duratec downgraded to 3-cyl

[ralph]

Noticed some rough running, low power, backfiring on my last lap around Nerdburgring.  

 

Infrared thermometer says cyl #1 is cold.

 

Ignition on, motor off, I'm getting 12v across #1's coil connector leads (I think something more like 0.35v is normal -- that's what the other three cylinders show). 

 

I should be looking for a short in the #1 coil wires, no?...is there any other reason nominal ignition-on/engine-off voltage across coil connector leads would be different?

[CBuff]

I have read issues with the 420 #1 wire connections shorting due to vibration. 
https://www.caterhamlotus7.club/forum/techtalk/420r-running-rough?page=2

 

[CBuff]

Calling @KnifeySpoony  I think he had a similar issue before his hesitation gremlin arrived. 

[KnifeySpoony]

Is the misfire consistent? Happening at part throttle vs WOT etc? I had bad coil wires to cylinder 4. Check your continuity first. Jiggle the coil loom while idling and listen. 

[ralph]

I would say it's consistent at idle and partial throttle; since it was running rough, I didn't attempt WOT for fear of damaging the engine. 

 

Continuity back to the ECU connector seems fine (white/purple are all effectively the same piece of metal by way of a crimp, and I can see brown white on the back side of the engine loom connector.  Coil loom does indeed appear suspect, but I'm not sure how to confirm what bit of it, or if/how to implement a workaround. 

 

I would expect a continuity fault to result in no measurable voltage, not more voltage.  In my admittedly limited understanding of engine electronics, this seems more like a short (like one of or both of the two #1 coil wires has had insulation fail near another wire it shouldn't be sharing electrons with).  Is it expected that there is continuity across all six pins on the engine side of the coil loom connector?...not the case on the ECU side...

[KnifeySpoony]

I have a spare coil loom you can try out if you want. 

[ralph]

OMG, yeah, for sure.  ping'd you my contact info...

[MV8]

14 hours ago, ralph said:

Noticed some rough running, low power, backfiring on my last lap around Nerdburgring.  

 

Infrared thermometer says cyl #1 is cold.

 

Ignition on, motor off, I'm getting 12v across #1's coil connector leads (I think something more like 0.35v is normal -- that's what the other three cylinders show). 

 

I should be looking for a short in the #1 coil wires, no?...is there any other reason nominal ignition-on/engine-off voltage across coil connector leads would be different?

It is key-on, battery voltage found between the positive coil terminal and chassis ground, not across the coil positive and negative terminals. Positive gets batt volts when the key is in run and crank, while the negative is the control side from the ecu and drives the tachometer, in general.

[ralph]

Makes sense. I'm getting batt voltage from positive to ground on all four coil connectors, but I'm only seeing that voltage across the #1 connector leads; the other three have a fraction of 1v across their pairs, which would indicate short affecting #1, right? 

[MV8]

If checking for voltage across the positive and negative terminals with the key on, you are shorting battery voltage through the meter (as a parallel path to the coil) to the ecu coil control leads. Generally much safer to use the continuity/ohms setting on the dvm if you wish to check for shorts with no power applied.

 

You can check the harness for shorts by unplugging the ecu then put the dvm across the harness wire and chassis ground using the continuity setting. Often the dvm lead must be moved to a different socket on the dvm.

Edited September 5, 2023 by MV8

[ralph]

Understood re:white/purple->meter->ground confirms 12v supply; thanks.  Curious why there would be different voltage across just the #1 cyl' coil pairs...suggests something is different in those wires, and different is probably not good.

 

Will be swapping in a loaner coil sub-loom tonight to see if that's it (odds are, from what I've been reading).  I verified continuity at the loom connector, and will take it a step further at the ECU connector.  Ordered a set of NOID lights to test the injector side, and I suppose if I do have spark on cyl #1, the next step would be to have a look at that injector.

 

[KnifeySpoony]

Did you try swapping injectors and coils yet? 

[ralph]

update:  swapping in spare coil loom didn't fix it.  I actually have good continuity, low resistance on either my loom or the one I borrowed, back to the ECU connector.  I had swapped coils 1&2 to see if the problem moved/cleared, and it didn't - more like it spread.  I now have *nothing* burning in #1, no spark at all, and intermittent/weak spark in #2.  I suspect the injectors are fine since plugs 1&2 are coming out wet, but I'm stymied as to why electrons aren't finding their way through the coils.  Car is now 2.5 cylinders...ECU coil driver?

[KnifeySpoony]

Yeah maybe coil 1 was bad, then whatever caused it to fail then caused coil 2 to fail after moving it to that spot. I don't know if that's possible though. Swap coil 3 in to see if the pattern continues...

[ralph]

swapped plug, coil, coil sub-loom, still no spark on cyl #1.  #2 seems ok now (?)...back to 3 cylinders...

[Drakman]

Hi Ralph,

 

Have you checked the compression?  Is a valve stuck/burnt.

[ralph]

No, but I definitely don't have spark on #1, so I figure I should fix that first.  Is there a failure mode where the ECU won't send spark due to some other condition?

 

 

[MV8]

It sounds like it started out as a bad coil but became ecu failure from "testing" ecu outputs.

The position of the crank and cam help determine which coils are discharging or charging so all the ecu coil trigger outputs would not be expected to be the same or necessarily the same potential as chassis ground when charging.

 

In general, the quickest, easiest thing to do is to swap components from one cylinder to another to see if the problem follows, indicating a bad component is likely.

[ralph]

Bad coil theory seems to align with symptoms, but so does bad coil driver in the ECU, which Lee (CC) postulated as something he's seen before.

 

RE:testing, I'm admittedly very much a noob at this, but shouldn't the DVM essentially be passive when in DCV mode? I could see that if I had it sending voltage (e.g. measuring resistance) maybe that would cause problems.  As for the differential between cylinders, it makes perfect sense that crank position would alter these readings, but I don't think that's a factor here unless my engine happens to stop in exactly the same crank position every time it's shut off; further, while I did notice variance from one cylinder to the next, it was never in the giant increment evident on cyl #1.  

 

The thing that's leaning me toward ECU/coil-driver here is that when I did swap the coils on #1/#2, #1 still didn't fire.  That was with the original sub-loom so it's possible there was a continuity problem with that wire at the time, but if so, that appears to have fixed itself.  In the handful of threads I've read on sub-loom issues, there does seem to be a pattern of longer, more intensive use before the wire-fracturing issues surface -- not something that would surface in ~1700mi of casual driving; that doesn't mean there couldn't have been some temporary impedance factor in how that wire was planted, I suppose...for good measure, I will probably get 3AL080A when it's back in stock.

 

[MV8]

Yes, if a dvm and not an analog meter but the check was in parallel, so the current at the ecu is more than it would be with just the coil connected. How much more and what is safe? Who knows. Worth it for comparison since we have no specifications? I don't know.