Mysterious SBC Oil leak

Mysterious SBC Oil leak

Author
Discussion

Nabbott

Original Poster:

294 posts

135 months

Tuesday 23rd December 2014
quotequote all
So - this is my last issue on the car and the one of the more frustrating!

I have an oil leak and diagnosing where it's coming from is proving tricky as it only happens when the engine is warm/hot and has been subjected to some acceleration (so revving the spuds of it in the garage doesn't show anything). Add it turbulence under the car at speed and it could be coming from anywhere....

After a 'spirited' drive I have engine oil (not gearbox oil as there's no smell) on TOP of the gearbox. The rockerboxs are well sealed, the valley/inlet manifolds are also well sealed and I also put a new sump gasket on the engine as that was definitely leaking at some point (is now good).

Having never built a SBC would it be possible for the rear crankshaft seal to have failed and throw oil out of the inspection hole in the gearbox? Find this hard to believe as the clutch was dry when I pulled the gearbox (to do the main input shaft seal) and the other side of the flywheel is also dry...

Answers on a post card OR let me know how you've diagnosed a similar issue......

Thanks Nabbott

V8Dom

3,546 posts

201 months

Tuesday 23rd December 2014
quotequote all
if rear seal, look at bottom hole and put finger inside to see if wet

is it arround the dizzy?

also oil pressure switch at back ontop too.


dom

Nabbott

Original Poster:

294 posts

135 months

Tuesday 23rd December 2014
quotequote all
Thanks Dom!

So negative on the Dizzy (checked this a couple of weeks ago) and I wrapped the oil pressure switch up (in a zip lock bag) and that's coming away dry.

Last time I looked there was a drip of oil hanging off the selector fork - but what I don't understand is how the flywheel and clutch would remain dry if it were the main seal...... but then having never stripped one down I'm 'in the dark'

The Italian

136 posts

121 months

Tuesday 23rd December 2014
quotequote all
I have had SBC & SBF, many, and they often do leak out of the rear main
SEEP more like it and often after a run after cooling down
Eventually you will need to repair it as to not saturate your clutch down the road
I have never had one fail except for big HP-emergency-no, 100% sure, no.
Also make sure that check valve/breather is working and not clogged

confusionhunter

448 posts

221 months

Tuesday 23rd December 2014
quotequote all
agreedm SBCs breathe fairly heavily so its worth revamping and checkining your breather and or PCV system. I has a leak from the sump but the breather system wasnt good enough so I added and PCV system and better breather with catch tank and that seeesm to have improved the situation. Again it was always after a run. It may be that the seal is fine its just back pressure thats pushing oil out?

Nabbott

Original Poster:

294 posts

135 months

Tuesday 23rd December 2014
quotequote all
So I do need to whip off the rocker box breathers to check they aren't blocked (but would be staggered were that the cause) to eliminate another possible issue - but this is losing oil when running and under pressure and is too much to ignore!

I'm starting to suspect the rear crank case oil seal - and believe it can be replaced with the engine/gearbox/flywheel still in place…. anyone tried it?

confusionhunter

448 posts

221 months

Tuesday 23rd December 2014
quotequote all
For the small effort involved I would say yes. Are both sides vented to free air or is it running a PCV system at all?

Storer

5,024 posts

214 months

Tuesday 23rd December 2014
quotequote all
I doubt it will be the main seal as the oil is on top of the gearbox and the flywheel is dry.

I would go with breather or part of it's plumbing. It fits with your symptoms.
Another possibility is the dip stick.

Might be worth doing a compression test too.


Paul


Stupid question - is the engine over full of oil?

Nabbott

Original Poster:

294 posts

135 months

Tuesday 23rd December 2014
quotequote all
So the engine is pretty 'trick'. It runs Kinsler throttle bodies and a dry sump. Thus no dip stick. Both Rockerbox pipes are clear and vent to the oil reservoir.

I'm going to make up a 'catch tank' to place over the top hole of the gearbox to either use this in or out.

V8Dom

3,546 posts

201 months

Tuesday 23rd December 2014
quotequote all
if its dry sump.. there should be negative pressure inside engine so oil blowing out not a problem.
suggest small oil leak rear main, dropping onto flywheel and flicking off

dom

confusionhunter

448 posts

221 months

Tuesday 23rd December 2014
quotequote all
Hmm Interesting. I did some digging on this a little while ago and on SBCs they seem to react well by having a slight vacuum in the crank case created by having a PCV on one head and to free air (via catch tank) on the other. I did a quick google on a breather system for a dry sump and the principle is the same only rather than venting to atmosphere you vent the non PCV side to the oil tank through a check valve.

This of course all depends whether you want to run a slight vacuum in your crank case. I did the digging and decided I wanted to and I felt it was the right thing to do.

Having said that you shouldn't have any issues with your current set up but I might suggest a PCV on one side and see how that reacts. I ended up getting one of these from Real steel:
http://www.billetspecialties.com/shop-by-departmen...

You might have another issue in finding a place to take a vaccuum from with your funky throttle bodies!

