1990 3.9L Range Rover random cutting out problem

My Range Rover has an odd problem. It always starts ok but it will occasionally (intermittently and randomly) cut out and stumble as if it's running out of gas or has a loose ignition lead. When the problem occurs, it will never actually stall out completely since it will return to idle fine but if I apply any throttle it will begin to stumble, buck and cut out. Sometimes if I floor the throttle through the symptoms, after a little while it will just stop and go back to normal. If I pull over and shut down, it always restarts ok, but the problem continues and then just goes away until the next time! I am not sure what triggers this. I have checked everything visibly nothing seems noticeably apparent. The fuel pump had been replaced recently by the previous owner. I imagine the fuel filter had been replaced at that time as well(?) Does this sound like a fuel issue? Has anyone else experienced this before? Please help! I have a complete Range Rover factory shop manual on CD up for grabs for someone who can help me through with this to get this solved! You can also email me at highresolution@cox.net. THanks!

I don't know if it is the same thing, but two things to check could be the crank shaft position sensor, and the gear-shift position sensor. That's two sensors which can wear out, and start giving symptons like you describe.

Don't know if this helps.

Check idle stepper motor (aka. idle control valve or idle by-pass). Back left side of pulentium (sp ? the air box center of engine). This valve controls idle, gets gummed up, clean with carb cleaner. Also clean base inside hole with Q-Tip.

I always start here if idle is issue.

1991 Range Rover - 173K miles

SORRY, I did read again and this is NOT your problem but you should know this anyhow.

It seems as though you should get "Check Engine" light. If so read out OBD codes with a reader (~$100+).

The odd thing is that it always starts up fine and runs really well 85% of the time...both with engine warm or cold. The stumbling and bucking only comes on randomly both with engine warm or cold and only above an idle. It's almost as if someone randomly turns a switch from running "good" to running "bad!" When I say bad, it will idle fine...but anything more than 1/8 throttle opening will cause it to buck and cut out as if its about to run out of gas. The intermittent randomness of this problem makes me think of a loose connection or something electrical triggering it to act up. What about the fuel pressure regulator?? That sounds like it could be the possible problem. How can that be tested and what happens when they fail? ..Could it fail and possibly allow fuel pressure in the rail to drop intermittently causing my random symptoms? Could it fail and still allow it to run fine 85% of the time? The mystery continues to elude! I'm just going to have to mull over the RR manual on CD again! Also...How would I go about cleaning the plenum chamber? PS. The air idler valve could be an issue with my occassional high idle at startup, but I don't think it would cause my symptoms. THanks for any advice here!

THere's no check engine light or OBD codes stored on the digital readout display under passenger seat either. I heard that when a crank sensors fails, it only results in a no start condition...true? Not sure about gear shift position sensor...where and what is that?

Right, idle contol valve just sets idle, bypasses throttle plate and controlled by ECU.

Cause of high or erractic idle, sporatic engine dying at idle, cold idle problems, and some hard start issues (corrected with tap on the acclerator).

A fault that doesn`t affect idling or starting sounds like an airflow meter fault, because most other faults(fuel blockage, ignition faults) would make idle and starting difficult. However, a faulty ignition amplifier module could possibly do this, as could the coil, but this is unlikely. Cheap to substitute new parts to rule them out though!

Mass Air Flow Sensor, hit the sensor when it happens and see if it clears up.

Hope this helps you out, (Long Story)

My 91 RR 3.9 was having some problems related to a faulty fuel pump relay (this part one is really weird).

I had no power reaching the fuel pump, I traced it to a faulty relay (under the passenger seat) The dummy previous owner had installed one of those home air fresheners driven by the heat provided by a small AA battery, and installed it under the passenger seat. The problem was the freshener vapors were gumming up the relay poles!

While trying to figure out that problem and having the car stranded I decided it was a good time to put a free flow filter I had just bought.

Then when the fuel pump was solved, another problem arise (Part 2).

The engine would start OK cold or hot, would idle very well cold or hot, but whenever the applied throttle was more than some mm of pedal travel it would misfire heavily and could not go more than 20 mph.

After about 2 weeks trying to solve this I found a Morgan Site (has the same engine and EFI) with this info:

http://www.gomog.com/allmorgan/headgaskets.htm#AFM

************************************************** ****
Morgan 3.9 Issue with the AFM

The 3.9 Hotwire Injection System has a tricky little issue more commonly found in its Morgan configuration than others.. It can drive the best batty and force the expenditure of huge sums fruitlessly.
It is described in a careful reading of some of the testing procedures found on this site, the Haynes Manual for the car and the Autodata Mechanic Worksheets.

It is absolutely imperative that the AFM (Air Flow Meter) not be grounded (earthed) to the car. Its bracketry will normally prevent this but its situs on a Morgan can easily circumvent this. If the AFM is grounded, it will give aberant readings and the ECU cannot successfully operate the engine.

Morgan places the AFM next to the heater and its hoses on the bulkhead. The hoses used by Morgan and leading to the heater have a high carbon content and, if they touch the meter, they ground it.

Symptoms can range from the car stalling or not starting or racing at 2000 rpm + continually with or without an ineffective throttle.

If you are in doubt, simple test the AFM with your multimeter to check if grounded. If it is, simply change the hoses for higher quality non-conductive hoses or silicone.
************************************************** ****

I just had to lift the AFM and put a rag under it, take it for a drive and prove myself those Morgan guys were absolutely right!

When installing the Free Flow filter I left the AFM touching the top shock mount and created this mysterious problem.

Hope this helps you as your symptoms look similar...

J.

Sounds like an intermittent earth problem, you need to check that the negative earth right through from the negative terminal of the battery to the engine block, to the fuel pump and all other earth points in your RR. poor earths will cause you all sorts of weird problems and can prove to be difficult to isolate. If nothing else come's out of this exercise you will know your earth system is good. Or instead re-connect your earth points with new braded earth cable. Your mass air flow meter does not like being earthed so check it is still isolated from earth
regards
Alan

I recently had a problem with the power dropping out while driving (dash lights would all go on as though starting) or no starting with turn of ignition key. Jumped it but still no power.

Loose battery wire on starter was problem.