Friday, 22 April 2011

307cc waterlogged ECU fix

The problem 

According to the Internet this seems to be a common issue with 307cc models from 2005-2007.
It happened to me with my 2.0L 2006 model (EW10A engine type). 

I was driving home from work and the car juddered and I lost engine power and the brakes. I was lucky enough to have a grass verge beside me and I pulled on to it. After that it wouldn't start and I had it towed to a garage.

The dashboard display was showing "Engine management failure" warnings and the engine management light was constantly on. The engine would start but had no power, I had my foot fully down but  it had as much power as it has when idling. 

After a while looking the mechanic identified it as waterlogged Engine Control Unit (ECU). He pulled it out and dried it with a hair dryer. That cost the best part of £200 but the car was running again. 

A few days later the same thing happened, luckily I was nearly home and I was able to roll most of the way down the road and park it up.

After searching various forums I identified the problem to be coming from a leaking coolant temperature sensor (do a Google shopping search to see what it looks like) on the side of the engine block. It turned out that the sensor had developed a crack and coolant was tracking through the sensor, through the connector, up the inside of the wire (between the metal of the wire and the plastic insulator (see Fig.0)) loom and gathering in the ECU. It was also leaking coolant down the side of the engine block.
Fig.0 - cross section of wire from CTS to ECU

If this happens to you STOP DRIVING IMMEDIATELY and try the method below. If you carry on driving you risk the ECU getting damaged by short circuits or chemical damage from the coolant. 
I was quoted anything from £400-£1000 to fix including new wiring looms, ECU and labour. Some people online paid up to £1200. I managed to get it done for about £20.

The fix

What you need: 
Coolant Temperature Sensor (about £15 from a Peugeot parts shop or Ebay)
Silicone sealant (gun type) 
Tools (knife, screwdrivers)
An airing cupboard
About 5 days use of another car while your 307 is out of action. 

Fig.1 - Layout of the 307cc 2.0L engine


  • Removed the ECU from its bay to the right of the battery as you look at the engine.

  • Remove the metal plate the ECU is bolted to.
  • Remove the 4 screws on the ECU and cut away the seal between the two halves.
  • Carefully prize apart the two halves of the ECU - not too much as you could crack the circuit board - just enough so you can fit a pencil in the gap - and leave it in there.
  • Put the ECU in an airing cupboard (or warm, dry area) for about 5 days - make sure one side is held open with the pencil for the whole time so the moisture can escape .
  • After the 5 days, reseal with silicone and leave to dry overnight. 

  • Remove the engine cover, air-box, battery and battery cover from the engine bay to give you better access to the coolant sensor, you may want to move some wires too but note where they go because they are all similar. (see Fig.1)
  • Remove the retaining clip holding in the Coolant Temperature Sensor (CTS), be aware that it could fall out and leak coolant.(see Fig.2)
Fig.2 - you'll need to move the wires from the area as there's not much space

  • Quickly swap the old CTS for the new one, be sure to find the old ring seal as you may need it for the new sensor (my new one didn't come with one).
  • Replace the retaining clip and dry around the area with a towel so you can easily see fresh leakage (feel underneath the CTS with a dry finger)
  • Dry the plug at the ECU end of the wining loom (its the one that plugs in on the right).
  • Put the engine cover, battery, cover, air-box and wires back in place.
  • Replace thy ECU and plug the wires back in.
You probably lost a bit of coolant while replacing the CTS so bleed the cooling system by driving for a few minutes, stop facing uphill, open the bonnet, open the coolant filler cap. 

Using something small - not your finger as it will be hot - press the valve (like a car tyre valve with similar screw-cap) in the area just above and to the front of where the CTS goes (see Fig.2). You should get some air coming out then water (like bleeding a radiator).

You may also need to top up the coolant if the level is too low.
Check the area around the CTS after every few drives to make sure there is no leaking.
Also remove the plug from the ECU and check no moisture has returned, if there is it will be remnants from the wire pre-fix (not much but enough to cause a problem should it get in the ECU circuits). If there is dry the ECU plug and/or suck some more coolant out of the plug.

Thanks to Peugeotlogic for the details.


  1. Never just disconnect the ECU without first shutting down the electrical system (google it) and then disconnecting the battery.

  2. Never just disconnect the ECU without first shutting down the electrical system (google it) and then disconnecting the battery.