Miss­ing Nis­san

Motor Equipment News - - DIAGNOSTICS -

This ar­ti­cle is a true de­scrip­tion of an AECS tech­ni­cal help desk prob­lem and how it was solved. By H.P. Lei­jen (trainer/re­search).

Ve­hi­cle

2005 Nis­san Ti­ida, MR18DE 1.8 petrol.

Prob­lem pre­sented to the help desk

This Nis­san Ti­ida was driven into our work­shop, with a com­plaint of lack of power. It pre­sented it­self with a ter­ri­ble mis­fire. The work­shop in­volved has AECS equip­ment and tech­ni­cal sup­port.

The essence of this ar­ti­cle is how to quickly and ac­cu­rately di­ag­nose which cylin­der is mis­fir­ing or if it is a ran­dom mis­fire. This knowl­edge has a ma­jor im­pact on di­ag­nos­tic ap­proach and speed.

Mea­sure

The first mea­sure­ment made was crank shaft sen­sor vs ig­ni­tion trig­ger while the en­gine was run­ning and mis­fir­ing. Ac­ci­dently the power sup­ply of the coil was mea­sured vs crank, with some in­ter­est­ing facts.

The RPM pat­tern live cal­cu­lated from the crank shaft sen­sor showed clearly a mis­fire ev­ery fourth beat.

The coil’s power sup­ply showed that there was a bad con­nec­tion be­tween the bat­tery and the coils, dip­ping the power sup­ply each time a coil was ac­ti­vated. It is easy to mea­sure the coil ac­ti­va­tion time from that pat­tern, you just mea­sure how long each dip lasts.

The out­come was that the coil just be­fore the mis­fire was only ac­ti­vated for three mil­lisec­onds, as op­posed to the work­ing coil at 7.8 msec. Is three msec per­haps not enough en­ergy in the coil to ig­nite the mix­ture?

Cam chain?

Could the ECU drive the coils in­cor­rectly as a re­sult of a crank/cam re­la­tion fault? Some Nis­san en­gines do have cam chain stretch is­sues.

A cam vs crank record­ing was made and no chain stretch of sig­nif­i­cance was de­tected, we look for more than two de­gree cam/crank phase vari­a­tion (chain slap) as an in­di­ca­tor that the ten­sioner has ran out of stroke. We came later to the con­clu­sion that the bat­tery volt­age vari­a­tions caused the er­ratic coil charge du­ra­tions.

Ig­ni­tion trig­ger

Time to find out which cylin­der was mis­fir­ing, and com­pare the coil trig­ger time with the find­ings in our first mea­sure­ment. In which di­rec­tion shall we go?

Even to the un­trained eye it is clear that the mis­fire was hap­pen­ing on one cylin­der con­tin­u­ously. Since the chan­nel 2 lead was con­nected to ig­ni­tion trig­ger of coil 4, it was also im­me­di­ately clear that the mis­fire hap­pened on cylin­der 4 alone.

Let’s look at the pat­tern in de­tail just to learn a lit­tle. Zoomed-in, you can see the crank shaft speed­ing up after ig­ni­tion of cylin­der 1,3 and 2. You can also see that the com­pres­sion is slow­ing the crank shaft down after each “speed up” (power stroke) event.

This in­di­cates, most im­por­tantly,

that the com­pres­sion of cylin­der 4 is there (more about this later). You can also see that the over­all en­gine is be­ing dragged down by the two suc­ces­sive com­pres­sions of cylin­ders 4 and 2. The fir­ing cylin­ders are mak­ing up for it (RPM line trend­ing up), to achieve “tar­get idle speed”. Rea­son for such a mis­fire is nine out of 10 times an ig­ni­tion prob­lem, so it makes sense to me to in­ves­ti­gate the spark qual­ity on cylin­der num­ber 4.

Spark

The pri­mary spark pat­tern can­not be recorded on these COP coils as the coil has got an ig­ni­tor build in. We deal with the con­struc­tion and how to prop­erly mea­sure and di­ag­nose the spark qual­ity in the AECS EMS 1-1 train­ing.

The di­ag­nos­ti­cian de­cided to swap the coil in­stead of mea­sur­ing the spark qual­ity. While hav­ing the coils out he found that the spark plug gap was around 1.7 mm. He re­placed the spark plugs and swapped coil 1 and 4.

He re­ported that the en­gine ran a lot bet­ter, power was back, but still a mis­fire at idle on cylin­der 4!

Fuel

Bet­ter look at crank vs In­jec­tion to see if the in­jec­tion was present and to see if the du­ra­tion was equal to other cylin­ders.

