Bad compression on piston #4

[fvittes]

I have a nissan 240sx 1997. I was wondering if anybody has faced the following issue, I am running a 80shot of NOS on it, and after one of the races the engine started shaking. The AFR in the NOS was tuned properly so that ther was no leaning. I took the car to a mecanic and this is what has happened:

1. Pistons 1-3 had 150 psi compression, piston #4 had 40 psi. There was no smoke comming out of the tail pipe, just the engine was shaking. The mechanic diagnosed that the intake valve(s) at piston #4 were nopt sealing properly.

2. A head job was done, all valves were checked and in some of them the valve seats were changed. Also head was machided 15 thousands of an inch. The head was put back in the car, and the psi on piston 4 is still 40 psi.

3. Before the head job, the mechanic checked the piston rings and the piston. he put oil on top of the piston and said it took a long tome for the oil to dissapear, also like i said there is no smoke of any color comming out of the tail pipe.

4. Now we tested piston number for by positioning the engine position so that piston 4 has both intake and exhaust valves closed. We blow air into the cylinder compartment via the spark plug hole using a piston leak device and the satrangest thing happens: Air does not come out from the intake or the exhaust. There is air coming out from the valve cover. Some air comes out from the air duct that connects the valve cover to the air intake pipe and some air comes out from the drain to poor the oil in.

So it seem as if there is some kind of whole somewhere. It does not seem to be the piston or the rings, does anybody out ther have any suggestions on what else to test or a diagnosis procedure we could use.

Thanks for any help you can provide.

FV

[JustinHallenbeck]

Sounds like you blew a hole in your combustion chamber, or cracked it. Also i hope he replaced the head gasket when he put the head back on. Take your valve cover off and see where the air is coming from, try to trace it.

[fvittes]

Thnas for your reply, i will go to the shop tomorrow and look in detail

[orion::S14]

No, I'd bet $50 you cracked the ring land and need a new piston...here's a shot of a #4 piston in a KA24DE after light detonation:



It's hard to crack an iron block, and since the head is good, it's not anywhere above the piston...

I think you need a new or better mechanic...hehe.

Good luck with the rebuild...I reused my rings, rod, wrist pin, and bearing...and it still runs like a champ, except I used the wrong grade piston (and a used one at that!) and that motor is no longer in my turbo car, but it still lives in my daily driver and runs great, with a hint a piston slap at idle due to the wrong grade thing.

- Brian

[sykikchimp]

You shouldn't just test leak down at TDC. As long as you are still on a part of the stroke with the valves closed you can test how the cyclinder is holding pressure anywhere in the stroke.

You could have cylinder wall damage further down that is causing problems.

[orion::S14]

...but if the leak down had air comming from the valve cover that would indicate bad valve seals? no?

(Don't know where this reply went, but I got it in the e-mail notification on the thread...?)

But to reply, no...air leaking from the valve cover is probably coming from the crankcase - Think, you pour oil into the valve cover, and it drains to the oil pan. If you pressurize the cylinder, and air leaks past the rings or past cracked ring lands, then it pressurizes the crankcase and air goes up the passages that oil usually flows down...

Hope that helps - Brian

[fvittes]

I appreciate everybody's reply. I will let you know what is the final diagnosis.

[JustinHallenbeck]

You can crack a iron or alum head preaty easy. I had a set of iron heads on my LT1, the head gasket went and the temp shock snaped the head right in half. But also i agree with sykikchimp, you could have cylinder wall damage.

[orion::S14]

Cylinder wall damage and cracked head damage = Not common on a KA.

Cracked ring land = Common on a KA with too much cylinder pressure (detonation), whether it's from nitrous or turbo.

I still have $$$ on the piston being hosed...

- Brian

[candypainted 240sx]

He set oil on top of the piston and waited to see how long it took to go away what kind of shit is that. Why didn't he just add oil in the #4 cylinder when compression testing it to see if the compression went up.

[fvittes]

Guys,

Thanks for all the replies, the piston is broken in the area that separates the rings. I will need to replace the piston.

[JustinHallenbeck]

Nice call Orion.