The V8 Owners Forum -

Generally, you only need a fan at all at low road speeds. On the move, there should be more than enough flow through the rad without any fan assistance.

There must be lots of cars like yours with the same engine, so I'd be inclined to see what the difference is with your cooling system and air flow.

You could also warm the engine fully until the thermostat opens and check the temperature in and out of the rad with an infra rad thermometer. That should show if the rad is doing what it should.

southernd wrote:Hi,

I've got a Mazda MX5 with a 3.5 modified Rover V8 which sits pretty comfortably in the engine bay. It benefits from a huge custom radiator and a bonnet scoop. It's generally quite a cool running car, even in traffic it takes a while for the fan to be needed.

This week due to time I dropped it off at the local garage and they put in semi synthetic 10w40. My normal oil is 20w50, any premium type. As soon as the oil was changed I could feel the heat from the engine bay, way warmer than normal. Oil pressure seems roughly the same, coolant normal but the vents on the dash were very warm.

Anyway I swapped the oil back to 20w50 this morning and what a difference. When I took the 10w40 it looked almost like water, I guess there simply isn't enough thickness to pump the oil efficiently.

So no actual empirical evidence at all of a temperature change ?

Only word that springs to mind here....is "imagination"

My car (P6 with a CB 4L V8) came with 10/40 in it, but somehow it just didn't sound right.

I bit rattly and not particularly smooth. I changed it to 20/50 classic oil and it transformed it. A lot quieter, pulled better and became super smooth.

Don't know about heat as to be honest, I didn't notice.

Richard

Don't think there's any doubt a thicker oil can help a worn engine.

Ian Anderson wrote:Can the air that came in through the radiator easily get out of the engine bay?

Very easy, I have wide flared side panels so it's not that, I'm afraid.

DaveEFI wrote:Generally, you only need a fan at all at low road speeds. On the move, there should be more than enough flow through the rad without any fan assistance.

Agreed.

DaveEFI wrote:There must be lots of cars like yours with the same engine, so I'd be inclined to see what the difference is with your cooling system and air flow.

A few but not usually as large an engine as mine. My rad was designed to give as much cooling as possible with a 75mm core.

DaveEFI wrote:You could also warm the engine fully until the thermostat opens and check the temperature in and out of the rad with an infra rad thermometer. That should show if the rad is doing what it should.

Fan (stage 1) is controlled by MS ECU with a calibrated sensor. Stage 2 is controlled via an inline Otter switch (97/85) that I know is correct via the MS ECU gauge plus I have a dash light telling me it is on stage 2.

Always useful ideas, thanks.

I am going to remove the nose cone grill to see if that makes any difference. Then try 10/40 oil (then consider oil cooler).

Is your fan turning the right way?
I know it sounds stupid but trust me it happens that they get wired wrong and push air forward instead of backwards!

Ian

Yep definitely, it's a big two speed 12" Kenlow 3,126 cubic meter / hr fan too.

Your thermostat not opening fully?

Or do you also have a Camino heater fitted?

Could be that either way you are not getting full flow through the radiator.

If you have a heater matrix fitted, try clamping the pipe to or from it (mole grips) and take it for a run. You may be getting so much bypass plow that the fluid would rather flow through that than through the radiator.

Ian

Way I see it is there are three possibilities.

1 The rad isn't up to the job.
2 The air flow though the rad isn't sufficient
3 The water flow through the rad isn't sufficient.

1 is unlike as I'm sure you've done your sums.
2 is unlikely given you have a powerful fan too.
3 seems to be a distinct possibility. MS already measures the temperature at the hottest part of the engine, so I'd want be see what the return flow temperature is too. And know what the flow rate in and out of the rad is.

I'd say the size and tune of the engine not valid, if it is overheating at cruise, as the heat developed then isn't going to be anything like as much as at full belt.

Could it be that the air scoop on the bonnet is pressurising the engine space and thereby resisting/slowing the air flow through the rad at cruise.

