Bad compression on piston #4 - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

fvittes · 04-26-2004, 05:53 PM

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

04-26-2004, 05:53 PM	#1
fvittes Join Date: Apr 2004 Location: Dallas TX Posts: 7 Trader Rating: (0) Feedback Score: 0 reviews	Bad compression on piston #4 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