Hi

I live in Sweden and recently got a 2006 Mondeo 2.2 tdci. It is equipped with an auxiliary heater  that is automatically started when outside temperature is low. It works fine and the engine temperature and internal heat is good. But it has a scary side effect: it makes the engine accelerate occasionally and since the engine is quite powerful it is scary. When I drive slow on town roads I often need to push the break since the car accelerates without me touching the pedal. On 3rd or 4th gear it can easily reach 60-70 kph if I do not use the break. If I cruise on the highway and engage cruise control at 110 kph it can sometime accelerate up to 140 kph unless i hit the break.I just need to gently touch the break pedal for the acceleration to stop.

Any suggestions on what could cause this behaviour? I have gotten suggestions that it could be due to a bad battery which makes the engine revs go up when the heater is working. Another suggestion is that it could be a slightly clogged fuel filter which might confuse the injections system when the preassure goes down when the heater consumes diesel. 

What are your thoughts? Any other idea? I will check the status of my battery tomorrow, I also have a new fuel filter but will need access to a garage to change it. (Really snowy and cold here.  ) At the moment I have removed the fuse for the heater and these problems go away. (But the car takes a lot longer to get warm)

I suspect the battery is to blame here, when I check it with a multimeter it reads 11.98v. At the moment it is -17° C here but it still feels low. I also saw that it was the original battery for the car, it has a sticker on it that says it was initially charged in november 2005. It looks like I should try to get a new one.

13 hours ago, P3anuts said:

when I check it with a multimeter it reads 11.98v

(I am assuming the heater is the electric PTC type, or an electric water heater, not the fuel fired heater)

If it reads under 12v with the engine running & aux heater off, then it sounds more like an alternator problem. You should see over 14v (up to 14.8v) with engine running and normal driving load of lights, wipers, fan. Heavy loads like screen & cabin heaters will drop it down a bit, but they should not drag it down below about 12v.

Also while running, the car should de-energise the heater (I think it is controlled via the BCM (aka GEM), it is on the Focus), before the heater overloads the system and starts draining the battery. But if the alternator is poorly, and the system voltage is low already, it may not do that, and the engine control system may get confused by the low voltage.

A battery that old, especially in your climate (brrrr, it is -2 here, and that feels cold enough for me!), is likely to be knackered, but a bad alternator would be more related to the symptoms you describe.