Lean combustion?

[KenS]

The manual vacuum gave interesting results with not having the periodic change in revs! Does this imply that those sudden change in revs are not caused by a lack of spark or bad injector? Or was it not quite low enough pressure to help initiate the upsets? Do you know if the right angle filter thingy feeding from the manifold to the MAP sensor pipe is OK? Wonder what would happen if it was no longer a filter and allowed transient pressure changes through to the MAP.
But ...
I have expanded one of the manifold pressure/rev pulses from '26 try fuel reg..' to see its time relation with Injector pulse width, IAC position, Air/Fuel ratio.

sml7-10-26.jpg

It shows the injector pulse width is significantly reduced prior to the manifold sudden pressure change. So I think the root cause cannot be directly related to the Manifold pressure but something that has required the ECU to reduce fueling. I guess this does bear some similarity to the original problem, so they may well be related.
ElanScan is giving ECU driving values, not necessarily values that are achieved by the IAC or fuel injectors, so its hard to know for sure what they are doing, maybe a fuel injector is occasionally overfueling causing the ECU to suddenly reduce fuel on all injectors, or maybe the IAC snaps shut occasionally, causing revs to drop, air/fuel ratio goes down causing the IAC to be immediately raise a few counts and fuel reduced. I'll have a deeper look at the O2 sensor voltages during these dips to see if there is any clue to what the injectors may be doing. (I am going away for a few days so I will probably go quite for a few days!). Good luck.

[Chas54]

Ive measured the resistance of the injectors at around 2.6ohms, all very close to this. I have just ordered an injector test light to see if i can spot my dip in revs corresponding to some intermittent flashing.

Also cleaning up every connection around the top wiring loom .

Chas 54

[Tuga2112]

Ive measured the resistance of the injectors at around 2.6ohms, all very close to this. I have just ordered an injector test light to see if i can spot my dip in revs corresponding to some intermittent flashing.

Also cleaning up every connection around the top wiring loom .

Chas 54

you can just use the timing gun light, and put the loop around each of the wires. mind you. that will tell you the power is there, but wont tell you if the injectors are opening and closing. that would require a different type of test tool

like this
https://www.ebay.co.uk/itm/317812260496

[Chas54]

Tuga

Im not convinced the timing light sensor will pick up the small amount of inducted current on the injector cable. The voltage of its pulses must be 12v or less?
As my 'noid' lights are taking some time arriving i may test the theory.

I still think i am seeing a missing intermittent spark on no.3 . It does correspond to a dip in revs when idling every 21 seconds . Its a bit weird really . I have swapped the coils round so what was 1/4 is now 2/3 . With the coil construction being it is exactly the same windings and and LV terminals that produce the spark for both 2 and 3 , it would leave the missing spark issue being from the coil HT terminal to the no.3 cylinder.

Ive done a compression test , all the same pressures.
Swapped the plugs about and swapped the HT leads about.

Ill try the spark plug and leads again today and now i have enough leads i should be able to identify one that alway works on other cylinders and use that on no.3.

But i am wondering if it is an injector problem. I can only think it throws to much petrol in every 21 seconds its too much for the plug and thoroughly wets it and shorts out inside the cylinder.
After i finish swapping my plug and leads around again and run the engine to idle and test noids, I think ill have to swap the injector no3 for say no 2. See if i can move the issue to no.2 cylinder .

Thanks for comments though
Chas54

[chrism]

Chas54 I'm having (what I think to be) f/inj issues.

Cyls 1+3 have died and I suspect a 'bad' injector has failed pulling the other circuit down.

In order to get ins breakdown service to relocate to my fave classic car repair man I had to have a breakdown van to pre-assess. He used these to verify a strong healthy spark on all cylinders and I was really impressed



I'd never seen them before...might save you swapping plug leads around and you can see all the sparks 'in real time'

ps I've found that injectors fire as pairs 1+3 then 2+4 and when one breaks it switches off the other in the pair
Ignition uses 'wasted spark. 1+4 then 2+3 fire simultaneously, which made me suspect f/injs rather than coil packs.
Plus, I gather there's a m/plug hidden under/close by the coolant overflow that governs f/inj electrics. It can get corroded with overspill and may need a bit of wrangling to get it going again as can the coil packs in the same area.

Hope any of this can help

[KenS]

I think the injector theory is quite possible (but probably best to rule out the ignition/spark first). Removing each injector plug one at a time may lead somewhere, it may run differently for each of the 4 plug removals and the pattern may imply something. If it comes to it, I have one spare new FJ559 injector from RockAuto that I could send/lend, which may help to confirm any findings, just swapping injectors is not an easy process but gets easier with practice! Good luck.

