causes for low compression? - Nissan Forum
GA16DE 1.6L Engine Engine Discussion: 91-99 Sentra, 95-98 200SX, 91-93 NX1600

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post #1 of 39 Old Apr 24th, 2004, 11:56 AM Thread Starter
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causes for low compression?

I tested the compression last year on my 96 200SX SE and i believe it was a little low (150psi across all cylinders, but less than 10psi total variation). the GA16 engine has 130k miles on it and i want to purchase the HotShot turbo kit but i'm hesitant because of the miles and compression that the engine has. i'm afraid i will either blow the engine or not experience max power/torque readings when i dyno it afterwards.

what are the causes for low compression? is there anything that i can do or check on the engine in order to bring it back to factory spec? i really dont want to buy a brand new engine or have to rebuild it... at least not this early.

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post #2 of 39 Old Apr 24th, 2004, 04:13 PM
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have you adjusted the valve lash on your valves?

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post #3 of 39 Old Apr 24th, 2004, 11:49 PM Thread Starter
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i have not.

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post #4 of 39 Old May 19th, 2004, 10:11 AM Thread Starter
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bump

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post #5 of 39 Old May 19th, 2004, 01:34 PM
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edit nm, adjust your valve lash and see what the #'s are after that.

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post #6 of 39 Old Nov 17th, 2004, 01:33 PM
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Bump. I have a problem with my compression as well and I read this and I wanted to know what the valve lash is and how do I replace it. I kinda figured out that more or less it regulates air pressure or something to that effect, but Im not sure.

Any help would be greatly aprreciated.
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post #7 of 39 Old Nov 17th, 2004, 04:30 PM
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you dont replace valve lash. You adjust the valve opening and closing, the sealing effect when they close on the cycle, sometimes they don't close at the right time or not tight enough, so they leak. You need to do a leakdown test on all cylinders first to test valves. Compression may be lower at high miles due to cylinder wear. You can only try a can of 4 cyl restore and see if that improves.

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post #8 of 39 Old Nov 17th, 2004, 08:38 PM
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Well thanks for the info. Once the valve lash is adjusted the compression should regulate, if not am I going to need to overhaul it? And if so should I overhaul it or just buy a new "used" motor and swap it out.
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post #9 of 39 Old Nov 18th, 2004, 12:30 AM
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Quote:
Originally Posted by MCHNHED
what are the causes for low compression? is there anything that i can do or check on the engine in order to bring it back to factory spec? i really dont want to buy a brand new engine or have to rebuild it... at least not this early.
Dirty ring packs can effect compression. I am currently using a product called Auto-RX to clean the engine to prevent seal leaks. It also is cleaning the ring packs. Do a search at their Website using the word "Sentra". Some recently brought neglected Sentra back to life using this stuff.

http://theoildrop.server101.com/ubb/...=001712#000004

My experience so far is very positive. I am definitely having better performance. This also may be due to a product I am using also to clean the combustion chamber and valves called Fuel Power. I was hesitant to mention it because I was afraid it may hurt injectors. After over 2 ½ months of using the recommended dose of 1 oz. per 5 gallons of gas my idle is smoother. At start-up I used to have a little bit of a rough idle. That has cleared up. On the highway I definitely have more pick-up and better economy.

http://www.lubecontrol.com/

Rebuilding or replacing an engine is a big and expensive project. I bet if you tore down your engine it would be filthy. Even if you adjust the lifters, if the valves are dirty, you will not get the best seal. So, I am taking a two-stage approach to getting my Sentra engine as clean as possible and am pleased with the results. This is just an option before rebuild or replacement to get your car as close to factory specs as possible. If you go to this website and do a search you can find a lot of additional information on these products:

http://www.bobistheoilguy.com/

I have been also using these products in a 99 Saturn I have for the same amount of time. Dirty oil control rings can cause oil consumption in a Saturn. This car is running better than ever, idle is smoother, and has more power and better fuel economy. I am able to do both cars with Auto-RX for about $60 a piece, buying oil, filters, and Auto-RX on sale. If you include the Fuel Power, I will spend about $80 per car in about 7,000 miles of driving. Pretty good for the results I am getting. Prices have gone up since I have purchased these products, but it is still a bargain considering the results. Good luck!

