D2 V8 MAF sensor issue?

**nobbyclrk** · 7th October 2011, 03:18 PM

What I was showing earlier was a lean fault in the MAF creating a reduction in performance. The actual complaint was 'surging at idle and blowing black smoke'. This time I can show you what differance a few millivolts will do to the engine management.
The faulty MAF was with in the specified voltage range. So specs don't help.

Again I'll show a good one first. Not much to see in the first image, more of an overall picture.
You can see the engine idling and then revved up to just under 5000RPM. The whole capture is taken over a 50 second time scale (along the bottom).

You can see in:
Black - Engine RPM
Blue - MAF output voltage
Green - injector switching
Red - Signal to ignition module from PCM.

Pretty easy really. You can see for referance the area I will zoom into.

**nobbyclrk** · 7th October 2011, 03:32 PM

And here's the zoom.

Again:
Black - Engine RPM
Blue - MAF output voltage
Green - injector switching
Red - Signal to ignition module from PCM.

Note the injector duration, steady RPM and MAF voltage. The oscillations on the MAF are caused by the valves opening and closeing increasing air flow as each valve opens and draws air out of the inlet manifold. You will see four dips in the whole distance between two ignition triggers (this is only one coil of two for two cylinders of four, so in reality if you included the other coil you would see a dip for each ignition event)

Also the time scale across the whole screen is less than 1/2 a second.

**clubagreenie** · 7th October 2011, 04:26 PM

Thats actually a really good sign in terms of the MAF flow vs pulse reversion from the valves. Shows the inlet/plenum design is working really well.

**nobbyclrk** · 7th October 2011, 05:48 PM

Thats actually a really good sign in terms of the MAF flow vs pulse reversion from the valves. Shows the inlet/plenum design is working really well.

This (Subaru) type of MAF, I'm not to sure about its operation, is the 'hot film' type (As opposed to the older style 'hot wire'). But I would imagine it's operation is the same as the VW type. That has (if I have my numbers right, not sure off the top of my head) two heater elements towards the outedge of the film and two thermistors in the middle. That way, depending on which thermistor is hotter at any particular time, the MAF can work out weather it is air being drawn into the engine or a high pressure pulse leaving. That way it gives far more accurate measurement of air mass entering the engine.
Whereas a traditional 'hot wire' can not differentiate where the movement of air mass occured.

But the result at the end of the day is the same. Volts for air mass.
Probably more info than you wanted, but in for a penny and all that.

**nobbyclrk** · 7th October 2011, 05:55 PM

And here's the poor MAF.

Again:
Black - Engine RPM
Blue - MAF output voltage
Green - injector switching
Red - Signal to ignition module from PCM

No rocket scientist required here to see the surging issue. So not only is the RPM unstable, the voltage output from the alternator is making it's self evident on the ignitor pattern. This vehicle had the problem for approx 80,000Km

**clubagreenie** · 7th October 2011, 06:34 PM

Too much info is barely enough. Wish you were around when I was developing the toyo V8's. We fluked out pulse reversion negation, just combined designs from Audi and Ferrari. Found some pics and ran them through a 3D modeling system to estimate the volume and worked from there. Had an even flatter line than that.

**nobbyclrk** · 7th October 2011, 07:58 PM

Too much info is barely enough. Wish you were around when I was developing the toyo V8's. We fluked out pulse reversion negation, just combined designs from Audi and Ferrari. Found some pics and ran them through a 3D modeling system to estimate the volume and worked from there. Had an even flatter line than that.

Thankyou for that.

That would of been way interesting doing that developmental stuff. Just love the way it all comes togeather. Very smart people out there who design and engineer this technology. I guess you would have done some really interesting research.
Do you remember the scope specs from that and how long ago? It would be interesting to see if and how much that has changed over a relativly short time. I'll check out what these are, but its damm fast.

Anyhoo......
I just spent the last of couple hours or so explaining the zoom and went to post. Unfortunatly the internet explorer decided to crash.

