Di­ag­nos­tics

Motor Equipment News - - CONTENTS - By Peter Lei­jen, BE(Hons), Ap­pli­ca­tion Engi­neer, for AECS Ltd.

The prob­lem pre­sented to the help desk: a 2008 Nis­san Vanette CR diesel with a mis­fire at about 1,300–1,500rpm. This ar­ti­cle is com­plex and needs de­tailed read­ing for full un­der­stand­ing – en­joy! I will run through all the data that was pre­sented, ex­plain­ing the di­ag­nos­tic steps that we take when run­ning through a com­pli­cated prob­lem like this seem­ingly sim­ple is­sue.

Where to start?

When pre­sented with a mis­fir­ing en­gine we want to dis­tin­guish be­tween two dif­fer­ent sce­nar­ios. 1. Fixed mis­fire on one (or more) cylin­ders. 2. Ran­dom mis­fire. First, we want to see an ATS scope record­ing of the crankshaft sen­sor sig­nal and an in­jec­tion sig­nal.

The in­jec­tion sig­nal gives us a ref­er­ence as to which cylin­der is hav­ing an is­sue, or if the is­sue is across mul­ti­ple cylin­ders. Record­ing the crankshaft sen­sor sig­nal also shows the qual­ity of the sig­nal i.e. is it cor­rupted with noise, etc.

Zoom­ing in on the crankshaft pat­tern showed that there is no noise on the pat­tern so this will not be the cause of the mis­fire. The record­ing of crank vs in­jec­tion with the cal­cu­lated rpm line shows that there are mul­ti­ple mis­fires oc­cur­ring which seem to be linked to two cylin­ders. Based on the fir­ing order we can see that cylin­ders 1 and 2 are mis­fir­ing.

EGR?

Ran­dom mis­fires are usu­ally caused by air flow is­sues i.e. an EGR valve that is not func­tion­ing prop­erly. We blocked off the EGR valve, but this made no dif­fer­ence to the mis­fir­ing of the en­gine. It must be noted that block­ing the EGR valve also dis­rupts swirl in the en­gine. The ig­ni­tion damp­en­ing ef­fect of the CO2 in the ex­haust gasses has been re­moved, so as a re­sult the en­gine sounded more rat­tily.

We can now safely say that the EGR is not the cause for the mis­fir­ing en­gine, and that we are happy with the gas flow through the en­gine. Just to be dou­bly sure a rel­a­tive com­pres­sion test was also per­formed which showed that all four cylin­ders were within two per­cent of each other, a per­fect re­sult.

Com­pen­sa­tion val­ues.

Hav­ing elim­i­nated the crankshaft sen­sor, EGR valve and com­pres­sions, it is time to look at the in­jec­tors. This Nis­san Van­nette (RF-T en­gine) is the re­sult of a joint op­er­a­tion be­tween Mazda and Nis­san. As a re­sult, we had with the scan tool ac­cess to the in­jec­tor com­pen­sa­tion valves, which is some­thing the de­sign­ers at Nis­san do not give you ac­cess to on, for ex­am­ple, their Navara.

The in­jec­tors are com­pen­sated at four dif­fer­ent pres­sure set points to en­sure that the en­gine is run­ning smoothly un­der dif­fer­ent con­di­tions. The graph in fig­ure 3 shows these set-points and the com­pen­sa­tion val­ues of each cylin­der.

These adap­tion val­ues were recorded with a Launch Scan­tool and sent to AECS for anal­y­sis with our own soft­ware.

Please note: this level of sup­port, and be­ing able to share data is some­thing that is unique to the AECS help desk.

Fig­ure 3 shows the four set-points at 35, 65, 100 and 140 MPa rail pres­sure. This graph in­stantly shows that there is an is­sue with in­jec­tor num­ber four be­cause its adap­tion val­ues lie so much higher than the other three in­jec­tors. How­ever, our crankshaft speed mea­sure­ment did not iden­tify no.4 as a mis­fir­ing cylin­der.

Clos­ing the loop

I re­gard the au­to­mo­tive sec­tor in New Zealand as be­ing highly skilled, and not sim­ply parts Swap­pers, so let us look fur­ther. Please put your think­ing cap on, brew another cof­fee, and step into our world!

All four in­jec­tors were re­moved and tested by a lo­cal diesel spe­cial­ist, who shared the­data with us, thank you.

A scanned copy of the re­sults was sent to us and the data was a real eye-opener! The ta­ble presents the re­sults.

