Car Mechanics (UK) - - Catalytic Converters & Dpfs -

With the car owner re­port­ing a loss of power and this Ford Fo­cus 1.6 TDCI'S en­gine man­age­ment light be­ing il­lu­mi­nated, the first step was to per­form a full scan to check for fault codes from all of the car’s ECU mod­ules.

The scan re­vealed a wealth of com­mu­ni­ca­tion is­sues from other mod­ules – but are they the cause or the re­sult of an en­gine man­age­ment fault? Never place all of your trust in fault codes.

Sure enough, an un­ex­pected DPF pres­sure dif­fer­en­tial sen­sor sig­nal has been de­tected, but fur­ther in­ves­ti­ga­tion is needed to es­tab­lish if it is a faulty sen­sor, duff wiring, or even cor­rupt soft­ware.

As cars ex­pe­ri­ence prob­lems in ser­vice, up­dates are ap­plied to the var­i­ous ECUS. It is worth check­ing that all soft­ware is up to date – for this, the bat­tery will need a back-up sup­ply. An un­ex­pected power cut could ruin one, or more, ECUS. A live data read­ing con­firms that the DPF pres­sure dif­fer­en­tial sen­sor’s out­put of al­most 2 KPA is too high, when the en­gine speed is var­ied and the read­ings from the MAF sen­sor and EGR valves are mon­i­tored in real time. To find out if the pres­sure dif­fer­en­tial sen­sor is faulty, or if there is an is­sue within the DPF, the DPF in­let pipe is re­moved at the sen­sor end. A me­chan­i­cal pres­sure gauge is con­nected to it and the en­gine started.

If the DPF were blocked, the pres­sure cre­ated could dam­age the me­chan­i­cal gauge, which is why the more sen­si­tive and costly dig­i­tal ma­chine is used af­ter­wards, to give a more ac­cu­rate read­ing.

The sen­sor can be tested by ap­ply­ing a pres­sure to it and not­ing if a volt­age dif­fer­ence is recorded. Al­ter­na­tively, con­nect it to an os­cil­lo­scope and com­pare the sen­sor volt­age read­ings with the pres­sure mea­sured across the DPF, as in Step 7.

The ac­tual pres­sure read­ing (pic­tured blue) does not cor­re­late with the sig­nal from the DPF pres­sure dif­fer­en­tial sen­sor (pic­tured red), which is at the top of the scale, in­di­cat­ing that ei­ther the sen­sor, or its wiring, is faulty.

It is worth not­ing that the pres­sure pulses, mea­sured across this DPF dur­ing the fir­ing stokes, are not iden­ti­cal. This might in­di­cate a fur­ther run­ning is­sue that could have con­se­quences for the par­tic­u­late fil­ter.

In this en­gine’s case, a fuel injector pos­sesses a slight com­pres­sion leak – a com­mon prob­lem with the DV6 en­gine range. This causes the af­fected cylin­der to run with in­suf­fi­cient air, in­creas­ing the DPF soot load­ing.

Apart from an ob­vi­ous vis­ual check for cor­ro­sion (pic­tured in­set – this con­nec­tor is in good or­der), you can back-probe the rear of each wire, pro­vided that you have suit­able data avail­able to iden­tify each wire func­tion and the ex­pected out­put.

Pres­sure dif­fer­en­tial sen­sors are ex­pen­sive, which is why it is worth test­ing them be­fore in­vest­ing in a re­place­ment. Be wary that aftermarket com­po­nent qual­ity can be vari­able and con­sider that iden­ti­cal-look­ing parts can be cal­i­brated dif­fer­ently. Blocked DPFS can gen­er­ate pres­sures high enough to dam­age sen­sors. Re­place­ment sen­sors are easy to fit, how­ever, tend­ing to be re­tained with only a few screws and the pipes/wiring be­ing the pull-off and push-on va­ri­ety. With a new sen­sor fit­ted, the sys­tem has to be re­cal­i­brated. Some ve­hi­cles per­form this au­to­mat­i­cally; oth­ers re­quire di­ag­nos­tic in­ter­ven­tion – this ap­plies even to parts sourced from main deal­ers. Com­par­ing the di­ag­nos­tic read­ings af­ter the sen­sor is fit­ted with that pic­tured in Step 5 re­veals that the sen­sor read­ing is now at 0 KPA (mean­ing be­tween 1-9 mil­libars – the pres­sure will never be zero), prov­ing the DPF is un­blocked. A 30-minute road-test al­lows the car to com­plete its own ‘ac­tive re­gen­er­a­tion’ cy­cle, sat­is­fy­ing the en­gine man­age­ment. Once the fault codes were re-read, the spu­ri­ous is­sues noted in Step 2 did not reap­pear.

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