Cylinder 3 low compression. Bad valve seal?

[Phantez]

My car has a stock s15 sr20. It's always burned a bit of oil on deceleration, but it started getting much worse towards the end of summer. I usually like to downshift through the gears as I'm slowing down and every once in a while it would make a big puff of smoke out when i grab a gear on decel. It clearly smelled like burnt oil. I didn't notice anything on accel.

The car runs fine, makes decent power. Doesn't smoke or smell like burning oil at idle or on start up. It does have some intermittent surging at idle but I'm pretty confident that's unrelated, probably TPS or something. It's been in storage for winter but i decided to run a compression test today. I let it warm up to temp, didn't smoke or smell burnt oil while idling in the garage. Results were 148, 148, 105, 148.

When i pulled the spark plugs, all 4 where a nice golden brown color at the tip. Coolant is clean and clear.

Only thing I can thing of is something with valves. Any ideas?

[Javi802]

you may have a cracked piston ring

[Javi802]

just pour some automatic trans fluid in the cylinder and see if that ups the compression.

Just a little, like a teaspoons worth

[Phantez]

Originally Posted by Javi802

just pour some automatic trans fluid in the cylinder and see if that ups the compression.

Just a little, like a teaspoons worth

I like this idea. I'll try it tomorrow after work. I'd be a little surprised if it's the ring seals only because it only burns on decel and I would guess a bad ring seal would burn all the time, but i'll see what the results are.

[Tom N]

A leak down text will answer this question.

[joe3180]

Originally Posted by Tom N

A leak down text will answer this question.

^^ what he said

[Gorman240]

Leak Down test will be your best friend, make sure Cylinder is on TDC. Heres a list of what could be broken with their respective symptoms.
Exhaust Valve- Air coming out of Exhaust Pipe
Intake Valve- Air coming out of Throttle Body
Piston Rings-Air coming out of Oil Cap on Valve Cover
Head Gasket-Air coming out of Radiator
Find your Leak=Find Your Problem! GL

[Scott@FontanaNissan]

Originally Posted by Gorman240

Leak Down test will be your best friend, make sure Cylinder is on TDC. Heres a list of what could be broken with their respective symptoms.
Exhaust Valve- Air coming out of Exhaust Pipe
Intake Valve- Air coming out of Throttle Body
Piston Rings-Air coming out of Oil Cap on Valve Cover
Head Gasket-Air coming out of Radiator
Find your Leak=Find Your Problem! GL

this right here is a nice little list of facts. good luck to you sir, Piston rings, gasket kits, all in stock!

[JSimpson]

Originally Posted by Gorman240

Leak Down test will be your best friend, make sure Cylinder is on TDC. Heres a list of what could be broken with their respective symptoms.
Exhaust Valve- Air coming out of Exhaust Pipe
Intake Valve- Air coming out of Throttle Body
Piston Rings-Air coming out of Oil Cap on Valve Cover
Head Gasket-Air coming out of Radiator
Find your Leak=Find Your Problem! GL

BOOM! exactly. Make sure the engine is warmed up or your leak down numbers will be crazy. Its done on a percent system. Process goes as follows:
1. warm up engine
2. 27mm socket on crank pulley rotate cylinder in question to TDC (1342 firing order)
3. fire up your air compressor and let it pump up to 120psi
4. close/unscrew your leak down tester and screw it into spark plug hole of TDC cylinder
5. connect air to leak down gauge
6. slowly crank up the pressure with the knob to 100psi on the pressure gauge
7. the other gauge will read a number from 0 to 100psi, this is your leak down percent.

My S14 SR at ~60k leaks down about 97-99% depending on the cylinder. Anything 92% and above is runnable. If it gets below that or is like 50-75psi your motor needs attention.

Gorman240's synopsis is spot on. It may be easier to pull off a crank case return hose to hear that leak as the oil cap is a bit close to the leaky air chuck in the spark plug hole... just making it easier to hear the leak. The intake or exhaust will be obvious and should echo through the exhaust and build up pressure if you cap them with ductape or something similar.

Good luck!

[liangsako]

just pour some automatic trans fluid in the cylinder and see if that ups the compression.

[SaUcEy]

Awesome advice in this thread. #3 is always the guilty party it seems like.

[Javi802]

The pouring of oil in the cylinder is time effective. If doing that ups your compression, you have a bad ring. If it doesn't change anything, you have a valve issue. Thats when i would leak down test it. To find whether its intake or exhaust.