KA24de(T) Compression issue

[jcarney1987]

Okay today I took my 240sx KA24deT out for a spin. It's running around 11lbs boost with 370cc injectors, enthalpy tune. It is a newly rebuilt motor with only 50 miles on it. I've kept it under 4k RPMS.

Now today where I was a 120ish PSI Compression on all cylinders (New engine), after the drive I took cylinder number 3 dropped to 60 psi and there is loads of blow by (Pressurizing the crank case).

Now I pulled the motor out of the car and working on taking it a part. I notice the oil is clean, no water leaks, so I'm going to assume I have a ring issue and not a head gasket issue.

So then I noticed my knock sensor was broke and not fully connected. So here is one of my question, could I have detonated with the broken knock sensor? I know some cars get put into limp mode with there is a problem with the knock sensor, is this the case for a 93 240sx?

Thanks for your advice.

[MeanGreenS13]

Start from the beginning here... When you say the oil is clean, did you open up the filter? Did you put a magnet in the drain pan? Also a mistake I made when I built my CA a while back was not getting file fit rings... Did you do this or just buy off the shelf oem rings?

[jcarney1987]

Start from the beginning here... When you say the oil is clean, did you open up the filter? Did you put a magnet in the drain pan? Also a mistake I made when I built my CA a while back was not getting file fit rings... Did you do this or just buy off the shelf oem rings?

Sorry, I meant the oil was clean as there were no metallic looking flakes in the oil. But I do plan on opening up the oil filter though.

The piston rings I bought were NPR brand and the rings where in spec when I put them in. I didn't have to file them.

I'll be pulling the head off today to get a better look.

[jcarney1987]

Okay, I got the head off and everything look fine, so I proceeded to take of the oil pan and disconnected the rod #3 and pull the piston. I have ring lands cracked in several places. So I'm going to assume I detonated.

Could a broke knock sensor allow this happen?

[biggie]

Just a quick thought, 11psi seems a bit high on 370s.

[jcarney1987]

Just a quick thought, 11psi seems a bit high on 370s.

Yea it is, I was going to tune it down to 7 psi, but to late now ha ha.

[MeanGreenS13]

Just a quick thought, 11psi seems a bit high on 370s.

I didn't even noticed that... 11psi is going to cause a nice lean condition on 370s... No Bueno

OP to answer your question, a knock sensor is there to retard the timing if it detects detonation or knock of any type in the engine. You did likely detonate which would be the cause of this. Sucks to hear man. I've been there.

[jcarney1987]

Just a quick thought, 11psi seems a bit high on 370s.

I didn't even noticed that... 11psi is going to cause a nice lean condition on 370s... No Bueno

Well I have my fuel pressure bumped up a few psi to counter that, and I just got to thinking. I forgot I bought cheap gauges because I wasn't going to run the engine to the limits, so my glowshift boost gauge could be a few PSI. Not a problem for running 7 PSI, but yea running I could be running a bit higher.

OP to answer your question, a knock sensor is there to retard the timing if it detects detonation or knock of any type in the engine. You did likely detonate which would be the cause of this. Sucks to hear man. I've been there.

Yea, I know what it does. But I know some cars go into limp mode with a faulty knock sensor. But all is well, another good day to slap in some new pistons and go with it lol.