compression leak

H

hgrewal

New member
Hi all,


I have a track day coming up this weekend and any opinions/pointers regarding this are greatly appreciated.


Some background!
During my last HPDE I miss-shifted from 3rd to 2nd instead of 4th which over revved the engine to about 8k rpm. Once the session ended, I checked the cylinder compression and got the following reading 1: 190psi, 2: 195psi, 3: 190psi, 4: 195psi. Since the compression seemed ok and the spark plugs looked good, I took it back on the track for the last session and the car ran fine except that the exhaust sounded a bit different then normal. Since that day I didn’t really drive the car but just started and warmed it up to normal running temperature few time in the garage.



The problem!
Till last Friday the car was starting up just fine and I loaded it on the trailer to take it for tech inspection but when I tried to start it to take it off the trailer, it just wouldn’t fire up. There was partial combustion but it never fully started.

So last night I removed the spark plugs and found that all the plugs were black with carbon residue. I also checked the compression again on cold engine and the readings were as follows
cyl 1 - 180psi
cyl 2 - 200psi
cyl 3 - 195psi
cyl 4 - 200psi

Then I installed the spark plugs after cleaning and adjusting the gap. After this when I tried to fire up the car, it started up just fine and I had engine running for 10-15 mins. The idle was smooth but the engine was shaking a bit and I could hear a minor clicking sound coming from around the cylinder head.

Today morning started and warmed up the car for 10-15 mins again and then took out the plugs and observed that the cylinder 1 plug had carbon residue again whereas the other 3 were just fine. I tested the compression again since the engine was at normal running temperate and here are the results
cyl 1 - 130psi
cyl 2 - 190psi
cyl 3 - 195psi
cyl 4 - 190psi

So it looks like there is a compression leak from cylinder 1 and I think the exhaust valve might have gotten bent when I over revved as initially stated.



The question!
Is it safe to run the car in the next HPDE without fixing this as I don’t think I will get enough time to fix before the weekend? And if I do run it the way it is, what is the worst that can happen? Also, is my assumption valid that it most likely is the exhaust valve that is bent since there is carbon buildup on the spark plug?


Thanks in advance.
 
Your scenario sounds very much like what happened to me last year. Missed a 3-4 shift, hit 2nd and zinged it to around 8k. Only difference is my car immediately went into "motorboat" mode and was obviously not running on all 4. Compression and leakdown tests confirmed # 1 and # 2 were gone - 0 compression and 100% leakdown. Both exhaust valves were bent, and both pistons had nice little dings in them from the contact. From what I've been told this seems like a pretty common result of over-reving these motors. From what you're saying about the compression tests it sounds like the problem may be getting worse, which is probably to be expected if you're running the motor with a bent valve.

I would run a leakdown test if you haven't already, but I'm betting it will confirm what you already think. If that is the case I wouldn't put the car on track again until I had it taken care of.
 
Earl, when this happened to you, did you just rebuilt the top end or did you re do the whole engine? Just trying to gauge what is the best thing to do in such cases.
 
Since the head was pretty fresh they just put in two new valves, replaced all the retainers & locks on all the exhaust valves, and checked the head for straightness. The dings in the pistons weren't bad enough to worry about, so I just knocked off the little ridge with a piece of emory cloth and put it all back together. The car ran fine after that.
 
finally i got around to taking the head off and here are some pictures

there are dings on all the pistons from exhaust valves hitting them but piston 1 ding seems to be little bigger than others and as the compression test suggested the leak was in cylinder 1. now, looking at the valves while they are still in the head, none of them seem to have any damage as you can also see in the pictures though i dont know if the valves normally get bent at the top or if its usually the stems.

i am not sure if i should just go ahead and replace all the exhaust valves along with the springs and retainers. if you have any advice on how i should proceed with this please let me know.

thanks
 
That's a drag...

