V8 3.9 EFI sort of mis fire

After a full upgrade and service on my V8 3.9EFI electrical system which was completed yesterday I took it for a test drive, everything was working fine except when I press the accelerator hard the engine lets say at about 50mph it appears to be running on 7 cylinders, no mis fire just loss of power/shudders until I back off on the throttle.

If I press the accelerator slowly, I can get to above 70mph no problems.

The engine was running ok before I upgraded the system, had a valley gasket leak, yes running rich, yes sometimes hard to start cranking over a long time, so hence the upgrades and service, but once started ran like a dream.

Engine oil and filters changed every 3000miles

New valley gasket (Just for info)

New steering pump (as I had the front stripped)

New fuel pressure unit and all set to the correct PSI

New in tank fuel pump, all new filters on the fuel system

New distributor, complete with new cap and rotor from RPI

New spark plugs NGK BPR6ES

New RPI amp, plus Bosch coil

New temp coolant sensor

New magncour leads

Timing set at 6degrees BTDC, and set up with a timing light, with vacuum hose disconnected in fact set up several times as I wasn’t happy with the first set up.

Base idle set with vacuum hoses blocked, 570rpm on the Rovergauge on the laptop, when vacuum hoses connected RPM is then sitting at 770rpm which sounds nice

No fault codes on the Rovergauge software when driving.

MAF out put voltages are ok as per diagnostic info found on this web site, and set at 1.5v due to the Adams ECU remapped.

TPS output voltage was high so I set the voltage as per the instructions, THIS was done last month before I changed all the parts. I plan to test voltage and adjust tomorrow but not sure if this is the fault?

After reading the long story above can anyone shed some light on my next course of action?

many many thanks in advance Ives

I’ve never had any issues with my 3.9 engine although just plugs and leads are the only ignition replacements in 22 years, but I’ve an idea where I would start fault finding if I had to.

So as your issue is associated with the throttle when accelerating, why not look at the throttle position sensor again.
The the sensor fitted to the 3.9 isn’t adjustable, the EFI ECU looks after the voltages so I’m wondering what modifications u have done. Its the 3.5 item which is adjustable.
Although perhaps it’s course and affect... the issue is the affect and course is an engine modification/ iffy or incorrect replacement part, such as the dizzy/cap.

discool

the TPS on my yoke has two screws when slackened i can adjust the voltage output, going to have play with the output voltages tomorrow, just seems strange that with the old distributor even with the bob weight problems it was ok

If it was ok with the old dizzy, put that back in and take it from there? Check timing again too. Is the tdc mark really tdc?

"Turn over the engine by hand with the dizzy cap until the rotor arm is pointing at roughly the 5'oclock position. That's roughly where the arm should point at tdc for number 1 cylinder. Then move the dizzy body until your ht lead running to the number 1 plug is lined up with the rotor then roughly tighten the dizzy bolt and give it a try starting while moving the dizzy slowly abit."

above is info I found on this site about rotor position

yesterday when i set the wagon to TDC before removing the old distributor my rotor was pointing to the 7 oclock position +/- removed old distributor inserted new distributor and the rotor was pointing to about the 10 oclock position, fitted the leads to suit the position of the rotor so No1 lead was near the rotor, all other leads fitted as per the firing order, as said fired up right ways set timing with strop light with vacuum disconnected, revved the engine and could see the timing advance. Then on the road test run all ok until I changed down a gear and dropped the throttle?? appeared to be off a cylinder, gently pressing the throttle (hard to do on a V8) got up to speed no problem

have i set the timing wrong???

kermit_rr said:

If it was ok with the old dizzy, put that back in and take it from there? Check timing again too. Is the tdc mark really tdc?

Click to expand...

Kermit

old dizzy had the damaged bob weights and springs

will double check the timing, yes TDC on the wheel was actually TDC with the rocket cover removed, although my wagon before ran at 6 degrees BTDC

Becks Bear said:

discool

the TPS on my yoke has two screws when slackened i can adjust the voltage output, going to have play with the output voltages tomorrow, just seems strange that with the old distributor even with the bob weight problems it was ok

Click to expand...

