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how to tell if motor is any good

Discussion in 'The Garage' started by bigbadchev84, May 5, 2005.

  1. bigbadchev84

    bigbadchev84 1/2 ton status

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    i am going to compression check a motor i picked up a moth or so ago. a buddy gave it to me out of a wrecked parts truck... i dont know anything about it. is there anything else that i should check? i am wanting to stick it into my blazer. the motor i currently have only has 25lbs of oil pressure at most while revving 0 at idle and extremely guttless to boot.
     
  2. Jagged

    Jagged 1 ton status

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    ever try replacing the sending unit or oil pump? (both are relatively easy)
     
  3. bigbadchev84

    bigbadchev84 1/2 ton status

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    i got 2 gauges mech. and elect. both are very low. i think this motor is on its last leg. what are the bare minimums on compression numbers?
     
  4. Jagged

    Jagged 1 ton status

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    i did a compression test on my 350 a few weeks ago before i replaced the heads, had about 100k miles on it... all cylinders were 180 while #1 and #6 were 160; 160 is getting a bit on the low side, but within acceptable limits imho.
     
  5. bigbadchev84

    bigbadchev84 1/2 ton status

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    is there anything else that i should check before i swap them out?
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    check the end play...

    First off, if it were me,I would NOT swap in a motor with oil pressure problems,if your sure the gauges are accurate,the bearings,oil pump or crank need work...and you'll be yanking it right out again!

    The compression test is a good guide to the condition of the rings,valves,and cylinder walls,but little else...I've had motors with only 90-100 lbs of compression that ran decent,but used oil rapidly,like 500 miles or less to a quart..

    Check for excessive end play in the crank by pulling in and out on the flexplate or harmonic balancer--if it moves more than .020 or,so,the thrust surfaces on the rear main bearing are worn--and that means new bearings and maybe crank work need to be done--

    An old motor might need head gaskets too--if it sat with no antifreeze a long time,the steel OEM head gaskets often rust thru and leak when the engine gets "woken up" after a long nap.. :(

    I was shocked to see my 400 small block I pulled out of my 74 K20 had almost an 1/8 of an inch of end play--I noticed it while removing the tranny torque converter bolts!--and the motor ran decent,and never made any noises!!--the oil pressure was always on the low side at idle for the 10 years I drove it,but it never knocked or anything.. :confused: --I'm selling it as a rebuildable core....I'd keep it but I need money more than another motor gathering dust... :crazy:
     
  7. bigbadchev84

    bigbadchev84 1/2 ton status

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    u must have mis-read my post....the motor in the blazer right now is on its last leg with hardly any oil pressure..........i have the other motor sitting on my shop floor that i know nothing about and niether does my friend......he bought the truck for its running gear.....i just wanted to make sure i didnt go thru swapping motors to find i put a bigger piece of crap in than what was in there before although i am starting to think even a 4cyl would be a better candidate than what i have in there now.
     
  8. 1977k5

    1977k5 3/4 ton status Vendor

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    Check the oil pressure with a gauge (if you can run it). A compression test is pretty much all that you can do, and don't worry about getting 180 psi at all cylinders, all you are looking for is that they are nearly the same (the numbers change with altitude, cam, weather, etc.)
     
  9. 79chevyk10

    79chevyk10 1/2 ton status

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    It would be eisier to run a leak down check rathter than a compression check. Assuming that it is sitting on you shop floor without a flexplate or starter on it. You do not need to crank the engine for a leakdown check and it will tell you percentage of cylinder sealing ability. I would also rec. putting it on an engine stand and pulling the pan off and look at the debris in the bottom of the pan, then pull off a main cap and rod caps to inpspect the bearings. At this time you can see a little of the cylinder walls on the bottom side and check for scoring. Then I would take the intake off and inspect for proper valve train operation and lift (check for a flat lobe on the cam). If all checks out okay then give it a try on the install and see what happens. It might not be a bad idea to tear it down for a complete inspection, depending on how much you like engine swaps and your skill level.
     
