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MG TD TF 1500 - Compression Test 53 TD

I picked up a compression tester form Harbor Freight the other day (Man I could live in that place) :) $25.00 Not too sure how accurate it is & have no way to test it here.

My TD runs very well, (baring a possible cam tappet follower issue) Plugs are not fouled. Seems to run nicley at 700rpm tickover. Pulls away well.
The numbers I came up with were for Cylinder 1, 90psi - 2, 89psi - 3, 85psi 4, 90psi

Engine was cold. It was rebuilt 8000 odd miles some 14 years ago.

What should I have expected?

Rod
R. D. Jones

Did you have someone hold the throttle wide open while you did it? And did you count about the same number of revolutions on each cylinder? They are awfully low - and should be at least in the 120 range, and ideally even higher.

Tom Lange
MGT Repair
t lange

I replaced the pistons, rings and big end bearings on my TD recently and have the following readings:
1 170psi
2 160psi
3 170psi
4 165psi
Are you sure you have measured it correctly?

Declan
D Burns

A couple of weeks ago I decided to have a look at Lazarus' compression figures while I was installing a new set of plugs. I took the readings the lazy man's way, engine cold and didn't open the throttle. Readings (#1 to #4) were 148, 152, 143, 146. Engine was rebuilt in 2002 and now has about 18,000 miles on it. Bud
Bud Krueger

Right - well I guess I did it the lazy man's way as well Bud - Engine cold and measured at tickover. Even so I am way under your readings

Will give it another go and see what the difference is.
Engine warm and throttle open

3000rpm I assume will be OK??
Rod
R. D. Jones

I assume that you're joking, Rod. Just pull the plugs (all of them), insert your compression tester into one of the plug holes, open the throttle. Crank the starter and watch for about for about 5 or 6 puffs on the gauge. Read the number. Go on to the next one. Bud
Bud Krueger

I have the same HF gauge and get the higher readings the other guys get so it probably ain't the gauge "quality" fyi.
Ed
efh Haskell

OK, never having done a compression test in 50 years of having the TD. I just went out and removed one spark plug at a time and put the gauge on ran it on the starter with the ignition off and no throttle - That gave me the low readings. You live and learn.
Did a quick Google on how to properly do a compression test and gave it another go.
Engine warm 75C, all plugs removed and throttle wide open. I ran the starter for 10 seconds on each cylinder.
4 was 120psi,
3, was 112psi
2, was 118psi
1, was 119psi
Not great but the TD runs well and is not burning Oil. Last year did 1000 miles and did not add any oil till I change it at end of year before the winter. No appreciable loss noted. Oil analyst came back showing a little dilution as it needed the carbs sorting out. Definitely running rich. That is now resolved I believe.
I left the pressure on each cylinder for a few minutes and it did not drop more than a lb. or so.
Am I correct in thinking this is acceptable?
Rod



R. D. Jones

Those numbers look fine for a stock head engine, Rod. My engine has a theoretical 9.1:1 CR. So the readings would be higher. Bud
Bud Krueger

Definitely low, but evenly matched - they should all be within 10$ of each other, and are.

A new engine will be in the 160-180 range, anything below a hundred is pretty tired.

I'd be surprised to see an engine with only 8,000 miles have such low readings - try to do the test again, squirting a tablespoon of oil into each cylinder first. Let us know what the reading is then.

Tom Lange
MGT Repair
t lange

Tom, I think you're being a bit optimistic on compression readings for a stock engine. Obviously, his 'with oil' readings will be higher, but there are still some laws of physics. Bud
Bud Krueger

My XPEG has 10.000 miles on it since engine rebuild 20 years ago. I have done a compression check, dry values160,160,155, 160. Pleased wtih the result. Supercharger next?

regards

Jan
Jan Emil Kristoffersen

Well Tom. That was a heart stopping moment :)
I repeated the test and using an eye dropper put a tablespoon of oil in each cylinder and repeated the test.
First cylinder came up 120 psi
The second cylinder came up 30psi and repeating the test after releasing the pressure it read zero??
Something went wrong with what looks like a valve just after the quick disconnect in the gauge hose. I cleaned it out with compressed air and the second cylinder now showed 121psi.
Third cylinder came up 25psi and again the valve was not working correctly.
Repeated the cleaning with air and then got a reading of 119psi
Forth cylinder showed 122psi
This is an XPAG engine but it does have TF larger valves. Other than that I don't think it is different form STD.

Bottom line - a very slight improvement was seen with the oil.

120 121 119 122 I don’t think the HF tester liked the oil mist in the line??

Rod




R. D. Jones

DO NOT DO ANYTHING until you get a different gauge. I went years thinking my compression was marginal. Got a friend to check the compression and they came back perfect. Bought a good compression gauge and the readings were very close to the ones my friend got.

Bruce Cunha

Agreed, Bruce!

Tom
t lange

Point taken
I was not going to change anything as the car runs very well. Starts OK first pull and pulls OK too. I was just curious as to what pressures I would find.

What would be a good pressure gauge? This one from HF looks fine and the quick release hose is where the problem seemed to be. There is a small valve in the end that does not like oil it seems. Can't explain it though. We are only talking pressure right, not flow. Some movement would be expected but very minimal I would think.
Realy strange. May have to take it to the friendly Garage and have them give it a go with their gauge to compare.
Thanks for the input.
Rod
R. D. Jones

Forget HF. Everything they get is from China. Don't buy their tap and die sets as the tooling is way out of spec. Go to the Sears store and get a Craftsman or hit up a Snap-on, Mac tool truck etc or just go to Napa Auto Parts if you have them in your part of the country. HF tools are junk. You get what you pay for and many times That is a one time use. Yes I buy quite a few things from them but have destroyed many tools from normal use. If you want quality you have to pay for it.
W. A. Chasser Jr

Roger that
R. D. Jones

Since a compression gauge is really just a pressure gauge, you can test it against your air compressor. pressure gauge (assuming that is accurate). Fill the tank to a specific pressure and connect the compression gauge to your air hose and compare the readings. That should give you an estimate on if it is off. If both gauges read close to the same, your compression readings should be believable.

Try a couple different pressures as some gauges will be ok at low pressure but off at higher ones.
Bruce Cunha

This thread was discussed between 15/07/2014 and 18/07/2014

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