The TPS for RR (flapper 3.5) and the early RR (hot wire 3.9) were adjustable, on later and also the 3.9 disco 1 it wasn't.
Voltage for the adjustable TPS is: 3.5, 290mv + - 20 millivolts and the 3.9, 325mv + - 35 millivolts, that's according to my RR workshop manual.
I believe the later non adjustable TPS replacements are difficult to find now.

I do wonder whether the issue is the TPS.
If you accelerate slowly & all is fine but if you put your foot down it doesn't do anything then I'm inclined to think the ECU isn't getting a 'foot down' signal fromthe TPS which should cause the injectors to pulse & provide the extra fuel & consequently you have a weak mix.
Going on from discool's post above I believe replacement TPS's are unobtainable. So your choice is either one of the workarounds on various websites or an alternative ready made item such as: https://www.actproducts.co.uk/produ...ing_wp_cron=1574420351.4062089920043945312500

Ratae said:

I do wonder whether the issue is the TPS.
If you accelerate slowly & all is fine but if you put your foot down it doesn't do anything then I'm inclined to think the ECU isn't getting a 'foot down' signal fromthe TPS which should cause the injectors to pulse & provide the extra fuel & consequently you have a weak mix.
Going on from discool's post above I believe replacement TPS's are unobtainable. So your choice is either one of the workarounds on various websites or an alternative ready made item such as: https://www.actproducts.co.uk/produ...ing_wp_cron=1574420351.4062089920043945312500

Click to expand...

Ratae
thanks for the link

Looking at my notes when i tested the TPS before i adjusted it i had 0.849v and i adjusted to 0.354v

will adjust it back, with the new dizzy and see what happens when i put my foot down

wee update

today removed the rocker cover and No1 plug rechecked timing marks TDC ok same as I had the last three times.

removed the new dizzy and repositioned so the rotor arm was at the 5 oclock position.

with dizzy out checked vacuum on the advance working as expected (new dizzy)

checked plugs and leads all ok, should be only a few hundred miles on them

reset base idle, rechecked the fuel pressure regulator (adjustable unit) set to 40psi with ignition on, drops to 34psi with engine running, stay at 34psi for a long time after the engine is stopped

checked output on TPS which was a bit low 0.189v?? I had set it before to 0.354v, set back to above 0.50v

checked all ignition wires to coils etc and renewed and crimped what was needed.

set timing to 9 degrees BTDC

test run and drove her hard, the misfire or hesitating on full throttle not as bad as the other day, 99% better but as I have OCD I need to get her to 100% but that’s for another day

fitting a new LOF powerspec clutch and master cylinder tomorrow, oh the joys of owning a landrover

thanks for everybody’s input

Interesting. My gut reaction would have been to suggest inlet manifold leak but if you have a new one, maybe not. Have you run a compression test? I think I would want to check all is good on cylinder compressions.
If you have one, try doing a vacuum gauge test, sometimes a vacuum gauge is a very useful diagnostic tool.
Ignition system, I think just look at ignition amplifier - if the misfire is noticeable when engine hot suspect ignition amp. BTW your TPS voltage is too high, this is likely to affect your idle control as the ecu will not see the correct voltage and stabilise the idle speed by adjusting the stepper.

now running like a dream 100%, fires up on the first tuen of the key
set TPS back to 0.315v
serviced and rebuilt my old lucas dizzy and installed
re-set timing to 10 degress BTDC
re-set base idle

happy days until the next issue

Glad to hear you got sorted!

Interesting read - how do you alter the voltage of TPS?

JJKM said:

Interesting read - how do you alter the voltage of TPS?

Click to expand...

Undo the the two screws slightly and turn the TPS to the voltage required then tighten the screws. Simples

@discool that is what i did, adjusted to get the correct voltage as you said simples