Compression: Turnover

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Compression: Turnover

Post by Pug_XUD_KeenAmateur »

white exec wrote: 16 Oct 2018, 16:01 ....when the engine is warm, try cranking it, but not allowing it to fire up (eg by disconnecting the 12v feed to the Engine Stop solenoid). If compression is down on one cylinder, you'll hear the cranking accelerate every fourth compression.
With reference to Chris' response elsewhere on the Forum, I thought I'd give this a go for myself, having noted a slight change in engine tone under acceleration.

I was a bit surprised how fast it cranks, warm. Do others think this is excessively fast, suggesting poor compression on all four pots? Mileage is high but we all know these XUD's are superb lumps, its na.


I do have spare engines, but I'm in no rush to fit them.

Start up also available


In other news, I've also just removed two fuses to attempt to trace down what appears to be a Parasitic Drain.
Last edited by myglaren on 17 Oct 2018, 17:16, edited 1 time in total.
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Re: Compression: Turnover

Post by MikeT »

Nope, sounds about right speed for a well-charged battery imo.
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Re: Compression: Turnover

Post by white exec »

That sounds like a really healthy cranking, and a start. No worries there!
Always did like the NA XUD.

If compression is down on a pot (or two), the cranking will have noticeable surges instead of even resistances.
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Re: Compression: Turnover

Post by moizeau »

Sounds like you've just fitted a new battery, purrfect.
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Re: Compression: Turnover

Post by Pug_XUD_KeenAmateur »

Thanks Guys, glad to realise its fine.

No, Battery's 4 years old Pete, but I was just back from town (10miles, mostly fast rural roads), so should've been in a great state of charge. Lost 0.16v so far in 2 hours with Fuses 1 & 15 out though, so looks like I need to 'dig' some more (or simply Isolate the Battery)
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Re: Compression: Turnover

Post by white exec »

Pux, have you measured the residual current drain in mA ? Should be not more than 50mA, and preferably less (typically 30mA on XM).
Wouldn't take too much notice of small voltage drops per se, as as soon as [heck, that a lot of ass] the battery comes off-charge, its voltage will start to settle. Persistent current drain (and where it's going) is what's interesting.
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Re: Compression: Turnover

Post by Pug_XUD_KeenAmateur »

Thanks Chris. That seems like a whole lot easier way of doing things than comparing Battery Connected / Disconnected readings as I have been.

I did try this back in the summer when after investigation I'd decided there wasn't a problem. However a flat Battery two weeks ago after sitting unused for 4-5 days changed all that; it was so flat that even the Ignition lights were dim and obviously turning the key had no effect.

Having forgotten how to do it, I reminded myself via Youtube (I used

....but I seem to be getting a nonsense reading approx 1000x higher than the 50mA you suggest.
20181018_100455a.jpg
Am I doing it wrong, or do I have a massive 6A Drain? ...surely its me?
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Re: Compression: Turnover

Post by white exec »

Check the accuracy of the meter (on that DC amps range) by trying it with a bulb or two, eg 21W (1.75A), 5W (0.4A), etc. Also try the meter on the 10A range; sometimes just one range on a meter goes screwy, but another will work. Check that the meter's own battery is ok; readings can go wonky if not good.

6A is one helluva residual drain, and enough to flatten the battery in less than a day. I have once found a drain of 3A (on a Toyota, caused by an idiot auto-electrician fitting alternator connections the wrong way round), and this flattened the lump in a couple of days.

If you still have that big drain, leave the meter in place, and pull fuses one at a time until the drain drops back (to a sensible figure). That will tell you where the offending toaster is.
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Re: Compression: Turnover

Post by Pug_XUD_KeenAmateur »

'Toaster' indeed.

The Fluke Multimeter managed a reading of 6.21A this afternoon, a new PP3 had no effect and I was a bit mystified about the Bulb Test idea. I tried connecting it as you would for Continuity, ie to test for good filament, but got nothing; I guess that's not what you meant, but no matter; the little yellow Meter has proved that reading to be just nonsense.

I hadn't considered the fact that the Meter might be at fault here, so particular thanks for that inspiration. Bodging a connection to the Yellow Meter using the Fluke Probes (its own probes are broken) I got a reading that fluctuated between 0.05A and 0.22A, when I could get a good connection; so a new Multimeter is on order and hopefully that'll permit me to get some reliable readings.

