My 4.2 v8 efi is running too rich??!!

Hi there, my 4.2 v8 efi 80" hybrid is finally running after lots of wiring issues. I now have an engine that seems to be running rich- can I adjust the mixture on an EFI engine?

The engine has got the same running issues it had before I removed it from a 1993 Rangerover LSE. It starts fine but is very "lumpy" on tickover and keeps cutting out. It always starts again straight away but you have to hold the throttle pedal at just above tickover to keep it running. When you rev it up the exhaust gases are a bit black. Basically if it was running on carbs I'd just adjust the mixture, but it's running using the original ECU/EFI system so my question is- Can I adjust the mixture somehow, if I can how do I do this? Or might it be some sort of a sensor not telling the ECU to do the right thing? A friend suggested it might be the fuel pressure regulator. I changed this for a brand new one and it has made no difference.

Any help would be much appreciated, thanks, Dave.

No u can't adjust the mixture, u could on the 3.5 but for a 3.9 and above the engine managment ECU looks after the mixture and tickover. The ECU engine temp sensor located behind the distributor can be faulty and give the symptom, also so can blocked cranckcase breather and pipes or an air filter.
Even the engine managment ECU could be faulty, may be a case for connecting to testbook or similar to see what is happening.

Thanks for the info, I'll look into these tomorrow, Dave.

If you think it is running rich, take plugs out and check colour to confirm.

discool said:

No u can't adjust the mixture, u could on the 3.5 but for a 3.9 and above the engine managment ECU looks after the mixture and tickover. The ECU engine temp sensor located behind the distributor can be faulty and give the symptom, also so can blocked cranckcase breather and pipes or an air filter.
Even the engine managment ECU could be faulty, may be a case for connecting to testbook or similar to see what is happening.

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Yes and no. The ECU does not alter the fuelling in the same way as a more modern system (as I understand it). If you are running rich it could be that there is a fault and the ECU is in limp home mode. Is there an EFI warning light that is coming on?

You can adjust the idle mixture on a hot wire injection system, you have to remove one of the tamper proof plugs (can't remember which one off hand), look on rave/workshop manual.

classic kev said:

Yes and no. The ECU does not alter the fuelling in the same way as a more modern system (as I understand it). If you are running rich it could be that there is a fault and the ECU is in limp home mode. Is there an EFI warning light that is coming on?

You can adjust the idle mixture on a hot wire injection system, you have to remove one of the tamper proof plugs (can't remember which one off hand), look on rave/workshop manual.

Click to expand...

On a 3.5 the idle speed adjuster is on top of the plenum chamber and the idle mixture adjuster is on the airflow meter.
On the 3.9 idle speed adjuster is now used only to adjust the base idle speed and there is no idle mixture adjuster fitted.

I have the same problem on a 3.9 with lucas hotwire injection system, its the stepper motor, a divice at the back of the plenum on the drivers side, it adjusts the air bypass at idle. easy job to replace 5 min job/one beer!. My problem is there like rocking horse shyte over here!!. Hope this helps

Are you using the oxygen sensors in the downpipes that the LSE would have had ? If not this is what is causing the rich mixture.You have 2 choices,first is to change the tune resistor in the efi harness - it about 12" from the ecu plug in the loom.Or you could fit the oxy sensors in the downpipes and connect them up.(Technically the better option.)

Thanks all, I've replaced the coolant sensor that feeds info back to the ecu and this has made no difference to the over rich mixture/idling problems. In answer to your suggestions- no I am not running the oxygen sensors, I've changed the tune resistor for the non cat one. I'll see if I can source a stepper motor and change that next- can't think what else it might be. I'll let you all know if that solves the problem.
Thanks again, Dave.

You may need to adjust the trimming of the Maf now its running open loop,a double,double check for any induction leaks is in order.Dont waste money on a stepper motor,its not going to help.

Go with adjusting CO Trim like eightinavee suggests but also check throttle potentiometer and the adjustment of the throttle disc.

Thanks, I'll check through the induction side for leaks and adjust the MAF- any tips for doing this? Thanks, Dave.

Thanks everyone, it's all sorted now, my Air Flow Meter needed adjusting- I removed the tamper proof plug and adjusted it, but also, my timing was out. I had timed it up using a friend's timing light, turned out it wasn't working properly!!! Another friend leant me his snap on one and the timing was a mile out!! Having adjusted this, it's now running so sweetly. Now it's wire up the lights and get it MOT'd. Thanks so much to all of you for your help and suggestions. Dave.

firstly start with the basics dependinf if or not you have the cats fitted.
1 disconnect the battery for 1/2 hour this will clear any faults you have.
2 running no cats fit a non cat tune resister if you run the lambdas you will cause engine issues of porus block as the lambdas will cause the engine to run lean for economey not good
3 remove the stepper motor and clean it and the housing.
4 replace the coolant injection temp switch pin sensor.
5 remove the afm plug and clean the terminals on the plugs
6 with a multi meter connect to the outer two pins on the afm unit set meter to ohms and you should have a ohms reading of 300 this is 1.5% co at idle turn the screw till you have this reading.
7 reconnect all the leads and bat and start engine and allow to run to temp
8 connect a serperate tacho unit with a brake pipe clamp,clamp the hose to the stepper motor housing and the idle will fall it should fall to 520 rpm +/-20rpm if it is all is ok if not adjsut the idle by the screw on the top of the plenum untill it is right then slowly undo the clamp and see how it idles should be 720rpm +/- 20rpm.
9 recheck the ignition timing at this point and see how it runs if still poor then it will need a ecu read and dynamic test to see what is playing up but this does give you a good base setting to start from