The V8 Owners Forum -

Evening folks!

I am finishing off a longstanding project, an ex-military Half Ton (Lightweight) Land Rover with a freshly built standard-ish 3.9 14CUX hotwire V8 lump. Thanks to everyone who had contributed to this forum, it’s been a huge help to me.

It has been a fun and satisfying project and my first full engine build, but I am at the ‘tearing my hair out’ stage now!

Basically I am chasing an issue with it starting really easily, running smoothly but cutting out every single time after the first fire - about 3 seconds in! Would hugely appreciate some moral support and technical ideas - hopefully I have come to the right place!

Having checked and replaced everything I can think of (see list below) and explored various theories I’m well and truly stumped…

The really interesting thing is that it will start and continue to run (albeit rougher and very rich) if I pull the connector off the MAF sensor, so I think this really narrows things down.

Initially I thought this was a duff MAF sensor. So I bought a replacement. Still the same issue! The voltage and resistance tests are all in range of what’s considered ‘normal’ online. Also no fault codes on the basic Rover display thingy and I guess the fact that it runs better (in very short bursts!!) with it connected suggests it’s not this.

So I then thought I would replace the ECU trigger wire and resistor between coil negative and ecu, despite it testing fine on the multimeter. No difference. With this wire completely disconnected it doesn’t fire at all, so it’s definitely doing something. Again, conclusion is it’s not this.

Any initial ideas given the above? I don’t know the details of the ‘limp mode’ map and what this is bypassing but clearly in this mode it all works. It’s as if the injectors are not firing after initial cranking and start but in limp mode they are - no idea how to test this.

Probably unrelated but my fuel pump is run off a separate circuit, not the engine loom. Could this cause issues?

Stuff I have checked or changed:

Battery - new and holding charge
ECU - have swapped with another one, no change
MAF sensor - have replaced with new (pattern part), voltages and resistances appear in range, no change
IACV - replaced and housing cleaned, resistance in range
Coolant temp sensor - replaced
Throttle position sensor - voltage and smoothness checked
Earths - engine very well earthed (all new and clean)
Fuel pump & filter - replaced
Vac lines - all new
Fuel - fresh and plenty of it
Dizzy assembly - new
Ignition amplifier - new, remotely mounted
HT leads - new 10mm silicone leads and NGK plugs
Coil - new, tested resistance, all good

Spec: Freshly built top-hatted 3.9, standard spec apart from a mild cam and K&N cone filter. Range Rover classic loom and 14CUX system.

Definitely out of ideas and money now!! Any suggestions greatly welcomed. Cheers.

Just one thing to add - with the MAF connected it can be kept running with some throttle pedal, but it’s a real challenge! Is this just something wildly out of adjustment perhaps??

Is your fuel pump continuing to get power when the engine starts running and is the fuel pressure maintaining?
I don't quite understand how it is working with the fuel pump on a separate circuit, normally the fuel pump is controlled (via a relay) from the ECU so that fuel won't flow if engine is stopped as a safety issue.

The feed from the coil to the ECU is very important, this is the trigger for the ECU to perform a fuelling cycle.

Rovergauge and a cable would be useful to see what the ECU is reading from all the sensors.

Just because a part is 'new' doesn't mean it is good. Infact these days with cheap and (not so) cheerful Chinesium parts you can nearly guarantee they are bad.

Thanks for you advice here, much appreciated!

I came to the same conclusion earlier today and ordered a cable so I can get Rovergauge hooked up as a next step. Will hopefully take some guesswork out of it.

Completely agree with your comment about new parts not always meaning they’re ok - I buy genuine wherever possible, but not much choice with MAF sensors and few other bits these days unfortunately.

Re the fuel pump, yes it continues to run and maintains pressure. It’s currently a separate switch and relay, but I plan to change this over to use the original harness and relay soon.

One more idea after hours of reading online… I found a really useful page here:http://www.g33.co.uk/pages/technical-fu ... ction.html
Especially the bit which states “One side effect of this system is if air leak develops any where in the inlet system, the engine will start and run for 3-4 seconds and then die. The initial over rich mixture will allow the engine to run, but once the air flow volume comes into play, (Now reduced because of the air leak) the injector pulse width is reduced to the point where the is insufficient fuel to keep the engine running, so it dies.”

Vac lines are all good, but maybe I have a poor seal between the plenum and trumpet baseplate? I don’t recall using any sealant… This would fit with it running in the overly rich default map when the MAF is unplugged, but not with it plugged in… Definitely something to investigate at the weekend.

I will report back on here in case it helps others!

Steve R wrote: ↑Tue Nov 11, 2025 10:27 pm
One more idea after hours of reading online… I found a really useful page here:http://www.g33.co.uk/pages/technical-fu ... ction.html

Yeah, that is a useful site. I've used it a lot over the years.
Basically any air which gets into the plenum that does not go through the MAF will not get fuel supplied for it so will cause a lean mixture. The larger the proportion of unmetered air to metered the greater the effect will be.

