK, 1 atmosphere = 14.7psi

Therefore 19.5 compression ratio = 14.7 x 19.5 = 287.8 psi

Minimum required for diesel auto ignition under compression is 14 : 1 so your minimum pressure is therefore 205.8 psi
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#3 John Orentas

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Posted 05 November 2007 - 11:07 AM

Yes the real situation is rather more complex, that calculated figure is all very well but an engine will never start or run with such a low figure.. You must understand that as the air is compressed in the cylinder it gets hot, very hot (remember the bike pump with your thumb over the end experiment), this of course expands the air and increases the pressure even more.

When the engine is running the pressure is more in the region of +400 psi, and you need that heat too.

Edited by John Orentas, 05 November 2007 - 03:27 PM.

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#4 RobinJ

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Posted 05 November 2007 - 03:23 PM

Yes the real situation is rather more complex, that calculated figure is all very well but an engine will never start or run with such a low figure.. You must understand that as the air is compressed in the cylinder as the air it also gets hot, very hot (remember the bike pump with your thumb over the end experiment), this of course expands the air and increases the pressure even more.

When the engine is running the pressure is more in the region of +400 psi, and you need that heat too.

Yes with a compression ratio of about 22 it comes out in the 400 - 450 psi range, so I would guess anything above about 370 would be pretty good!
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#5 JohnT

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Posted 05 November 2007 - 06:49 PM

Thank you for your input so far, if I recall correctly some of the suppliers of the test equipment used to supply a book with the normal or new figures and a midrange to knackered figure, if any of you have any direct experience of testing this engine and the pressure range it would be of great assistance.

What many of you have written is good theory, although in reality of little use if you do not take into account valve timing as well. it is not really practical to work out a theorectical figure. They are usually based on the manufacturers/importers actuall readings.
The only real way is perhaps to contact Thornycroft and very nicely ask for the info!

Thanks again for replies so far.
JohnT
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#6 Yoda

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Posted 05 November 2007 - 07:02 PM

You won't get any of those figures when cold being turned over by battery. That's why we start on heaters. If you get a screw in guage and run the engine you will get a better indication.
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#7 onthecut

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Posted 05 November 2007 - 07:23 PM

Hi JohnT.

Must disagree a little with Yoda --- battery cranking will give usable figures, but I would do it with a warm engine. Take out all the injectors, or glow plugs, depending which orifice you intend to test on. Broadly speaking, 400 psi + --- nice. 350 - 400, very typical and quite acceptable for a used engine; 300 - 350, not untypical for a well worn engine, but will still do the biz with a bit of cranking; sub 300, I'm going to disagree with DaveR here --- time to be reaching for the spanners.
Make sure the battery is well charged and gearbox in neutral. Also make sure you have a good seal on the test adaptor. Don't forget of course, a low reading may only be telling you of a simple problem --- leaking gasket, poor valve seats. If it's combined with high oil consumption, then it's more likely to be bad news. Good luck.

Mike.
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#8 JohnT

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Posted 06 November 2007 - 01:02 AM

Thanks guys, mike I am aware it needs to be done with a warm engine and in my case all glow plugs out! also the engine stop out, will be using a screw in adapter type.
The engine uses very little oil for its age, about a pint and a quarter in 100 hours running.
It will only start with 20-30 secs of pre heat from cold, starts readily for the rest of the day, white diesel smoke when cold for 5-10mins (everyone says this is normal, but few if any can say why) the injectors were serviced last year(before we bought her).
The head has never been off in 27 years (of light private use) so the mechanic in me says it must be due for a decoke and valve seating plus new springs, although I am wary of spending money and effort when it may need replacing in the next couple of years.
Not a common engine and not many experienced guys around with this engine, although it was fitted to the FX4D London taxi for about 10 years.
Spare parts are sparse and at a premium, 64 for a set of spring and over 100 per piston replacement.

