my '90 XJS V12 coupe starts normal when cold but with a hot engine the starter cranks significantly slower to the point that the engine does not fire up again (sounding like a drained battery). With the engine cold, I can do multiple starts back to back without any issues…
Here is what I did/discovered so far: There is no battery drain when the engine is off. The voltage at the poles is around 12.6-12.8V without engine running, after starting the engine the voltage at the battery poles is around 14 V when cold and 13.3V when hot. In a “slow crank situation” jump starting immediately causes the starter to work normal and the engine fires up. I do not have the tool to measure the alternator Amp output but the voltage at the battery poles drops to about 8.5 V when cranking.
I went to Autozone to have the battery tested - “bad”. I went to another store, they stated “nothing wrong with the battery”.
My next step is to check the electric connection at the alternator. I already visually inspected the main positive terminal at the firewall which seems to be fine. I rule out a battery ground issue as the jump process (see above) with the jumping clamps at the battery poles eliminated the problem.
8.5 volts while cranking sounds a bit low to me. I’d be looking for at least 10, I think.
Eliminated a problem, or masked it?
On principles clean the cable-to-body connection on the battery “-” cable. Then look for the engine ground and clean it as well, at both ends. On your XJS V12 it’s probably right next to the LH engine mount.
“Seems to be fine” isn’t very reassuring :-). Remove the nut and cables, clean everything until you have shiny metal.
Anyhow…
I suspect a weak battery or a starter problem. Sometimes a failing starter will turn slowly (while absorbing tons of volts and amps) after absorbing engine heat.
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The slower hot cranking is somewhat inexplicable, Stephan…
When comparing hot and cold charging voltage; take into account that charging is higher in enhanced cold idle - increase hot idle with the pedal to the cold idle rpms. The battery readings 12,6 - 12,8V is normal for a reasonably charged battery - but voltage readings do not necessarily reflect the battery’s ability to deliver ample current for cranking. Also, the battery voltage before cranking should be the same for both a cold and hot engine, but bear in mind that a hot engine has been running - charging the battery…
8,5 V while cranking is too low (except in very cold climate) - reflecting either excessive engine resistance, or simply low battery cranking capacity. The latter denotes a bad battery - which doesn’t make sense if hot cranking is OK. While bad connections interfere with cranking power delivery; it will also reduce the current drawn (and slow cranking) - which will cause less battery voltage drop. All highly bewildering.
That hot cranking is restored with a jump battery is, at first glance, certainly an indication of a bad battery. But there are three ways of connecting a jump; ‘jump’ battery post to ‘jumped’ battery posts (not recommended), ‘jump’ battery neg to ‘jumped’ engine block - and’ jump’ neg to ‘jumped’ car chassis. The latter uses the jumped car’s grounding straps and internal connections. The two others bypasses the grounding straps, but use the others…so…?
Difficult to see how engine temps can affect connections, and a bad battery should affect cold cranking more than hot. The easy ‘test’ is of course to replace the battery - but cleaning connections is not a waste of time in this puzzling situation…
thanks a lot for your comments. Your last sentence summarizes my thoughts very well. I will check the ground connections but I am quite sure that this is not the issue here. My thought process about “elimination”: If the jumper battery is connected to b both battery terminals than the current flows through all internal wiring including ground connections while a connection to engine or chassis would bypass potentially faulty wires.
The battery had issues before (it twice was completely drained) so I think it is a good measure to replace it anyway. I am still suspicious about that starter, though…
With a jumper battery connected you’re introducing a lot of extra amperage…which overcomes the resistance of poor grounds and connections, allowing the starter to turn normally, thus masking the problem caused by the faulty grounds/connections. A clue would be the battery cables getting hot to the touch.
A clue that might help condemn (or exonerate) the starter would be to observe what your interior lights do when the difficulty occurs. Assuming a strong battery and good connections you’d typically see the interior lights dim just a wee bit when cranking. However, a failing starter will often draw excessive amperage and cause the interior lamps to nearly extinguish. Unfortunately, lack of nearly extinguished interior lights doesn’t prove that the starter is good.
As DIYers with limited test equipment on hand we’re left with running down one clue and moving on to the next. But even a pro with tons of test equipment must rely on a known-good battery and known-good connections before the test results can be trusted.
just a follow-up on my post: I finally replaced the starter motor and all is fine! Upon inspection of the old starter I could see that the main bearing was completely disintegrated causing increased friction resp. a lock situation, probably when hot.
Thanks for all comments
That’s interesting, my X305 V12 recently started doing that as well. I did replace the diying battery, which did help to the point where I have no issues anymore, but cranking is still sihnificantly slower when hot.
The starter is relatively new, I replaced it three years ago because the old one didn’t release properly and made a horrible noise after the engine started. But maybe the exchange part was not peoperly reconditioned. I’ll see if it gets worse - I don’t particularly fancy replacing it again, it is too hard to ger too…
If this was an off-the-shelf unit I wouldn’t be surprised if it was poorly reconditioned.
Increasingly bad luck with store bought rebuilt starters and alternators prompted me to have all such work done at a local specialist. Zero problems over the last few years.
I think you’ll find a compression ratio doesn’t change at all ever. Hot engines struggle to start only because the oil is thicker, think 15w/50 its 15 cold and 50 once hot, simple!
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Nevertheless, Mark - cranking rpms are overwhelmingly governed by internal engine friction; which is very high with a cold engine…
During cranking, the air in the cylinders are heated substantially by the compression - whether the engine is hot or cold the compression temps and pressures are little changed; the air spends very little time in the cylinders to absorb engine heat. And all heat related computations on temp differences refers to the Kelvin scale…