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Discussion Starter · #1 ·
Hi guys,

I got a junkyard motor and I am trying to perform a compression test out of the car to check the condition of new motor.

I have flywheel attached and went to attach starter. I have no spare trans so am trying to run the test without trans attached.

My issue is that I cannot get starter to seat properly and make contact with the flywheel without pulling on bottom of flex plate, simulating where transmission would bolt up and bring it closer. However, i can’t keep my hand here during test and want starter to be secure and not risk damaging the teeth. Any suggestions??

Thank you,
-Braden

 

· SDSU Alumnus
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Why not do a cylinder leak-down instead? It will reveal more anyway, imo.

-sent from my Galaxy S22 Ultra
 

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Discussion Starter · #3 ·
Why not do a cylinder leak-down instead? It will reveal more anyway, imo.

-sent from my Galaxy S22 Ultra
I like the idea but I do not have the leak down tester or compressed air source and would have to buy these. Motor was only $300 and if it’s bad, I’ll probably get rid of car, so trying to not spend a ton just to run the test.

This may be the route I go if nobody else has ideas on how to get starter to align properly. I would be surprised if nobody had ever compression tested one of these motors out of car.
 

· Science, bitch!
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I like the idea but I do not have the leak down tester or compressed air source and would have to buy these. Motor was only $300 and if it’s bad, I’ll probably get rid of car, so trying to not spend a ton just to run the test.

This may be the route I go if nobody else has ideas on how to get starter to align properly. I would be surprised if nobody had ever compression tested one of these motors out of car.
Would you be working on the car yourself? Go buy the proper tools or get another hobby (not trying to be harsh, it's just reality). You can use an air compressor for all kinds of things. And you can rent a leak down tester from Auto Zone.
 

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Discussion Starter · #5 ·
Would you be working on the car yourself? Go buy the proper tools or get another hobby (not trying to be harsh, it's just reality). You can use an air compressor for all kinds of things. And you can rent a leak down tester from Auto Zone.
Thanks for the tip about renting the leak down tester. I may get one of those cheap $70 compressors from Harbor Freight to do the job.

Anybody else ever compression test out of car? Would still like to go this route if possible and appreciate any input
 

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93 NA ATX 3000gt DOHC
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Anybody else ever compression test out of car? Would still like to go this route if possible and appreciate any input
Never actually tried to do it, but seems to me if you could wedge proper size piece of wood between the engine block and other end of starter. That would push that end of starter outwards from engine, forcing pinion gear end towards flywheel and deepen teeth engagement at same time while starter is securely bolted to motor plate.
 

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Discussion Starter · #7 ·
Never actually tried to do it, but seems to me if you could wedge proper size piece of wood between the engine block and other end of starter. That would push that end of starter outwards from engine, forcing pinion gear end towards flywheel and deepen teeth engagement at same time while starter is securely bolted to motor plate.
Thanks for the advice again. You've been very helpful on multiple of my posts and I really appreciate the assistance!

My Uncle and I were able to complete the compression test the other evening, and I am pretty pleased with the results we obtained (see chart below). The motor was cold since it was sitting out of the car on a hoist when we performed the test. So these results represent the worst-case scenario, and are likely 5-10 or so psi higher on each cylinder.



Due to my lack of understanding on how a starter engages the flywheel, I didn't realize that the engagement gear actually jumps out when the starter receives power. Therefore, once we had the starter bolted up and aligned with the teeth on the flywheel, it worked once we supplied it with power.

I did not get to finish the wet test because I didn't want to wear the flywheel unnecessarily. It wasn't lining up perfectly well and chipped a tiny bit of metal off of a couple of teeth of the flywheel. I know it will still work for my purposes, but ideally I recommend folks use a used flywheel that you don't care about too much and try your best to get the starter engagement angle right if you don't have the transmission available. I used some shop rags wedged in between the block and the starter to help firm it up, but it was still moving more than I would have liked. Best to try this if you have the transmission attached to the motor, but now I know that it CAN be done without it in a pinch.
 

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93 NA ATX 3000gt DOHC
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Thanks for the advice again. You've been very helpful on multiple of my posts and I really appreciate the assistance!
.....................................................................
I used some shop rags wedged in between the block and the starter to help firm it up, but it was still moving more than I would have liked.
You’re more than welcome for any assistance that I’ve given.

Starter alignment and movement would have been better if you’d have used something more rigid to stabilize the free end of starter, instead of shop rags. But none the less those dry compression numbers are acceptable, being above the factory minimum of 139 psi and within the recommended variation of 14 psi.
 