I'll update when it arrives, hopefully with news that I've at least traced which Fused circuit is at fault.

(naturally, all three sets of Probes have different size fittings to the Meter, so aren't interchangeable).

Multimeters L-R
Yellow: Broken probes, but appears accurate. Attempts to bodge new ones using a wire wrapped around an insulated Precision Screwdriver failed.

Fluke: wildly inaccurate

Draper: gives a bizarre reading also '220' in this case.
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Re: Compression: Turnover

Post by white exec »

Meters! Best of three!

The bulb test
Connect a 12v car bulb to a 12v battery, but insert the meter into the circuit to show the current the bulb is taking.
Compare what the meter reads with what (you know) the bulb should be taking:
eg 21W bulb....1.75A
eg 5W bulb....0.4A
eg 55W headlight bulb....4.6A
[ Amps = Watts ÷ Voltage ]

Can recommend Uni-T meters, if you're buying. You've probably ordered by now anyway.

The old analogue (Draper) meters tend not to develop faults, and can remain accurate more/less indefinitely.
The way they go awry is if too large a current is shoved through them on the Amps range, and the internal resistors get cooked before any internal fuse blows.
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Re: Compression: Turnover

Post by Pug_XUD_KeenAmateur »

An update with a result, with thanks for all your advice guys.

Things have got a whole lot more 'sensible' with the new Multimeter.

A Residual Drain reading also came about about right, nowhere near the rather silly 6A reading I was getting; instead where it ought to be between 0.05A and 00.7A
....albeit on initial connection it usually reads approx 0.27A, falling within maybe 5 seconds to the aforementioned. I guess that's normal though.

The Battery's losing between 0.07v and 0.1v a day, no matter what I do with it, connected or not and even though it'll show as much as 13.40v immediately after a run, that'll fall within a couple of hours to IRO 12.8v.

so if I leave it stood for 2 days I'll have only 12.6v
3 days 12.5v
if I was to leave it a month I'd have no chance of starting it with about 10.3v

So it seems I can reasonably confidently say I don't have a Parasitic Drain, but the Battery's not 100%; coz surely even 0.1v a day is rather more than is ideal.
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Re: Compression: Turnover

Post by white exec »

A battery that settles down to 12.5v shouldn't be a problem. As you say, lower than that is not good, and likely the result either of continual residual drain, or just the battery self-discharging through age.

Interested in the residual drain current though, which you say settles to 0.05∼0.07A.
That's 50∼70mA, which I think is rather on the high side. Would be better if it was around half that.

Can you track down where that current is going?
With that showing on the meter, try pulling a fuse or two to things like radio, alarm, etc. and see whether the figure can be made to drop.
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Re: Compression: Turnover

Post by Pug_XUD_KeenAmateur »

just revisiting this post with an Addendum re Batteries.

Despite the healthy cranking demonstrated in the clip above, and the Battery regularly holding 12.8v (sometimes as much as 13.1v), it delivered me approx 10 seconds of Cold Cranking fresh from a charge not 12 hours previously

I've since replaced it (and the GP Relay, which was the reason for the unusually long cranking) it and normal play is restored.

Just coz it gives a good reading on a Multimeter don't necessarily make it a good Battery.

PS: utterly underwhelmed with the Exide brand, this one expiring at 4-ish years old. Suggest if you want a half-decent Battery that'll you buy something else; I did.
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Re: Compression: Turnover

Post by white exec »

Your GP diagnosis was right - given enough cold cranking without the GPs working, the air temperature in the cylinders will increase enough to fire up the fuel. Compression will raise cylinder air temp to 600°C+, but this heat quickly dissipates if there's lots of cold metal surrounding it!
Glad the new battery is doing its stuff, Pux.
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Re: Compression: Turnover

Post by sparksie »

Hi
Sorry, coming to this too late to help the OP, but a battery that settles to 10.5V, or thereabouts, has a dead cell in it.
This is caused by sediment in the bottom of the cell shorting out the plates. Usually, in regular use, they'll keep going for a while like this, because the sediment gets agitated enough to stop it making a sound connection in the bad cell, allowing the cell to charge.
Over time, when left sitting, the cell gets shorted out and flattened. Sometimes the act of trying to start "blows" the connection and allows the cell to accept a charge again, for a while, but really, once there's enough crud in a battery to cause this, it's beyond redemption.
It's one of the first tests, when faced with a flat battery. 10.5V is a red flag!
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