Thread: Rover 3.9i Running Rich troubleshooting

One of the most common causes of rich running is that the engine temperature sensor has a high resistance or simply a dirty connector.

The sensor is on the RH corner of the inlet manifold and has an injector plug.
At operating temp it should read 300 Ohms AFAIR.

Sometimes a low temp thermostat at 82C will cause enrichment as the last 5% of enrichment cuts out at 82C. 14CUX should have an 88C thermostat.

The MAF sensor could also be rich and should read about 0.5 volts at idle.
Or all three!

Regards Philip A

Originally Posted by PhilipA

One of the most common causes of rich running is that the engine temperature sensor has a high resistance or simply a dirty connector.

The sensor is on the RH corner of the inlet manifold and has an injector plug.
At operating temp it should read 300 Ohms AFAIR.

Sometimes a low temp thermostat at 82C will cause enrichment as the last 5% of enrichment cuts out at 82C. 14CUX should have an 88C thermostat.

The MAF sensor could also be rich and should read about 0.5 volts at idle.
Or all three!

Regards Philip A

Just discovered one of your previous posts about the operating temp range and your memory is on the money!

http://www.aulro.com/afvb/discovery-...ical-data.html

G'day Drover,


I've a thread on the same issue going in the RRC section of the pinnacles. Some good input from the guys there and I'm working through the suggestions.


Also with the gearbox the ZF 4 speed has a torque converter lock up at just over 80km. But if you get hard gear changes might be a good time to put a service kit through the gearbox. Cheap as chips and easy to do.

Engine temp gauge appears to work fine

The temperature gauge is a very approximate measure of temperature and is not run by the same sensor as advises the ECU. You will see a single contact sensor on the front of the manifold. That is for the gauge.

Does this car have Gas?
If so it is possible for the injectors to seize open if not used enough.
I recall selling a set of injectors to a bloke in Melbourne .

Regards Philip A

As advised to fisherX on the RRC forum...

One further easy thing to check that doesn't seem to have been mentioned yet is the adjustable resistance in the MAF hotwire. With ignition OFF, withdraw multipin plug on side of MAF. Place probes of an ohmeter ($20 digital type will do) one on each of the outermost pins. Resistance from factory setting is around 280 ohms which gives a rich mixture across the range - 4% CO (apparently to help with cold starts in the UK I was once told). The resistance can be adjusted by the Allen head screw on the side of the MAF. Clockwise increases resistance and enriches mixture. Anticlockwise the opposite. In practice 80 ohms gives a good lean mixture, or go a bit more if performance suffers. I've found this settles a lot of tuning/idling issues when all the other checks are ok.

I've found on my '90 3.9 litre that over time (couple of years) the resistance tends to drift off higher again resulting in black soot in exhaust pipe, fumey exhaust smell and heavier consumption. Obviously this setting is independent of all other ECU inputs etc.

While you're at the MAF remove air cleaner and duct forward of the MAF and check out the hotwire. It can get coated with contaminants. Give it a spray with electrical circuit board cleaner to clean it up - helps ensure accurate and quick reading of temp of airflow. Do not touch it with anything solid or brush it.

Good luck.

Thanks guys - hope all had a good Aus day.

Since the last post I've got rovergauge working (finally - lesson learnt - get the genuine FTDI device (i.e. not cheapo chinese knockoff)). No fault codes in the ECU.

Swapped the thermostat - Also spent a fortune on coolant as I didn't clean the surfaces on the thermostat housing as well as I should have...!!!
Now runs at a steady 90/91degC & idles at approx 690/700rpm (which I believe is high - should be closer to 600).

New NGK plugs all round.

Cleaned the MAF with appropriate cleaner - no idea if it was dirty to begin with - regardless, it should be as clean as it can be.

Still to do:
1. Check/tweak the MAF resistance as per aRRon's advice.
2. Check the temperature sensor - although I think this may be redundant if the ECU is reading the correct temperature.

Cheers

What condition are the O2 sensors - ultimately it is these that will tell the ECU how the engine is running mixture wise and the ECU will try to adjust accordingly.

If the o2 sensors are playing up and telling the ECU the engine is lean when it is not the ECU will tell the engine to richen up.

Originally Posted by garrycol

What condition are the O2 sensors - ultimately it is these that will tell the ECU how the engine is running mixture wise and the ECU will try to adjust accordingly.

If the o2 sensors are playing up and telling the ECU the engine is lean when it is not the ECU will tell the engine to richen up.

No oxygen sensors on a 3.9 in Australia.