Truth alone triumphs, not untruth. :)
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post #10 of 39 Old Nov 18th, 2004, 01:18 PM
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Thnax for all the information Catman, I greatly appreciate at.
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post #11 of 39 Old Nov 18th, 2004, 02:53 PM
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The compression of an engine is the pressure that is measured through the spark plug hole when the engine is turned (usually by the starter). On the compression cycle of the engine, the piston is at the bottom, and both valves close. The piston rises to the top compressing the air that is trapped. It is a measure of whether there are any leaks in the cylinder. Leaks can occur if the valves are not sealing properly and/or if the piston rings are not sealing properly.

As an engine ages, the forces and temperatures of the combustion process cause wear. Usually the exhaust valves are the first to leak because they run very hot, and oxidation wears the valve seat and valve. The piston rings move up and down against the cylinder walls, and cause the cylinders to slowly enlarge (usually more in one direction causing the bores to become elliptical) so the rings cannot seal properly.

If you turbocharge a worn engine, two things happen. The erosion of the valves accelerates, and combustion gases blow by the rings into the crankcase. This contaminates the oil quickly since the combustion gases are acidic. These gases also pick up oil mist as they escape from the engine wherever they can. This oil usually ends up in the intake via the PCV valve. Also, the engine is down on power compared to a tight engine, and usually runs rougher because all the cylinders are not sealed the same.

Another problem with worn engines is leaking valve guides. If oil leaks into the combustion chamber, it is burned along with the fuel. This has two deleterious effects. It causes deposits in the comustion chamber which can cause hot spots which cause pre-ignition. Second, it lowers the octane of the combustion mixture causing detonation in a turbocharged engine.

Lew
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post #12 of 39 Old Nov 18th, 2004, 07:37 PM
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Thanks for the information, that really helps to explain a lot of what happens and what it is that goes on in the the enigne.
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post #13 of 39 Old Nov 18th, 2004, 07:49 PM
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I'm not sure how well they work, but certain high-mileage oils and engine cleaners are supposed to help some of the rings and seals work better.
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post #14 of 39 Old Nov 18th, 2004, 08:02 PM
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Quote:
Originally Posted by Ninety-Nine SE-L
I'm not sure how well they work, but certain high-mileage oils and engine cleaners are supposed to help some of the rings and seals work better.
Yeah well Catman said that used some product called Auto-RX and thats what Im thinking of using and hoping that this will help out and also Im going to adjust the valve lash and hope that all this is going to fix the problem, cuz frankly I dont have any money to be spending on a new motor or overhauling it.
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post #15 of 39 Old Nov 18th, 2004, 08:19 PM
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Quote:
Originally Posted by Psychsal
Yeah well Catman said that used some product called Auto-RX and thats what Im thinking of using and hoping that this will help out and also Im going to adjust the valve lash and hope that all this is going to fix the problem, cuz frankly I dont have any money to be spending on a new motor or overhauling it.
Before you use these products, measure the compression. Then see if they help by measuring again after 10 tanks of gas.

"Dirty ring packs" are caused by carbon build-up in the ring grooves of the piston usually due to an engine which burns oil. This makes the ring stick in the groove and not expand to seal against the cylinder wall.

As I mentioned above, deposits in the combustion chamber (including the valves) can cause pre-ignition which robs the engine of power. These deposits do not form on the valve seats (they form on the valve face and underside of the vale) because the seats are constantly being battered by the valves and swept by the hot combustion products which keeps them clean. Additives can reduce these deposits, but if the valve seats are worn or the cylinders worn out of round, they can't help.

You've got to take the hype on a product's web site with a grain of salt. Of course they want you to think their product is the best thing since sliced bread.

Lew
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