Just a suggestion... smile

confusionhunter

448 posts

221 months

Tuesday 23rd December 2014
quotequote all
here we go... Can of worms warning.....some links from my previous digging:
http://www.burtonpower.com/tuning-guides/tuning-gu...

'On dry sump systems, the scavenging action of the pump should evacuate any excess blow by gases in the crankcase and, in an ideal situation, maintain pressures at or below 2 inches of water. Depending on the practicalities of individual engine types and installations, both ‘open’ and ‘closed’ systems can be adopted with some tuners preferring the closed system. Providing a closed system (incorporating a PCV or similar check valve between the engine and oil tank) can be seen to function efficiently, it can offer added benefits. The closed system allows the scavenge pump to reduce crankcase pressures to a minimum, in some cases as low as zero or even a slight vacuum. In such situations a small bhp gain is achieved by eliminating combustion chamber contamination and reducing any residual oil drag (clinging to crank, rods, etc.) to a bare minimum.'

And about crankcase pressure:
http://www.aa1car.com/library/pcv.htm

for SBCs:
http://www.chevytalk.org/fusionbb/showtopic.php?ti...


And lastly for the sake of irony, this fabulous forum:
http://www.pistonheads.com/gassing/topic.asp?t=357...

Steve_D

13,737 posts

257 months

Tuesday 23rd December 2014
quotequote all
The famous David Vizard, a well known SBC expert, claims HP gains by running a vacuum in the crankcase. He has even used a vacuum pump.

I had a number of small oil leaks on my SBC that all went away when I installed a PCV valve.

Steve

Nabbott

Original Poster:

294 posts

135 months

Tuesday 23rd December 2014
quotequote all
V8Dom said:
if its dry sump.. there should be negative pressure inside engine so oil blowing out not a problem.
suggest small oil leak rear main, dropping onto flywheel and flicking off

dom
So when I spoke with the engine builder he said it would be a mistake to assume you had a BIG vacuum in the pan at high revs - there will always be blow by past the rings due to the working tolerances of the moving parts. My engine is heavily stroked at 6.7 liters and has large breathers from the Rockerbox covers back to the oil reservoir.

I think (but could be wrong) that the notion of a PCV with an engine running a dry sump is a dead end. At the end of the day I have quite a lot of oil passing through a seal or gasket when under load - something isn't right.

Regards the breathers from the rocker boxes 'leaking' I very doubt these are the cause. There's too much 'splatter' to the oil on the top of the gearbox - which implies (to me at least) a high pressure issue......

I've read a bit about rear main seals failing on SBCs and they do fail. I'm suspecting this as the same thing happened with the main seal on the gearbox. The car had only done 700 miles in its 11 years so the seal may well have gotten hard and is now leaking like a mo fo!

So the plan now is to make up a 'box' to fit over the hole in the top of the gearbox. That way I can see whether or not the oil is coming up through the hole or lying on top of it. Make sense?

Steve_D

13,737 posts

257 months

Tuesday 23rd December 2014
quotequote all
Nabbott said:
......So the plan now is to make up a 'box' to fit over the hole in the top of the gearbox. That way I can see whether or not the oil is coming up through the hole or lying on top of it. Make sense?
Get some cardboard and some kitchen roll.
stick the kitchen towel to both sides of the cardboard and tape it all down over the hole.
After your test run one side or the other of you kitchen towel will be wet.

Steve

confusionhunter

448 posts

221 months

Wednesday 24th December 2014
quotequote all
Sounds like a plan! Id make sure evrything around it it spotlessly clean too so it can all add to the evidence! If you were local I'd loan you my 'dogcam' which is a fiarly robust (but old) remote camera to watch whats happening! Im sure if you had a go pro you might not want to shove it in the engine bay!

I'm getting excited for you; getting to the point you can drive it reliably, with most of the niggles gone. Looking forward to getting to that point myself! smile

Also.... I'd love to hear what that engine sounds like on those Throttle bodies..... any vids?

Nabbott

Original Poster:

294 posts

135 months

Wednesday 24th December 2014
quotequote all
So cleaned up and ready for a dry road day....

|http://thumbsnap.com/8IYuHs9S[/url]url]

Place your bets as to where the oil ends up.... inside (rear crank seal) or outside (somewhere else.... that I've already checked 100 times before)

|http://thumbsnap.com/nr3Lrm9z[/url][url]

confusionhunter

448 posts

221 months

Wednesday 24th December 2014
quotequote all
Going for a drive now then? What a cliff hanger....laugh

Nabbott

Original Poster:

294 posts

135 months

Wednesday 24th December 2014
quotequote all
confusionhunter said:
.... I'd love to hear what that engine sounds like on those Throttle bodies..... any vids?
I've not done many videos (too busy spannering), but couldn't resist recording the final run of the (hub) dyno when i had the car tuned. Apologies for the quality (my poor old iphone's microphone was somewhat overwhelmed) and check out the small flames from both pipes at the end of the run:

http://youtu.be/1ga4-1vc79o

confusionhunter

448 posts

221 months

Wednesday 24th December 2014
quotequote all
Wow, 640 with out forced induction is pretty impressive! smile sounds good!!!!