Compression/Leak down test results.

[Chris28]

Ok, I'm posting in the ka-t section because you all are a thousand times more knowledgeable than 240 gen chat, and I'll be boosting soon haha. So a few weeks ago I did a compression test, 180 psi on the first 3 cylinders but 120 psi on number 4. I had no clue what it could be so I just performed a leak down test...which showed nothing. We hooked up everything right, let loose the compressed air and nothing. We didn't hear air leaking from anywhere. I then re-hooked up the compression gauge, and got a reading of 150. I'm not sure a) why I got a higher reading this time, and b) why the leak down test was inconclusive. The engine isn't smoking, revs normally, it just has really low compression in one cylinder. Also, it takes me about 18 seconds to get from 0-60. Haha not sure what that has to do with it, but everyone that has driven my car says it's really slow for stock. Thanks for reading my wall of text haha, it's just I'm not sure what is wrong and I want to make sure i know what is wrong before rebuilding it. Thanks.

-Chris

[red_rain]

more than likely your headgasket is starting to wear down or piston rings are going bad.....do a few more compression test see wat ya got for that cylinder.....

[Chris28]

Ok, but if the head gasket were going bad wouldn't at least one cylinder have bad compression also? The fact that the others were so good leads me to believe it's something wrong with just that cylinder, but i'll keep checking. I guess a rebuild is in order then.

[red_rain]

yes if it were headgasket, another cylinder would be at fault with it....more than likey that cylinder has bad rings......if the tests still shows low compression, i would recommend doing a rebuild......good luck.....

[480sx]

When did this start happening?

Very unlikely that your other cylinders would have perfect compression and it would be as simple as number 4's rings were trashed so bad that they would give you 120 psi.

Idk im leaning towards ringlands or something, but your not boosted. Its not going to be anything in the head or the leak down test should have caught it. Would be kinda helpful to do a professional leak down test.

Have you dropped a cap full of oil down into the cylinder through the spark plug wells? Do that and see if your compression goes back up. Go drive your car around and get everything hot, then compression test cylinder number 4. Then drop the cap full of oil into number 4 cylinder and retest. Make sure your throttle plate is open when your doing your compression test.
Modified by 480sx at 11:41 AM 10/9/2008

[Spinn3r]

I'm pretty sure bad rings, I guess oil ring is still fine thats why you dont see any smoke, drop a cap of oil to #4 after engine is still hot and see if comp ratio change, if headgasket im pretty sure it would be blue smoke.

[Chris28]

480sx, it never really "started" happening, I've had the car fro about a year and it's always been this way. I never really thought about it, i just thought well the engine is like 16 years old so it's going to be slow. It was after I let someone drive it about 2 weeks ago that they said something was wrong. That person used to have a sohc in his 240 before going sr, and he said even the sohc seemed faster than mine. That's what prompted me to do the comression test in the first place. I'll see what the cap-full of oil does, i'm really not sure what happened to my compression haha. Thanks for the responses guys.

-Chris

[WDRacing]

Here's what I would do, get a can of sea foam and a can of engine flush and engine restore from Auto zone. Use all three, then run some 10-50 generic oil after the flush with the restore. Drive car for 1 week, retest.

Let us know the results.

I say the above because it will eliminate a lot of problems caused by bad lubrication and or clogged intake/head/exhaust/sensors/etc. I'd swap the plugs and wires, because it will need to be done anyway once you add boost.

This along with the fuel and air filter is where you should be taking your baseline numbers from.

WD

[Chris28]

That's pretty much the first thing I did. I went to Advance Auto and got 2 cans of seafoam, one for my gas tank and a third of the other for the intake vacuum hose. After that was all out of the system I cleaned the maf with maf cleaner, then installed new plugs. Oil was changed about 250 miles ago after seafoaming the crankcase, and I refilled with royal purple. The only thing that I didn't do was the engine flush and restore. The compression test was done after everything above. I don't really want to have to change the oil again so soon, but do you think the engine flush and restore would really help?

[WDRacing]

I've pulled the valve cover off of cars before and after the engine flush...TOTAL difference. Before had carbon buildup and sludge ALL over everything. After was shiny clean...FTW.

It also loosens up rings and generally breaks up gunk in the lube system...far better then seafoam. DO IT!!!

Then swap in some elcheapo dino oil and retest. No need to run expensive oil unless you're setup with an aftermarket PCV system, the stock one is crap. But that's a whole different story. Expensive oil is good for racing and engines that see sustained high rpms...other then that, cheap oil ftw.

WD