[chrism]

pps if you take off the plug of the 'suspected bad' fuel inj the other of the pair will start firing again and you'll be back to 3cyls...
So...it's a 50-50 guess using injector pairs 1/3 or 2/4 to find the culprit.

ppps our injectors are low impedance of ~2ohms. They can vary between ~1.8->2.5 but no higher. This'll further isolate a bad player.

pppps try getting new f/injs in UK at this moment in time - they all seem to be 'on order' - and imo be careful not to buy Ultra-power MFI31's despite some assurances that their '1991 ISUZU Impulse 1.6L Turbocharged' flow rate matches ours. (I've verified they have 2ohm impedance tho'...)

ppppps I'll imminently have a used spare that you can try, rather than break into Ken's new part. And it'd be available for you to keep if it cured your problem...just a thought...

[Chas54]

I started today by testing with the strobe and convinced myself the intermittent spark at No3 cylinder was there still.
I swapped the lead and plug for some old ones I had around. Still on number 3.

I then swapped the position at the lower coil so the no.3 lead was now going to no.2 and the no.2 lead was going to no3 cylinder. The missing spark problem moved to no.2 .

So it looks like the problem is with the normally No.3 output on the lower coil. How this can be I have got no idea. I didn’t notice this when I swapped the coils round a week or two ago.
Unless its something to do with the LT connections and ive just made a better fit ?

Ive lost one of the rubber seals from the lower ignition plate plug .

Ive now swapped the lower coil for an old one I had and the missing spark seems to have gone now although the engine was smoother idling at its best periods before on the other coil.

I tried the noid light tester on all 4 injectors in turn . The noid light is not very bright but can be seen flashing evenly on all injectors. Incidentally, I couldn’t get my ignition strobe to detect the injector pulses.

I think it might be time to solder all the wires back onto the ECU plug and O2 sensor and see how it runs..

Kens Thanks for the offer but I do have a spare set of injectors I had cleaned and tested. I havent used them since as I had the fuel rail sat on top of the engine with newsapaper underneath and when the fuel rail was under pressure I noticed one built up a drip. At the time it put me off using them and I bought a new set from US .

I do like the look of the spark plug test inserts an at around £12 a set of four looks a good investment – I have two other cars with magnetos and its always a bit hit and miss to tell if theres a spark sometimes..

The plugs on the ignition module plate are susceptible to any coolant spillage and I have found a bit of a drip from my header tank bottom hose . Its bin off and on so many times in the past few weeks, its getting a bit tires.

Ive attached my two elanscan traces from today

First was whilst I was swapping the leads and testing all the injectors with the dismal noid light. Im wondering if the noid light being so dim is a clue? . I think ill have to test it on a 12v battery and if its brighter I will have to check the voltage at the injectors when running.

If I have proved my issue is coil failure I cant picture what the fault is unless I am completley miss understanding the coils construction.

Thanks again for all advice comments and suggestions.

swapping lead and coil position noid.zip

after lower coil swap.zip

Chas54

[KenS]

Is good that you have found the cause of the regular drop in revs, that last plot after the coil change does look far better in that respect. The plot still shows the O2 sensor voltage as odd, slow to start-up and maybe lean, so reconnecting the wiring around that does make sense, then establish a new baseline plot. You say you have fitted new US fuel injectors - have they worked well prior to this problem of dropping out of closed loop and 'lean' CEL fault codes?

[Chas54]

I bought the US injectors in 2021. I think they have been OK until this problem appeared

[Chas54]

Ive soldered up the O2sensor cables at the socket under the bonnet and at the ECU plug end where i had cut the cables to patch in my jump leads that supplied a direct battery +ve and _ve and the external sensor feed back .

Unplugged all the HT cables to reroute them and ensure connections good. They have been all over the place as i added HT cables in to test. I haven't yet re-inserted the injector plug paper clips locks.

As soon as i started the engine it felt better, quick rise in revs then drop back to a very smooth idle . The elanscan traces look good to me at the moment, maybe the IAC a litle to far closed , I think the targets around 20 counts but i haven't bothered adjusting the bypass screw yet. I thought I would give it a run on the road first as it seems to be running so well.

At the moment the one thing i can say has improved my problem is replacing the bottom coil that had the intermittent no.3 spark . As far as i can see, the two spade connectors on the ignition plate or the HV output to the cable connection turret no3 are the most likely culprits. I had cleaned the male part of the spades but its not easy to find something to dress the female sockets on the coil, maybe it has an issue in there ?

I attach my latest elanscan trace . It will be a day or two before i do my road test.

Ive been looking on the internet for any details of how the wiring inside the coil is arranged but cant find drawing at all.

Thanks for all the comments and thoughts . Any thoughts on this latest engine idle scan welcome.

soldered up O2 sensor cables.zip

Chas54

[Chas54]

coil.jpg

Ive just found a drawing of a wasted spark coil and I dont see how mine can be faulty on no3 only as no1 sits on one end of the HV windings and no3 the other.

[KenS]

For the wasted spark, the high voltage is produced across the coils 2 spark plug outputs, if one was to breakdown and connect to earth within the coil pack, then the other will see the full high voltage potential, so still spark, possibly with higher than normal voltage since it is not shared across two spark plug gaps.

