3.9efi started to back fire after engine service

A brief history and sorry for the long winded story, but just wanted to inform as much as possible so i can find out what i did wrong

3.9efi non serp, no cats, with mark adams tuned ECU running green resistor.

The motor has been running good, yes has always been running rich

New dizzy rebuilt professionally less that 1000miles, new power amp at least two years old, checked still good spark. HT leads all new and good again about 2000 miles, engine oil and filters every 3000miles+/-

All injectors stripped, new filters and cleaned and bench tested maybe 600 miles ago all giving good spray pattern


So serviced the wagon yesterday as it was due

Oils and filters

Replaced the spark plugs NGKBP6ES 7811 which I always use and set gap 0.8mm

Old plugs removed where all black (but this due to her running rich and has always ran rich been trying to sort this for ever)

Removed K&N filter (it was old and I had no K&N cleaner left to service it) and replaced with standard new air filter

Replaced dizzy cap and rotor arm with a new Lucas

Firing order correct

Set base idle

Checked throttle pot, 0.35 volts

Checked MAF volts 1.5vdc

Checked fuel pressure 38psi (adjustable regulator so can always tweak this)

Removed and cleaned the stepper motor (not dirty when removed and it was replaced with a new one maybe three years back)

Checked the timing 10 degrees before TDC

Changed the engine coolant temp sensor (connected to the correct wiring loom, so fuel and coolant not crossed wired). The reason for changing, the one I fitted some time back I was not confidant it was good quality it was very cheap, the new replacement states its Lucas

Issues

After the service started the yoke and its back firing slightly and the smell of petrol fumes in the garage is worse than it’s ever been.

Test drove and the backfire is when I boot the throttle and she is also down on power.

So again rechecked all of the above, advanced the timing slightly and test drove again, slightly better power but still a slight backfire when accelerating and deaccelerating. Replaced the removed dizzy cap and drove again just the same.

Tested all on the rovergauge computer and no faults, tested the stepper motor with rovergauge opening and closing all ok, fuel pump working, spark at all plugs.


So the question is what did I do wrong as she never ever back fire before?????

Also the new spark plugs (from a reputable dealer and not cheap) look like ****e, the old ones removed are stamped as made in Japan, the new set have what looks like Russian writing on them.


I have taken pictures so can post if required


Any genuine suggestions or advice greatly welcome still working on it this morning so wish me luck


Cheers Ivan

Dodgy plug or plug lead?

kermit_rr said:

Dodgy plug or plug lead?

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Just about to pull the plugs for a look

If all the spark plugs look crappier than usual, Id be tweaking the timing a bit more. Have you tried running it with the air filter removed.

Col

Start by checking the leads are correctly fitted. It's very easy to mix them up.
Then, starting with the rotor arm, remove the new bits & refit the old ones one part at a time & see what happens.
Based on it running fine - albeit you say a bit rich - before you did the work you've either disturbed something or one of the new parts is faulty.

As far as the rotor arm is concerned you MUST press down on the reluctor - the star shaped thing under the rotor arm - as you pull the rotor arm off or there is a very good chance you will disturb/disconnect the auto advance/retard mechanism in the bowl of the dizzy. Quick check is to turn the rotor arm as far as you can with your fingers & then release it. It should spring smartly back to its original position.

Ratae
Must admit had a wee chuckle when I read ur post “check leads are connected correctly”
Was about to pull the plugs to have a look when I seen lead 3 and 5 swapped, school boy error so off to spec savers

all good now no back fire, plenty of power

but still over fueling me thinks but she has been like this since I bought her back in 2014

I was going to ask what emissions figures you were getting at MOT time, but I see you're in NI & I don't know if your test includes emissions?
Are your fuel figures in the ballpark of what everybody else gets?
Does the tuned ecu alter the fuelling?

Ratae said:

I was going to ask what emissions figures you were getting at MOT time, but I see you're in NI & I don't know if your test includes emissions?
Are your fuel figures in the ballpark of what everybody else gets?
Does the tuned ecu alter the fuelling?

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Yes we do emissions as or MOT centers are government controlled centers
They must be ok as I have never had any problems however going for the test I give her a good run just before going into the MOT center

as for miles per gallon I read somewhere I should be getting 20mpg
I know I am not getting that but must check again

These are the plugs removed they look black so I am thinking over fueling?
Only 3000miles +/-

Other than plug deposits, what are you basing your running rich on?
I was curious as to what the emissions figures were.

NGK part finder suggests your plugs should be BPR5ES.
Hotter than 6 & I wonder if that might be a contributory factor to the sooty deposits.
Part Finder - NGK Spark Plugs

I don't know what effect your tuned ecu might have on the fuelling so I think that needs looking into.

Is the vehicle used for short journeys & never really gets up to temperature?

For what it's worth I've never used anything other than Champion plugs.
Yes, I know NGK's are listed for the RV8 but I've always stuck with Champs. & Never had a problem over a 20-year period & four RRC's.
Doesn't explain the apparent over-fuelling though

I have had engines that ONLY run correctly with the right NGK (or whatever) plug - some engines are very specific. My 2003 RS6 V8 - well documented that anything other than the correct NGK plus is asking for trouble - people have tried. Same goes for my old saab. Nevertheless, in general terms, and certainly for cooking motors like the Rover V8, it's like oil. It'll all work and it'll all do the job just fine, but we all have a favourite brand.