Cheers guys
JohnT
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#9 Yoda

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Posted 06 November 2007 - 01:52 AM

You might get better figures without running it as I suggested, suckit and see. One thing though, if it aint broke dont fix it. Leave well alone.
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#10 Tony Brooks

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Posted 06 November 2007 - 07:50 AM

Thanks guys, mike I am aware it needs to be done with a warm engine and in my case all glow plugs out! also the engine stop out, will be using a screw in adapter type.
The engine uses very little oil for its age, about a pint and a quarter in 100 hours running.
It will only start with 20-30 secs of pre heat from cold, starts readily for the rest of the day, white diesel smoke when cold for 5-10mins (everyone says this is normal, but few if any can say why) the injectors were serviced last year(before we bought her).
The head has never been off in 27 years (of light private use) so the mechanic in me says it must be due for a decoke and valve seating plus new springs, although I am wary of spending money and effort when it may need replacing in the next couple of years.
Not a common engine and not many experienced guys around with this engine, although it was fitted to the FX4D London taxi for about 10 years.
Spare parts are sparse and at a premium, 64 for a set of spring and over 100 per piston replacement.

Cheers guys
JohnT



John

From the Taxi comment I surmise you have a BMC 2.2 or a BMC 2.5. If it is a 2.5 then there is a manual for this engine on a Dutch website (link from bottom of my first page (tb-training.co.uk). If it is a 2.2 then with the 2.5 being an upgrade the compression pressure will be very similar. If I am correct these engines are so simple any semi-competent "engineer" should be able to overhaul it as long as he has the manual.

However my personal view in the light of how you describe the engine is that you really should leave well alone and only thunk about stripping it when it becomes difficult to winter cold starts and you have ensured the glowplugs are working. If you have a 2.2 you can even get modern 12v glowplugs from ASAP supplies.

I am not sure the 2.5 manual lists a compression reading, but if you look across a range of indirect compression engines that do give this data you will find the 400psi mentioned by Robin is a good working figure. Valve timing does not really come into it because the figures are so similar across a range of engines,so please KIS before the less practical thinks compression testing is harder than it really is.

No one has mentioned that the normal allowance for difference between cylinders is about 10% and if you have more comparing the wet and dry readings for each cylinder should indicate if the problems is valves or pistons (email off group if you want more info on this).

Tony Brooks
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#11 larkshall

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Posted 06 November 2007 - 01:37 PM

John

From the Taxi comment I surmise you have a BMC 2.2 or a BMC 2.5. If it is a 2.5 then there is a manual for this engine on a Dutch website (link from bottom of my first page (tb-training.co.uk). If it is a 2.2 then with the 2.5 being an upgrade the compression pressure will be very similar. If I am correct these engines are so simple any semi-competent "engineer" should be able to overhaul it as long as he has the manual.

However my personal view in the light of how you describe the engine is that you really should leave well alone and only thunk about stripping it when it becomes difficult to winter cold starts and you have ensured the glowplugs are working. If you have a 2.2 you can even get modern 12v glowplugs from ASAP supplies.

I am not sure the 2.5 manual lists a compression reading, but if you look across a range of indirect compression engines that do give this data you will find the 400psi mentioned by Robin is a good working figure. Valve timing does not really come into it because the figures are so similar across a range of engines,so please KIS before the less practical thinks compression testing is harder than it really is.

No one has mentioned that the normal allowance for difference between cylinders is about 10% and if you have more comparing the wet and dry readings for each cylinder should indicate if the problems is valves or pistons (email off group if you want more info on this).

Tony Brooks



Thought about doing a 'leakdown test', its more accurate than a straight compression test, any competent diesel spanner monkey should be able to facilitate. The results properly interpreted will tell you the state of valves as well as piston rings/bores.
The other vital thing is that the deviation in cylinder results must not exceed 10% of each other, ideally 2-5%.
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#12 JohnT

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Posted 07 November 2007 - 10:20 PM

JohnFrom the Taxi comment I surmise you have a BMC 2.2 or a BMC 2.5. If it is a 2.5 then there is a manual for this engine on a Dutch website (link from bottom of my first page (tb-training.co.uk). If it is a 2.2 then with the 2.5 being an upgrade the compression pressure will be very similar. If I am correct these engines are so simple any semi-competent "engineer" should be able to overhaul it as long as he has the manual.However my personal view in the light of how you describe the engine is that you really should leave well alone and only thunk about stripping it when it becomes difficult to winter cold starts and you have ensured the glowplugs are working. If you have a 2.2 you can even get modern 12v glowplugs from ASAP supplies.I am not sure the 2.5 manual lists a compression reading, but if you look across a range of indirect compression engines that do give this data you will find the 400psi mentioned by Robin is a good working figure. Valve timing does not really come into it because the figures are so similar across a range of engines,so please KIS before the less practical thinks compression testing is harder than it really is.No one has mentioned that the normal allowance for difference between cylinders is about 10% and if you have more comparing the wet and dry readings for each cylinder should indicate if the problems is valves or pistons (email off group if you want more info on this).Tony Brooks

