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Hey guys, I’m going to perform my first compression test, as I’ve noticed blow by from the engine. 92,000 original miles, 1961 98, 394, 10.5:1 compression.
What is the proper way to test? Will I need to unplug / disconnect anything more than all the spark plug wires and remove all plugs to test each cylinder? Looking for some guidance for my Olds. I’d like to see what kind of condition the engine is in, thinking it’s not the best, since there is blow by.
If you disconnect the power (small wire) to the coil and protect it from grounding OR remove the high tension lead out of the center of the distributor and ground it you won't get bit electrically.
Keep the throttle open.
Run a dry test first, followed by a wet test.
Post your results. The focus is more toward variation between cylinders than higher readings but both matter.
Also post pictures of the spark plugs keeping them in order.
Last edited by Sugar Bear; April 29th, 2024 at 05:51 PM.
If you can create a modest/simple table (example below) such as my dry/wet compression test results - makes it easy to decipher. Remove all the spark plugs, no sense running down the battery and creating additional wear on the IGN system e.g. starter.
It would be a good idea to have another car on hand that you can hook a set of jumper cables to. Cranking 8 cylinders 2 times can drain even a healthy battery.
Ideally, you want the engine warm, and the battery fully charged. Remove all the plugs, disable the ignition. Wire the throttle wide open. Put the compression tester in, and crank the engine. Crank the engine, note what the reading is after the first puff, and then the final reading.
The textbook results: the first puff should be at least half the final reading. There shouldn’t be more than 20% variation between the highest and lowest reading. Put a couple teaspoons of oil in each cylinder, run the test again. If there is a dramatic improvement, the rings are worn. If the results are even but low, the timing chain is probably stretched. If you have a cylinder that’s really questionable, it’s time for a leakdown test.
Ideally, you want the engine warm, and the battery fully charged. Remove all the plugs, disable the ignition. Wire the throttle wide open. Put the compression tester in, and crank the engine. Crank the engine, note what the reading is after the first puff, and then the final reading.
The textbook results: the first puff should be at least half the final reading. There shouldn’t be more than 20% variation between the highest and lowest reading. Put a couple teaspoons of oil in each cylinder, run the test again. If there is a dramatic improvement, the rings are worn. If the results are even but low, the timing chain is probably stretched. If you have a cylinder that’s really questionable, it’s time for a leakdown test.
How long should I be cranking? So far, just started, but been cranking for about 6 seconds. All cylinders, dry, after the 6 seconds and hand tightened gauge into the cylinder, reads 150 psi so far….3 cylinders to go
What reads 150 psi so far? Be specific. Cylinders have numbers. You MUST identify each numbered cylinder with its unique psi. Create a table by cylinder number for both dry & wet. Read Post #4.
BTW, you perform both the dry & the wet compression test on each cylinder one-at-a-time. You DO NOT perform the compression test dry on all eight cylinders then backtrack again performing the same test wet on each cylinder. You do both dry & wet on each cylinder one-at-a-time.
Here is the data. I had to go back and re-do each cylinder like you said Norm. Dry and then wet. I cranked for about 6 seconds on each cylinder, for both wet and dry tests. Looks like an issue with #'s 5 and 7. Thoughts??
Cyl. #7 demonstrates 27% difference in compression between dry & wet.
Cyl. #5 demonstrates 9% difference in compression between dry & wet.
Overall difference in compression between lowest (dry) and highest (dry) = 27%
Overall difference in compression between lowest (wet) and highest (wet) = 6%
Overall difference in compression between lowest (dry) and highest (wet) = 31%
Greatest difference in compression between two neighboring cylinders (#7 & #6/#8) = 27%
Suggest leak down test next. Cyl. #7 is significantly losing compression. Cyl. #5 is most likely OK.
Cyl. #7 demonstrates 27% difference in compression between dry & wet.
Cyl. #5 demonstrates 9% difference in compression between dry & wet.
Overall difference in compression between lowest (dry) and highest (dry) = 27%
Overall difference in compression between lowest (wet) and highest (wet) = 6%
Overall difference in compression between lowest (dry) and highest (wet) = 31%
Greatest difference in compression between two neighboring cylinders (#7 & #6/#8) = 27%
Suggest leak down test next. Cyl. #7 is significantly losing compression. Cyl. #5 is most likely OK.
Thanks Norm. This will be my first leak down test as well. Any suggestions for leak down test kits?
It's possible (perhaps likely) cyl. #7 (requires) rings; but, a leak-down test should be performed to assist in narrowing this down. In the grand scheme of things, the compression test is an overall measure of engine health and the psi/cyl. you evaluated in a first step in suggesting where might an issue reside. Generally speaking if the compression increases by 40 PSI (generally 20%) or more (between dry & wet), the problem is a poor ring to bore sealing & the most common/likely cause is poor rings making contact with the cylinder bore/sleeve. Sure, there's the off-chance the bore/sleeve/cyl. bore is gouged or a ring is broken, but often times it's a worn ring or set of rings. A leak down test would help to diagnose further if this might be poor valve seating, a worn/blown head gasket or piston rings.
