Crankcase Ventilation 3.9

[aRRon]

Guys, wondering what the correct set up is with the hose from the flame trap on the rocker cover to the throttle body on a 89/90 3.9? Mine has a single hose which was bought new but doesn't fit well on the flame trap. Looking at some parts catalogues online I see a two hose set up with a T-piece containing a "restrictor" with a third iose leading off somehere else. Am I missing the T-piece/3rd hose or are there different set ups or doesn't really matter?

Always been paranoid about excess pressure in the crankcase since I had two blocked flame traps on an old 3.5 and blew oil out everywhere. I have got a good oil leak from the rear main seal through the transmission bell housing but think it's becasue I trimmed the ends of the cruciform oil seals when I fitted the sump gasket. :(

[bee utey]

The Tee piece is made of plastic and is prone to shattering if fiddled with. You can make your own out of brass fittings but the restrictor should have a hole of 2 or 2.5mm diameter. The small branch of the tee goes to an 8mm nipple low on the plenum near no. 1 cylinder.

[MacMan]

I had to make a new unit out of brass and went to all the trouble of sourcing the bits before I had fully disassembled the broken plastic crumbs and hoses. I laughed out loud after threading the inside of the 1/4" adapter (small one running to plenum) that bee utey mentioned, only to find that the broken plastic stub stuck in the hose had been blocked with a screw. I suspect that's part of the reason I'm one of the lucky ones who's LPG conversion doesn't backfire.

Total cost to remake was about $25. I'll see if I can find the packaging from the original bits, but I'll take a picture of what I made. Brass is the way to go. The plastic part just fell to bits when I stripped my engine.

[bee utey]

Just a reminder to everyone on the breather issue:

Having the breather connected normally does not cause LPG back fires.

This is an "old mechanics" tale. I always leave the LR V8 breather as standard. I have done dozens of LPG conversions on these things and they don't generally backfire until the plugs or timing are messed up by "knowledgeable" mechanics. Or something goes wrong with the Lucas ignition amp or the engine is chockers with carbon deposits.

The purpose of the small bleed hole in the tee is to allow the blowby gases to travel direct to the manifold without first passing through the IAC (idle) stepper. Blocking this branch of the tee will just cause your stepper to jam up with carbon all the sooner.

[MacMan]

Interesting! I'll take your word for it bee utey. Still made me laugh. I might live dangerously and remove the screw :D

[aRRon]

Thanks Bee Utey and Macman for your valuable information and advice.
Mine is on LPG (Impco 1999 vintage) - never had a backfire as long as I remember to start on petrol. 35000km following a full recondition.
So most of the blow by gases get sucked into the plenum - I wondered what the blanked off forward port was for! Without the T-piece etc would I be losing significant manifold vacuum when the throttle opens given there's a relatively large diameter hose going straight through to the flame trap or does the restrictor only come into effect when the throttle is closed?

[bee utey]

Thanks Bee Utey and Macman for your valuable information and advice.
Mine is on LPG (Impco 1999 vintage) - never had a backfire as long as I remember to start on petrol. 35000km following a full recondition.
So most of the blow by gases get sucked into the plenum - I wondered what the blanked off forward port was for! Without the T-piece etc would I be losing significant manifold vacuum when the throttle opens given there's a relatively large diameter hose going straight through to the flame trap or does the restrictor only come into effect when the throttle is closed?

With the standard tee breather fully connected at the plenum and throttle and "mixer" type LPG fitted:

At low throttle opening, manifold vacuum sucks blowby gases AND some gas/air mixture directly into the manifold via the small branch of the tee. NO gas /air mixture travels toward the rocker cover.

At high throttle openings the manifold vacuum is low and blowby gases are dumped directly before the throttle plate. The IAC stepper sees little of the fumes as they are sucked into the inlet and the IAC hose is connected to the opposite side of the inlet.

With the small hose blocked:

At low throttle opening breather fumes are dumped in front of the throttle plate and can travel across to the IAC channel.

With the large hose routed to the air cleaner side of the LPG mixer:

At full throttle blowby gases are dumped into the air box and MAF causing all sorts of troubles with scum and stuff wrecking the MAF.

In all cases remember there is a flame trap on the RH rocker cover, it needs occasional washing out and prevents that mythical beast, the backfire caused by "flames" travelling around the breather system. If you have actual flame inside your crank case, your engine has major issues, eg blown out head gasket.

