G'day people, I am seeking some mechanical advice. I own a '99 Landrover Discovery Series 1 with a 3.9L V8 which I have converted to LPG. I bought this vehicle 18mths ago when it had about 95k on it and it now has about 140k. It has been a very reliable vehicle except for a persistent engine noise that started at about the 120k mark.
When the noise first started, it was identical to the traditional ‘lazy lifter’ noise i.e. a light tappet noise would just clack for a second or so when the engine was first started from cold, then totally disappear. I assumed it was a lifter and did nothing. It progressively worsened over the next 10k to the point where it started appearing at all engine temperatures when the engine was put under heavy load (hard acceleration or towing a heavy load) from about 2800rpm upwards.
After worsening a little further I got the stethoscope out to determine the exact location of the noise. At this point I could make the noise appear just by giving the engine a quick rev. It appeared to be the top of the engine (it sounded obviously like a tappet) at the rear of the engine on the right bank. At this point, I dismantled the top of the engine including the inlet manifold and the rockers, pushrods and lifters. I inspected:
• the tops of the valves – no excessive wear
• rockers & shaft – no excessive wear
• pushrods – all straight and no excessive wear
• lifters – no excessive wear (which should indicate no excessive wear to camshaft lobes??)
I did note, however, that there was a fair build up of sludge in the top of the head and the rocker covers which indicated that the engine may have suffered from a lack of oil changes and perhaps short runs. I replaced all the lifters assuming that one had either failed or was blocked up with sludge. Upon reassembly I was disappointed to find that this made absolutely no difference to the engine rattle.
I drove the car for another 10k and though it worsened slightly, it didn’t appear to make much difference. The rattle is very obviously at the back of the engine on the right bank and is absent if the engine speed is increased slowly (no load) and if it is suddenly decreased from high speed, the rattle is quite loud. It also appears now at a much lower engine speed than it used to when under load and is louder than before. I considered the possibility of broken or weak valve springs so I recently removed the right rocker cover, tappets and pushrods and re-inspected.
Slight wear was apparent on the rear most rocker where it contacted the valve was apparent but not excessive. I removed every valve spring on the right head and inspected carefully. There were no broken valve springs or any that were significantly weaker than the others. The only discovery of note is that there were two oil feed holes in the top of the head for oil feed to the rocker shaft. They are under the front and rear most rocker shaft mounting posts. When compressed air is applied to the front one, it escapes into the sump through the oil gallery. The rear one, however, appears to be blocked. It doesn’t appear to be clogged; it just doesn’t continue past the head. The Haynes workshop manual I have has an oil feed diagram and only shows an oil feed to the front of the rocker shaft - none to the rear. Therefore, I must assume that the rear hole is not supposed to go anywhere.
I cannot explain the tappet noise. Numerous other mechanics have listened to the noise and agree that it is actually a tappet.
The only thing I have not done is remove the camshaft and inspect the lobes for wear, but I must assume them to be ok since the bottom of the old lifters showed no significant wear. There is no obvious wear in the rockers, valves, pushrods and the lifters have been replaced. I don’t know
where to start looking now. Can anyone help?