I have a 1990 240SX, with a KA24E and 138k miles on it.

Since last year I'v been hearing a slight hissing noise when I turned off the car, coming from the back of the engine, as if the compression in the 4th cylinder was being "let out" after the engine has been shut off. I've checked my compression, and it was 11.5 on the 4th cylinder vs. 12.5 on the 3rd(and the 1st and 2nd). I also have a bit of blue smoke (oil leak/burning somewhere) under very hard/rapid acceleration (when I floor the car hard, WOT).

Also, my rocker arms tend to clatter for a few seconds upon immediate startup of the car, is this related to my problem, or normal, since there is no oil supply to the rocker arm shafts until the oil pump has pumped oil into those places?

From what I understand, these are symptoms of either my 4th cylinder piston rings going bad, or my valve seals going bad. If you think there is another cause for this, please let me know.

I am wondering how I can figure out exactly whether it's my piston rings or my valve seals on the 4th cylinder.

Another question, related to this one:If I decide to swap out the piston/rings assembly on JUST the 4th cylinder, can I simply take another piston with rings, (together with the rod, bearings etc), from another KA24E I have lying around, which has 70k miles on it, and just swap it over? Or must it be a whole NEW piston/ring set?

If it IS the valve seals, can one replace those without taking off the head? I've taken off a head of a KA24E before, I am just wondering whether there is a way to keep the valve from falling into the cylinder chamber, while the valve seal is being replaced.

Thanks,

sil80

the hiss may be leaking compression, do a leak down test on it, it will tell you more accurately then the compresion test

for the rocker arms, are you using an OEM oil filter?

i dont see why you couldnt use just 1 piston although i would replace the rings

i dont know if you have to remove the head, but if you put the piston at TDC it should keep the valves in the the head.

i would reccomend all new rings, but if you are a ghetto mechanic(like me!) then slap that sucker in there and see if it works. valve stem seals are easy. just pull the rocker shafts, get something to hold valves up(special tool with compressed air, or fill cylinder with string and raise piston to tdc), pop of the valve springs, retainers, and keepers, then take a spark plug boot pliers and yank the valves stem seals off. install is the reverse. be carefull with the rocker shaft bolts. do not overtighten them as they love to rip threads out. if you do rip the threads out, they are easy to fix with a heli-coil. if you are putting a piston and ring in it anyway then you can do the valve seals on the bench(easier!). be sure to check the cyl for excessive wear. no good fixing rings in a bad cyl.

the reason i have all this on the top of my head is that my sr has low compression on cyl 4. so low that the blowby is coming out of my breather filter and making my turbo sound really good! still fast on 3 1/2 cyls though.

s13sr20chris wrote:the reason i have all this on the top of my head is that my sr has low compression on cyl 4. so low that the blowby is coming out of my breather filter and making my turbo sound really good! still fast on 3 1/2 cyls though.

Oh I can so relate to your mindset Chris.

We know whats wrong with the car,We know what it will take to fix it,But we will drive it till it blows up. Cause in the end even after it does blow up,We know we can still fix it in about the same amount of time. AHHHH the beauty of being an auto tech. If it dont put cash in our pocket why waste time with it.

I used to use OEM filters, but for now I've used the FRAM filters for about 3 oil changes, since they are cheap. But I'll go back to OEM soon. Thanks for tips on the piston/valve seals. Also, does it matter WHICH piston I replace my 4th with? If I use the 1st (2nd/3rd) piston/rod from the other engine, to replace my 4th with, will it have any negative effects/incompatibilities?

sil80

you need to match the grade piston with whats in that cyl. there should be a number stamped on top of the piston 1 through 3 make sure you get the right one. it would be best if you used a feeler gauge to check your piston to cyl clearance from the deck and not the bottom of the engine. this way if you need to step up a grade you can. otherwise you could throw another piston in there and have the same problem.

In the end, a bad ring only costs me a loss of power and a loss of oil, no other damages, right? (I check my oil ever week, it usually goes down a quart of a quart bottle, so I fill it back up).

Also, what are the grade numbers for, and what would be the consequence of mis-placing the different grades?

sil80

grade 1 is the smallest and grade 3 is the largest. they have very minor diameter differences. a grade 3 in a cylinder graded for a 1 could cause substantial premature cyl wall wear, a 1 in a 3 cyl could make your problem 10 times worse.

so all in all, i can run around with 11.5 compression on the 4th and not have any detrimental effects on the motor, with the exception of power and oil loss?

sil80

blowby fuel could contaminate your oil. mine too. thats why i will eventually fix my sr. just not this weekend.

mmm sounds like premix did anyone say "rotary"?

sil80

heh, too bad its not a two stroke eh?

Bringing back up this topic to see if anyone can help me. well my ka24e had ran great until one day the wiring harness was cut by the clutch fan. Dont ask me how haha. But anyway this fried the ecu. I got a new ecu and am trying to get down the timing just right. Unfortunately now my motor is appearing to be burning oil and making this hissing noise after i shut it down. ( Scary loud) Anyone have any ideas.?

im in idiot i was running low on coolant from a leak and needed to rebleed the radiator. Should be ready to roll again!