At no stage during normal operation is there gas vapour travelling from the mixer through the flame trap into the rocker cover. This can only happen to engines with a PCV system fitted, eg Fords, Holdens, Chevs and sundry others.

This LR V8 does not use a PCV system and therefore any change in the breather will have negative consequences somewhere else.

[milld]

Just a quick question. The passenger side rear of the motor on the rocker cover there is another little breather. In the past I've tried to search for a replacement with out luck. Only found the foam part but not the assembly. So now it simply has a thin rubber hose connected to it and flaps away hidden at the back somewhere. Where can I reroute this hose or get the correct breather assembly?

[bee utey]

Just a quick question. The passenger side rear of the motor on the rocker cover there is another little breather. In the past I've tried to search for a replacement with out luck. Only found the foam part but not the assembly. So now it simply has a thin rubber hose connected to it and flaps away hidden at the back somewhere. Where can I reroute this hose or get the correct breather assembly?

On plastic air cleaner boxes there's a little nipple above the element on the guard side of the box. On metal air cleaners I've run the hose to a 3/16" nipple screwed into the can. You could use a small plastic inline petrol filter on the end of the hose instead. I've seen all sorts of small filters used for this job, eg personal eye protection mask filters, scraps of foam etc.

[bee utey]

Brass breather with 2.5mm jet (sawn off bolt) in the small barb:

https://www.aulro.com/afvb/

Pic of a 1993 RRC 3.9 I am working on.

[MacMan]

Brass breather with 2.5mm jet (sawn off bolt) in the small barb:

http://www.aulro.com/afvb/attachment.php?attachmentid=48941&stc=1&d=1341968784

Pic of a 1993 RRC 3.9 I am working on.

That's pretty much the same deal as I have. Bit came from local Repco (the main T, approximately $22) and the other adapter that screws in to replace the standard 5/8 one came from Bursons (about $6).

[MacMan]

Bee utey, just for the sake of interest I removed the screw that was blocking the plenum vacuum tube to the brass tee. Idle impossibly rough and the exhaust was popping and farting. With it reblocked it resumed perfectly smooth running.

[bee utey]

Bee utey, just for the sake of interest I removed the screw that was blocking the plenum vacuum tube to the brass tee. Idle impossibly rough and the exhaust was popping and farting. With it reblocked it resumed perfectly smooth running.

Yeah but is the big hose connected to the throttle or the air cleaner? If the air cleaner it will run lean and do exactly as you said. You know, sucking air instead of gas/air mixture? Also how big is the hole the screw was in? More than around 2mm and it'll suck much way too much air. If you have a pic of your setup it would help.

When changing breathers you may have to readjust idle mixtures and base idle speed, because it was wrongly connected before. Not rocket science.

[MacMan]

Another update.

The big hose is connected to the LPG mixer not the air cleaner. I drilled a 2mm hole down the middle of another 3/16" screw to go in the small port on the tee (the port itself is threaded for 3/16" so too big). The idle was smoother but even with the idle screw adjusted to smooth it out, it doesn't drive as well off the bottom and the idle speed too high, especially with the timing advanced to where it is. I've gone back to the way it was - blocked. The conversion was done by LPGAS1 here in Melbourne and it's as it was when I bought the Disco, and the previous owner said it was left as installed.

[bee utey]

Another update.

The big hose is connected to the LPG mixer not the air cleaner.

Precisely. It's connected to the air only side of the mixer, where it is getting no LPG but is dropping fumes into the mixer. Open it up one day and have a look inside. They look real ugly after a few years of oily fumes.


I drilled a 2mm hole down the middle of another 3/16" screw to go in the small port on the tee (the port itself is threaded for 3/16" so too big). The idle was smoother but even with the idle screw adjusted to smooth it out, it doesn't drive as well off the bottom and the idle speed too high, especially with the timing advanced to where it is. I've gone back to the way it was - blocked. The conversion was done by LPGAS1 here in Melbourne and it's as it was when I bought the Disco, and the previous owner said it was left as installed.

Look if you're happy with it I am not going to argue with you but all of my comments from earlier still stand, with the exception that your MAF is probably safe. Your mixer and your IAC stepper are still going to get gunked up. The OEM location (on the throttle intake) for the big breather hose is still the correct one for this vehicle, whatever LPGAS1 have done.

Oh and if the idle speed is too high after fixing the breather, stick a 3/16 allen key in the manual idle adjuster on top of the manifold, and screw it down a bit.