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MG TD TF 1500 - Conpression test - Check me on this.

The TD's on jacks for some winter work which should include valves and rings... probably. It runs pretty well, but I'm getting a fair amount of blow-by and think there's more power to be had.

The engine hasn't been run in about a month. I did a compression test tonight and got 30, 30, 50, 50. I'm pretty sure this engine won't run like this - but OK.

I poured oil into the chamber to isolate the valves and did the test again. On #1 5 turns put the tester over 300 - it's now suck at 50. I think It's broken now.

What the heck? I've done this plenty of times over the years. What happened?
MAndrus

Mitchell you should only put a squirt of oil from a pump oilcan in the hole. All plugs out, throttle wide, ignition off and turn over until highest number reached. Should be 4-5 revolutions. But do the test with dry cylinders first. Did you forget to zero out your gauge? That doesn't make sense. And your engine wouldn't likely run with only 30-50 psi
W. A. Chasser

Dry first: 30, 30, 50, 50. Gauge zeroed. I've done this before, on this engine about 18 mos ago.

I'm going to pick up a new gauge before doing anything.
MAndrus

Sounds like you may not have opened the throttle when you took the test. ?
Been there done that.

Jim B.
JA Benjamin

JA, that doesn't explain the 300+ reading.
MAndrus

My guess is maybe the tester got drowned in oil Mitch? Cheers
Peter TD 5801
P Hehir

If the cylinder was filled with oil, the pressure at TDC would be extreme.
Steve Simmons

Too much oil in the cylinder I suppose. ....can't pressure test liquid like that.

Well, the engine is original specs (uncut) and at 68K miles it's due so valves and rings will be done anyway. When I first go the car last year the compression tests were 105, 110, 110, 115.

Except for a fault in the tester and its valves (it holds pressure just fine)I just don't get the 30, 30, 50, 50 readings just 1,800 miles later in a running car.
MAndrus

I would likely say your gauge is at fault or you didn't have a good seal on the dry test. Again the test needs to be done with throttle plates wide open. The car simply wouldn't run or have enough power to move the car at those dry readings. It would smoke so profusely as to drown the plugs. Unless you have stuck valves or burnt valves but then again the engine wouldn't run.

Bill Chasser
TD-4834
W. A. Chasser

sorry to put a damper on this, but if you put a hydraulic lock on the engine it would be a good idea to check you have not bent the rods.
Oil is as near as dammit incompressible, I bent a rod just push starting not knowing I had water in the bore.
Ray TF 2884
Ray Lee

Mitchell
Are you by any chance using a Harbor Freight Compression tester?
Just asking, as I have one that does really strange things doing a test dry and then wet. I think it is a P.O.S. and you can't buy it now - least wise I don't see it in the catalog any more.
IF any oil or oil mist gets to the gauge? It will not read correctly.
Rod.
R D Jones - Ex Pat

This thread was discussed on 19/11/2016

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