Mysterious SBC Oil leak

[Nabbott]

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

[Nabbott]

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'

[Nabbott]

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?

[Nabbott]

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.

[Nabbott]

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?

[Nabbott]

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]

[Nabbott]

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

[Nabbott]

Davrianman said:

Is this your car? It was a 6.7l with a kinsler setup at the factory a few years ago...

Yep - that's the one 😃

[Nabbott]

So had 10 minutes and (almost dry roads) today so..... drum roll.....

Here are the results from running the car down the road:





So unless anyone can explain how I can get engine oil out of that hole in the gearbox I think we can conclusively state that the rear crank oil seal is knackered.

[Nabbott]

Yes 100% not gearbox oil. Spins the wheels deep into 3rd gear, so car is in rude health 😃

None of the parts places are open and Peter Knight appears to be shut for Christmas (can't say I blame him), so I'll have to satisfy myself with draining the sump and popping it in readiness. Having investigated rear main seals from Fel-pro there appears to be several options 😒 so will hold out to speak with the guys at Knight - don't want to do this sump removal more than I already have done!

[Nabbott]

V8Dom said:

Might find flywheel casing side is wet..:0)

I'd be amazed if that weren't the case. I'm assuming at this stage that the oil is only coming out of the main seal under duress (big revs and the oil in the scavenge pan being pressed up against the rear seal), it then hits the crank 'spider' and it flung up from there. Given enough time i suspect that the fly wheel and clutch would all be soaked in the stuff.

The postie delivered a 'standard' seal today but all I've read says it's not suitable for 'race' applications so will have to see what colour/type the seal is when I've stripped it.

Happy New Year to All!

[Nabbott]

ROWDYRENAULT said:

Nabbot: Unfortunately rear main oil seal on the SBC was one of several weak points that the LS corrected. If yours is a two piece seal there is a conversion to a one piece seal that my engine builder says is a much better solution. I always imagined that at some point there was a meeting at G.M. A gaggle of engineers got together and started making a list of items to be corrected in the next generation V8 which we know as the LS. Rear main seal was probably in the top three things on the list. Lee

Yes I know

Trouble is that's probably an engine out job (or at the very least 'box, clutch and flywheel). The two pierce looks like it lasted 10 + years (probably hardening through lack of use accelerated the failure). So all being well it's a 'do it once/do it well' job. Fingers crossed......

Had I had my time again LS3 would be the way I'd go but time and money are always gonna be a factor.

[Nabbott]

Great and well thought through response - thanks very much!

I do get a little oil 'sweating' on the valley plates (for all the reasons you outline in your response) - and have resigned myself to this. It isn't a modern engine afterall.

The fact remains that my rear seal is definitely leaking and if not addressed will toast the clutch

I've disconnected the scavenge pipes and drained the sump - new two piece seal is the only option I can retrofit with the engine and box still in the car (limited garage time and an impending house move being chief causes). More on this later

[Nabbott]

I'm fortunate in so much as I have a pitt in my current garage (that I've used every week since I bought the car!). It makes doing the sump so much easier. I've already done the job once (the sump pan seal was gone), so the most inconvenient part is having to jack the block off the chassis to get clearance for the pan to clear the chassis.......

I've seen the one piece seals but have been advised away from them by both the pan manufacturer (stefs) and the engine builder.

All in all Just one of those jobs that isn't fun - tea in an almost endless supply

[Nabbott]

Roj - go back to work before the oldtimers desease takes full effect ))))

[Nabbott]

Will do and thanks. The block is a GM aluminium 'performance' unit - so I'd like to think that the dimensions should be on spec !

The engine builder should be back to work today so I can his advise on which seal I should be using..... Coincides nicely with the fact that the sump is off - just needs extracting from the chassis.

[Nabbott]

So patience being a virtue - having left the sump for about a week to drain - the removal was (fairly) straightforward, especially as I've had a fair bit of practice. Absolutely ZERO evidence that the sump gasket was leaking (which is reassuring as I only did that a couple of months ago ) BUT the flywheel is suspiciously dry........

I talked with the (ever helpful) Tim at Peter Knight Racing and ordered the rear main seal (Fel-Pro 2909), once he confirmed what it was. So the only issue that I have is the last two torx bolt holding the end cap on - any ideas? It's slightly larger than a T50..... I had no idea that imperial TORX existed.

[Nabbott]

[Nabbott]

Oh man - were it not for my pit with this car I'd have been driven mad a long time ago!

[Nabbott]

Storer said:

Don't think they do exist in imperial (I could be wrong though).

Is it a T55?


Paul

Defo NOT T55