[928uk] Slow cranking on hot restart issue

[Paul R Smith]

If it was an auto, I would say check your thrust bearing clearance asap, 
as slow hot cranking is a symptom of TB wear.
But I don't think it is an auto.
A hot engine will have higher compression pressures, and I know some 
engines (e.g. big twin BM's) struggle to turn over more when hot, but 
I've never noticed it on a 928, they usually start quite quickly anyway.

I suspect that the fast turnover with the external temp sensor was 
probably caused by the engine firing a bit.

I think the most likely issue is a dying battery, either test your 
battery voltage during cranking or get the battery tested.
If the battery is perfect, check your connections: battery terminals, 
battery ground, engine to body ground, battery connection to starter, or 
check voltage at starter motor during cranking.

Smiffy

On 02/06/2024 10:04, jonathanrackowe at gmail.com wrote:
>
> Hi Simon,
>
> I’ve started a new thread for your ‘Slow cranking on hot restart 
> issue’, as I can’t see why Temp Sensor 2 should affect cranking speed.
>
> The speed of the starter motor (unloaded) is affected by battery 
> voltage and condition, but if anything the battery should be in a 
> better state at hot restart. The load on the starter motor is of 
> course the engine and ancillaries, but I don’t really see why a hot 
> engine should be harder to turn over – if anything it should be easier 
> because the oil is hot, although perhaps some clearances could tighten 
> up when hot… ?
>
> Temp sensor 2 affects fuel mixture enrichment and ignition timing 
> through warm-up from cold. I can’t see fuel mixture affecting engine 
> load on cranking. And ignition timing also seems doubtful to me unless 
> so far out it was firing way before TDC.
>
> Smiffy, Paul, anyone, have you got any ideas on this?
>
> Best regards,
> Jonathan
>
> 1991 928 GT Amethyst Metallic
>
> *From:*Simon Painter <simonpainter101 at gmail.com>
> *Sent:* 02 June 2024 08:40
> *To:* jonathan at rackowe.net
> *Cc:* Paul <ukkid35 at gmail.com>; 928 List <928uk at lists.928.org.uk>
> *Subject:* Re: [928uk] Cheap LH-Jetronic temp sensor II NTC2
>
> The FAE Temp2 sensor reads:
>
> 20C - 5.3k
>
> 32C - 4k
>
> 46C - 2.4k
>
> 60C - 1.5k
>
> 70C - 0.93k
>
> I was quite surprised how the Temp2 sensor affects starting. I have an 
> ongoing issue with re-starting when hot, eg re-starting after filling 
> fuel. It will turn over quite slowly compared to a cold start. One day 
> after a fuel stop, I unplugged the Temp2 connector and just plugged in 
> a new FAE sensor and left it hanging loose. The engine turned over 
> really quickly! It didn’t want to start as it was probably running 
> with a super rich mixture as the Temp2 sensor was cold and engine hot.
>
> I’m pretty sure my original Bosch Temp2 sensor is causing my hot 
> re-start issue but I have yet to find a good replacement that is in spec.
>
> Thanks
>
> Simon.
>
> Check me out on Instagram and Facebook @oldcarfixer
>
>
>
>     On 1 Jun 2024, at 23:41, Jonathan Rackowe
>     <jonathanrackowe at gmail.com> wrote:
>
>     
>
>     What does your FAE one measure Simon?
>
>     Best regards
>
>     Jonathan
>
>     1991 928 GT Amethyst Metallic
>
>     On Sat, 1 Jun 2024, 3:55 pm Simon Painter,
>     <simonpainter101 at gmail.com> wrote:
>
>         I’ve now bought two (one an FAE) and both measure way off what
>         they are supposed to be. I’m trying to get gold of an original
>         Bosch one.
>
>         Thanks
>
>         Simon.
>
>
>