One think I've used in the past to test port sealing when the head's off (to check the quality of my valve-lapping job) was to fill the port with water, and see if any leaks past the valve. This might help you determine, prior to disassembly, just which ones are bent...

PS - wanted also to ask the group - what's the consensus on re-use of the pistons? Just clean them off, file off any sharp edges, and roll with it?
 
I am NO piston guy, but,,,,

At the stresses we see, in most cases, yes. Clean and reassmeble.

However if the car is known to run aggressive timing, ir fairly high compression, or has thin piston tops, then I'd think twice. I would imagine that if the ding were significant, the piston could heat more in that area, causing early detonation, or at worse, it could lead to more catastrophic failures.

It's too bad the engine didn't have a leakdown test wehn assembled, as that would have indicated where, and how bad things are.

Again, just some general thoughts from a non expert.
 
Harp - I didn't take any pictures, but I don't believe any of my pistons were as badly dinged as yours. Some of that was probably due to the fact that I had the lower compression dished pistons in the engine, which gave the valves a little more space to float in, but still I can't imagine that something didn't get bent with them hitting that hard. For me it was simple - took the head the shop and let them run a pressure test. That confirmed what the compression and leak-down tests had told me. It was very difficult to see by eye - the valves looked fine to the naked eye - so you really won't know until you have it tested.

As far as the springs & retainers - it was my engine builder who noticed that that exhaust valves had "pulled through" the retainers a little, and suggested I replace them. In talking with a couple of other 240 drivers, I've come to believe this is a weak point in the engine; one guy I talked to was in the habit of replacing the retainers/locks every few races just to be safe (along with carrying extra valves/springs in his toolbox). I've added that to my maintenance list now - about every 3 - 4 races I'll be replacing them. Those parts are cheap compared to new valves & shop time :D

Oh, and as far as re-using the pistons - I never had any trouble with the old pistons after I just knocked off the raised part of the 'crater', and eased the sharp edges.
 
Somebody take me to school. I thought the 240SX motor was a non-interference motor. If that's the case then how did the pistons hit the valves?

David
 
My pistons have those dings too, and I haven't had any ill effect reusing both pistons and the valves, after i checked everything for straightness.

David, these motors are not a "non interference" motor. I haven't torn down my motor yet after it grenading in June, and I am hoping when it blew it didn't whack the valves hard.
 
Ok. So under normal conditions the pistons don't hit the valves. Why did they in this situation? Is it because the motor revved so fast that the piston reached the top before the exhaust valve completely closed? Is that a function of weakened valve springs or would it have occurred even with brand new springs? I know this is probably engine 101 stuff.

Tristan, hope your head is salvageable. That was a nasty blowup. I wonder if something happened to the oil pump with the contact.

David
 
David, I'm a neophyte on motor work as well, but as I understand it:

1. On a non-interference motor, there really is no way for the pistons to hit the valves. It's just mechanically not possible.

2. On an interference motor, you can have the great calamity of the timing chain/belt breaking and all kinds of banging going on, or you can have an overrev and have pistons and valves collide as a result.

Tristan indicated that the 240 8v is an interference motor; that's what's happened to these folks. An overrev resulting in smilies on the pistons and bent valves. It be suckin'.
 
The 12v motor is definitely an interference motor, I can attest to that. And yes David, that's exactly what happens; when you over-rev this engine the exhaust valve springs aren't strong enough to close the valves before the pistons catch up to them. As near as I can tell this becomes a problem when the revs get up around 8K, although that's just an educated guess. And it doesn't matter whether the springs are old or new; when it happend to me the head had four races on it after having been recon'd with all new Nissan springs, lifters, etc.

You have to remember that Nissan originally built this motor for their trucks, so they probably never thought one would see anything close to 8K rpm.
 