Tony as I stated in my original post it is a 2.52ltr, I do allready have the manual from the dutch site thanks to a previous posting by you. No info re pressure readings unfortunately.Semi competent I is, a trained mechanic although not worked as a mech for 25yrs also my experience is mostly pertol engines until now!The valve timing reference was in reply to a previous respondant who was attempting to work out a theoretical figure from the compression ratio.So your recommendation is to leave the lid on - cant disagree really!Any suggestions how to reduce the white/grey (unburnt diesel) smoke for the first ten minutes on cold starting?Thanks to all respondants so farJohnT

Thought about doing a 'leakdown test', its more accurate than a straight compression test, any competent diesel spanner monkey should be able to facilitate. The results properly interpreted will tell you the state of valves as well as piston rings/bores.The other vital thing is that the deviation in cylinder results must not exceed 10% of each other, ideally 2-5%.

Twas my next step if any real issues showed up from the pressure test.Thanks for the deviation figures and your input.JohnT
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#13 onthecut

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Posted 07 November 2007 - 10:38 PM

Hi John.

Ideally, I'd get the injectors tested. If you don't want to go to the expense, you could try, while it's in the unburnt vapour stage, cracking the injector pipes one at a time to see if any particular one makes a significant difference. If so, I'd swap the injector for one of its neighbours and see if the problem moves with the injector, or stays with the cylinder. If there's nothing to choose and you're satisfied the timing is OK, you could cheat a little and amend the glowplug wiring to accommodate a glowplug control box from a one of the modern automotive diesels, where the glowplugs continue to function for a short while after the engine starts, to promote early smooth running. (Suppose you could just fit a manual switch for the same effect !)

Mike.
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#14 JohnT

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Posted 08 November 2007 - 11:25 PM

Hi Mike
As I said in one of my earlier posts the injectors were serviced/exchanged last year(before we bought her) although i know and trust the yard where the work was done, i know not of the diesel engineers who actually serviced/exchanged the injectors.
The ignition switch is one of the old fashioned originals and, turn it to the right, ignition, turn it further and it turns the engine over, turn it to the left (i'ts on a spring return) heater plugs, turn it further and it turns the engine over, you can then let it back and hold the heater plugs on(i assume). Only problem is by turning it to the left and starting it does not excite the alternator and does not charge(not a big problem)although i am concerned it may damage the alternator or charging system. I have to admit to starting it to the left when i first got her and it took me a couple of days (and very near flat starter and domestic batteries) before it dawned on me what was happening! fortunately no harm or damage done! I will try starting it this way and holding on the heat to see what happens(i am sure Snibble will be on here to give me the gypsies warning if needed)
Thanks for input and i will certainly keep your suggestions in mind when i go to the boat next week, if its still there, she's on the Norfolk Broads and tonight the outlook with the weather is non to rosy to say the least, but i'm sure she will be fine!(fingers crossed and everything else) this forum certainly has the best knowledge and expertise i have found, the Broads boaters seem to pay someone else to everything!
Cheers
JohnT
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#15 Tony Brooks

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Posted 09 November 2007 - 08:46 AM

Hi Mike
As I said in one of my earlier posts the injectors were serviced/exchanged last year(before we bought her) although i know and trust the yard where the work was done, i know not of the diesel engineers who actually serviced/exchanged the injectors.
The ignition switch is one of the old fashioned originals and, turn it to the right, ignition, turn it further and it turns the engine over, turn it to the left (i'ts on a spring return) heater plugs, turn it further and it turns the engine over, you can then let it back and hold the heater plugs on(i assume). Only problem is by turning it to the left and starting it does not excite the alternator and does not charge(not a big problem)although i am concerned it may damage the alternator or charging system. I have to admit to starting it to the left when i first got her and it took me a couple of days (and very near flat starter and domestic batteries) before it dawned on me what was happening! fortunately no harm or damage done! I will try starting it this way and holding on the heat to see what happens(i am sure Snibble will be on here to give me the gypsies warning if needed)
Thanks for input and i will certainly keep your suggestions in mind when i go to the boat next week, if its still there, she's on the Norfolk Broads and tonight the outlook with the weather is non to rosy to say the least, but i'm sure she will be fine!(fingers crossed and everything else) this forum certainly has the best knowledge and expertise i have found, the Broads boaters seem to pay someone else to everything!
Cheers
JohnT



John

1. I would also say get the injectors tested at a specialist who works for TRUCK FLEETS and not the local "marine engineer" who happens to posses an injector pop tester.

