Can you compressions test an unattached motor? Please Help?

Question

Is it possible to do a compressions test on a motor that is completly out of a car?
I need to know if a motor is good before bought, is there any other way to check check if a motor is good short of a dyno? If the compressions test shows the motor compresses fine, what other could be wrong with the motor that would not be worth fixing or is unfixable?

Answer 1

As long as the motor has a starter and oil in the pan, and you have access to a well-charged battery and some cables, you can do a compression test on an uninstalled engine. If the engine is out of the car and you do not know if it is good, I would do a compression test to confirm compression. Then I would replace all the basics, such as timing belt, balance belt (if it has one), water pump, thermostat, front and rear main seals, valve cover gaskets and probably the oil pan gasket. If I had the oil pan gasket off, I might replace the rod bearings and main bearings.

Any number of things could be wrong with the engine, but you won't know about many of them until you try to start it. There could be fuel injection problems, leaky/blown manifold gaskets, ignition/timing problems, sludge and other things. (You'll probably see sludge if you take the oil pan off.)

Answer 2

As long as the starter works, you can compression test the engine. Although a compression test is better performed with warm/hot engine. With the plugs removed, turn the engine by hand using a ratchet on the harmonic balancer - listen for unusual noises and "hard" spots while turning. Using the starter, check the compression in each cylinder and write down the results. They should be within 10% of each other. Next, squirt oil in each cyl. and do another compression test. Significantly higher readings indicate the rings are worn and need to be replaced. You can also remove the valve cover and make sure each rocker arm moves freely and there are no bent push rods, etc. Reputable businesses should give some type of warranty/guarantee on a used engine. Good luck.

Answer 3

Hello,

others told you about compression metering, so here's some piece of advice before fitting the engine:
- cylinder head (no leaks, cracks, no oil/water mixing, valves, valve seats and spring, camshaft wears, cam chain, arm rockers/tappets (pushrod engines), inlet/exhasut ports)
- engine block (cracks->block is a junk, sealing pans, intact connecting surface with the cylinder head(s if V6/V8))
- electronics (ECU test, error code readout, sensor failures, idle regulator)
- fuel system (fuel pump, fuel filter, fuel rail, pressure check, fuel lines leaking or not)
- electrics (starter motor, alternator)

The most important is the block and cylinder head, they aren't reasonable (mostly impossible) to repair.

1. Don't fit it without ripping apart and check.
2. Always change when engine is disassembled:
- driving belts (alternator, cam(s))
- oil
- gaskets
- filters(oil, fuel and air filters)

3.Clean the block and the major parts see the damages.

4.When the condition of engine is unknown:
- check comp. (you know how to do it :D.)
- change the water pump
- do water drill/radiator flush with appropiate chemicals (eg. Kleen Flo)
- change fuel and water lines if possible
- cracnkshaft bearings, cam bearings, piston bolts, con rods (I would weigh-scale them if they have the same weight (in grams!) and match the weights. But basically every moving/rotating part should have the same treatment ! A MUST if you want to tune the engine!)
- counter-weight the crankshaft with the flywheel on it and mark the mutual position
- look at the inner surface of the belt pulleys (on burnt surface the belt will slip, so clean/polish it a bit)

5. Generally, as a rule of thumb, when the engine repaired is assembled properly (I mean correct cam phasing and stuff :D), should be possible to turn by hand without stucks. When the engine doesn't seize, basically it can't be nothing terrifiying ;D

6. When your block is intact and cylinder head is all right, the other parts can be repaired, only the matter of time, nerves and money. :D

The list is endless, but I hope you can go on with these ...

Regards

Answer 4

Yes you can check compression on an unattached motor. Some of the better engine rebiulders do it on exotic engines to check compression and oil pressure before releasing the engine to the customer. They spin the flywheel on a special stand with an electric motor at various rpms to do their checks. You can do it by attaching your starter and check compression by turning over the starter with a good battery.

Answer 5

With the engine out you may examine to be sure if there is compression stress. It fairly couldn't be appropriately checked. F.I.Y. If the engine runs it ought to be warmth and function gas/air blend contained in the cylinders. If it in basic terms cranks over all you would possibly want to do is study readings in the course of the cylinders, it really is really a lot an same for a warmth engine.

Answer 6

First of all, yes you can...but it seems you know little if anything about engines, so you should get a mechanic or someone you trust that knows such things to help you decide if a particular engine is what you're looking for. Good luck.