Thanks guys for suggestions and to summarise ..

1. Thermostat not opening fully? Have tested it so yes it does and it’s relatively new anyway

2. Heater is fitted and has separate on / off control. There is also a small bypass fitment (from memory a 10mm bypass pipe) so some flow when heater turned off.

3. Full flow through the radiator? Possibly but I took (cut) the bottom off and cleaned each pipe so that’s not it.

4. Is rad up to the job? Should be as I spent a long time getting it right and had it tailor-made.

5. Air flow though the rad isn't sufficient? Possibly.

6. Air scoop on the bonnet is pressurising the engine space and thereby resisting/slowing the air flow through the rad at cruise. I have big flared side panels each of some 100mm x 300mm so in theory there is no problem.


Current thoughts:
1. I’m wondering about air turbulence. I’ve taken off my rad grill so will try it this weekend.

2. I still think my oil gets too high a temp and so the rad cannot cool enough.

3. I do need to get my ECU re-mapped as I’m using too much fuel – had problems with traction control unit seriously sooting up plugs 7 & 8. Now all sorted but need to visit Dale in Telford but that’s a long trip for me.

4. Easier to try 10/40 oil.

Hi Richard,
I have basically the same car as you with basically the same engine (5.0 vs 4.6) but also with the parasitic load of the supercharger and the reduced air flow through the rad due to a large intercooler blocking half of it and my car doesn't overheat when cruising or in traffic. temps sit at arround 90 when cruising, creep up to arround 100 when in traffic, oils seems to be arround the same temp as coolant. I have the crank driven oil pump and use a quality 10w40 oil.

My setup:
. Dax Ally rad
. Puller electric fan (don't know what brand or current/air flow but not a high power one)
. Custom made winged sump which probably gets some airflow when moving
. Heater circuit, using the GEMS inlet manifold.
. Using remote thermostat positioned after the rad outlet, with a 'sensing' coolant hose connection, as used in MGF's and Elise and new range rovers, so it bleeds the cooler rad water with the bypass circuit but also closes of the bypass circuit when required so you get full flow through the rad. Supposed to reduce thermal shock?
. MS2 with coil packs triggered by bosch coil switching module

1. Due to the fact I have an intercooler I really doubt that your grill is causing your overheating.
2. Are you sure you're not getting belt slip? Seem to remember to have a custom belt routing and not very much water pump belt wrap?
3. Normally too lean rather than too rich would be associated with over heating.
4. What ignition timing and AFR are you running at when cruising? I can compare with mine.

Cooling problems which I had/cured:
5. I found that due to a wiring fault my fan relay was vibrating on and off so full current wasn't getting to the fan
6. I changed my fan from pusher to puller and although I ensured the fan was rotating the correct way to pull air, I didn't realise I also had to reverse the blades due to their aerofoil shape so my fan was pulling but the aerofoil was working against it!
7. My fan had just touched the rad so flattening half of the fins which obviously restricted flow.
8. My fan wasn't sealed very well to the back of the rad so was able to suck air from the sides and not directly through the rad.
9. I think my bypass circuit was flowing too much coolant so I changed to the new thermostat detailed above.
10. My header tank wasn't quite high enough, (should be the highest point of the cooling system) so I raised it 15mm

Tom.

Too much water bypassing the radiator.

So many it just circles without ever getting cooled.
Also this is free flow whereas the radiator has some resistance to flow, guess which flows more? Not the radiator.

I had similar problems so on my bypass which was open I fitted a blank in the. Pipe and drilled a 1.5 MM. Hole in it to bleed air through and then only a trickle of coolant.

The temperature now does not make the wild swings

Ian

Two more things come to mind
If you are running serpentine do you have the correct rotation pump for the rotation being done? Slow revs will move some water but cavitation at higher revs

Impeller spinning on water pump shaft. So again some movement on the impeller but not keeping up at revs

Ian