Had a quick look at the plots, they still show signs of the original problem, the O2 sensor is starting up in reasonable time now, but quickly starts indicating 'lean' combustion (still at 35 degC coolant at 145sec) but then just about recovers (around 400sec). The O2 sensor (integrator) also shows it saturates (value of ~ 200) so the ECU has no remaining headroom to get the fuel mixture richer. As things warm up, the integrator comes back into range (just) and things look a bit better but I think it is right on the edge of going lean if the engine is put under load. It will be interesting to see plots when driven.

17-10-2026.jpg

17-10-2026b.jpg

17-10-2026c.jpg

[chrism]

I'm comparing that latest plot with my own - and sensor values seem a lot better and quite close to (my) 'normal'.

What seems most obvious (to me) is that the O2 heater circuit regularly seems to 'peter out' and it seems that engine temp starts it up again. First thoughts? - naff heater circuit/sensor/supply from regulator.

Yes as Ken says it'd be interesting to see if engine load weakens the mix more esp when warmed up cos, second thought, might be naff flow rate? - Not naff inj cos it takes out 2 cyls - 1/3 or 2/4 - simutaneously.

(BTW your RAuto sensors aren't these Ultra-power MFI31's that they claime to support turbo's are they? Loadsa contributors reckon their flow rate isn't sufficient for turbo applications, although their 2-ohm impedance is 'spot-on' for ours.)

[Chas54]

The O2 sensor is from a UK supplier and is a Walker 350-33022

The sensor output drops during warming up occurs even when the sensor is supplied with a direct cable +ve and –ve from croc clips on the battery .
The sensor output is being fed direct to the ECU plug . All soldered joints and run through the driver door gap and over the dashboard.
Although now all replaced to original and soldered up..

The battery voltage shows between 14v and 14.5, so it looks like the alternator regulator is good.

The only explanation I can come up with for the warming up engine showing the sensor stopping working is that the cool exhaust gas passing through the system has lost some heat to the engine top metalwork mass and then passes over the heater plate cooling it a bit to much so it stops functioning. Why it doesn’t cool it straight away is a bit baffling. After all, the heater plate doesn’t warm up until the battery is charging.

The ‘noid’ test light is dim when I test the injectors , but if I put it across the 12v battery it glows bright. This is a common thing though looking on the internet.
Im guessing the square edged wave pattern doesn’t have a long period with 12v supplied to the injectors so doesn’t allow the bulb filament to reach optimum temperature.

So Kens description of the coil could explain one of the two HT lead turrets on the coil not working well , whilst the other still being good ?. The HV winding being tethered to the turrets at each end . If one end was down to earth I think both the sparks would be affected but if one was going open circuit at the end of the HV coil where it connects to the turret internally, then one would spark an the other may be more reluctant. . Having said this I am old enough to remember spark plug leads having a deliberated gap in the condutctors . The thinking behind it being it raised the high voltage at the plug more rapidly than allowing it to raise over a longer period.
So to stop the spark on my No3 the gap would have to be larger than I can think it could be realistically. If I can prove definitvley this coil is faulty I will cut it open to look why. It may mean I have to put it back on for a test once ive cured my issues .

Yesterday, as soon as I started the engine it felt far smother and better than it has for a while. The proof will be the road run , hopefully tomorrow.

I haven’t put the awkward injector plug clips back in yet.

Thanks again for comments ,

Chas54

[chrism]

Chas54 here's my own Walker 350-33022 trace overlaid with your latest 'soldered' scan both from cold.



Being like-for-like it crystallizes our earlier thoughts that your O2 sensor seems to be starting later, running for a bit then crashing out as the heater fails, then (I reckon) to be restarted by sufficient exhaust gas heat.

I have the impression that you've verified that the sparks are okay (that Laser test kit would absolutely verify it; I've seen it work) so I'm wondering if f/inj flow rate might now need to be considered - which is why I asked about your RAuto f/injs in my last post.

Do you know for certain that they're not the infamous 'Ultra-power MFI31s'?

That dim noid light could be showing that the injectors are 'buzzing' - but might any (or all of them) not be delivering enough fuel due to a compromised or insufficient flow rate?

Our injectors are very low impedance and I wonder if the dim noid can be explained by eg 5V being used to 'power' their led from the ecu rather than your 12V battery?

I'm not as forensic as Ken - just chucking out possibilities and trying to notice patterns And yes, also looking forward to an on-road EScan trace - which somehow might 'inform' about flow rate?

ps have you tried a different O2 sensor? I strongly suspect you might have?

[chrism]

I tested Walker, Bosch 15701 and Denso sensors a while ago and found that the Walker was the highest-powered of the three. It gets mine going within ~40s (Denso ~120s)

It might be an idea to see if you have a different make of sensor which gives you a longer start rate than your Walker - which might be significant and 'iron out' both your heater supply and your O2 sensor?