Toyota missing injector/coil pulse

[pman92]

Probably a bit off topic here but thought I would see what you guys thought anyway.
We have a 2001 Toyota RAV4 in at work this week.
1AZ-FE engine, manual (the same model that has PCM failures that lead to burnt out auto transmissions).
Drives fine, idles fines most of the time. Decent compression (just slightly below spec). Fuel pressure is spot on.
It's problem is it will intermittently stumble at idle and sometimes stall. Sometimes idles perfect for half an hour, other times will not idle stable at all, have to open the throttle slightly to brings the revs up and stabilise it.
Have cleaned out the throttle body and idle control motor. Have substituted the MAF sensor with another known working one with no change. Have replaced both post cat 02 sensors as the heater circuits had failed, have checked both pre cat sensors. Good cross counts and the fuel trims are bringing it to perfect lambda at idle from readings on our gas analyser. Idles slightly rich with O2 sensors disconnected to force open loop, as expected.

After trying all this I've put the scope on it. 4 channel scope, measuring from the top:
-injector/coil voltage supply
-injector pulse (from cyl 1 injector)
-coil trigger pulse (from cyl 4 coil)
-idle control motor control/duty cycle. Obviously time scale is a bit slow to see it properly in the pictures.
Here's a pic of it running fine:

image.jpg

Here's a pic of it running 650 milliseconds after that pic (when it stumbled):

image.jpg

As you can see in the first pic, it has an injector pulse and coil trigger approx every 3 and a half divisions. In the second pic, it's simply missed one. No fuel and spark for that engine revolution. And then the next rev it has picked up and started fueling and sparking again.

Then put the scope on the cam and crank angle sensors, I'll put some pics up of them aswell

[pman92]

Ok so turns out I haven't got pics of them. But they were working, however the signals became quite weak as it stumbled. there both inductive type sensors and I'm pretty sure that's a side effect due to the reduced engine speed.
As I'm writing all this I'm now thinking maybe the bigger gap between injector/coil pulses is also just a side effect of reduced engine speed.
However the gap virtually doubles from 170mS to 450mS within one cycle. If that's the case the engine speed would have to drop from 700 rpm to about 250rpm within that 620mS, is that even possible?
Also just noticed the first cyl 4 coil pulse on pic 2 is late compared to cyl1 inj pulse and compared to all the ones in pic 1, so the engine speed had already started dropping by that point.
Also the last injector pulse width on page 1 seems wider.

Probably things I should of noticed before posting this topic lol.

I'll have a further look tomorrow and see what I come up with

[Gareth]

This job looks like fun

I think one of the fundamental things to think about is how much do you trust your data? Knowing what scan tool you are using, I don't... That tool has cost me plenty of wasted time and money, particularly on Toyota's...

Anyhow... I don't think that engine speed loss in the time you are talking is possible, so you're onto the data there. Don't ever forget the basics of diagnosis, how about battery charge rate at idle? You mention having to crack the throttle to keep it running, have you checked for vacume leaks? don't forget to check when the engine is hot and cold...

I got caught with one of these that did a similar thing when the canister purge soleniod stuck open... randomly...

I will be watching with interest

[pman92]

Not actually using this scan tool as a scan tool, just as a scope. We have a VeDiS11 scan tool. I always try to be conscious not to trust any scan tool data too much.
Just out of curiosity what scan tool do you use/recommend?

It doesn't want to play up as much today (bloody thing). However I've found it does play up every single time the engine cooling fans come on. Disconnected the alternator to confirm its not the electrical load of the fans causing it, still does it as soon as the fans come on.
It seems to play up a lot worse here though. Stalls almost every time.

I've confirmed the cam and crank signals are good. They do get quite weak in comparison to idle at 700 rpm but they are definitely still there. I'm sure it's just engine speed.
As for the 'missing' signal in my photos above, I've also established that's just the reduction in engine speed also. Looking at the delay between cyl 1 injector and cyl4 coil pulses, it's usually 20ms or there abouts. Directly before the stumble the coil pulse is late, 40 to 50ms after. But the injector pulse wasn't late compared to the last cycle. So it lost speed sometime between cyl 1 injection and cyl4 firing.

Done a lot of calculations, which I'll post up later, and worked out engine rpm was at about 400 when cylinder 4 fired, and engine speed was lost during the middle part of cylinder 3 power stroke, indicating a cyl3 missfire in them photos. Which makes sense.
Was also able to work out no other cylinders subsiquently misfired based on the time it took to get to the next cyl 1 injection after the stumble, and that engine rpm was back at or close to 700 rpm by then.

It's a random missfire. Oddly enough drives fine and doesn't carry on or missfire under load or on the road.
New plugs are on the way, even though they look fine, electrodes arnt corroded at all.
I'll also make sure all the coils, especially cylinder 3 can jump a 25/30mm gap in open air.

[Gareth]

Just out of curiosity what scan tool do you use/recommend?

use a G-scan for day to day stuff, use genuine software via a drewtech pass through for more in depth testing...

from what you just mentioned about fans, i'd be looking at earths now...

[pman92]

Coils are fine. Completely agree about the earths, I'll give them a check.

Our VeDis11 has pass through. I've never used it though. Wouldn't actually know how.
How much does the diagnostic software cost on average? It's a subscription service right?
Might have to look into it and talk to the boss man

[Gareth]

isn't it 'VerDis' by snap-on? They are ok i suppose, i tested one for the Ballarat uni years ago and wasn't thrilled by the loading speed - windows type software?

Yes the snap-on unit is a pass through, last I checked it wasn't an SAE approved one, the drewtech is, so is the Bosch.

Yes each manufacturer requires purchasing of a subscription, they all cost different amounts, some have short term and long term subscriptions.

[festy]

Looks like a lot of dwell in the ignition event prior to the missed one - had it already missed a tooth/teeth and badly miscalculated the engine speed?

[pman92]

Nah it's not the snap on one.
http://www.autolandscientech.com/v2015p/en/product-3
I find it pretty good to use. Does everything you would need for day to day stuff. It's user friendly and reasonably fast at loading. Far better than the scan pro which we only use as a scope now.
Having said that I've only ever used it and a scan pro (I'm only 23).

Festy, I've worked out the 'missed one' is in fact not a missed one at all. There's just twice the time gap as the engine speed had dropped so much there. You can see the delay between injector pulse on cyl 1 (2nd channel) and coil pulse cylinder 4 (3rd channel) is about 20mS, until the last pair before the big gap, where it's almost 50mS. Because the engine has already started slowing there due to a cylinder 3 missfire.

Got a diagram and checked all the earths in the engine bay. And BINGO. Found one unplugged, the main head to body one above the alternator, was tucked nicely away under the alternator so I didn't notice it.
Thought beauty this will fix it for sure, and it kinda has. Cooling fan operation no longer makes any impact on the engine.
However the intermittent miss/stumble at idle even when the fans weren't running that it originally came in for has returned after it disappeared this morning. And it seems to have got a lot worse. Missfires even when driving which it didn't do before. I'm unsure if fixing this earth has contributed to it yet.

Haven't looked much further yet, but I quickly scoped the earth from the loom for the coils, which has the 4 coil earths in it. It bolts to the end of the head.
Found that as it missfires, the earth point becomes very noisy relative to the negative battery terminal. As much as 110mV peaks. When running fine it sits at a steady 10mV.

[pman92]

image.jpg

There it is running fine

image.jpg

There it is playing up

Didn't really have the voltage scale set as I should but you can see how noisy it becomes.

Hopefully have time to keep looking tomorrow