I'd look for something in your affordability range. There is a good chance Autozone or another parts store will allow you to rent (for free) a leak-down test kit. Call a couple of them and ask. Many have them to loan out for free (sometimes rent). If you want to own one, price ranges are all over the place, but I have a Pittsburg - it's w/in my price range for as often as I need to perform a leak-down test and it works well. I mean, this one is worth a look anyways. A professional mechanic test kit (used on a daily/weekly basis is going to run $100+)
Before buttoning it back up or pulling it apart I'd do a chemical ring soak with something like Marvel Mystery Oil on all of the cylinders for a few days then run it hard afterwards and re- test #'s 5 & 7. Since 5 & 7 are on the same side raise only that side of to the car to partially level the piston, it won't completely level it but it helps.
After the ring soak, get it hot and accelerate hard from 20-60 mph and let it coast from 60-20 repeating this approx 10 times. This will work the rings back and forth in the grooves. If the rings are stuck badly this won't work but if they're just sticky there is a fair chance it will work.
Last edited by Sugar Bear; April 30th, 2024 at 06:12 PM.
Greatest difference in compression between two neighboring cylinders (#7 & #6/#8#5) = 27%21%
Originally Posted by Sugar Bear
Before buttoning it back up or pulling it apart I'd do a chemical ring soak with something like Marvel Mystery Oil on all of the cylinders for a few days then run it hard afterwards and re- test #'s 5 & 7. Since 5 & 7 are on the same side raise only that side of to the car to partially level the piston, it won't completely level it but it helps.
After the ring soak, get it hot and accelerate hard from 20-60 mph and let it coast from 60-20 repeating this approx 10 times. This will work the rings back and forth in the grooves. If the rings are stuck badly this won't work but if they're just sticky there is a fair chance it will work.
Jeff - Those are good suggestions. Reading your advice allowed me to re-read (validate) & correct my data (above). The greatest difference in compression between two neighboring cylinders is not #7 & #6/#8 (but is instead between #7 & #5 (21%).
Jeff and Norm, at this point, plugs are out still. How much Marvel Mystery oil should be poured into each of the cylinders? How long to let that sit? After this soak, I can re-test the compression, and then if things still haven't improved, do a leak down test on #5 and #7 cylinders? Does this sound ok?
About a teaspoon per cylinder: but, visualize the angle of the piston because of the V shaped engine block, most of the Marvel Mystery Oil (MMO) will go toward the bottom of the piston/cylinder edge. Try to squirt the MMO toward the back of the cylinder wall if possible, jacking up that side of the car will help get more of the MMO to more of the ring area as the angle is lessened. This isn't an issue with most inline engines as the pistons face straight up. After installing the MMO and with the plugs out, crank the engine a few turns by hand to work it in and then let it sit for a couple of days if you can. Crank the engine again with the plugs out using the starter to blow out any excess oil and gunk. Install the plugs, start the engine and do the 20-60, 60-20 procedure run it for a few more days, "re-test and hope for the best."
Don't crank the engine using the starter with the plugs installed until after the MMO has been blown out by cranking it with the plugs removed.
The car will smoke a lot on startup, this should be done outside.
Do an oil and filter change afterward.
If the compression comes up, great. If not, you did all you can and it is time for rings.
The biggest hassle with using a leakdown tester is getting the piston exactly at TDC. If it’s off, air pressure will force the piston down.
The easiest way I have found to get around that problem is to get the piston close to TDC, and clamp a couple vise grips to the flexplate. Position the Vice grips in a place where the engine rotation is minimized.
Just remember to remove them BEFORE trying to start the engine!!
Matt, good point about the air moving the piston/crank assembly; however, I don't think he will be able to access the flywheel/flex plate on that car as mentioned. Going to have to find another way, maybe a bar secured to the balancer?
Actually, @joe_padavano recently mentioned in another thread that you can use a tissue in the spark plug hole to find TDC. This is an excellent trick. You know you are at TDC when the tissue blows out of the spark plug hole. 🤔
Actually, @joe_padavano recently mentioned in another thread that you can use a tissue in the spark plug hole to find TDC. This is an excellent trick. You know you are at TDC when the tissue blows out of the spark plug hole. 🤔
Exactly. I used wadded up paper towel, but same thing. You can stand outside the driver's door, bump the key, and watch for the paper to blow out.
The tissue blowing out method is used to determine when a piston is beginning the compression stroke moving toward top dead center. It isn't accurate to determine whether or not a piston is at top dead center for conducting a leak down test.
The tissue blowing out method is used to determine when a piston is beginning the compression stroke moving toward top dead center. It isn't accurate to determine whether or not a piston is at top dead center for conducting a leak down test.
Of course not. Once you know the #1 cylinder is on compression, simply align the timing mark at zero. Done. Now rotate 1/4 turn at a time for each subsequent cylinder in the firing order. Don't make this harder than it needs to be.
Of course not. Once you know the #1 cylinder is on compression, simply align the timing mark at zero. Done. Now rotate 1/4 turn at a time for each subsequent cylinder in the firing order. Don't make this harder than it needs to be.
This is the OP's first compression/leak down test. What may be obvious to an experienced mechanic probably isn't obvious to him; hence, the detailed clarification.
Certainly not trying to make it harder, rather clarified for the OP.
TP, tissue paper, paper towel is great for determining compression stroke. A plastic straw, chop stick or BBQ skewer (metal or wooden) works well to locate TDC.
This is the OP's first compression/leak down test. What may be obvious to an experienced mechanic probably isn't obvious to him; hence, the detailed clarification.
Certainly not trying to make it harder, rather clarified for the OP.
Attempting logic w/ Curmudgeon? A kick to the curb screaming get off my lawn can be an effective alternate.
April 29th, 2024, 05:29 PM