I've always been a bit confused about the crankcase ventilation system as this seems to give a path for air to flow from one rocker cover, through the engine and into the plenum after the MAF. I can only guess that this flow is kept in check by orifices in the various components and this is taken into account in the tune in the ECU.


IIRC the 14CUX system can detect a disconnected MAF (or a non-viable signal) and uses the TPS signal to guesstimate the airflow or, as you say, it may use the limp map. This may be different if the MAF signal is not available prior to start or if it goes out of spec' while running.

The interface between the plenum and trumpet base and trumpet base and manifold are a bit odd as there is no gasket as standard, I don't know if these are supposed to be assembled dry or with liquid gasket, a product a really don't like. I made up some gaskets on a laser cutter last time I assembled mine which gave me some more confidence.

Coincidently I'm a bit concerned about an air leak on mine and was intending to do a smoke test on the inlet over the winter. When I get around to buying a smoke machine.

Good luck with the RG, it really is a useful tool for working out what the 14CUX is doing.

Thanks - fingers crossed for Rovergauge giving some clues!

Re air leaks, I have found a place doing plenum gaskets, so I will give these a try - I also don’t like using sealant everywhere! https://www.classeparts.com/products/ra ... gasket-set

I also might get myself a few smoke pellets (used for checking chimneys) as a cheap test!!

Shall let you know how it all goes, thanks so much for the advice and suggestions!

Ok, here’s this weekend’s update…

Intake system carefully sealed (manifold to trumpet base, trumpet base to plenum, and IACV housing). All vac lines attached and appear to draw a vacuum (i.e. no split hoses or diaphragms). Throttle butterfly gap set with feeler gauge.

Hooked up Rovergauge - have confirmed no fault codes, temp sensors working, IACV working, throttle pot working and smooth operation, MAF appears to be working as expected (and voltage checks within sensible range). Running Tune 2 (uk no cats) with MAF plugged in and it goes to Tune 0 (default/limp) with MAF unplugged.

End result still the same as last weekend - starts and dies after 3-4 seconds with everything plugged in. No fault codes. With MAF unplugged and in default/limp mode it starts and continues to run, but less smooth (as expected). Data logs all look sensible. So I don’t think it’s anything in the 14CUX system.

So… I think this leaves 3 options beyond the 14CUX system…
1. Still an inlet air leak somewhere.
2. Fuel pressure incorrect (low, but sufficient to run in limp mode due to longer injector pulses?)
3. Spark starting strong but becoming weaker (difficult to explain limp mode behaviour)

To narrow this down my plan is:
1. Smoke test plenum (have ordered smoke pellets)
2. Pressure tests on fuel system (have ordered a testing kit)
3. Further ignition system tests

Initial tests on coil, amp, battery and wiring all looks fine. Fuel pump, filter and pressure regulator all new, but could be duff.

Any other suggestions welcomed!! I won’t let this beat me!

The black wire to the airflow come disconnected?

I only disconnected the MAF sensor to see if it would run in limp/default map. The connections appear to be fine when it’s plugged in. Voltage as expected and I can see it working on the Rovergauge display.

Do you know what tune you are running? RG will give you a tune number & ident, of course that's no guarantee that the chip may not have been altered since.

Fuel pressure would definitely be my next check so it's good you've got that on the go. Is it a standard regulator mounted on the fuel rail or a remote/adjustable one?

Are you running the standard/Lucas injectors? If not what are they?

I'd be interested to know how your smoke test goes, I want to do one on my car and was going to buy a smoke machine but if these smoke pellets do just as well (or well enough) that may be a better route.

Yes, the RG is telling me I’m running Tune 2 (uk no cats), which lines up with the tune resistor fitted.

ECU is standard and not tampered with as far as I can see, but there’s no guarantees.

Fuel regulator is a new Genuine Land Rover part (standard RR part and position). Pump was new earlier this year, but most likely a Chinese made item, hence my suspicions. Injectors are standard Range Rover items but condition unknown. So all pretty standard stuff. Will see what the pressure tests show…

I will let you know how successful the smoke pellets are - might be a disaster but it’s only a few quid and I needed some anyway!

Thanks for confirming that I’m heading in a sensible direction next…

Hi

Do you disconnect the multi plug or also the black wire?

From memory (2006) it goes MAF to distributor and being black just disappears against other wires and looms.

My car did something similar and the black wire had come off

Ian

Ian Anderson wrote: ↑Mon Nov 17, 2025 6:27 am Hi

Do you disconnect the multi plug or also the black wire?

From memory (2006) it goes MAF to distributor and being black just disappears against other wires and looms.

My car did something similar and the black wire had come off

Ian

Now you're worrying me. My MAF (Standard 5AM) only has the multi-plug attached to it no additional black wire, hope I'm not missing something.
Also don't have anything going to distributor. There is a wire going to the coil, is that what you mean? System won't work at all if that's not connected.

Around 8:45 to 9:00 minutes in

Ian

Hmm, I don’t have this black wire. Will be double-checking (thank you!) but haven’t seen anything or spotted it in the wiring diagram…