ticking/knocking with low compression

[Villain3g]

Recently I've developed a very loud tick/knock that responds to engine rpm. It started after a day of heavy driving. I had also let the synthetic oil go about 7k. After searching the forums, it seems that my symptoms mimic those that have an issue with one of the valves. I've begun performing some of the recommendations. The first step was an oil change. I stared with a high mileage engine flush then changed the oil and added seafoam. This didn't fix it. I then performed the 60k mile service with the hope that it was the VTC springs. This also didn't work.

And this is where I'm at now. Ive checked the compression of the cylinders and two showed bad news. The rear driver side cylinder read only 120psi. The front driver side cylinder read only 55psi!

Where do I go from here? Is it possible to perform another test to further narrow down a solution. Is there any way to determine if the valve is sticking or in fact bent? How would I cure either of those situations?

Anything I do on this car is a "first time" for me. This forum is the only way that I can own this car. Thanks everyone for any help or advice.

[evildky]

step 1, drain tha sea foam oil mix out of your engine, replace the filter refill with oil no thinning addetives this time! run it for a couplple of hours drain and repeat, these thinnign addetives are to loosen up gummed up buildup, they are nto to be left int he engine they are to be run shrtly before changine the oil you basically sis 2 engine flushes in a row, you could damage bearing surfaces playing that game!

then retest you compression, might improve it might not, but at the very least you need the propper weight f oil without thinning to get accurate results

when you retest and that cylinder is low again as it likely will be just hpefully not as low, after testing add a teaspon of engine oil agian just regular oil no addetives, then retest that cylinder, if the compression comes up significantly you have bad rings, if the readings don't change or barely change then it's a valve, head or headgasket issue in any even the head has to come off

[Villain3g]

Can I rule out a spun rod bearing as a source for the sound?

[Villain3g]

can I pull the heads with the engine in?

[evildky]

if the oil solves the compression and the noise persists then it might well be bearings, if the oil does not solve anythign tt could still be the bearings, yes you can pull the head with the eneigne in the car, but if you do have a rock knockign then it's gonna have to come out

the noise, does it feel like a rev limiter? does it sound like death at 3k rpm?

[Villain3g]

it doesnt feel like its lost power, and it does sound like death at 3k. Should i drop the oil pan to see if i can jiggle any of the rods?

[evildky]

if it feels like hitting a rev limiter at 3k rpm then it's a rod and you might as well go ahead and pull the motor and plan on taking it to your local machine shop

[Villain3g]

do you know of any head removal write-ups? i've been searching but only found one for a plenum pull. Thanks

[Villain3g]

I've got the pdf manual. It doesn't look that bad.

http://www.nicoclub.com/FSM/300zx/1990/em.pdf

just wondering if there was a more in depth writeup with pictures

[evildky]

once the plenum is off, you have to pull the timing belt and intake manifold then you can take off the exhaust and the heads,time consuming pita like everything else on these cars