  10. bigbadchev84

    bigbadchev84 1/2 ton status

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    how would i go about performing a leakdown test? i have swapping engines down to a science now-about an 1hr-2hr to remove, replace and finish tuning, just dont want to bother until i know what i got this time, i hhave had bad experiences in the past.
     
  11. Fierospeeder

    Fierospeeder Banned

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    The leakdown test is a tool that takes (air compressor) air and puts it in your cylinder, while at TDC. There is a gauge that will indicate how much air leakage is present in your engine. Also, you will hear where the air is escaping which will inform you what is worn on the motor.
     
  12. 90blzr

    90blzr 1/2 ton status

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    Yes, I am curious about a leak down test also! Was gonna do a compression test, but realized the motor must be under the power of the starter to crank over fast enough to get an accurate reading.
    Never thought of a leak down test.
     
  13. 90blzr

    90blzr 1/2 ton status

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    What parts can you tell are worn from a leak down test?
    Can you get the tool from autozone or kragen? Any idea on cost?
     
  14. Fierospeeder

    Fierospeeder Banned

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    the cheapest i have seen them is around 50 dollars. I dont know if autozone carries them. You can call and ask if they have a rental tool... leakage tester.


    You apply 90psi of air from your compresser which is constant. Then you will see the gauge drop and they have it labeled indicating how much is leaking. This is because the engine isn't sealed.

    You diagnose leakage by ear. If you hear the air escaping into the coolant jackets, then that means your headgasket is bad.
    Air escaping into the bottom and you can hear it from a pcv hose, then your rings are bad.
    Hear air leaking into the valve cover then you have a bad valve.

    If you have a bad enough crack, you can hear that too.


    You might be able to make one yourself. They sell a tool where you can hold the valves in the engine while changing valve springs. Buy one of those and get a regulator and get another gauge. And see how much air is leaking. Or just use it to see if theres anyplace where a lot of air is leaking from.
     
  15. 90blzr

    90blzr 1/2 ton status

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    so if you hear air leaking from the bottom (assuming the engine is on a stand) the rings are bad? Does the tool tell you what is 'normal' leakage or? I mean its not gonna hold pressure indefinately right?
     
  16. 90blzr

    90blzr 1/2 ton status

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    nevermind, just re-read your previous answer. :crazy: :grin:
     
  17. 79chevyk10

    79chevyk10 1/2 ton status

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    It's call a leak down tester that can be bought at a tool truck like Snap-on or a parts store like Napa. Its a two gauge manifold. You must have compressed air. Plug the air hose into the gauge and regulate it to lets say 100 psi, the other gauge will read 100psi also if nothing is pluged into the other gauge. Then screw in the compression tester hose into the spark plug boss. Put the cylinder on top dead (both valves closed with piston on top of its stroke) plug the hose from the cylinder head into the leakdown gauge and it will tell you how much of the 100 psi the cylinder is holding. If the engine is healthy it should be about 90 psi holding (10% leakdown). If the leakdown is below 80psi, squirt some oil into the cylinder then check again. 80 psi holding is not that bad. Don't bother with the new motor is the leakdown is above 30% leakdown. This test will also tell you wear the leakage is coming from. If you hear air coming from the intake port then it the intake valve thats leaking, if you hear air coming from the exhaust port then its the exhaust valve that leaking and all you need is a valve job. if you hear alot of air from the crankcase then the rings are leaking. Be aware that no engine is 100% effiecient, there will always be a little air getting past the rings. :grin:
     
  18. Fierospeeder

    Fierospeeder Banned

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    i have to make this short

    the rings will have leakage. But the gauge will indicate how much leakage is bad.

    l8er
     
  19. 79chevyk10

    79chevyk10 1/2 ton status

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    You guys replied while I was still typing. Damm I'm slow. :D
     
  20. 90blzr

    90blzr 1/2 ton status

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    SLOW POKE!! :laugh: Naw thanks for the explanations! Much appreciated!! I was just asking another member about this same situation! Funny how there always seems to be somone else doing the exact same thing at the same time!

    I assume the heads have to be on and sealed (as in new head gaskets?) I already pulled the heads :crazy: :mad:
     

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