My Renault has started to 'miss' at low revs (below 2000rpm). Upon taking it to my local garage the fault was diagnosed as a worn valve, no. 1 cylinder. This was diagnosed through it having a lower compression rating than the other cylinders.
When I asked why the engine doesn't seem to miss above 2000rpm I was told that below 2000rpm No.1 cyclinder takes more of the load. Therefore, as the revs increase so more effort is taken up by the other cylinders.
Can anybody tell me whether this is correct?
Many thanks.
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Had this on a Datsun(?!) Bluebird.
The car's idle would be very lumpy and up to about 1500rpm the engine would be audibly only firing on 3 cylinders but above this everything sounded okay.
Similar reason given though the garage did give several other possibilities.
What happens is that once the engine is above a certain speed say 2000 rpm, the momentum of the engine via the flywheel hides the fault in the cylinder. In fact, if the cylinder is weak then it will probably be half firing at this point. There is much less pressure on the cylinder to turn the engine because the power in the flywheel/momentum in the car drowns the effect out if that makes sense. The cylinder will most probably still be not firing correctly and you'll have less power than if the engine was working correctly.
Not entirely sure why one particular cylinder would be taking up more of the load below a certain engine speed - maybe someone else can explain?
Dan
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Dan you're right. Not sure of the exact engineering theory but a duff cylinder will most often show at idle then smooth out at speed.
If it's really duff, and with a lower power engine in particular, it will still show when you go faster.
Harder to detect one missing on a V8 Rover.
David
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David W wrote:
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> Dan you're right.
Phew - I did wonder for a moment after I posted :-)
> Harder to detect one missing on a V8 Rover.
Interesting you should mention that. When my dad was rebuilding a Rover V8 to put in his kit car, the engine had originally sounded fine, but on dismantling several of the cam followers were found to be completely worn out and we reckoned it must have been missing completely on at least one cylinder. Just goes to show that a knackered engine can sound fine - must make an interesting job of checking bangernomics used V8 cars in the US!
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I cannot believe that the engine is designed to have one cylinder taking more of the load than the others. Even if it did, this would be unlikely to affect the wear rate of a valve. Someone has got their wires a bit crossed here.
It can happen that one end of an engine can receive inferior cooling and thus exacerbate valve wear, though I would have thought that even this is most unlikely in a modern engine.
The most likely reason for the 'missing' to be less obvious at higher engine speeds is that there is less time for the gas to escape past the valve during each compression/expansion event.
Incidentally, low compression could also be due to leakage past the piston rings. The garage should have carried out a second compression check after squirting some oil into the cylinder to seal the rings -- if the reading was roughly the same as before, the leakage is probably past the valve, if it is higher then the leakage is probably past the piston. Did they also check for a leaking head gasket (less likely, but possible)?
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I had this on a Laguna late last summer, misfire on one cylinder which would even out as revs increased. Being an engine tuner of the 'old school' I hung a vacuum gauge (remember them?) onto the inlet manifold which confirmed a valve problem. Checking the valve clearances showed an inlet valve on #3 cylinder had virtually no clearance. Had the vehicle not been under "warranty" I would have tried reshimming but I suspect that the problem was a little more deep seated and would have preferred to remove the head to examine condition of the seats.
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Dan J
Rover V8s are notorious for this problem. Mine sounded fine apart from a slight misfire, especially when cold. I was going to buy a new set of carbs, but a specialist advised me to inspect the camshaft first. Two of the cam lobes were worn almost completely flat. It goes much better now, with a new camshaft and followers. The Rover V8 is actually fairly sophisticated compared to most older US engines, which is probably why Buick stopped making it after a couple of years and sold the design to Rover - too expensive to make, especially as it is a fairly small motor by American standards.
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All cylinders should have equal load. A missfire of say 10% will always be more noticable at low revs. Assuming the garages diagnosis is correct. Having carried out both wet and dry compression tests, cylinder leakage test and checking the valve clearance. I would pull the head off and have a look. If the garage think that the load should be different on No 1 cyl then I would go to another garage.
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If you are buying a car, you will often find that the seller (garage or private) has turned the idle speed up a hundred or two revs. to cover up any uneven running.
It is a good idea to listen to it with the idle set as low as possible before parting with the dosh.
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