Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[king cj]

got aproblem with my chev pick up, the engine is sucking through its rocker breathers. anyone know what is likely to be causing this? ive never known them suck through a breather, blow yes but not suck!

[Warthog]

Faulty Positive Crank Ventilation Valve (PCV)  or pipe work too and from faulty/blocked?


The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. Sensing a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway.

PCV valve closes to prevent reversal of the exhausted air back into the crankcase/rocker covers again.


Hope that helps?

Should look like one of these?
http://www.google.co.uk/products?hl=en&q=Chevy%20V8%20PCV%20valve&um=1&ie=UTF-8&sa=N&tab=wf

[Bubba]

that sounds plausable is it 6.5 or 6.5 turbo diesel i am onlt thiking that as if memory serves its a bit new to be a 6.2..i think

[king cj]

350 petrol, idles well, revs freely but under load get it up to 25/30mph and all it will do is backfire continusly. doesnt mater how you drive it.
pvc valve needs looking at me thinks (thanks warthog) cause it sucks like a dyson at idle

[Warthog]

Hey Matey,
Your running LPG right?
The common reasons for backfiring are
a)Lean mixture
b)Spark problems
c)Low Compression

a)Lean mixture can be caused by incorrect miture adjustment (your system in not electronicaly adjusted but simply switch with a switch with built in safety car RPM sensing). As it has run for some time without problems I doubt this is the issue but the tractor like bedini regulators are not much cop at giving acurate fueling. Weak mix caused by leaking air is quite possible - you can check this by spraying carb cleaner or similare around the manifold to head joint around the hoses and other connections. If the revs pick up while being sprayed in any area then gasket/hose will need replacing.

b)Spark problems. Plugs, leads, cap and rotor the most common causes of backfiring on LPG converted engines. If the timing is out and delivering the spark at the wrong time this can also cause backfiring.

c)Low compression and exhaust gas is igniting the mixture in the manifold.

I think it's to do with valve overlap and flame speed, at full throttle and
low revs (ignition retarded) and if the mixture is weak the exhaust valve
can open while the gases are still above the autoignition temperature of
the mixture. as the gases don't have the inertia they would at high and in
a multi cylinder engine there will be another cylinder during its valve
overlap phase so there is the chance of a flame path from the exhaust
manifold through the fresh mixture in the cylinder and out through the
inlet valve.

At higher revs the natural momentum of the gases in the exhaust scavenges
the system.

Mixer problem?

The mixer where the gas is intoduced to the air inlet. Inside that should be a venturi (hour glass shaped ring) that creates the vacuum to draw the gas in. If that has popped out it would cause lean off ilde running.

The main flow valve when used with a OMVL R90 is normaly between 1/4 and 1/3 open. With a lesser unit like your I would expect about 1/2 or just over depending on venturi size. It is possible it was to far open but if that was the case then lean mix is not likely to be the cause. The only problem with regulators like yours is they struggle to supply enough fuel at higher RPM's.

Also if the air filter is removed (or the pipe blown and leaking air) the mixture will be leaner as there is not as much resistance to draw the air.

The opposite problem where the mixture doesn't fire and carries
through to the exhaust and is subsequently detonated there by the next hot
exhaust stroke.

Probs writing stuff you already know mate? But have had problems similiar on me Rover V8.

ATB
Mark

[king cj]

thanks mate i,ll try some of that, had the manifold off and there is evidence of fire in the intake. gona rebuild and trick up the motor soon including injected lpg and mega squirt, would like to fix this tho so i can run it untill i,m ready and got the parts to do the build

[Bubba]

i realy hope you sort it dood its a funny one that 

[Asylum]

Glad you guys know what your talking about  

[Bubba]

what happens running on petrol i seem to recall phill walker had this problem on chev powered cj10 it was somthing to do whith the lpg and its timing

[ZJ Brad]

Check your timing mate. Small block chevy engines are pretty finiky about timing. a few degrees either way and it will raise havoc. if memory serves correct (double check please as it's been almost 20 years since I've played with one) timing should be about 8 degrees BTDC.

[king cj]

pretty much sorted it, was a inlet manifold leak

[Bubba]

sweet