Compression Testing - Hot or Cold?

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Forkie
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Compression Testing - Hot or Cold?

#1 Post by Forkie » Sun Sep 29, 2019 7:46 pm

I know this is widely discussed generally and I have seen a thread on here regarding the testing procedure but not so much the ' hot or cold' bit so just thought I would see which way you guys have done it ? I ought to add at this stage the car in question is the Dolly Sprint, which of course has an Alloy head, although the question could be asked for our heads also.


So today having now purchased a new Sealy tester with all the relevant fittings and extentions,off we jolly well go in search of some actual readings. Here are the results which quite frankly stumped me. Any comments are welcome :D

Brand new tester
Brand new 038 battery
All spark plugs removed
Throttle wide open
Coil and fuel line disconnected
COLD engine as purchased as non- runner.

All 4 cylinders pushed the gauge up to its high reading in 4 crank overs. I did each one twice - and i got 130 for each cylinder over two readings,give or take a couple either way. I was not expecting that. Father in law mentions 'Perhaps as it is cold it may not read correctly' - so i have spent several hours now trawling the net and it realy seems to be 50/50 with the rights and wrongs of doing this hot or cold.

It has been quite interesting - with some saying when cold the engine is at it's worst for sealing so this is the correct situation to test, some say they would NEVER do a hot test on an ally head for fear of stripping threads removing a hot plug. Etc Etc !!!! Any thoughts?

So regarding the Dolly - with all four generally around 130 - cold - am i right in thinking that even though this may not be the correct pressure reading, there is not one or two way diferrent to another - would that indicate the head and gasket is ok? That is the impression i have, based on what I been reading this afternoon.. I have not found in any of what i have read to say on the lines of ' a cold reading MAY NOT find issues of head or gasket failure. So on that, things are a little more upbeat, i just wonder if i get any replies that agree. Interesing - i think :!: The only thing that still concerns is the truly shocking state of the oil that we drained out - it was way overfull - this was before turning the engine over. On the dipstick it looked pristine. On draining a good litre or so out to bring the level down ,what came out was awful. Water sinks to the bottom....So this is very puzzling :roll: And this MUST of been the reason for the high level in the sump.
I forgot to add that all four plugs looked good - a little sooty but not wet, and looking through plug holes all four crowns looked ok and dry. Basically it all SEEMS to check out?
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Phil T
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Re: Compression Testing - Hot or Cold?

#2 Post by Phil T » Mon Sep 30, 2019 7:57 pm

I'll stick in a fairly inexperienced thought.

Did you try it wet and dry (although as yours were roughly the same you may not need to)? Firstly take the pressures dry then add a drop of oil into the bores and try again. If the pressures increase (the oil should seal the bores) then there are issues there.

I'm sure others will have other thoughts on the matter.

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Re: Compression Testing - Hot or Cold?

#3 Post by Forkie » Mon Sep 30, 2019 9:39 pm

Hi Phil. Thank you for the input. As you said yourself ,at 130 evenish over 4 bores I saw no need to do a wet test. So I have not done one.
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Re: Compression Testing - Hot or Cold?

#4 Post by johnnydog » Tue Oct 01, 2019 11:40 am

I have a Gunsons compression tester which I have had for years, and the instructions with it state to run the engine for 10 minutes to normal operation temperature before carrying out the test. It also states to remove the supply lead to the coil (+ or SW), and to remove one plug at a time rather than removing all and then doing the test.
I have used it reasonably frequently over the years, and admittedly, I have checked compressions both hot and cold.
But the instructions state it should be done at normal operating temperature with the throttle wide open (this wouldn't be applicable to modern cars without a manually operated throttle) turning the engine over for about 5-10 seconds.
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Forkie
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Re: Compression Testing - Hot or Cold?

#5 Post by Forkie » Tue Oct 01, 2019 8:23 pm

Ok thanks for the input guys. Well brother -in - law has finally joined the dolly forum so over to them now!!! Hopefully no more need to ask Dolly questions on a 2000 forum :D
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Re: Compression Testing - Hot or Cold?

#6 Post by Mike Stevens » Wed Oct 02, 2019 3:52 pm

As all 4 cylinders gave much the same reading, I think that is probably more important than an absolute value. Like johnnydog, I've done compression readings both hot and cold - I don't see what it should make much difference - assuming that rings and valves are sealing OK.

In a way, it's a shame that you didn't do a wet reading as that would tell you if the rings/bores were 'OK'.

Are you now washing your hands of it completely - and letting the Sprint forum take over? I'd be very tempted to replace the engine oil and try for a start. If the engine is OK then you shouldn't do any more harm. If not, then you're probably not going to make it any worse!

It's a real shame really - as you say, it's not doing the classic car fraternity much good.

Good luck - keep us posted!

(I must get back out into the garage myself and do some more on the '49 Land Rover. It's a bit chilly there today though....)

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Forkie
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Re: Compression Testing - Hot or Cold?

#7 Post by Forkie » Thu Oct 03, 2019 7:43 pm

Hi Mike. Well i'm not washing my hands of helping him with the car :) I just thought i was being a bit cheeky asking questions on here regarding 'another car' (albeit it is a Triumph :D ) whilst I was waiting for him to get his A into G so to speak with joining his own. But then again there are some sound people on here that hold valuable knowledge,and plus I am really intrigued to try to get to the bottom of what is wrong,but quicker than he is :roll: So if you happy for me to carry on quizzing your expertise than i will :lol:

Regarding the wet reading - i can do that at some point no problem - but as i was happy that all four plugs looked fairly normal - a little sooty from 'trial' start ups no doubt over the last five years - but no drastic evidence of appearing dripping wet with oil or sludge etc ,and also shining some light down the plug holes to view the piston crowns I saw no wetness or anything like that. So with the similar comp readings,and my initial viewings of plugs tips and piston crowns,everything SEEMS ok there - so I thought a wet test unnecessary ? Please tell me if I am missing something !!!!!

Regarding replacing the oil - that was my initial thought on Sunday when I did these simple tests - a pretty much even reading over 4 cylinders on a Sprint sold with 'overheating issues' was nothing short of a miracle :lol: . Excitement kicked in. Lets go for it. Time i drove home sense kicked in - we have not found the reason for the sludgy horrible oil. So water MUST be entering the sump somewhere. Until we have sourced that fault,surely any re-plumb of coolant and oil change and subsequent trial start up will just add more water in.....so I shelved that idea for the time being.Again, tell me if I am missing something !!!!!

My money - I hope - is a water pump issue. That is a whole different scenario,and possible post!!! The only thing I dread,however, is a cracked block between waterjacket and bore - surely this though would show on the piston crown and plug of the suspect cylinder...

One more thought for the reader - what thoughts on the bearings ( main and big) running in the contaminated oil for an unknown time / speed etc....
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1975 TRIUMPH 2500S AUTO ESTATE IN BRG!

DON'T TIDY UP - LEAVE IT WHERE IT FELL!!

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