2. White smoke for a short while after starting a BMC - situation fairly normal as long as it starts well from cold and there is little smoke once it has got a bit warm. Ever seen a bus depot at about 05.30 AM?

3. You will not damage your alternator using the switch as described. Do a cold start and then turn it clockwise to the run position.

If you could damage it that way every alternator would fail within half an hour of first fitting because the regulator does exactly what you will be doing except many times a second.

Its disconnecting the MAIN CHARGING CABLE(S) that may damage them, not the warning lamp wire.

By all means try loosening the injector pump mounting bolts (I recall the inside one is a right pig, I think I got a very long 1/4 inch square drive socket and very long extension bar) and try gently twisting the pump to see if a slightly different position improves the situation, but I doubt it will and I doubt it will turn out to be worth the effort.
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#16 John Orentas

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Posted 09 November 2007 - 10:06 AM

Hi John.

Similar threads have come up several times in the past couple of years, all I would add is that to improve starting, make a small modification to your heater circuit by adding an extra push button to energise the heaters.. In this way you can keep them energised whilst the engine is cranking and even for a few seconds after starting until the engine is running reasonably smoothly.

Use fairly robust wiring and switch, the heaters will draw up to 50 amps.

P.S. Just noticed from your post of yesterday, you seem to have a similar facility already, might be an idea to check that it is functioning though.

Edited by John Orentas, 09 November 2007 - 10:12 AM.

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#17 Sir Nibble

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Posted 09 November 2007 - 10:24 AM

(i am sure Snibble will be on here to give me the gypsies warning if needed)
JohnT

Not a lot to add really. That type of switch will give you glow when cranking, "post heat" use of glowplugs on modern engines is an anti emissions function but I doubt it will make much difference to your engine, and your plugs won't like it. Just as well your alternator doesn't cut in right away, it is common for Vauxhall/isuzu engines to hold the plugs on and destroy the alternator with the excessive load.
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#18 smileypete

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Posted 09 November 2007 - 10:31 AM

Hi,

Worth checking voltage between glow plug and engine block, a small drop from the wiring/switch etc will give a much larger drop in heat output.

cheers,
Pete.

Edited by smileypete, 09 November 2007 - 10:34 AM.

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#19 JohnT

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Posted 11 November 2007 - 05:13 PM

Not a lot to add really. That type of switch will give you glow when cranking, "post heat" use of glowplugs on modern engines is an anti emissions function but I doubt it will make much difference to your engine, and your plugs won't like it. Just as well your alternator doesn't cut in right away, it is common for Vauxhall/isuzu engines to hold the plugs on and destroy the alternator with the excessive load.

The issue with the charging is that if you use it to the left (Glowplugs&start) it does not charge when you put the switch to the ignition/run position, the only way to get it to charge is stop the engine and re start to the right. I presume this must be so that the alternator is exicted before it turns?
The engine actually runs very well and smoothly once its warmed up with just a haze of diesel smoke when ticking over, is this another sign of the injectors not being upto scratch?
Many thanks for your help
JohnT
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#20 Tony Brooks

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Posted 12 November 2007 - 09:53 PM

The issue with the charging is that if you use it to the left (Glowplugs&start) it does not charge when you put the switch to the ignition/run position, the only way to get it to charge is stop the engine and re start to the right. I presume this must be so that the alternator is exicted before it turns?
The engine actually runs very well and smoothly once its warmed up with just a haze of diesel smoke when ticking over, is this another sign of the injectors not being upto scratch?
Many thanks for your help
JohnT



Sounds very odd to me. In fact I suspect there is some peculiar wiring or the switch has been incorrectly wired.

If the charge warning lamp does not come on with the switch to the left the alternator will not energise (well it might if you reved the nuts off it), but if the warning lamp comes on with the switch in the first clockwise position (auxiliaries on - gauges etc) then as soon as you turn the key to this position the alternator should energise. It may require a very modest rev up though.

Another thought. Some engines used a rising oil pressure switch to energise the alternator. If yours is like that is it working?
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