Compression test

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trmorrisminor
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Compression test

Post by trmorrisminor »

Hi all, going to do a compression test on the engine at some point in the next month. As I understand it, I have to warm the engine first and bring the engine up to temperature, label the HT leads eg the order they are currently located in the engine and then remove leads and plugs from the engine, disconnect the HT lead going into the coil, leave the ignition on, leave the throttle open - what do they mean by this please? Is that pushing the accelerator pedal right down and wedging it in place? Sorry for the daft question but not done this before and want to make sure i do it right.
The type of compression test I have is a Gunson’s HI Gauge - with a 14mm and 18mm screw thread. Reading some earlier posts It seems to suggest you need the 14mm to be screwed carefully into where the spark-plugs are located - making sure there is no dirt in the recess where the spark plugs sit. Do you agree its a 14mm that will fit the thread where the spark plug should be?
Any assistance would be greatly appreciated.
I have the red winding handle in which to turn the engine over with.
Many thanks Richard
nutmegct
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Re: Compression test

Post by nutmegct »

Hi Richard - you're looking for four similar readings, not four "perfect" readings. PSI of 140 to 150 is the goal. PSI differences between cylinders of 10% or less indicate problems.

When I do mine, I have someone hold the gas pedal to the floor, which opens the throttle. You can also do that at the carburetor. Also, remove the air cleaner so there's no air restriction. On some cars, there's a button under the starter solenoid, which you can use to turn over the engine. Be sure it's out of gear!

Spark plug hole is 14mm.

You don't use the hand crank; you crank the engine using the starter. I do ten turns and then take the reading.

Hope this helps.
Tom M.
trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Hi Tom, thats a huge help thankyou, pleased you mentioned about not using the starting handle, i know the button you mean on the starter solenoid, I replaced mine recently so unsure if the new one has a button? - will have to check. If i haven't got a button on the solenoid, is it just a case of turning the key in the ignition 10 times and turning the engine over that way (obviously making sure the car is in neutral! ).
Would having the choke out fully be the same as holding the accelerator pedal down or is the accelerator pedal the best thing to keep to the floor and wedged?
Thanks Tom.
Richard
nutmegct
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Re: Compression test

Post by nutmegct »

Hi Richard. Glad I could help.

If you can't use a solenoid button, just turn the ignition key to crank the engine. You can count ten turns - or ten seconds. Either way, you want to test each cylinder for the same time length.

Choke won't do the job. You need to physically have the throttle throat fully open, which you can do with gas pedal to the floor, or holding the throttle linkage fully open.

Main thing is that you want to test each cylinder exactly the same way.

Tom M.
unclealec
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Re: Compression test

Post by unclealec »

Based on my experience with other cars of similar age, I use the starting handle to perform a "compression test".
For a rough n ready survey of an engine, I just give it a few twirls either on the starter or the handle, then deploy the handle and give four measured pulls. If all four pulls of the handle need equal effort, I assume that the engine is in good basic shape compression-wise.
You don't get definitive readings of course, but as a roadside investigation it passes muster.
Alec Gatherer
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simmitc
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Re: Compression test

Post by simmitc »

14mm adapter for the spark plug thread.

Remove all four spark plugs. You do not need to remove the HT lead from the coil, but you can remove the one of the low tension wires. This is simply a precaution to prevent the coil overheating if you happen to leave the ignition on and the points are closed. If you do not have the ignition on or are very quick, then you do not need to worry about this.

If you have a button on the solenoid, then you can leave the ignition off. Even with no button, you can still operate the starter without the ignition: Connect a small lead to the terminal with the white/red wire on the side of the solenoid. Touch the other end to the non-earthed terminal on the battery. This will operate the solenoid and turn the starter.

You can use a brick or similar weight to hold the accelerator pedal on the floor.

Safety wise, handbrake on, gear in neutral, keep clear of the fan and other moving parts.
philthehill
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Re: Compression test

Post by philthehill »

When you do a compression check do as above but do the first check with dry bores.
Do a second compression check after putting several squirts of oil down each spark plug hole.
If the compression goes up by a noticeable amount that is a good indicator of worn bores/rings/pistons.
Write the compression reading down for each bore both dry and wet and when completed compare the readings. Any indications of wear will be quickly noticeable.
If the compression reading is low and putting oil down the bores does not improve the compression figures it may be that a valve is not seating correctly or the seat of the valve is burnt.
Good luck.
Phil

nutmegct
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Re: Compression test

Post by nutmegct »

Phil - When testing compression, should the engine be warm, or cold? or does it matter?

