Di­ag­nos­tics

Motor Equipment News - - CONTENT -

This ar­ti­cle is a true de­scrip­tion of an AECS tech­ni­cal help desk prob­lem and how it was solved. By Her­bert Lei­jen, trainer/re­search for AECS Ltd

Ve­hi­cle

2002 Nis­san Sylphy (Blue­bird) 2.0-litre di­rect in­jected QR20 (DD) en­gine.

Prob­lem pre­sented to the help desk

This car has been hes­i­tat­ing on ac­cel­er­a­tion, pinks un­der load, and the en­gine check light comes on in­ter­mit­tently. We checked for fault codes; the fault codes logged so far were: P0120-Throt­tle po­si­tion sen­sor cir­cuit. P0605-ECM. P1320-Ig­ni­tion sig­nal-Pri­mary. P0400-EGR sys­tem. Be­cause the codes, ex­cept P1320 (a non-OBD code), did not re­ally re­late to the driv­abil­ity prob­lems the car had, the di­ag­nos­ti­cian de­cided to mea­sure the in­jec­tor sig­nals at the ECU on all four in­jec­tors.

Three-wire in­jec­tors

The in­jec­tors used in these Nis­san en­gines are of the three-wire type. As dis­cussed in the EMS 1-3 train­ing, the in­jec­tors have two coils, one with a very low re­sis­tance to open the in­jec­tor very quickly, and one “nor­mal” coil to hold the in­jec­tor open.

A lot goes wrong with those di­rect in­jec­tion in­jec­tors, any garage can vouch for that!

Dur­ing the EMS 1-3 I show pic­tures of an ul­tra-son­i­cally cleaned and flushed in­jec­tor, made with a mi­cro­scope, of the swirl chan­nels in­side the in­jec­tor. The chan­nels still fill with car­bon, formed as a re­sult of the high heat and pres­sure the fuel is ex­posed to in­side the in­jec­tor.

This first mea­sure­ment the di­ag­nos­ti­cian made re­vealed that the num­ber 1 in­jec­tor scope pat­tern looked very dif­fer­ent to the rest of the in­jec­tors.

Mea­sur­ing

The dual chan­nel mea­sure­ment made with the ATS scope showed im­me­di­ately that there was a prob­lem with how the ECU was “driv­ing” the in­jec­tors, specif­i­cally num­ber one.

The record­ing of in­jec­tors 3 and 4 looked the same as the record­ing on in­jec­tor 2.

ECU faulty?

This left us at the AECS help desk puz­zled. We had never seen that be­fore. Both coils of one in­jec­tor need to be ac­ti­vated at the same time, not with a 180 de­gree off­set as recorded at in­jec­tor 1. This looked to us like as if the ECU was “con­fused”. If you com­bine that with the fault code P0605 (ECM), any­one would start to lean to­wards a “magic fault” (=ECU fault)

Think

Just a lit­tle bit of think­ing tells you that the ECU IS ca­pa­ble of switch­ing both coils of in­jec­tor 1, would you not agree? That tells me straight away that there can­not be too much wrong with the ECU. It is still pos­si­bly faulty, do not get me wrong, but be­fore we get an­other ECU we cer­tainly need to check an­other few items!

What the record­ing did tell us is that in­jec­tor 1 was in­ject­ing twice per two rev­o­lu­tions, in­stead of once.

This would cer­tainly make the en­gine run in­cor­rectly, to say the least: First, when the in­jec­tion should take place dur­ing the com­pres­sion stroke (strat­i­fied in­jec­tion mode), not enough fuel will be in­jected, as only one coil ac­ti­vated the in­jec­tor. Sec­ond, in­jec­tion will also take place dur­ing the ex­haust stroke. Since there is no pres­sure to speak of in­side the com­bus­tion cham­ber dur­ing the ex­haust stroke, the pis­ton rings will not seal. A lot of that in­jected fuel will end up in the sump mak­ing the mix­ture richer for all cylin­ders (through the PCV). In ad­di­tion, a lot of that fuel will go down the EGR chan­nel, back into the in­take man­i­fold.

Later dur­ing this job, it was found that a lot of fuel was present in the sump, be­cause of a leak­ing fuel pump seal.

So is the ECU faulty?

An ECU can­not com­mand the in­jec­tion or ig­ni­tion prop­erly when its in­put sig­nals are in­cor­rect.

The main in­put sig­nals for in­jec­tion are crankshaft/ca shaft ac­tiv­ity. If the en­gine is not turn­ing, the ECU does not know that in­jec­tion has to take place, it is as sim­ple as that.

If there is some­thing wrong with in­jec­tion or ig­ni­tion phas­ing, it is usu­ally a re­sult of ECU cal­cu­la­tion trou­ble, where it does not see the ris­ing or fall­ing slope of one of the sen­sor sig­nals when ex­pected.

On top of that, many Nis­san chain­driven en­gines suf­fer from silent chain stretch. Again, any garage can vouch for that.

A log­i­cal next step is to record cam vs crank sig­nal.

At first glance, this sig­nal is wrong! The ris­ing slope of the sin­gle pulse of the cam should no co­in­cide with the last teeth be­fore the ref. point of the crankshaft.

I have seen so many be­fore. How­ever, this seems to al­ways give ig­ni­tion faults, but I have never seen in­jec­tion faults like this. To make sure I even looked up a pat­tern recorded on an­other Nis­san, not a QR20 (DD) en­gine.

I said with­out too much fur­ther thought that he had to re­place his tim­ing chain and guides, and thought that this would be the end of it.

St­ing The st­ing is as al­ways in the de­tail! This is why we use high sam­ple rate scopes.

The re­mark of the di­ag­nos­ti­cian af­ter the cam chain re­place­ment job was; “it runs sweet but the cam crank pat­tern

Ad­vance

Of course, when I saw the pat­tern, we dis­re­spect­fully asked the di­ag­nos­ti­cian “are you sure the marks line up?” How­ever, even be­fore the ques­tion was ex­pressed we re­alised that the cam had to ad­vance with new chain and guides, some­thing it ob­vi­ously did.

Con­clu­sion

The car is fixed, it runs per­fectly now, that is for sure. Yes, the prob­lem was the tim­ing chain and not the ECU. How­ever, this case has taught us here at the AECS help desk that we have to be more care­ful with com­par­ing known good pat­terns with prob­lem pat­terns. No com­pla­cency al­lowed!

The com­pare sig­nals HAVE to be of the same en­gine. As I said, the st­ing is as al­ways in the de­tail - de­tail which can be achieved with good qual­ity equip­ment like the ATS scope.

is very dif­fer­ent from what I ex­pected”

ATS 5000 scope record­ing of cam vs crank.

ATS scope record­ing of cam vs crank af­ter re­pair.

The four three-wire in­jec­tors can be seen In this wiring di­a­gram.

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