Jumpy Jeep

Motor Equipment News - - NEWS -

This ar­ti­cle is a true de­scrip­tion of an AECS tech­ni­cal help desk prob­lem, and how it was solved.

By H.P. Lei­jen (trainer/re­search)

er num­bered the in­jec­tion events in fir­ing se­quence, not cylin­der num­ber. A time mea­sure­ment be­tween the last two in­jec­tion events shows that the en­gine is do­ing 2,964 rpm just be­fore it dies com­pletely.

This means that in­jec­tion did not stop as a re­sult of the crank shaft sim­ply stop­ping with ro­tat­ing. In my view this may in­di­cate a cat­a­strophic prob­lem with the crank/cam shaft sen­sor sig­nals.

How­ever, this AECS trained di­ag­nos­ti­cian had al­ready looked at those sig­nals and saw noth­ing un­to­ward, I trust his opin­ion.

Best op­tion is to look at the above pat­tern again but now in more de­tail. A much closer look at the same pat­tern as above shows a prob­lem:

When zoomed in on in­jec­tion event three and com­par­ing it with event four a big dif­fer­ence in in­jec­tor ac­ti­va­tion time can be seen. In­jec­tion du­ra­tion vari­a­tions should not be jump­ing around this much.

Please note: in­jec­tion event one looks dif­fer­ent as the scope is con­nected to that in­jec­tor. We cover also that in the DMS 1-3 train­ing. We sug­gested hav­ing a look at the adap­tion val­ues of each in­jec­tor/ cylin­der with the scan tool, as we did not trust that these val­ues were even.

We show a sam­ple pic­ture of an adap­tion in­jec­tor value screen. This pic­ture has been made with the scan tool con­nected to a bare four­cylin­der MB ECU (no sen­sors nor ac­tu­a­tors con­nected). The adap­tion val­ues in this Launch Pro3 screen shot are shown in mm³ per stroke fuel quan­tity and are the cor­rec­tion quan­ti­ties the ECU makes to keep the en­gine run­ning sta­ble (equal strength power beats per cylin­der).

These val­ues are on top of the in­jec­tor in­di­vid­ual de­liv­ery vari­a­tions which are coded into the ECU with the Launch when an in­jec­tor gets re­placed.

When an ECU finds that that adap­tion val­ues can­not be ad­justed any fur­ther to keep the en­gine run­ning smooth, this sys­tem will shut down. The in­jec­tors are no longer be­ing ac­ti­vated, and the pres­sure con­trol valve will now do all it can to drop the rail pres­sure.

The adap­tion val­ues were 1.8, -2.5, -0.2, -0.6, 2.3! The ve­hi­cle data stated that the adap­tion value lim­its are -2.0 to +2.0. It was de­cided to pull the in­jec­tors out of the cylin­der head and in­spect them on a test bench.

The di­ag­nos­ti­cian was so kind to send us a photo of the num­ber 2 in­jec­tor, be­side a good in­jec­tor. The in­jec­tor noz­zle re­tain­ing nut was split and de­formed.

The in­jec­tor seal­ing washer al­lowed com­bus­tion pres­sure to leak past the washer erod­ing the noz­zle re­tain­ing nut, weak­en­ing it un­til it split.

The leak­ing in­jec­tor washer was let­ting high pres­sure Diesel leak­ing from the split nut into the com­bus­tion cham­ber dur­ing in­take and ex­haust stroke, caus­ing the knock and white smoke. The di­ag­nos­ti­cian re­set the adap­tion val­ues af­ter five brand new in­jec­tors were fit­ted, and recorded a pat­tern again.

To an­a­lyse and com­pare the in­jec­tor ac­ti­va­tion du­ra­tion of each in­jec­tor you have to zoom in down to just a cou­ple of in­jec­tion pulses.

The mea­sured in­jec­tion du­ra­tions do not jump around any­more and showed on the scope: Cyl 5 – 1.763 ms Cyl 4 – 1.763 ms

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