1995 Disco 1 3.9 Injection cut-off / Hesitation

Hi Guys,

After looking through many posts in this forum I decided to join to ask all the experts to solve a mystery I have with my 1995 - Land Rover Discovery 3.9 v8 14CUX. Manual Transmission non-cat.
I am a land rover enthusiast that is mechanically inclined and about a year ago I bought in Colombia South America a very clean Disco 1 with 100k that I planned on fixing up and possibly taking back with me to the US.

Here is a bit of background and the things that I have tried to remediate to solve the current issue.
The car last year blew the right passenger side head gasket where only one side was replaced. After the repair, the car started with some hesitation on heavy acceleration and that felt like the ECU was cutting injection causing the car to jerk where acceleration was gone, the odd thing here is it was happening once the disco reached normal operating temperature. This is driving me nuts !! Here are the most recent items replaced:

1. AICV - (Air Idle Control Valve).
2. TPS - New Brit-Part version. (Unfortunately on the new one I had to replace connector as mechanic (idiot) pulled connector and internal cables disconnected).
3. Plugs and Ignition cables (Magnecore 8.5 mm).
4. Replaced remote amp - pertronix. (Have spare Coils put back the original Bosch as Lucas one feels cheap).
5. Fuel Pressure Regulator.
6. Injectors cleaned.
7. Changed several female connectors (Fuel Pressure and Temperature).
8. New Bosch Battery.
9. ABS speed sensors (ALL).

I even ordered from the UK the 14CUX cable to review several metrics that I have researched online, such as MAF and TPS current settings. Some of my current issues, when I bought the car the previous owner put in a Brit Part MAF, and still provided the original the funny thing here is the Brit-Part MAF does not adjust the volts as it is pegged at 2.5 and the old one I read somewhere in this forum that the non-cat should be set between 1.0 and 1.5 the old allows adjustment.

Also when I look at the error codes I have the following turned on (red):

(17) Throttle Pot
(19) Throttle Pot lo MAF hi (Not clear what this one means?)

I reset these errors and the come back after a rescan.

Another issue I have is, the oil temperature sensor has no lead cable, haven't been able to find it at the moment, I can really use some direction as to what color or what part of the loom this comes out of to hunt this connection. Wondering if this could be the cause of hesitation or could it be timing a worn cam or loose worn timing chain, not allowing timing correct setting, suspecting also a bad fuel pump or fuel rail pressure sensor?

I've also read that the car's immobilizer can cause some electrical gremlins, this disco has an aftermarket alarm which I hate...

Along these lines I am also trying to run the ABS blink procedure without much luck finding K103 the ABS dash light relay to bridge the connector and put it in diagnostics mode. As I replaced all the speed sensors.

Please help a new member out, as I am about to either run this truck off a cliff or spend some more money on a top end rebuild.

I can provide logs from rover gauge, and pictures if need be the case to help diagnose these electrical gremlins.

Looking forward to solve this with your help !

The ECU defaults to 2.5V for the CO trim, if the MAF is giving no readings. The Britpart MAF is probably junk (seems to be par for the course, with Britpart!).

If you unplug the MAF, you'll get the same reading.

Plug in the OLD MAF, that *might* still work. Put a digital multimeter set to DC volts in the front-most wired pins on the MAF (pins 2 and 3 - 1 is empty). You should get about 1.7V at idle and hopefully about 0.35V after turning on the ignition but not starting. It should initially shoot up and quickly settle, if it's OK and your meter has good resolution. If shouldn't linger high for more than a second or so.

You can try the same on the Britpart one, to compare. If they are OK, you can fit an external CO trim pot, as it doesn't do anything IN the MAF, just sends the CO trim signal to the ECU.... but I'd start with the original, for preference.

In Rovergauge, you should be idling on the second (mostly) and third row down on the fuel map table at idle, maybe more second on a manual trans. Maybe somewhere at about 30% or a bit lower on MAF in Rovergauge, set to direct. Under heavy acceleration, it should move down to the bottom row of the fuelling and the MAF reading should climb dramatically and get close to max near full load at 5000+ rpm.

If you leave the multimeter in the MAF plug, it should get up into the mid to high 4 volt range under load, 4.6+

The TPS readings in rovergauge should be smooth, from near 0 to near 100.

Have you driven it with the MAF disconnected? It will read 2.5V for CO trim and give you the errors you list, plus the "MAF" fault code...

DiscoHotMess said:

car last year blew the right passenger side head gasket where only one side was replaced.

Click to expand...

Did you skim the head?...if you did you must do both not just one...

AllanWorms said:

The ECU defaults to 2.5V for the CO trim, if the MAF is giving no readings. The Britpart MAF is probably junk (seems to be par for the course, with Britpart!).

