Ominous noise quiz

[Keith Turner]

You haven't fitted a new timing belt recently have you? on the larger engines the petrol pump cam can hit a con rod if you don't position the auxiliary shaft pulley correctly to the book, this makes a distinctive knocking noise from starup. just a thought. Unfortunately running low on oil sounds like your problem you think you have got away with it as the engine sounds OK but the damage is done & the bearing fails some time later.
Even with a damaged piston the other 3 should fire & start - check compressions these engine return pretty high readings on a compression tester 150 - 180 is normal my 1600 showed 200 -210 - an odd one to check is the distributor shaft turning? is the fuel pump pumping? or is an 1800 camshaft driven? not familiar wth that engine.
Engine wear is a strange thing I had a Trevi 2000 at 50000 miles the bore wear was horrendous but my volumex did 325000miles with 1 piston ring set change & a couple of head overhauls in the end it failed because the ring slots in the piston had become so wide that the rings twisted gouged the bore & broke, it still kept running but rev it over 3000 & it dumped the contents of the sump into the aircleaner much pinking & lots of smoke thats what you call crankcase compression!
Thes engines are pretty tough & will keep working in awful condition but low oil is a killer. If you rebuild it use a semi synthetic oil not fully synth tooo thin! good luck with getting parts, oilpumps are hard to find & pistons.

[Randy Adams]

Hi Andy:

You are correct. A cracked piston should yield lots of smoke.

Concerning the matter of noises, rumbling is what you get with worn-out main bearings. With worn out big ends (connecting rod bearings) you get a knocking sound but it tends to be a bit heavy sounding because two pretty heavy metal items are whacking against each other (the crank pin and the connecting rod). With a piston, you have a light hollow aluminum piece. When it is whacking something, it is usually a lighter sound. Think of the difference in sound between a snare drum and a tom tom. The piston is the snare while the conrod bearing is the tom tom. The difference is not as radical as that but you get the idea.

The timing belt idea is a good one but that noise should have been apparent from the moment you changed the belt.

[andy]

And the winner is........
probably Keith, with the petrol pump cam theory, but nothing's been taken apart yet, so I could be wrong.
Yesterday I finally managed to grab a mechanic for 10 minutes, who was able to confirm one glaringly obvious thing (well, glaringly obvious when you're not sitting inside the LHD car holding the ignition key): the timing belt's not moving, although it hasn't snapped. The starter motor is turning over the engine, or at least the crankshaft, but there's not much else moving. So, would the following uneducated guesses be correct?
(1) The timing belt has lost a few teeth down below, so the toothed wheel on the end of the crank shaft is spinning without gripping the belt.
(2) The last time I drove the car, when I could hear a knocking sound that was still fairly quiet, the belt had already lost at least one tooth, so something was already making contact where it shouldn't have been.
(3) The next time I tried to start the engine, I probably ripped a couple more teeth off the belt, which is why it no longer moves at all.
(4) I have almost certainly bent at least one valve, and probably more.
(5) Since the cam wheels no longer turn and any exposed valves have already been bent out of the way of the pistons, the noise I now hear when cranking the engine is the petrol pump cam which is out of phase with the aux shaft.

My mechanic friend suggests that we start by changing the timing belt and checking compression, to see what valve damage has occurred. Should I assume that the petrol pump is knackered as well?
Regards,
Andy

[Randy Adams]

Ah, Andy.

You have been introduced to the joys of timing belt failure.

I don't think the belts ever actually break. Instead some teeth grind off. I had one perfectly good belt fail when the tensioner bearing froze up (on the freeway at speed with a car full of passengers heading to a concert, of course).

On the low compression U.S. version of the 1995 cc engine, there is actually no interference between the pistons and the valves and a belt failure can be repaired by simply resetting the cam/aux drive/crankshaft wheel timing and popping on a new belt. I do not know if this is true for any of the higher compression European variants.

Whatever you do, learn from my experience: replace the tensioner bearing along with the belt.

Best of luck

[Hamish]

Yep, sound advice. You should really change the tensioner every 2 timing belt changes - safer that way!
If you don't have at least one bent valve you are a seriously lucky fella... good luck, will be interesting to see how it pans out.