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TheRookie

Car losing power on motorway

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The car has started to lose power on motorway when driving fast (say, 70mph). The symptoms are the same in each case and are like that: the car suddenly loses power, decelerates rapidly and if I press accelerator is slows down even more. If I put the car into neutral, the revs will drop to about 500 and stay there no matter what I do. Then, suddenly after 10 seconds or so the engine will spring back to life and the car wuld go as normal.

I've ran Forscan check and there are no DTCs apart from door module A (it managed to jam during recent freeze, then became unstuck).

This happened several times already and I am quite scared that this can put me into dangerous situation (what if someone is talgating me when I am overtaking and the car suddenly loses power?).

Any ideas what can cause this?

The car is Focus CC Mk 2 2.0 TDCI manual 2010.

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1 hour ago, TheRookie said:

If I put the car into neutral, the revs will drop to about 500 and stay there no matter what I do. Then, suddenly after 10 seconds or so the engine will spring back to life and the car wuld go as normal.

I've ran Forscan check and there are no DTCs apart from door module A

If the engine is idling normally when in neutral, but ignoring the throttle, then that rules out a lot of faults like fuel starvation or crank sensor. Though most of this type of fault will also bring up DTCs that Forscan can read.

The accelerator pedal has duplicated sensors, one wired to the PCM, one to the IC. A significant deviation between them should raise a DTC, but it might take the safer reading (the one that drops to zero), and drop the engine rpm to idle.

The simplest common fault that can affect both pedal sensors, and avoid a DTC, is odd system voltages, out of spec or too much ripple or noise. Faulty batteries and alternators can sometimes do this.

 

 

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Thanks. CCs are known for electric gremlins and my car killed two batteries in 3 years (thanks god for  RAC battery warranties). I can try recharging the battery using external charger and see what happens.

I also thought of a blocked fuel filter? Do you think it can cause the issue?

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29 minutes ago, TheRookie said:

I can try recharging the battery using external charger and see what happens.

I also thought of a blocked fuel filter? Do you think it can cause the issue?

Before charging, test it with headlights on for a few minutes. See if its voltage drops below 12v, and does not quickly recover to over 12v when the headlights & ign are then turned off.

Then monitor the voltage before and while charging. It will be 12v to 12.3v probably after this discharge test. If it jumps up to over 13.5v as soon as the charger is connected, then it is duff (unless it is a very high current charger, > 20A say). It should take quite a bit of charge and time to recover to full charge voltage of 14.4v or so. Headlights & Ign take at least 12A from the battery.

If the voltage jumps up quickly, the battery is not accepting charge properly, and will allow ripple and voltage variations onto the system.

Certain alternator failures can cause bad voltages also. My neighbour has a van (not Ford) that went into a stage of sometimes ignoring the accelerator after start-up, this was a faulty alternator.

I thought filter unlikely. I would expect severe blockage to stall the engine, or at least make it run very rough when in neutral. Also the high pressure fuel rail is monitored, and an unexpected severe drop in pressure should raise a DTC. Also the sudden onset, stable symptoms and sudden end seem to indicate an electrical problem.

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Okay, ran the test you suggested. If I turn headlights on wile vehicle not started, the volage drops to about 10.5V and then recovers after 10 minuts or so to 12.3V.

Connected the charger (el cheapo one, from Lidl). It analysed the battery, gave it "All OK" and started to charge it. During the charge the voltage went gradually (in a matter of hours) from 12.3V to about 14.5V and once the charge has been complete, it went back to about 13V.

What can be concluded from this?

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Now put it back in the car, and turn the headlights on for a couple of minutes to take the surface charge off. The voltage should not drop below 12V (or at least significantly).

Do you have a scan tool (either OBDII or elm327) that can do live data? If you can reproduce the problem then the live data for the engine/throttle position/MAF/MAP and other sensors may give a clue.

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8 hours ago, TheRookie said:

If I turn headlights on wile vehicle not started, the volage drops to about 10.5V and then recovers after 10 minuts or so to 12.3V.

Connected the charger (el cheapo one, from Lidl). It analysed the battery, gave it "All OK" and started to charge it. During the charge the voltage went gradually (in a matter of hours) from 12.3V to about 14.5V and once the charge has been complete, it went back to about 13V.

If the engine was stone cold, then the glowplugs would have come on, and this will pull a good battery down to about 10.5v. Once the plugs time out, then the battery voltage should slowly recover (ign & headlights still on), so those results could be fine.

On the other hand, if the engine was lukewarm, or warm, then with no glowplug drain, 10.5v with ign & headlights on is an almost flat battery. However the re-charge part seemed to work ok, though without knowing the charge current, and reasonably exact timing, it is difficult to be certain.

Like Ted says, a further headlight discharge & recharge test would help confirm this.

A modern alternator can maintain charge, even with dipped headlights on, at very low engine speeds. So if the battery was discharged, it suggests an alternator problem.

I suggest you keep charging the battery between journeys, and see if that prevents the problem. If so, then the alternator goes up to top suspect.

If the problem continues, then again as Ted suggests, one way to try to narrow it down would be to use Forscan to do a live log, and hope to capture an occurrence. If you are unfamiliar with the Forscan data acquisition part & would like a bit more advice on how to do it, post here. Have a look at the available PIDs: Use the read PID data section and the setup (spikey wheel at the bottom), see if both APP sensors show up, and the FRP (Fuel rail pressure).

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Took it to a garage today. The guy was trying to reproduce the problem and luckily it reoccured just as he was about to give up.

The DTC showed up "CAN bus fault". Strangely, when he turned the car off and then on, the DTC didn't show up.

He is convinced that there is something wrong with the CAN bus somewhere and his advice was "part-exchange it now as finding the problem will cost a lot".

Is he right (this is a small local garage and they don't sell cars, just repairing it)?

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12 hours ago, TheRookie said:

The DTC showed up "CAN bus fault". Strangely, when he turned the car off and then on, the DTC didn't show up.

Unfortunately, that is not much help. There are pages of CAN bus related DTCs, some specific to a module, some rather general. Also they are not usually regarded as critical (on their own), and often vanish after one or two ignition off / on cycles. If it was U1900, that quite often comes up as a side effect of the diagnostic process, and should be ignored unless repeated often or in conjunction with other DTCs.

If it is a CAN bus fault buried in the wiring looms somewhere, then finding it could be very hard. But if the fault is in a module, it should be a bit easier. Forscan reports the module that the DTC was stored in. But you may need to run it soon after an event, ideally before switching the ignition off.

So it does come down to how much you want to keep the car, and if it has any significant value as a PX.

 

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