If you unplug the MAF, you'll get the same reading.

Plug in the OLD MAF, that *might* still work. Put a digital multimeter set to DC volts in the front-most wired pins on the MAF (pins 2 and 3 - 1 is empty). You should get about 1.7V at idle and hopefully about 0.35V after turning on the ignition but not starting. It should initially shoot up and quickly settle, if it's OK and your meter has good resolution. If shouldn't linger high for more than a second or so.

You can try the same on the Britpart one, to compare. If they are OK, you can fit an external CO trim pot, as it doesn't do anything IN the MAF, just sends the CO trim signal to the ECU.... but I'd start with the original, for preference.

In Rovergauge, you should be idling on the second (mostly) and third row down on the fuel map table at idle, maybe more second on a manual trans. Maybe somewhere at about 30% or a bit lower on MAF in Rovergauge, set to direct. Under heavy acceleration, it should move down to the bottom row of the fuelling and the MAF reading should climb dramatically and get close to max near full load at 5000+ rpm.

If you leave the multimeter in the MAF plug, it should get up into the mid to high 4 volt range under load, 4.6+

The TPS readings in rovergauge should be smooth, from near 0 to near 100.

Have you driven it with the MAF disconnected? It will read 2.5V for CO trim and give you the errors you list, plus the "MAF" fault code...

Click to expand...

Hi Allan thanks for the detailed response. I had a chance today to switch the old MAF and set it to 1.7 Volts,
the MAF on direct setting using the Rover Gauge was spot on 30% at idle. I decided to take a chance and take her for a spin, when the car is reaching temperature, all numbers seem ok even Idle RPM was green at times.
When she reached 160 degrees all hell broke loose, MAF reading jumped to over 75%. It seems like the car loses its parameters when it warms up and almost attempts to stall, the EFI MAP dropped to the bottom very rich, and heavy fuel smell at idle and was running rough. I went ahead and disconnected the MAF and drove her home, almost in limp mode I suspect.

I am starting to think that the old MAF is bad as well? Can you suggest a good brand to replace with?

Also thinking It may have been repaired incorrectly as the right head gasket was replaced only, and feel like that can also be contributing to the hesitation when the gasket reaches temperature and loses pressure? Also, old head bolts were re-used, heard these stretch with time.

The devil side of me says, rebuild top end both sides, including cam. The other side says it's the MAF, anyone experience similar issues?

Shippers said:

Did you skim the head?...if you did you must do both not just one...

Click to expand...

I believe the prior owner skimmed one side only.

DiscoHotMess said:

I believe the prior owner skimmed one side only.

Click to expand...

You cant skim one side only as the inlet manifold wont seat properly and the manifold gasket will leak...the fact it goes haywire when it gets to temp indicates that...I snapped a sparkplug on my 101 V8 and had to take the RH head off specialist removed plug but said dont need to skim head as it wasnt a gasket problem so put it all back together no problem...skim head and it changes compression value...youtube Barum Engines is worth a watch on engine tolerences...

DiscoHotMess said:

Hi Allan thanks for the detailed response. I had a chance today to switch the old MAF and set it to 1.7 Volts,

Click to expand...

This bit confuses people. I wasnt talking about the CO trim, thats the one you set with the pot.

If you are measuring the FRONT two pins, you are measuring the airflow signal, which you CANNOT adjust - its just what the Flow Meter puts out. and it should rise directly with increased airflow.

If it's jumping around, like into the 70's, then it's probably faulty. It's what mine did. It fluctuated between 3 and 70+ at idle and run like crap.

I'm not aware of any trustworthy NEW replacements, so I got a good used one.

FWIW I would have a close look at the O2 sensors. You mention Rovergauge, have a look at the fuel trims.

The thing is you are saying it gets all out of shape when it gets up to temperature. A 1995 car should have lambda probes but the ECU ignores them during warm up. Once warm the ecu looks at the O2 sensors to correct the fuelling. If the probes are failing then the fuelling will be all over the place at idle and cruise. The ECU ignores Lambda at WOT. What resistor is in place? If the loom has a catalyst tune resistor then the ECU is looking for the lambda feedback. If no resistor is in place I believe the default for the ECU is a catalyst map.

The other option related to temperature is the ignition amplifier. If it is failing you will see it most when the engine is up to temperature. Remember that the fuel injection pulse is triggered by the ignition; the ecu looks for the run signal off the coil. If the ignition signal is failing then the ecu won't fire the injectors. This is why when looking at the hotwire injection system it is important to understand if the issue is an injection misfire or an ignition misfire. If you search the V8 Engines section, there is lots of material covering how to test and set things up. HTH.