Evaluating a Used Engine

[Bxx1]

Assume you were going to buy a used Cobra what method(s) would you use to assess the engine condition and what would be the best method and why.

Also... I would be interested in what tools would be needed to perform an engine analysis... assuming a professional mechanic/garage was not available.

Thanks in advance for your responses.

As a side note... I drove my early model Superformance to the Monterey Car Week... almost 600 miles of trouble free driving.

[MOTORHEAD]

Is the alleged engine in a vehicle?

[Igofastr]

Minimum, do a compression check, better still, a leakdown.

A compression check will give you a decent idea of the condition of the engine, but a leakdown test is much more informative as you get a better idea of how (bad ?)the wear is, and a pretty good idea of where the problems are (ie valves, rings, guides, etc).

The tools are cheap, except you'll need a compressor to do the leakdown.

I always look at the oil..at least pull the dip-stick and run the oil through my fingers. Look at the bottom of the engine...is it dirty, where, has it just been cleaned?

With a factory car, look for maintanance records...probably won't get much with a cobra...who built the engine...can you talk to them, are they still in business?

That's what I'd start with.

[Shootnride]

So Ron, are you trading up

I agree with the previous poster that a leak-down test is about your best bet. Do a good visual examination looking for oil leaks, coolant leaks, fuel leaks, condition of wiring and hoses, etc.
Here's a pretty good article on leak-down testing : How to do Cylinder Leak Down Testing- Car Craft Magazine All Pages

Left you a phone message a few days ago. Hope you're having fun in Monterey.

Ted

[Bxx1]

Hey Ted, Thanks for the Car Craft link... good reading. I arrived home on Monday at 1pm. Nice weather and lots of car viewing.

[MOTORHEAD]

OK, I missed the Cobra part of the question ! My bad .
All as mentioned above, plus, if it's a high end motor ($$$$$) send an oil sample out for analysis. They will give you the amounts of different metals in the oil and show if they are abnormally high. From this you can deduce what parts may be showing excessive wear.
I don't remember the Co.'s that do this, a google search should ferret them out.

[tcabece427]

Leak down is a good test to do as others have mentioned. If you can't get the compressor you might do a dry/wet compression test. It's a bit old school but effective. You look for the difference between the two not necessarily a number. So the dry test should register a number (use the screw in type) and that number across all 8 cylinders shouldn't vary by more than 10% between the high and low reading. Then put a few drops of oil in each cylinder (just before you wet test that cylinder). The new reading shouldn't be more than 10% higher on that cylinder than the dry test.

[BMEP]

Excellent advice above. I'd also run it long enough to get it fully up to temperature and then see what noises it makes and watch what the oil pressure does.

[Jaydee]

If you do any compression or leak down test. Loosen the plugs 5 or so turns, then retighten and run the engine for 5 minutes. I have seen false readings from carbon deposited on the spark plugs threads and as you remove the plugs, this carbon can get under the valves as you crank it. Giving you a low reading.
Another test to do , Is decelerate the car down a hill, then blip the throttle to check for smoke, indicating valve seals. And the opposite is accelerate in a really low gear to load the engine and check for smoke, indicating rings. Check the colour of the spark plugs. moisture in the oil.
Good luck with your buy.
JD

[eschaider]

As others have alread said a leakdown test and an oil analysis by someone like Blackstone labs.

The leak test will tell you the condition of the cylinders and rings and will find problems well in advance of a compression test. The oil analysis will tell you what if any parts of the engine are eating themselves or being eaten.

When you do the leakdown test you want to use one of the old Sun Leakdown testers or one of the new Snap-On testers. They are the only testers sensitive enough at low leakage percentages to show you what you actually have for cylinder/ring leakage.

Parts store and internet units that are cobbled together from air pressure regulators and various pressure gauges seem to be the same but lack the low leakage calibration and measuring precision of the Sun and Snap-On units that is necessary to accurately measure cylinder leakage.


Ed

[vector1]

Look at the coolant, look at the oil, look through radiator fins to see if they look normal. Pressure test the cooling system. Fire it up, watch for smoke, drive it, observe brake and clutch operation. After you get back look for any oil or fluid leaks. If any doubts after that do a leak down.

[slider701]

I'm curious if anyone would actually let a stranger do a compression or leak down test on a car you were selling. In theory it sounds great but I find it hard to believe that you would just trust someone to take your car to a complete stranger and say have at it.

[Bxx1]

Originally Posted by slider701

I'm curious if anyone would actually let a stranger do a compression or leak down test on a car you were selling. In theory it sounds great but I find it hard to believe that you would just trust someone to take your car to a complete stranger and say have at it.

You have nothing to lose by asking...

[Igofastr]

Sometimes the owner will supply the results of these tests.

On a Porsche forum that I frequent, this is commonplace. In fact, without them (or recent documantation of a rebuild), engins are often valued as a "core" regardless of whatever other stories the seller may be telling. Can they be faked, sure. But you'd have to have steel cajones to do that, and think you could pull it off.

[eschaider]

A leak down test is no more invasive than changing sparkplugs. If the owner of a performance vehicle is hesitant to do this he is probably hiding something he does not want you to know about.

With respect to the oil analysis, the use of what amounts to a test tube's worth of oil from the engine for analysis should not create heartburn. If it does we are back again to the question of what it is the owner is attempting to hide and why.


Ed