Tom M.
philthehill
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Re: Compression test

Post by philthehill »

I would suggest that doing a compression test when warm/hot is much nearer to the conditions that the engine is normally operating at and will show wear faults/problems in a more realistic way.

trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Many thanks to you all for your help, very grateful for all your inputs, will have a go and put the compression readings on here, and maybe you could help me discifer them?
So warm engine, plugs out and labelled, take lead off coil, connect gauge into first cylinder, start car and turn over for 10 seconds or 10 turnovers of engine (with noise of starter motor, with accel pedal flat to floor) take reading, reset compression test and do same for cylinders 2,3 and 4. Then do wet test and put a few squirts of engine oil in each cylinder (through the spark plug hole) and do the same test again for each cylinder? - sound ok?
Thanks again
Richard
nutmegct
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Re: Compression test

Post by nutmegct »

also label the plug wires, and ... have gearbox in neutral!
Good luck - and do let us know the readings.
Tom M.
trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Many thanks Tom. Have a good weekend.
Be in touch once i have been over to dads to do it.
Richard
trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Should have said the reason why i want to do it as it dosent seem to have as much power in the engine as my other Morris Minor when you accelerate, the van has been in bits for the past x years so now trying to iron out all the issues as i progress with the rebuild. R
don58van
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Re: Compression test

Post by don58van »

I see that you are referring to a van engine. If it is a 1098, it would have left the factory with low compression pistons (they have a larger dish). So the power would be expected to be a bit lower than for saloons--although I have seen comments from other owners that they haven't noticed much difference. Anyway, if it does indeed have low-compression pistons, that should be taken into account with the readings you get from the compression test.
Don
trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Many thanks Don, I had wondered that given it was a van whether it was a low compression engine, the original diff has been replaced with a traveller / saloon diff, so I was wondering if the engine was original? The engine number plate when I bought the van was no longer on the engine so I have no way of telling if this is a high or low compression engine, any ideas? I am hoping to do an engine comp test tomorrow.
Many thanks
Richard
don58van
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Re: Compression test

Post by don58van »

I have no way of telling if this is a high or low compression engine, any ideas?
If the engine number is no longer on the block, it could be because the engine has been reconditioned. It is removed when the deck is skimmed. So even if it is a van engine, there is a good possibility that it has had the original pistons replaced.
I can't think of a definitive way of knowing whether low compression pistons are fitted without taking the head off. Even if you have one of those cameras with a 'snake' lens that can be fed into the spark plug holes I think it would be difficult to know for sure whether you are looking at low or high compression pistons.

Don
trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Evening all, spent an afternoon with my father in law working on the van yesterday, first job was to do a compression test, these are the results:-
Cylinder 1. 2. 3. 4.

Dry test. 170. 172. 168. 172.

Wet test 180. 190. 172. 165.

Am a bit confused about cylinder 4’s wet result vs the dry result, in hind sight I should have redone the wet test again but time wasn't on my side.
I think they look ok don't they?

With regards to the timing, I rung up the service dept of ESM and spoke to a guy - extremely helpful (as they always seem to be) and he suggested setting the timing to somewhere in between 6 to 10 degrees from TDC - 6 degrees for a newly rebuild engine and 8 to 10 for a worn engine. Having set the timing around 9 degrees and reduced the fuel to bring the idle speed down on the carb, the engine sounds great and ticking over nicely. The timing was originally set at around 3 degrees TDC. The sluggishness of the engine seems to have disappeared and really feels now it has a bit of a nippy engine under the bonnet!
So a good result I think, thanks for all the above comments from members.
nutmegct
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Re: Compression test

Post by nutmegct »

Congratulations on getting all that work done.

More experienced MM owners will comment on your findings, but I would be very happy with those cylinder PSI readings. Doesn't sound like a low compression engine to me.

How do the plug tips look? Light ash, dark carbon, oily, etc.?

Tom M.
trmorrisminor
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Re: Compression test

Post by trmorrisminor »

Thanks Tom, yes was very pleased that all the results were all pretty consistent with each other, the spark plugs were very black and sooty, probably because they have probably done no mileage but what mileage they have done has been on full choke as the van has been started but has hardly left my dads drive. I have put 4 new spark plugs in the engine after the compression test was done but before we did the timing so atleast i know its been timed on clean plugs.
Hope the days gone well.
Richard
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