delayed cooling fan shut off

[Dean]

Evening all

I've been playing around over the last few weeks with my undertray, a rather highly disliked item in our circles to say the least.
It does appear that along with its other benefits it does aid cooling giving a complete front to back air flow through the engine bay that does not occur without it, one thing that does concern me however is when the engine is shut down it becomes fiercely hot in there with very little air flow.

So cutting to the chase I have decided I want to install some additional wiring to supply the fans with power for up to something like 4 minutes after engine shutdown as with other larger engined vehicles I've owned, is this a standard feature on V6 engines? What are your thoughts on the best way or in fact any way to go about doing this because I don't, are there relays that have a delayed switching after signal power is lost or will it require something more complex?

D

[Dieselman]

A timer relay should suffice, but in all honesty, I can't see that it is worth doing. Once the engine is switched off, there is no more heat being generated and your engine isn't one of the high output engines anyway.

For the reasons you have stated, and for aerodynamic efficiency and stability, I will be reinstalling my under-trays, when I get chance.

[White Exec]

Have to agree, Dean, the run-on fan function shouldn't really be necessary with that engine, and such a large under-bonnet space. It would be possible to devise a circuit for this (most simply by an additional timer/relay/manual push button on the dash; more complicatedly by similar items wired into the "engine running" relay), but not sure it would be worth the effort.

The undertray not only smooths airflow under the very unaerodynamic engine underside, it is also equipped (at least on the 2.5, it is) with a single rear-facing louvre at the front. With forward motion of the car, this will act as an extractor of air from the engine compartment, so removing it would remove this function.

[xantia_v6]

Which engine types were originally fitted with under-trays? I thought they were only added to the noisier (diesel) engines to pass the EU drive-by noise approvals. Even Citroen call then sound proofing. If they had any noticeable aerodynamic benefits, they would have been fitted to the V6s.

[xmexclusive]

When I had the V6's the temperature rise rose very rapidly for a short while after the engine was switched off.
I put this down to the stopping of the water pump and then heat transfer from block to coolant.
It quickly dropped again.
In my opinion over-running the cooling fans will do very little to modify this effect.
The 2.5's with aircon have fan over-run but I suspect it mainly cools the sensors on the front of the engine block.

John

[Dean]

It must improve aerodynamics and reduce drag to some extent, its a rather messy hole without it, i know it has improved air flow to the back of the engine area with it fitted too.
The temperature does rise sharply after engine shutdown for the reasons you have highlighted John, i want it more to keep everything plastic based a cooler life, im thinking a few minutes of fans running with the undertray fitted with gives a good blast of hot air to the underside of the vehicle to help dry it off nicely, removing residual heat from the coolant would be an added benifit, if a pointless one.

John how is this achieved on the 2.5's is it as simple as replacing the standard relay for i timed one, im guessing not as the coolant temp sensor would require power also?

D

[xmexclusive]

Hi Dean

Not ever looked at the reason for the over-run.
Just a difference that you notice when you run both 2.5 versions.
Will see what I can find out.

John

[White Exec]

Have read that the undertray was indeed added to provide sound insulation (for the benefit of pedestrians) on the diesels.
Lavish attention was paid to body aerodynamics, and that would surely have included the underside.
The tough panel also provides considerable protection for the engine underside (sump, pipework, radiators, belts, wiring) from grit, grot and flying stones and rocks. Many vehicles have similar.

On the 2.5 the fans are, IIRC, controlled entirely by the engine ECU (fed by coolant temp info), which sends out just two fan commands: 'low speed' or 'high speed', each via a separate supply cable. These operate the three fan relays, kicking the fans into either series (low-) or parallel (high-speed) running. After-run occurs only if coolant temperature is exceptionally high, with the fans kicking in sometimes some minutes after parking up, if conditions dictate. Manuals carry warnings to beware of such unannounced fan start-up when working on a hot engine.

[Years ago, when petrol pumps lived under-bonnet and were engine-driven, larger engines could produce high under-bonnet temperatures when switched off, sufficient to cause vapour-lock in the pump and fuel lines. Renowned culprits were V8s with chunky cast iron exhaust manifolds, likened by some as having a pair of Aga cookers clamped to the engine sides. Mods included abandoning the mechanical pump (in favour of an electrical one at the rear), and installing Kenlowe fans, which always did provide "run-on" via their top-radiator-hose-mounted thermostat.]