4.6 Engine rattle on load and high RPM

**Old Rangie** · 19th March 2011, 10:52 PM

I have searched the various threads and I am not seeing anything familiar.

I did a couple of quick trips along the Pacific Highway recently with my old carb 4.6 V8 LPG Rangie and keep hearing this noise on hills. It starts on long hills as though it is pinging but the noise I don't think is pinging.

I have a programmable MSD CDI and there is not enough advance on LPG 100 plus octane, and its not leaning off. A/F ratio seems to be about right. The other interesting thing is the increase in oil pressure at the high rpm as though the relief valve is sticking.

My suspicion is that oil relief valve is sticking and lifters are pumping up and the rattle is the lifters bouncing on cam or some where in the valve train. The engine does not miss so valves seem to close.

Once the hill has passed and oil pressure has returned to more normal level, ie engine rpm back to 3000 rather than 4500-5000 the noise is gone.

I am fairly confident the noise is not detonation but a wise technical person once told me that it was difficult for human ear to hear detonation in the alloy V8 over 3000 rpm due to all the other noise.

Anyone had such experience and suggestions as how to isolate the possibles. A spark plug change is my next move and only change one thing at a time.
regards Old Rangie

**Blknight.aus** · 20th March 2011, 06:28 AM

whats the condition of the plugs and does it pop the dipstick?

**bee utey** · 20th March 2011, 07:13 AM

It sounds a bit like your head gaskets are leaking out the sides. My P76 engine used to do this. Use an inspection mirror and a light to check along the block/head interface for black spray marks. Possibly also loose exhaust manifolds.

**RoverP6B** · 20th March 2011, 07:14 AM

Is the oil pressure on the hills in the order of 60psi? If so, that could substantiate your theory of lifter pump up.

Ron.

**PhilipA** · 20th March 2011, 08:52 AM

In my experience the pressure relief valve nearly always jams open, causing no oil pressure, and it happens just after start up.

Unless someone has changed the spring for a high pressure one.

Regards Philip A

**RR P38** · 20th March 2011, 10:40 AM

If you dont get it tracked down with road testing.
Getting it put on a Dyno might help you identify the problem a lot more easily.
Sounds like pre ignition to me just the same.

4500-5000 rpm hell i hardly every reach those numbers with my 4.6

**Old Rangie** · 20th March 2011, 11:14 AM

I have new plugs sitting there just trying to get time to change. But with 50 K volts from the CDI they seem to be working.....Dipstick is fine, however it does seem to develop an appetite for engine oil but thats not surprising for flogging it like I have done recently. This is an open loop gas system so no back fire issues.

**Old Rangie** · 20th March 2011, 11:17 AM

Good point, shall look with lots of light (poor eyesight in this region) but from memory a recent look only showed a couple of coolant weepage spots. Has the capped cylinder liners done with 4 into 2 into 2 big bore extractors.

**Old Rangie** · 20th March 2011, 11:22 AM

Pressure normally 21 kPa - 24 kPa. Giving it a few revs it gets up to 40 plus.
Engine coolant temp 80-90 Cel. Engine oil temp 110 Cel. Also has an appetite for engine oil further supporting relief valve issue.
What I am not sure of is what happens when the lifters pump up I assume the valves are eventually held off the seat. They seem to still be sealing as no miss as yet.

**Old Rangie** · 20th March 2011, 11:26 AM

Has not always done this so unlikely some one has packed the spring. And agree normally the thing sticks where the ridge normally is. If stuck open (big orifice)then low oil pressure at low flow? where as stuck at smaller opening (small orifice) would give normal pressures until high flow? Does that sound sensible?

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