Aston Martin DB9 V12 No Compression on 2 cylinders...

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Aston Martin DB9 V12 No Compression on 2 cylinders...

Postby FioranoCars » Mon Mar 21, 2016 11:50 am

After some thoughts and comments please!

This is a V12 Aston Martin DB9, the psdata file has
chA - battery +
Ch B - Amp clamp to Negative cable
Ch C - Positive as close to starter as we could get (a positive post in the engine bay)
Ch D - Engine Block

This is annotated of just amps, showing that :
- 1 bank is lower compression that the other (even numbers)
- That 2 cylinders are probably no compression (8 and 12 by arbitrary numbering)
- That cranking is very slow and very high amps (44rpm +/- and >400amps)
Aston DB9 Crank Amps annotate3.png


The car can run for a few minutes, but then dies. There is oil in the air filter, it's not overfilled, and the PCV valves have been changed. I'd rather not run the engine unless we're going to get some very good data.
No exhaust readings (HC's etc)

Yes, I could test again with a spark pick up and pressure transducer, to get actual compression and real numbering of cylinders, so moving past that ... :)

We also plan on:
- checking the oil, but apparently recently changed, looks "ok" on dip stick
- checking water system
- leak down test
- bore scope down spark plugs (need to remove inlet manifolds which is a pain and disables the car)

Before we remove things (like inlet manifold) or the engine and one head, maybe both, what else can/should we check ... I fancy diagnosing the last degree out of this, as it looks like a great opportunity to learn ...

Ideas/comments most welcome regards tests, and maybe what you think we'll find inside!!

Richard Lukins
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Attachments
DB9 C7 SAH - Crank 1.psdata
(7.17 MiB) Downloaded 38 times
Last edited by FioranoCars on Tue Mar 22, 2016 10:41 am, edited 1 time in total.
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Re: Aston MArtin DB9 V12 No Compression on 2 cylinders...

Postby volrem » Mon Mar 21, 2016 12:21 pm

Hi


I did measure rpm to be 97 not 44 but that is still low and amps are indeed high.

Possible starter problem or engine mechanical.

I would measure crankcase pressure along with starter current and cylinder sync to see if this low compression goes to sump.

Cam and crank relation would be also good idea if there is something to compare to.
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Re: Aston MArtin DB9 V12 No Compression on 2 cylinders...

Postby FioranoCars » Mon Mar 21, 2016 12:54 pm

Thanks, Volrem
Engine mechanical rather than starter is my belief (98% sure), but will confirm at some point (against a known good car)
Crankcase pressure, will try to measure, need to see how best to get a good reading, close to source, and yes our gut feel is sump, but stopped all tests after this to take a moment and think!

Cam and crank I'm told hard to reach, but we will, to confirm timing (will get a good car in too, to benchmark)

RPM, calc was a rough quick and dirty one, and in doing so my maths was truly screwed up too but here's my measured result:
measuring 3 sets of 12 = 6 crank rotations
Starting at 560ms ending 4360ms = 3800ms or 3.8 seconds
Maths is:
Code: Select all
Rotations Per second = time measure in secs / number or rotations
For RPM x 60 (seconds in a minute!)

3.8/6 = 1.578947368421053
then * 60 = 94.7RPM

Thanks for pointing out my error, big difference from 44 :oops: , but clearly still some way off the 200rpm we'd like/expect. Not sure what a good DB9 spins at, will try and measure, but doubt its <150.

Any other comparative readings/tests you think will shed more light?

Forgot to say, with the "guessed" low compression, we should see lower resistance and faster spinning of engine, not slower and higher resistance (amps), so not just a blown head gasket ... Anyone care to comment?

Thanks
Richard
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Re: Aston MArtin DB9 V12 No Compression on 2 cylinders...

Postby volrem » Tue Mar 22, 2016 6:17 am

FioranoCars wrote:
Any other comparative readings/tests you think will shed more light?

Forgot to say, with the "guessed" low compression, we should see lower resistance and faster spinning of engine, not slower and higher resistance (amps), so not just a blown head gasket ... Anyone care to comment?



Well it all depends what what parts have easy access. In-cylinder pressure capture would be good, also intake pulsations.


And you can't rule out starter as a reason for slow crank and hing amps. But if anything is seized inside engine then it would cause both. Wheter it is seized oil pump, bearings, piston or something else.
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Re: Aston MArtin DB9 V12 No Compression on 2 cylinders...

Postby STC » Tue Mar 22, 2016 11:46 am

Richard

Engine mechanical rather than starter is my belief (98% sure),


With a cranking speed that low, my first port of call would be the Starter and its Circuit, which you seem to have covered belt & braces.

It is Clear that one Bank has little or no compression. Any evidence of misfiring when it does run ?

Jumped Belt / Chain ???

The car can run for a few minutes, but then dies.


Surely that rules out the starter motor, if its turning it just enough for it to catch, at this point the starter motor becomes redundant, all else being well it should run forever ?

Before we remove things (like inlet manifold) or the engine and one head, maybe both, what else can/should we check ... I fancy diagnosing the last degree out of this, as it looks like a great opportunity to learn ...