The in­jec­tion du­ra­tion on all cylin­ders was the same at idle when the en­gine was mis­fir­ing… Hmm, that did not help!

Back to draw­ing board

Bet­ter start again as we might have missed some­thing. We wanted to see ig­ni­tion qual­ity on cylin­der 4 al­most right from the be­gin­ning.

We had to mea­sure the in­duced ig­ni­tion pat­tern, to see the spark qual­ity. The coils were re­moved and a wrap wire was placed around the coil stalk. The fol­low­ing pat­tern was recorded – see pic­ture cap­tioned “In­duced ig­ni­tion vs crank record­ing on 2 chan­nel ATS 500XM scope”.

The pat­tern needs a lit­tle ex­pla­na­tion – please read in de­tail if you like to learn.

The crank sig­nal is first used to trans­form into a Delta N (RPM) pat­tern, then the RPM pat­tern is mea­sured for speed changes. The high­est speed changes (steep­est slope in the RPM line) are just after com­bus­tion and just be­fore the end of com­pres­sion.

The peaks of the slope line in­di­cate the in­ten­sity of en­ergy de­liv­ery after com­bus­tion and the troughs of the slope line show the en­ergy re­quired to achieve com­pres­sion, no mat­ter what the RPM is.

In the slope line of the above pat­tern, is clearly vis­i­ble that the com­pres­sion on each cylin­der is vir­tu­ally the same, so the mis­fire is not a com­pres­sion prob­lem. To ver­ify this, a com­pres­sion test was done by the di­ag­nos­ti­cian. They were all equal.

It is also vis­i­ble that the en­ergy de­liv­ery is not present in cylin­der 4 (slope peak just after coil 4 fires).

Spark again?

Let’s zoom in and look at the ig­ni­tion qual­ity.

Zoom­ing in on the spark du­ra­tion in the same record­ing showed a spark du­ra­tion of 1.12 msec. That is not a great spark but cer­tainly not cause for mis­fire.

So what next?

So we had com­pres­sion, we had ac­cept­able ig­ni­tion, and we had in­jec­tion. The coils had al­ready been swapped, time now to swap the in­jec­tors from #1 to #4.

Bingo!

Even though the en­gine sounded ex­actly the same after the in­jec­tor swap, the mis­fire moved to cylin­der 1 (with the in­jec­tor). This clearly in­di­cated an is­sue with in­jec­tor #4. A sec­ond-hand in­jec­tor was fit­ted, which made the en­gine run per­fectly.

Tail end

After about four days of run­ning the car was pre­sented back to the work­shop, run­ning on three cylin­ders again. A quick two minute mea­sure­ment, us­ing a scope, pre-set the pre­vi­ous record­ing and, in­di­cated that it was cylin­der 2 this time which was mis­fir­ing. Be­fore the di­ag­nos­ti­cian or­dered one more in­jec­tor he ad­vised the cus­tomer that it was bet­ter to re­place all as it was very likely that the rest of the in­jec­tors would fail too.

Wa­ter

The di­ag­nos­ti­cian, out of his own cu­rios­ity, started hunt­ing for why the two in­jec­tors had failed, as it is not nor­mal for in­jec­tors to play up like this. He took a fuel sam­ple and found a large quan­tity of wa­ter in the tank. The cus­tomer was ad­vised of costs of in­jec­tor re­place­ment and tank re­moval/re­fit.

Con­clu­sion

The write-up of this fault took ac­tu­ally longer than find­ing the fault. It is just the think­ing be­hind the diagnostics, we as a tech­ni­cal equip­ment provider and train­ers, like to ex­plain to our cus­tomers and prospect cus­tomers. I guess it is our pas­sion.

Over­all an ef­fi­cient job, good for work­shop turnover and cred­i­bil­ity. An ATS scope, an ex­pe­ri­enced di­ag­nos­ti­cian, and a lit­tle of AECS tech sup­port, made this job quick and prof­itable.

ATS 500XM scope record­ing of ig­ni­tion trig­ger vs crank sen­sor, with a live cal­cu­lated RPM trace in­di­cat­ing the crank shaft speed changes (Delta N) at idle.

ATS scope record­ing of coil power sup­ply vs crank shaft sen­sor.

Scope record­ing of ig­ni­tion trig­ger vs crank sen­sor zoomed in.

Ig­ni­tion trig­ger 4 vs crank record­ing after in­jec­tor 1 and 4 have been swapped.

In­duced ig­ni­tion vs crank record­ing zoomed in.

In­duced ig­ni­tion vs crank record­ing on 2 chan­nel ATS 500XM scope.

RPM vs in­jec­tion record­ing.

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