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Due to my lack of understanding on how a starter engages the flywheel, I didn't realize that the engagement gear actually jumps out when the starter receives power. Therefore, once we had the starter bolted up and aligned with the teeth on the flywheel, it worked once we supplied it with power.
Sorry I didn’t explain starter operation, assuming you already knew that. Should have suspected something since first picture showed starter bendix pinion gear touching the flywheel teeth, verses second picture showing pinion gear in about correct position/alignment to engage flywheel when it slide out. I just knew when it engaged the flywheel and starter began turning egine, the pinion would have an outward force maybe flexing the motor plate and allowing opposite end of starter to toque towards engine. That’s why I suggested something rigid back there to stop that movement.

If you still don’t quite understand starter solenoid operation, when ignition key turned to start position the solenoid receives power on small wire “S” terminal. That draws solenoid plunger towards that end if solenoid, the plunger is attached to a lever that causes bendix pinion gear to slide out into flywheel teeth, before a disc inside solenoid makes contact to the 2 large wire terminals, which then sends 12 volts from large wire from battery to large terminal wire going into starter motor causing it to spin turning the engine.
 

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Discussion Starter · #10 ·
You’re more than welcome for any assistance that I’ve given.

Starter alignment and movement would have been better if you’d have used something more rigid to stabilize the free end of starter, instead of shop rags. But none the less those dry compression numbers are acceptable, being above the factory minimum of 139 psi and within the recommended variation of 14 psi.
Thanks for the explanation on the starter. Always learning new things here! Where did you get the 139 psi minimum for the SOHC? I thought I had read that it was 125 psi, and was just curious which is correct.

Just pulled the upper and lower intake, exhaust manifolds, and vacuumed out the pile of turds, acorns, and leaves sitting on the coolant pipe in between the block. Time to get into this project! Glad the compression test went well. If I get the chance to borrow my friends air compressor, I plan to do a leak down test just to make sure the rings are in decent shape.

I did find that the oil pan is dented in pretty good. It is possible that it could have been damaged during moving, but it is something I forgot to check right when I got the motor, and now I see that it is clearly dented. I am planning on either banging it out, or replacing the whole pan and oil pickup tube with the one from the current motor in the car. I know sometimes if the pan is dented, it can damage the pickup tube/screen. Curious how to check this, or if it is pretty obvious to the naked eye.
 

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93 NA ATX 3000gt DOHC
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Thanks for the explanation on the starter. Always learning new things here! Where did you get the 139 psi minimum for the SOHC? I thought I had read that it was 125 psi, and was just curious which is correct.

Just pulled the upper and lower intake, exhaust manifolds, and vacuumed out the pile of turds, acorns, and leaves sitting on the coolant pipe in between the block. Time to get into this project! Glad the compression test went well. If I get the chance to borrow my friends air compressor, I plan to do a leak down test just to make sure the rings are in decent shape.

I did find that the oil pan is dented in pretty good. It is possible that it could have been damaged during moving, but it is something I forgot to check right when I got the motor, and now I see that it is clearly dented. I am planning on either banging it out, or replacing the whole pan and oil pickup tube with the one from the current motor in the car. I know sometimes if the pan is dented, it can damage the pickup tube/screen. Curious how to check this, or if it is pretty obvious to the naked eye.
I just used the naturally aspirated DOHC spec’s as it and SOHC engines has same compression ratio of 10:1. I’ve not seen the actual compression spec’s of SOHC engine, but with the same compression ratio, the min/max should be in same range with each other. The DOHC engine having 2 extra valves wouldn’t chance compression readings.


Not sure you need to do leak down test with dry/cold compression in 150 psi range and a spread of only 7 psi, By you only mentioning piston rings check in leak test, makes me think you don’t completely understand all the advantages of that test.

Cylinder being tested has to be on compression stoke, preferably at TDC. Depending what air pressure leak tester requires or suggest, sometimes if not at TDC the air pressure will push piston down (rotating engine). When air is applied, you’ll not only see the percentage of air lost but you can tell what is causing that loss. If by rings you can hear/feel air at oil fill hole on valve cover. If intake valve leaking you can hear/feel air at intake plenum, in your case at head intake port with manifold off. If exhaust valve leaking you can hear/feel air at exhaust pipe, again in your case at head exhaust port with manifold and piping off. It’s a great test to determine what’s exactly wrong with a cylinder especially with engine still in car, to determine if engine even needs to be removed or torn down.

For sure you want to correct that dented pan or change it. The pickup tube can mostly be visually inspected for obvious damage to bottom (screen area) and any bends of tube that looks not normal.
 
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