Indeed our testing should where possible be with everything intact with as little intrusion as possible but I think you have gone as far as you can and it could be time to get the spanners out. With it running for a short time you have various test options scoped & serial data to satisfy that learning opportunity. Engine Vacuum, First Look in the tailpipe(s), Ignition Primary(s), Cam & Crank backprobed at ECU, MAF, MAP, Knock Sensors, Oil Pressure, Coolant Pressure, Variable Camshaft angles .......
O2's & Injector durations if you can get it closed loop.

It would be great, dare I say a small miracle, if you did manage to spot the weakness that causes it to cut out. The fact that it will run puts the starter on the back burner in the short term but does point the finger at some sort of internal seizure, which explains the starter struggling to turn it but not the lack of compression on one bank.

I would suspect at least 2 different faults, the second may or may not be consequential of the first.

Seized Camshaft and consequent chain / belt jump, snapped Pulley ???
Last edited by STC on Sun May 15, 2016 9:49 pm, edited 2 times in total.
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Re: Aston Martin DB9 V12 No Compression on 2 cylinders...

Postby volrem » Tue Mar 22, 2016 3:29 pm

Auxiliary drive belt components are also a thing to consider and check.

I once got sidetracked with extremely high amps and very low speed. Checked that engine turned fine by hand so blamed starter. It turned out later that when ignition was turned on then A/C clutch was energized and starter didn't have enough power to run.

Very simple and stupid thing.
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Re: Aston Martin DB9 V12 No Compression on 2 cylinders...

Postby busjockey » Wed Mar 30, 2016 4:19 pm

I would agree you need to do a good visual inspection and make sure you don't have any pullys binding before moving on.
If that's ok I would then drop a pressure transducer in a cylinder and have either another tranducer in the intake at a centralized location and crank the engine over and analyze it. Then I would do the same thing to the opposite bank to compare. I would also try starting the engine and then look at your exhaust plateau and see if you have excessive back pressure, maybe one of the banks is restricted real bad causing this issue, you said it will only run for a few seconds and then dies. Just some thoughts. Post your captures please, I have never looked at a Aston Martin, way cool!!!!! :D
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Re: Aston Martin DB9 V12 No Compression on 2 cylinders...

Postby FioranoCars » Wed Mar 30, 2016 7:20 pm

Thanks for the comments and please keep them coming. I will post back some captures, but this is on the back burner for another week, as workflow means we can't get to it just yet.

Pulley's etc all free and clear, A/C will check, maybe we'll release the Aux drive belt and recapture to rule out alternator/power steering etc etc as well, just for complete freedom from the external loads.

We have a "First Look" and single pressure transducer, not sure I can stretch to a second :cry:

Will probably try a compression test first and identify the bank and cylinders we're concerned with, so we can endoscope via the spark plugs. Would like to keep rotations to a minimum if possible, but will try and get an idea of crank case pressure and exhaust activity too.

But if you think of a test worth doing, post it up and we'll add it to the list, in for a penny and all that !!
Results will be uploaded to the library and posted here, so best shout now!

Thanks
Richard Lukins
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Re: Aston Martin DB9 V12 No Compression on 2 cylinders...

Postby Fat Freddy » Wed Mar 30, 2016 11:35 pm

Hi Richard

I have no idea what this engine is so I may be way outside the box. Or I could be stating the bleeding obvious.

Can't explain the Amps yet

My take is it has a bank problem because of the alternating High/Low peaks. Also the lower peaks are not 360* but 330* after the preceding peak. I would keep that in mind. Timing on one bank????

And a least one cylinder has a major issue. I would sync up that low cylinder with an ignition pick up. Then WPS that cylinder but personally I would do a leak down on it then maybe WPS.
Then perhaps WPS one good right bank and one left preferably with crank and ignition.

As for the start and stall. If this ran a MAP it may see high manifold pressure, calculating a high load from bad timing or a leaking inlet valve and then inject more fuel till it stalls. I'm basing that on symptoms of a leaking EGR on C14SE ??? engines.

Anyway Richard that's my 2 cents. Not knowing what I'm dealing with or set up I can't come up with much else.

HTH
FF

EDIT:
My take is it has a bank problem because of the alternating High/Low peaks. Also the lower peaks are not 360* but 330* after the preceding peak. I would keep that in mind. Timing on one bank????


Brain fart - The duration is 60* (not 360*) but 55* (not 330*). So peak current draw occurs 0*, 55*, 120*, 175*
Last edited by Fat Freddy on Thu Mar 31, 2016 10:12 pm, edited 1 time in total.
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Re: Aston Martin DB9 V12 No Compression on 2 cylinders...

Postby House5 » Thu Mar 31, 2016 12:34 am

Hi Richard,

Ahhhhh, Aston's do adore spending time with bonnets up doing technicians head in, the fix for your symptoms as follows -

pour 1 litre of soviet aviation fuel on Aston plastic interior (hotter burn rate for less fuel...cheap as chips too!!), 1 redhead match lit coolly off worry stubble then flicked accurately into fav getaway car, then Guinness time!

ohhh... you did remember to remove lovely pico equipment and park car suitability distanced outside

:D i'm doing some motorsport travels again so will make a point of calling in on a stop over in London soon

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