Disco 1 misfire,1995 3.9 V8

discool said:

Some time ago some owners of the 3.5 RR found out that the TPS on their engine wasn’t immune from wearing out, and the 3.5 TPS was obsolete, so they purchased the 3.9 item which was available, of course they has to elongate the two screw holes to enable the item to be adjusted to suit there vehicle. That’s how the modification of the screw holes originally came about.

My 3.5 RR had 110,000 miles on the clock and the TPS was never touched, my disco has done a lot less and so far never been touched, so as the link gives you the voltages to expect so why no test and instead guessing that’s its faulty.

Click to expand...

Not guessing, just time, I’ll get round to checking voltages today and see what’s going on.

TPS voltage is 0.00484 millivolts with ignition on. So 4.84 Volts. Does that sound about right?

I suck bigtime with electrical issues so kind of relying on you guys here.

AFM Reading of 7.89 ohms, no adjustment that I can see on the AFM itself other than a large flat brass acre on the underside which has no affect to the continuity.

Edit: The above reading is with ignition ON.

Is this a duff AFM?

What I found in my Efi troubleshooting notes

To troubleshoot the Throttle Position Sensor, first disconnect system power and then disconnect the EFI Cable Harness from the ECU.
Using an Ohmmeter, verify that resistance between terminals 3 and 25 is between 4000 and 6000 Ohms.
Next, reconnect the EFI Cable Harness to the ECU, and turn the ignition key switch "on".
Take voltmeter readings from pin 20 to ground.
With the sensor in the throttle-closed position, you should read 0.085 to 0.545 volts.
With the sensor in the throttle-open position, you should read 4.2 to 4.9 volts. In between these extremes, turning the throttle position sensor should produce a smooth sweep of voltage readings.

So 4.84 V is about spot on. N.B. 0.00484V = 4.84mV not the other way round.
As above, a 3.9 TPS will fit but you'll need to elongate the holes to get more movement.

capo said:

What I found in my Efi troubleshooting notes

To troubleshoot the Throttle Position Sensor, first disconnect system power and then disconnect the EFI Cable Harness from the ECU.
Using an Ohmmeter, verify that resistance between terminals 3 and 25 is between 4000 and 6000 Ohms.
Next, reconnect the EFI Cable Harness to the ECU, and turn the ignition key switch "on".
Take voltmeter readings from pin 20 to ground.
With the sensor in the throttle-closed position, you should read 0.085 to 0.545 volts.
With the sensor in the throttle-open position, you should read 4.2 to 4.9 volts. In between these extremes, turning the throttle position sensor should produce a smooth sweep of voltage readings.

So 4.84 V is about spot on. N.B. 0.00484V = 4.84mV not the other way round.
As above, a 3.9 TPS will fit but you'll need to elongate the holes to get more movement.

Click to expand...

So with 4.84v it is pretty safe to assume that the TPS isn’t the issue then?

I should also add here that I’ve removed the plenum and cleaned thoroughly, I did do this a couple of weeks ago but only gave it a half hearted attempt. This time around the idle seems to be more stable, sitting just above 750rpm with very little movement on the rpm gauge.

Any idea on the AFM readings?

A cut & past from Rave for the D1 3.9 so leave the TPS alone.

Throttle position sensor
The throttle position sensor is mounted on the side of
the plenum chamber inlet neck and is directly coupled
to the throttle butterfly shaft.
The throttle position sensor is a resistive device
supplied with a voltage from the ECM. Movement of
the accelerator pedal causes the throttle valve to
open, thus rotating the wiper arm within the throttle
position sensor which in turn varies the resistance in
proportion to the valve position. The ECM lengthens
the injector open time when it detects a change in
output voltage (rising) from the throttle position
sensor.
In addition the ECM will weaken the mixture when it
detects the throttle position sensor output voltage is
decreasing under deceleration and will shorten the
length of time the injectors are open.
When the throttle is fully open, the ECM will detect the
corresponding throttle position sensor voltage and will
apply full load enrichment. This is a fixed percentage
and is independent of temperature. Full load
enrichment is also achieved by adjusting the length of
the injector open time.
When the throttle is closed, overrun fuel cut off or idle
speed control may be facilitated dependant on other
inputs to the ECM.
The throttle position sensor is ’self adaptive’, which
means that adjustment is not possible. It also means
the throttle position sensor setting is not lost, for
example, when throttle stop wear occurs.
CAUTION: Do not attempt to adjust throttle
position sensor.




Sent from my iPad on a train.

capo said:

What I found in my Efi troubleshooting notes

To troubleshoot the Throttle Position Sensor, first disconnect system power and then disconnect the EFI Cable Harness from the ECU.
Using an Ohmmeter, verify that resistance between terminals 3 and 25 is between 4000 and 6000 Ohms.
Next, reconnect the EFI Cable Harness to the ECU, and turn the ignition key switch "on".
Take voltmeter readings from pin 20 to ground.
With the sensor in the throttle-closed position, you should read 0.085 to 0.545 volts.
With the sensor in the throttle-open position, you should read 4.2 to 4.9 volts. In between these extremes, turning the throttle position sensor should produce a smooth sweep of voltage readings.

