Hot engine - slow cranking starter

Hi All

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.

Any advice?

Thanks
Stephan

unless you have a good reason why not to, I would swap/replace the battery.
Only a load AND hygrometer test will prove out a bad battery

it can only really be battery, wiring or starter, unless something is making yr engine harder to spin when it reaches operating temp

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.

Cheers
DD

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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…

Frank
xj6 85 Sov Europe (UK/NZ)
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All

thanks a lot for all your input. I am going to check the wiring again and will replace the battery.

And BTW: Sorry for accidentally posting this to the XJ forum. I was getting used to it (owning a XJ12 as well)…

Stephan

Doug

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…

Best
Stephan

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.

Cheers
DD

Good point! Thanks Doug

Stephan

The older ACDelco starters were inclined to have hot crank issues. The architecture is similar to the Lucas product.

I had similar issues with two others. One in my venerable 69 Toyota Land Cruiser. But not often. Once an application of an ice bag “fixed” it.

My old “lumped” Willys 4x4 pickup suffered it on occasion. Time “fixed” it. Just wait a while.

And, my 79 IHC Scout II. ACDelco starter. A switch to solenoid relay ala Jaguar fixed it.

Some have added heat shields of one type or another.
Proximity to a hot exhaust seems relevant.

So, a starter with great wires may over come the heat issue,
One with weaker wires may not. And an aged starter, even with good wires may not.

But, for sure, an aged starter and weak wires will not.

The above conclusions with a good battery. If ?, compound the issues.

So, make all as god as one can, and stuff will work.
Carl

Carl

thanks for that. I will replace the battery and then “wait a while”.

Stephan

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Replacing the battery is positive action, Stephan - if the problem still persists; you have a readily available jumpstart battery…

…but honestly; a starter that have hot start issues should be replaced…

Frank
xj6 85 Sov Europe (UK/NZ)
**

Hi All

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

Stephan

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…

Cheers,
Harald

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.

Cheers
DD

Yes it was. I didn’t want to leave the car in bits for too long, but I guess I will recondition myself next time.

The simple explanation for slower cranking when hot is that it takes more effort to crank a hot engine. Why? The compression ratio is higher.

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Slow cold cranking is definitely caused by cold engine friction, Harry - never had an engine cranking slower when hot…:slight_smile:

Crank compression is also affected by cranking speed - the slower it turns the lower the measured compression…

Frank
xj6 85 Sov Europe (UK/NZ)
**

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!

Really? What then is the effect of drawing in cool air and then heating it in a hot engine, given that pV=nRT?

A 15w/50 engine oil is about 10 times thinner at 100C, than it is at 20C.

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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…

Frank
xj6 85 Sov Europe (UK/NZ)
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