. All gone.

Being the worlds slowest typest, I'm going to leave it until after the weekend. But I'm glad you guys appriciate it.

Cheers folks. Have a good weekend.
Andrew

**nobbyclrk** · 11th October 2011, 06:25 PM

Here's the zoomed in portion of the previous screen.

The first thing to notice is the MAF voltage, sitting only 0.18Volts above the previous capture. This is well within specified limits. If this was checked with a multi meter and compared to a voltage table, it would of passed and diagnosis would have gone wrong.
You can immediatly see the result in RPM stability. In the previous screen it was almost perfectly stable, where as here we can see the RPM is varying 127RPM in less than half a second. This is also noticeable by the amount of screen time shown to cover three injector events. Previously it was 0.455 seconds while this time it has taken considerably longer for the same number of events to occur.
In the previous screen we could see one of the complaints, the surging RPM. In this screen the second, blowing black smoke, also becomes obvious. Injector duration (on time) is 5.12mSeconds in the above screen. That is almost three times the amount in the known good example. You can also see the PCM has lost fuel control, as the RPM falls the PCM increases the amount of fuel in an attempt to save from a stall. It will also open up the IAC and advance ignition timing. On some systems the PCM will actually increase the number of injections per cycle.

All this with a variation of only 0.18Volts and it's in spec. Still want to mess with your MAF?

**nobbyclrk** · 11th October 2011, 07:43 PM

At the very start I demonstrated the effect the MAF had on the Oxygen sensor during WOT (when operating correctly, it should go full rich >0.8V) and during cruise/light throttle (the sensor switching between 0.1v and 0.8v showing the PCM is in fuel control and keeping the fuel to air ratio correct).

Here's an image of what a minimal change in MAF voltage can do to the fuel ratio. I have used the faulty MAF for this.

In the first half of the image I have used a resistor to bleed off a minimal part of the signal voltage that is going back to the PCM. Reducing it from 1.37V down to 1.19.
The PCM is fairly happy about this. We can see it add and subtact fuel as the Oxygen sensor switches backwards and forwards between rich and lean. It's in fuel control.

Half way along the screen you can see where the resistor is disconnected and the reported MAF voltage shoots up a whole 0.18Volts. Immediatly you can see the Oxygen sensor climb to in excess of 0.8Volts. The Oxygen sensor is no longer switching. The PCM has now lost fuel control.

There is a couple of other things to see in this image. Back in the first half of the image, it's actually borderline fuel control. You can see when it switchs rich, but it's barely reaching 0.67Volts. It's slightly biased lean.
In the second half although the Oxygen sensor shoots up to close to 0.87volts as the resistor is removed. If you look closely you can see the line slope down. This is the PCM removing fuel (negative fuel trim) in an attempt to get control of the fuel again. It doesn't.

That about concludes it. I hope I have managed to show how sensitive the MAF can be and how much that sensitivity can effect the rest of the system. And with any luck I have shown why you get those performance issue and other strange symptoms and perhaps will you come up with ways of isolating these faults.
If measuring the Oxygen sensor output it is probably safer to use an analogue multimeter or a scope. I've been told digital meters can damage Oxygen sensors.

I do have a couple of other things to chuck up. Mainly for kicks. But some of you may find it interesting. And I have an audience.

I'll try and keep them shorter.

Just to get my head around the MAF....it only works with petrol and does not do anything when running on LPG?

And don't forget diesels. I don't know to much about LPG but I'm fairly sure on injected gas systems the gas computer will use the MAF outputs, although indirectly (as in the gas PCM is using the outputs of the petrol PCM which is based on the MAF).

Andrew

**nobbyclrk** · 12th October 2011, 08:08 PM

Here's a couple just out of interest. Not too exciting. But interesting all the same. Looking at signals from a PCM with a fast scope is very much like slowing down time.
Here's an injector pattern from a petrol injector on a Volvo. This type is very similar to the type found on at least older petrol landrovers and most other vehicles.

