Triumph Repair: GT6 wont fire!, internal combustion engine, vacuum gauge

Question
QUESTION: Hi Howard
I had to move my beloved '73 GT6 over the winter. It overheated 10 miles down the road,lost power and made a jingly/raspy sort of sound when I tried to rev the engine. I think it overheated due to an issue with the radiator which had boiled over a few times previously. Probably needs a new cap or thermostat but that's not my main concern at the moment.
What damage could there potentially be to the engine?
There is a spark and fuel delivery seems ok. Starter motor fine, engine turns but won't fire.
What should I be looking at next?
I am trying to diagnose the fault myself and am fairly inexperienced in the mysteries of an internal combustion engine. The engine is a recently fitted fully reconditioned tr6 block with the original gt6 head. Has about 5000 miles on the clock with no noticeable problems other than the previously mentioned radiator issue.

kind regards
A. Kirton

ANSWER: Hi Adam,

I learned early working in dealerships that when an engine will not start or it starts and runs poorly you MUST follow a test procedure and you must do it in the proper order. Symptoms can at times lead you to a fault but many times they lead you down the wrong path.

So don't try to guess what is wrong and forget your symptoms for now.

All engines require only three items to run. Compression, Fire and Fuel. They need to be tested in that order not to waste time and effort. It only takes a few tools and they are not expensive.

In the dealerships I worked in we had a scope with an exhaust anylizer attached. Most people don't have that equipment available but you don't need that.

All you need is a compression gauge, a fuel pressure and vacuum gauge, timing light and a set of hand tools to do the tests. It would be good to have compressed air available but it is not necessary.

First and most important is compression. You need to test all 6 cylinders and you want to see at least 125 PSI to 170 PSI on all cylinders with little difference between cylinders. Be sure to hold the throttle wide open when testing and turn the engine over at least 4 or 5 revolutions on each cylinder test. On an engine that has been overheated it is very important to run a second test called a "Wet compression test". After the first test is done and the results written down you squirt a few shots of engine oil in each spark plug hole just before running the second test. Throttle wide open as before and about 4 or 5 revolutions. The "Wet" test will be higher then the first test but it must NOT be more then 10% to 15% higher.

If it fails any of the Dry or Wet compression tests, it is no use continuing. If it does ok on the first compression test but fails the Wet test you have a ring problem probably due to the over heating. If it fails the first compression test you probably have a burned valve, badly blown head gasket or failed valve springs due to over heating.

If you had an engine over heat for any reason, head gasket, crack in a cylinder head, water pump, radiator, thermostat or leaks, there is a sequence of events. Usually if it is caught early no damage it done but if driven for more then a few minutes while over heating the following takes place.

First to be damaged is the valve springs loose there tension, especially on the center few cylinders, then the rings loose there tension causing loss of compression. (tested by a "Wet" test). Some gasket leaks will cause coolant to leak into the crankcase and cause the oil to turn brown then light brown and finally tan and foamy and at this point bearing damage has started.

So, run the dry and wet compression tests and look at the color of the oil then remove the valve cover and try to push a valve spring down on the front and rear valve springs with both thumbs and note how hard that is to do. Then do it to the valve springs of the center two cylinders. If the center springs are easy to push, you have failed valve springs due to the over heating.

If it don't fail either compression test and the oil looks good you most likely have no damages due to the over heating. It still does not mean you don't have a blown head gasket as they can blow fire into a coolant jacket and not show up on a compression test.

let me know the results of these tests and I will give you the next set of tests.

Howard



---------- FOLLOW-UP ----------

QUESTION: Hi Howard

Have been very busy with work but managed to spend some time with the GT6 over the weekend.
I firstly ran a dry compression test. The results from 1 to 6 were 4.4bar, 1.5, 3.25, 3.6, 5.8, 4.4bar.
Removed rocker cover. The springs seem to have the same resistance. Oil seems ok.

What comes next?

All the best

Adam Kirton.

Answer
Hi Adam,

As I remember a bar = about 14.5 PSI and if your gauge is correct and you held the throttle open when testing, Your engine failed the compression test and it is a waste of time to continue until the compression is corrected. Generally any gasoline engine that drops to or below 100 PSI,(about 7 bars) that engine is not going to run. A normal compression on a TR-6 should be 8 to 10 bars or 120 to 150 PSI on a dry compression test with the throttle open.

However, it is very unusual for an engine to loose compression on all cylinders as yours has. So I would check your compression gauge on a known good engine to see if the readings are correct on the gauge.

If your gauge is correct and the readings were that low, you should run a "Wet" compression test too and use about a table spoon of engine oil in each cylinder just before each test. If the "Wet" test shows more then 10% to 15% higher it is a ring problem and if there is little difference then you have a valve problem.

Generally when you see an engine starting to overheat, you only have a minute or two to shut it off before major damage is done by the overheating. If your gauge readings are correct you need an engine overhaul. If you already heard noises, then you are too late to save the engine from an overhaul.

Howard