Ratae said:

Other than plug deposits, what are you basing your running rich on?
smell of petrol fumes in the garage not a contusive test, black sooty exhaust, on start up she chucks out soot and moisture, then after that nothing

I was curious as to what the emissions figures were
cant remember the figure and cant find the pass sheet, but she pasted on the analyser .

NGK part finder suggests your plugs should be BPR5ES.

spoke Friday with RPI and they assure me i am using the correct spark plugs

Hotter than 6 & I wonder if that might be a contributory factor to the sooty deposits.
Part Finder - NGK Spark Plugs

I don't know what effect your tuned ecu might have on the fuelling so I think that needs looking into
ECU tuned to suit the engine by Mark Adams on a rolling road.

Is the vehicle used for short journeys & never really gets up to temperature?

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5000+/- miles over 6 months (work rotational 28 on 28 off) she gets up to temperature that's for sure

anyway was away today for a long drive at max warp speed and on mountain roads, no other issues fingers crossed

maybe the over fuelling is in my head, need to do an accurate MPG to get true figures again, last time i did a MPG was pre covid driving from my home around the NC500 and back, was getting 17MPH

for now so far so good

FWIW I would say your throttle pot value is a little high. Try backing it off a little so the base value is say 0.27v - 0.30v, check resistance and scaling just to be happy.
What value is your CO trim?
The remapped ECU might also be associated with slightly richer mixture. I vaguely remember reading somewhere the standard 14CUX fuel map has a bit of a hole in it resulting in a lean mixture at cruise; a remapped ECU is likely to increase fuelling to correct the lean spots and improve power delivery (or something like that).
I would also be wary of your "power amp". The ignition system doesn't like "performance" parts, standard ignition amplifier, coil, plugs, rotor arm and cap work just fine. One other thing, you don't mention if you checked your vacuum advance unit; if this isn't working you will always have a compromised ignition system and this can be felt as a slight hesitation when going from idle to WOT but also, when coming off the throttle you might get the occasional stall and unsteady idle because a failed vac unit can introduce a unmetered air although the effect is small and easily missed. Your ignition timing at 10 degrees is well enough advanced, depending on what cam you have and exhaust manifolds you might be able to dial in a bit more advance but ultimately the set up of CO trim and ignition timing is best done with the aid of a gas analyser. Vacuum gauge testing is also a useful indicator of how well your engine is set up.
Hope that is useful but always bear in mind before messing about with it you have a running and driving motor, sometimes it is best to leave it alone until attention is necessary. Enjoy the noise!

Becks Bear said:

maybe the over fuelling is in my head, need to do an accurate MPG to get true figures again, last time i did a MPG was pre covid driving from my home around the NC500 and back, was getting 17MPH

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Full tank to full tank, preferably from the same pump at the service station to make sure the 'pump cut-off' point is the same.
Try to fill at the same rate ie watch the pump display characters, same time in seconds for each litre don't just blast it in.

The last time I did this on a 300mile run several years ago on motorways & A class roads I got 23mpg from my factory standard 3.5efi auto Vogue & this was driving at the legal limit, wherever that was practical.
My average throughout the year (rural & town driving) is 17mpg.

nb. Years ago, on a similar run/condition as above, I got 21mpg from a 3.9 auto.
That was before I f....d it up by converting to LPG

my OCD kicked in again
yesterday changed to my spare non tuned ECU (going to run this for a month and see the difference if any)
rechecked TPS, set voltage 0.3, checked scaling full throttle I get 4.83v and ohms resistance is good
rechecked MAF and reduced the trim voltage to 1.2v (i used to keep this at 1.5v with the tuned ECU) found last years MOT paperwork idle C0 % was 0.450 and HC 779PPM with the tuned ECU, really not sure if this good or not but she pasted the MOT. Will get another gas anyliser test done when I find someone with the correct tester just to compare.
ignition timing reset static as per procedure, all connected and vac hose disconnected advance timing to 9 degrees, also checked that the advance was still working all good
reset the base idle, at 630rpm using rovergauge software, with hoses reconnected now sits at 750rpm, this is the best idle I have ever had
checked all vacuum hoses all good (only replaced last year)
reset fuel pressure regulator 36psi with engine running in idle, removed the vac hose and pressure increased
stepper motor working as it should, tested using rovergauge software opening and closing it works
did about 15 miles as a test, then pulled the plugs and had a look, for sure not sooty, I know they are only new
I honestly don't smell that much petrol fumes in the garage when I start her compared to before.
took her for a 70 mile road trip today and calculated was getting about 16mpg.

As Classic Kev said above now just enjoy the noise, and I say no more f******g tweaking leave it alone and just drive it.

Becks Bear said:

As Classic Kev said above now just enjoy the noise, and I say no more f******g tweaking leave it alone and just drive it.

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