10 replies to this topic

[clubman_dan]

Hi all,

 

Last weekend i replaced the head on my 998 (a+ high compression) with a Janspeed modified 12g202 head which is similar to the ones used on the old cooper conversions. My car has a new radiator, new water pump and the cooling system was working perfectly on the standard setup, just nudging up to the top the normal range if sat in traffic for very long periods, otherwise sitting bang on normal all the time. 

 

Since changing the head it now seems to be running a lot hotter, the needle tends to sit towards the top of normal, even nudging slightly higher at points, and although it sounds odd the engine bay 'feels' hotter when the bonnet is open. 

 

When I refilled the coolant it took quite a while for the heater to start blowing consistently hot so I think there were some airlocks in there but I am pretty confident that's solved now (purely based on the heater being consistently hot)

 

I did replace the temperature sender so I wonder if maybe it's related to that - but as I said it does 'feel' hotter than it used to. 

 

I have an 82c thermostat in there the moment so I could go down but I am not sure if that is the right action or it's just a sticking plaster. 

 

I've got the timing set at 7 degrees with the vac disconnected and aroud 1000rpm 

Plugs are BP5ES and I did bung some BP6 in there just in case the plug temperature had anything to do with it but there was no obvious difference there.

 

When driving it sits aroud the middle of the gauge, but seems to randomly creep up and sit there before settling later... Maybe I still have some air stuck in the system? 

 

Any words of wisdom appreciated! 

 

Dan

 

On the plus side, the car goes well now! 

[Earwax]

Perhaps the new head needs more advance?

 

Exhaust valves not fully seating?

 

Sucking air to make it run leaner.

 

Just guesses , but scatter-gunning around things that were moved or changed (or need to change) in the process.  I also would get a second reading - as it could all be just a slightly dodgy gauge. Good luck

[sonscar]

Changing the thermostat will not help the engine run cooler.As above it probably needs some tuning.Steve..

[Spider]

Before going any further - measure the running temp with another gauge - you may well end up chasing your tail otherwise.

[gazza82]

I did replace the temperature sender so I wonder if maybe it's related to that - but as I said it does 'feel' hotter than it used to.

Swap it back ...

[clubman_dan]

Thanks all - it runs well. I have a wideband gauge and my Afr readings are nicely around 13-15 depending on situation. Idle Afr is around 13.5-14.5. So fuelling wise I have no concerns.

I’ll bung the old sender in and also try and get some other measure of the temperature too.

I need to be confident in it by brands hatch next week so I have my work cut out!

[viz139]

Have you checked the chamber volume and compression ratio.  As its a Janspeed head I presume it has been on other engines and may have been skimmed for use with dished pistons.

[clubman_dan]

Stuck the old sender in and left the car running on the drive for a good 30 mins or so. Seems to be ok but I’ll give it a bit longer to see for sure, it’s promising though

[cal844]

AFR is slightly lean (needs to be 14.7 to be perfect) but not massively. Check it through the rev range and under load

Edited by cal844, 23 July 2025 - 06:37 PM.

[clubman_dan]

Afr if 15 is crusing dual carriageway/motorway speed so I assume that’s ok for fuel economy.

Sits more like 13-14 range on wide open throttle

Putting the old sender back seems to have cured the issue. I think I’ll need to add a proper gauge to my shopping list

[cal844]

Afr if 15 is crusing dual carriageway/motorway speed so I assume that’s ok for fuel economy.

Sits more like 13-14 range on wide open throttle

Putting the old sender back seems to have cured the issue. I think I’ll need to add a proper gauge to my shopping list

15 is slightly lean (iirc off the top of my head) and will suffice BUT a professional tuner/builder would like it spot on

Edited by cal844, 23 July 2025 - 09:21 PM.