Jeff, an non-interference motor will not have piston to valve contact if the timing chain/belt breaks. However, if you float the valves all bets are off...it then becomes possible because the valve/piston relationship is disrupted. Chuck
 
thanks everyone for all the replies and suggestions.

i also called Robello Racing and Dave mentioned to just replace all the exhaust valves and i'll be good to go and said i shouldn't worry about the dings on the pistons. i forgot to ask him if i should also replace the metal head gasket as well or will it be ok to reuse the existing 1 thats already there since engine hasn't seen much race action. from the looks of it the gasket seems to be in good shape. any suggestions?

also, any recommendations on where to get the various parts including valves/springs/retainers and gasket set?
 
For the parts, just go to your local Nissan Dealership. IIRC, the 4 exhaust springs, valves, retainers and locks all cost less than $35. I used a factory head gasket, and replaced it each time the head came off. I think that was $42 or so, but it has been a while.

Your engine builder is dead on with when he noted the exhaust valves were starting to pull through the locks. That is not uncommmon. I would check that as routine maintenance after every race weekend. If you notice any decrease in height from the retainer to the top of the valve stem, change out the whole set. This will prevent the exhaust valves from floating prematurely(<6000 rpm) Buy an extra set of exhaust parts also, and yes, I always kept a spare set. If the engine started to feel "flat" up top, use the air in the spark plug hole trick and change the exhaust side springs, retainers and locks. That may help prevent having to pull the head. It might not help after an 8000 rpm over-rev has already compromised the parts though.

As everyone else said, knock the rough edges off the eyebrows in the pistons and put them back together. Look at the bright side, some people actually pay a machine shop to clearance the pistons. :)

(yes, I still lurk over here in Nissan-land)
 
I've always used the Nissan head gaskets too, so I'm not sure about re-using the metal ones. I also always replace the head bolts whenever I pull the head. Cheap (well, sorta) insurance. For the internal parts I just go with the factory stuff - there isn't that much difference in the prices and you know what you're getting.
 
I would without question replace the pistons. I had a problem with a head - warped cam, which broke the end off in the paddock - almost no load and very low rpm. I stopped immediately - the result when the cam stopped turning, was light contact on each piston.

Here are pictures...

http://www.timelapseracing.com/page4.html

Doing as you are planning - replace/repair head and go out with the damage - is what I attribute the next problem too.

Scroll to the bottom - http://www.timelapseracing.com/page6.html

The ding most likely caused a hotspot on the piston which caused the piston to fail completely, allowing the rod free access to the side of the block - This was not my favorite race (really first hot lap in the first practice).

So - you might be safe with the dings, but it may be a grenade with the pin pulled.

REPLACE THE PISTONS - its a lot cheaper, and easy when you have the head off. And yes - 12v SOHC - definitely interference motor.

Jason.
 
If you have a built motor that has been balanced and blueprinted and you replace the pistons, then you no longer have a B&B motor, correct? So at that point do you just get a new motor (at least bottom end) built?

David
 
I gotta agree with Jason here, thats a really serious ding on that #1 piston. Here's the question to ask yourself: If I was putting my motor together and found that I had pistons with a ding like that, would I put the motor together or would I replace the piston?


I would without question replace the pistons. I had a problem with a head - warped cam, which broke the end off in the paddock - almost no load and very low rpm. I stopped immediately - the result when the cam stopped turning, was light contact on each piston.

Here are pictures...

http://www.timelapseracing.com/page4.html

Doing as you are planning - replace/repair head and go out with the damage - is what I attribute the next problem too.

Scroll to the bottom - http://www.timelapseracing.com/page6.html

The ding most likely caused a hotspot on the piston which caused the piston to fail completely, allowing the rod free access to the side of the block - This was not my favorite race (really first hot lap in the first practice).

So - you might be safe with the dings, but it may be a grenade with the pin pulled.

REPLACE THE PISTONS - its a lot cheaper, and easy when you have the head off. And yes - 12v SOHC - definitely interference motor.

Jason.
[/b]
Click to expand...
 
You must log in or register to reply here.
Back
Top