Green - Injector volts.
Brown - Injector Amps

The green line is voltage (measuring potential) on the switching side of the injector, the PCM side. This way we can see when the PCM switchs the circuit to ground and allows current to flow (assuming circuit integrity).
The brown line is current. This can be measured anywhere on the circuit as this is measuring the flow of electrons. Preferably you want too measure on part of the circuit that isn't shared by any other componants otherwise the current other componants consume will be added into the equation. (This may be what has happened in the above image, as the base line is sitting at 160mA, although I suspect it is more likely operator error by not zeroing the Amp clamp before making the measurement)

Inside a petrol injector is a pintle (needle), containing Iron, that seals a seat that if open allows fuel to flow under pressure from the injector. This pintle is held in place by a spring that sits on top of the pintle forcing the pintle into the seat. Surrounding this pintle is a winding of very fine wire. When a current is passed through this winding it creates a magnetic field which acts on the pintle trying to pull it into the centre of the magnetic field. To do this it has to over come the spring pressure forcing the injector shut, which takes time (al be it very small). But once it has acheived this the pintle lifts off the seat and fuel will now leave the injector to eventually arrive in the combustion chamber.

So above we can see the supply voltage is sitting at system voltage of about 14volts. Then the line falls straight down as the PCM completes the circuit, it is switched to earth.
At exactly that moment we can see current starting to flow, shown in the brown line, and get up to speed. The current ramp rises ( I shall now avoid the physics here as I'm not 100% sure on it all here) the longer the PCM keeps the injector switched to ground. Then after 2.506 mS we can see the PCM just as quickly turn it off again.
Immediatly after the injector is switched off we can see the voltage suddenly shoots up to over 70Volts before returning back to system voltage. This voltage is actually created/induced in exactly the same way the spark is created in an ignition coil, by the magnetic field collapsing inside the injector.

Ok that's the boring bit over and done with. Now the little interesting bits.

If you look carefully at the little inductive kick, it has a little flat top to it. This is no accident and not necessarily on every injector waveform. It has been 'clipped' by whats known as a zenar diode located within the PCM. So anything over 70Volts goes straight to ground.
Without this diode that inductive kick would climb to well in excess of 100volts.
And another bit of useless information. In electronic diesels (I suspect this will include the TD5) the injectors are run at 80volts not 14V. In short that 80V is created by grabbing lots of those inductive kicks and storing that voltage in at least one (usually two) capacitors. Even storing the kick from the last injector event ready for the next.

One last thing of interest is the actual open time of the injector. As the pintle moves through the magnetic field, it in it's self will influence it. Which in turn effects current flow. As I said before it takes time for the magnetic field to build up enough strengh to overcome the spring and lift the pintle. And just as true is the magnetic field has to reduce enough once switched off that the spring overcomes it and closes the injector once again. In the above images we can see the current flow being altered as the pintle moves up. And in the voltage waveform we can see the change in the voltage curve as the inductive kick collapes. Just incase you can't, I have circled those areas in green.
This gives us the actual true open time of the injector. Which in itself is of no use to anybody. But what is useful is that we can see the injector actually mechanically worked - the valve opened. The main image shows us the PCM activated the injector.
Those little dents/bumps tell us the injector actually opened physically. If the injector was siezed those curves would be nice and smooth.

Anyway, as I said a little boring but hopefully give's an insight into injector operation and diagnosis.

I want to show you guys still hanging in,

an ignition module (from a V8 Land Rover Discovery I) fault giving a long crank/hard start and intermitant cut out issue. Way more interesting as I think most of you have had hard starting problems. Which I'll do next week. Quite a bit to it.

Here's one last image. This is a waveform taken from a TD5. Note the higher current, the supply voltage and the different voltages measured at the same injector. A whole differant set up. Which I'm not going to explain. But looks very differant.

Red - Injector supply voltage
Green - Injector signal voltage
Brown - Injector Amps

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