v8 will not make the music 4.2 to 3.9

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craykent

Active Member
Posts
100
Location
abergavenny south wales
Hi well i am a bit slow but two years ago my LSE broke down [ split liner]
I bought a spare car 3.9 . changed the hardware left the software unchanged
now i have tried to get it running and it will not run it fires intermittently
but not pickup on all 8 cylinders . I have checked the timing seems ok checked the plug gaps .fuel is getting up . out of ideas cannot see the wood for the trees :( any help gratefully received .
 
Hopefully someone smarter than me chimes in on this one. I too have a 4.2 LSE and a spare 3.9 lump and have wondered about doing this. I do know that there are differences in the engines' electrickery. For instance, the 4.2 doesn't have a crank position sensor and the 3.9 does.
 
West Slope Rover said:
Hopefully someone smarter than me chimes in on this one. I too have a 4.2 LSE and a spare 3.9 lump and have wondered about doing this. I do know that there are differences in the engines' electrickery. For instance, the 4.2 doesn't have a crank position sensor and the 3.9 does.
Click to expand...

This is not quite right. The 3.9 and 4.2 are mechanically similar, the difference being the stroke and the compression ratio, but neither has a crank position sensor because the ignition system is looked after by the distributor and the injection pulse is triggered by the ignition coil so there is nothing for a crank sensor to do. Nor is there a cam sensor. The only sensors for engine management are throttle pot, afm, coolant temp thermistor, fuel temp thermistor, lambda sensors (if running cats) and the speed transducer.

In terms of the injection and ignition systems they are the same.

To the OP are you saying the engine hasn't run for 2 years? The fact it fires suggests that all the fundamentals are there so you need to clean and fettle the ignition system first then start looking in to the injection side.

Do a search for non-starting, poor idle etc there are loads of posts that will tell you possible causes and their fixes. HTH.
 
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Please excuse my disinformation campaign. I was relaying info I had received from a parts guy on the phone. He said that the 4.2 was the only Rover power plant that operated sans a CPS.:eek:
 
thanks the engine was in a standard range rover [ ex MOD ] i did not buy it running [ mistake ] was assured it ran well .I have been slow due to ill health and a disco that got bought an repaired and is in use . but now i need to get the LSE running and moving cos its a auto and cannot be towed far .
 
craykent said:
thanks the engine was in a standard range rover [ ex MOD ] i did not buy it running [ mistake ] was assured it ran well .I have been slow due to ill health and a disco that got bought an repaired and is in use . but now i need to get the LSE running and moving cos its a auto and cannot be towed far .
Click to expand...

It can be towed: just put the transfer box into Neutral. That way the auto box isn't being turned by the wheels. Alternatively, take the prop shafts off.
 
craykent said:
it will not run it fires intermittently
but not pickup on all 8 cylinders . I have checked the timing seems ok checked the plug gaps .fuel is getting up . out of ideas cannot see the wood for the trees :( any help gratefully received .
Click to expand...

In no particular order:

1: Is there fresh fuel in the tank, is there enough fresh fuel in the tank?
2: Check condition of HT leads, might be better to just get a new set of leads.
3: Check condition of dizzy cap and rotor arm, replace if necessary but please, please, please USE GENUINE PARTS!!!!!!!!
4: Check for air leaks - perished/split vac hoses and main air pipe from AFM to plenum and of course, air leaks around the plenum itself.
5: Make sure the battery is good and strong
6: Check ignition amplifier is switching correctly
 
Have you just taken the 4.2 out and put the 3.9 in?

What about AFM and ECU?

Try using a 3.9 AFM - they are different

ECU fuelling map is also different - I recently checked with Mr Adams of Tornado Systems - because had just put a freshly-rebuild 4.2 into a Challenge racer and he told us we had to. Was great.

Same with Gems and Mems - 4.0 and 4.6 ecus not interchangeable without reprogramming.

And he knows these things.
 
toally second above...once you checked the basics,(plugs, leads.cleaned injectors etc) one MUST use correct ecu etc for your engine....it may run with wrong ecu..but not well, at best worse mpg and power figures..at worse...you may even feck it..different fuel and spark maps etc..
 
well i thought that if i just change hardware it should be ok ! .
I read the book of lies and summised it shoulk work as the block & heads are the same
on the 3.9/4.2 crank & conrods is the main difference .i toyed with changing the internals
but wimped :rolleyes:
 
griffman said:
Have you just taken the 4.2 out and put the 3.9 in?

What about AFM and ECU?

Try using a 3.9 AFM - they are different

ECU fuelling map is also different - I recently checked with Mr Adams of Tornado Systems - because had just put a freshly-rebuild 4.2 into a Challenge racer and he told us we had to. Was great.

Same with Gems and Mems - 4.0 and 4.6 ecus not interchangeable without reprogramming.

And he knows these things.
Click to expand...

That's interesting. As far as I know, the 14CUX doesn't have different mapping between 3.9 and 4.2 except that early 3.9's are non cat so run a different fuel map but all versions (3.5, 3.9 and 4.2) running cats run the same map. The 14 CUX running cats essentially ignores the air flow meter for fuel trim and looks at Lambda until it gets to about 75% throttle then uses air flow only (or something like that)

I did read somewhere that the 3AM and 5AM air flow meters cannot be interchanged but I have also seen something in the dim and distant that said they could so it is a bit up in the air really.

When all is said and done the 3.9 and 4.2 are almost identical engines using the same hardware and the same ECU. That said, from what I read, the 14CUX does tend toward running lean in the mid range so what the Tornado chip does is to alter the fuelling to give a slightly richer mixture that helps the engine breathe better.

It would be different if the 14CUX was also controlling the ignition and used adaptive mapping which "learns" as in the case of the 4.0 and 4.6 where the ECU is most definitely an engine management computer.
 
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the main cause of slipped liners is thought to be lean running to get some mpg and for emmisions..
so to use electronics from a 3.9 to run a 4.2 seems rather silly...only 300cc but as on the edge anyway......
 
classic kev said:
That's interesting. As far as I know, the 14CUX doesn't have different mapping between 3.9 and 4.2 except that early 3.9's are non cat so run a different fuel map but all versions (3.5, 3.9 and 4.2) running cats run the same map. The 14 CUX running cats essentially ignores the air flow meter for fuel trim and looks at Lambda until it gets to about 75% throttle then uses air flow only (or something like that)

I did read somewhere that the 3AM and 5AM air flow meters cannot be interchanged but I have also seen something in the dim and distant that said they could so it is a bit up in the air really.

When all is said and done the 3.9 and 4.2 are almost identical engines using the same hardware and the same ECU. That said, from what I read, the 14CUX does tend toward running lean in the mid range so what the Tornado chip does is to alter the fuelling to give a slightly richer mixture that helps the engine breathe better.

It would be different if the 14CUX was also controlling the ignition and used adaptive mapping which "learns" as in the case of the 4.0 and 4.6 where the ECU is most definitely an engine management computer.
Click to expand...

the fuel maps are different and the AFMs are not interchangeable

I checked with Mark.
 
WOHOOO it lives i finaly got the engine started bit lumpy at first .
blow me down the LPG is working as well .considering its been stood 2-3 years not bad
the 3.9 sounds a bit diffrent a bit roarty if ya know what i mean .
oil and filter change next & fit the rear propshaft :D
 
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