The ta­bles need some ex­plain­ing. The ac­tu­a­tion time, pres­sure, mea­sure­ment time, set in­jected quan­tity and set re­turn quan­tity are all spec­i­fied by the man­u­fac­turer.

For ex­am­ple if these in­jec­tors are tested for a full load test they need to be ac­ti­vated for 860us at a pres­sure of 160 MPa for 120 sec­onds, and the ac­tual in­jected quan­tity should be be­tween 51.8 and 58.8 mm3/H (ccs per stroke), with a fuel re­turn quan­tity be­tween 0 and 86 mm3/H.

These tests have con­firmed what the adap­tion val­ues have al­ready told us. In­jec­tor num­ber four is faulty. As a re­sult, the di­ag­nos­ti­cian re­placed just in­jec­tor num­ber four.

Based on the scope pat­terns it is need­less to say that re­plac­ing just in­jec­tor no.4 did NOT im­prove the run­ning of the ve­hi­cle.

What is in­jec­tor com­pen­sa­tion?

To an­swer this ques­tion we need some more data from the ve­hi­cle. The record­ing shows rpm, Inj4 and suc­tion con­trol valve duty-cy­cle.

When the suc­tion con­trol valve duty-cy­cle in­creases, more fuel en­ters the rail, when the con­trol valve duty-cy­cle de­creases less fuel en­ters the rail. Fig­ure 4 high­lights three im­por­tant ar­eas.

#1. The duty-cy­cle in­creases af­ter a mis­fire has oc­curred. The ECU sees that the crankshaft has slowed down and now wants to speed up the crankshaft again. This is done by in­ject­ing more fuel in the fol­low­ing cylin­der.

#2. The duty-cy­cle de­creases af­ter in­jec­tor four in­jects. This is be­cause in­jec­tor four has higher than nor­mal leak­age rate (see test sheet ta­ble). #3. This is the same cylin­der as in #1, but now we don’t see the in­crease in duty-cy­cle. This is be­cause the cylin­der be­fore­hand fired prop­erly so there is no need to speed up the crankshaft with

ad­di­tional fuel. In this duty-cy­cle pat­tern, we can see two mech­a­nisms at work. We can see the ECU com­pen­sat­ing for a sud­den de­crease in crankshaft speed and we can see the ECU com­pen­sat­ing for an in­jec­tor with high re­turn flow (rail pres­sure de­crease).

Con­clu­sion

Look­ing back at the in­jec­tor ta­bles we can see that the re­turn flow of in­jec­tor #2 un­der the leak test is more than twice that of in­jec­tor #1, but fur­ther from the set-point. Sim­i­lar con­clu­sions can also be drawn with the other in­jec­tors.

As a re­sult, we re­placed all four in­jec­tors. The car was re­turned to the cus­tomer who took it for a 1,000km drive, and then bought it back into the work­shop to have the in­jec­tor com­pen­sa­tion val­ues re-checked. Fig­ure 5 shows the be­fore and af­ter re­sults. We can see that In­jec­tor one did not need re­plac­ing, be­cause the com­pen­sa­tion val­ues did

not move as far as the other three in­jec­tors.

It also needs stat­ing that we get more and more cus­tomers who pur­chased other brands of scopes, ask­ing for our tech­ni­cal sup­port. We try to help as much as we pos­si­bly can, but this usu­ally ends up in a DNF (rac­ing terms for “did not fin­ish”) job. This is most frus­trat­ing for us and our cus­tomers.

Please make sure you choose your knowl­edge and equip­ment part­ner wisely.

Both you and AECS are deal­ing with more com­plex prob­lems daily. AECS has the abil­ity and re­sources to sim­plify these com­plex prob­lems in an easy to un­der­stand man­ner for your ben­e­fit.

Fig­ure 1: ATS scope record­ing of crank vs in­jec­tion show­ing mis­fires.

Fig­ure 3: in­jec­tor set-points and adap­tion val­ues.

READER RE­PLY 0140841

Fig­ure5: be­fore and af­ter re­place­ment in­jec­tor com­pen­sa­tion re­sults.

Fig­ure 4: ATS 5004d scope record­ing of inj, SCV duty cy­cle, and RPM.

Fig­ure5: be­fore and af­ter re­place­ment in­jec­tor com­pen­sa­tion re­sults.

Fig­ure2: rel­a­tive com­pres­sion test.

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