So 4.84 V is about spot on. N.B. 0.00484V = 4.84mV not the other way round.
As above, a 3.9 TPS will fit but you'll need to elongate the holes to get more movement.

Click to expand...

Very confusing, I assume that this information is for the 3.5 RR as it looks familiar.
You say “ first disconnect the system power” assume that’s the vehicles battery, so with out any power how can any voltage readings be taken.
Also a 3.9 TPS will fit of course the vehicles engine is a 3.9 so why do the screw holes have to be elongated?
So errors in your method statement that u may or not wish to rectify.




Sent from my iPad on a train

V8demon said:

Any idea on the AFM readings?

Click to expand...

When I get home I'll dig out the test manual and post it here.
Mines a 3.5 Hotwire from a Disco1.

discool said:

Also a 3.9 TPS will fit of course the vehicles engine is a 3.9 so why do the screw holes have to be elongated?

Click to expand...

Ok it's a 3.9.
The idea of enlarging the holes is to turn the sensor a little to override any worn track which usually occurs at the near off position.

discool said:

You say “ first disconnect the system power” assume that’s the vehicles battery, so with out any power how can any voltage readings be taken.

Click to expand...

Power off for resistance -ohm -measurement, power on for voltage.

Ahh, hang on a moment, I tested my TPS with ignition ON and got the reading above.

Any advice on my AFM results?

These might help; tests 15-16 tps in Check procedures for the 3.9 engine
The base idle setting pdf helped me solve a poor idling issue once, and more detailed info in the 14CUX install and troubleshooting pdfs.
Good luck.

V8demon said:

Ahh, hang on a moment, I tested my TPS with ignition ON and got the reading above.

Click to expand...

Test 1 ign off

test 2 ign on


I also found this in my notes from years ago; I cannot be held responsible for any errors and omissions

A very interesting read Capo, thanks for that buddy. I may go ahead and check the base idle as a matter of course, it hasn’t been tampered with before but I guess over time tolerances change and it may want adjusting.
I’m very tempted to go ahead and replace the AFM, I’m pretty certain it isn’t working as it should, I get the odd occasion where I can smell excess/Unburnt fuel. I’m also not quite convinced that the stepper motor in replaced is working properly either, cold start ups and the idle hunts for a few seconds before stabilising and then gradually picks up to just under 1000rpm. It’s usually okay when warm but the odd occasion it will hunt when warm too.

The road speed sensor section seemed a very likely issue when I read it, that may be the next thing I look at. It cuts out at any temperature, any rpm and any speed but usually when warm (not optimum temp) and between 35 & 50 mph.

This started off with me thinking, ‘ah all it needs is a good service’, now it’s really got me thinking and ‘in deep’! Haha.

Thanks for the input so far folks, very helpful indeed.

If any of you are local to Romsey (Hampshire/Wiltshire border) and fancy casting an eye on it feel free to drop by, the kettle is always on!

After testing voltages yesterday, cleaning the plenum and cleaning the AFM today it has ran as sweet as a nut! Not once has it cut out or idled iratically. Maybe i disturbed a wire at the AFM or TPS, or maybe the third time cleaning the AFM and plenum just did the trick, who knows?!

For now (I won’t hold my breath) it appears to be running well.

I’ll keep you all posted as to how it goes for the rest of the week.

Great news!
V8s are like thoroughbred race horses...brilliant at what they do best but highly strung, skittish and respond well to lots of attention

capo said:

Great news!
V8s are like thoroughbred race horses...brilliant at what they do best but highly strung, skittish and respond well to lots of attention

Click to expand...

I can relate to that very well indeed as I breed thoroughbred race horses for a living haha!

V8demon said:

I can relate to that very well indeed as I breed thoroughbred race horses for a living haha!

Click to expand...

Unsuccessfully it seems otherwise you would own a £110 grand RangeRover, instead of £1 grand with issues Discovery 1.

discool said:

Unsuccessfully it seems otherwise you would own a £110 grand RangeRover, instead of £1 grand with issues Discovery 1.

Click to expand...

Haha, well I don’t own horses, it’s my job, I’m not the gaffer just yet lol.
However I did have a very tidy L322 RR and sold it, repair bills were horrendous and it was in and out of the garage every month with suspension issues. I’d buy Toyota over RR next time I think.

Back again guys, still with issues!

Hasn’t cut out on me so much, only once infact.
I’ve cleaned the AFM and the vacuum advance which was seized solid!
Now I have a slight misfire from idle to 1200rpm and an iratic idle when cold, idle improves when the engine is warm.

My next step is to test the stepper motor that I replaced and check the timing after fitting the new vacuum advance unit.

Also, do UK models have a knock sensor fitted? If yes, where is it located?