Classic Misfiring at 2-3000rpm

ukadamwest · Jan 23, 2017

Doo said:
If you've definitely ruled out all the electrical possibilities then next it has to be valves....

Measure twice, cut once. MAKE SURE the coils, leads & plugs are perfect, and if distributor, electronic or points are fine, check capacitor and anything else that may cause this electrically.

Failing that, I would be checking the camshaft, valves, exhaust manifold (for blockage) and any O2 sensors which may have have a bearing on the firing & burning of fuel.

camshaft, valves were all done 500 miles ago, exhaust manifold blockage definitively not... i cant see how anything could have blocked it. it ran absolutely spot on one day, the next not.

it starts on the button idles happily, its only when it gets above 2,000 rpm it starts misfiring.

ukadamwest · Jan 23, 2017

Pete1987RRC and Bike Teacher Dave may take the crown here..

pete1987RRC said:
My 87 efi had a similar issue which was solved by an ecu swap

Courtesy of Bike Teacher Dave .. he sent me this link http://www.carelect.demon.co.uk/rr396.html

RANGE ROVER/DISCOVERY WITH 3.5/3.9 LITRE ENGINE
(CAT & NON CAT) and HOT WIRE AIRMASS SYSTEM (from 1990)
- Possible ECU/airmass meter failure symptoms

The Lucas 14CU 'hot wire' airmass system ECU has proved to be more reliable than the earlier 'moving flap' airflow system 4CU ECU. Please click here for ECU photo. A common ECU problem we see is loss of 1 (or both) injector banks (usually following work in engine compartment) This failure is relatively easy to diagnose (check for injector pulses).

Early 3.9 ECU type number PRC7081 (superceded by PRC9061) develops a misfire at 2,800rpm (most noticeable at motorway speeds when overtaking). We now have a fix for this.

14CU ECUs as a group can suffer from intermittent running problems - such as running rich or cutting out for short periods - which are very difficult to diagnose so when there are vehicle problems we STRONGLY recommend road testing with a substitute before condemning your ECU.

Air mass meters are also causing running problems as vehicles age. Again, because diagnosis is so difficult, we STRONGLY recommend road testing road with a substitute before condemning air mass meter.

Doo · Jan 23, 2017

I wasn't suggesting your's had sucked in the filter.. I think you'd have noticed

So it's electrics or airflow then. Gotta be one or the other. The only experience I have of dodgy ECU's are if they got wet or fried.

Looks like you're onto a winner though

ukadamwest · Jan 23, 2017

yes that's what BTD and I arrived at.. electrics or airflow..

i got a new sh*tpart MAF through today.. put it on and it ran like a box of hammers.. wouldn't even idle which it does with the OEM Lucas MAF.

So that determines two things.

1. Sh*tpart QC is poor and they don't manufacture decent MAF's
2. If the sh*tpart MAF makes it run badly, then that would determine that the OEM MAF is good (or better) which then leads to electrics.

spark is good and strong timing is spot bollock on, we can remove ignition from the equation.. which then i suspect leads to the Fuel ECU.. or the OEM Maf has failed in the range where it meters more air...

i have the ECU refurber and a MAF refurber... i guess i'll be on the phone tomorrow. whats the betting that both say it'll be the part they refurb

hmmmmmmmmm

Doo · Jan 23, 2017

ukadamwest said:
yes that's what BTD and I arrived at.. electrics or airflow..

i got a new sh*tpart MAF through today.. put it on and it ran like a box of hammers.. wouldn't even idle which it does with the OEM Lucas MAF.

So that determines two things.

1. Sh*tpart QC is poor and they don't manufacture decent MAF's
2. If the sh*tpart MAF makes it run badly, then that would determine that the OEM MAF is good (or better) which then leads to electrics.

spark is good and strong timing is spot bollock on, we can remove ignition from the equation.. which then i suspect leads to the Fuel ECU.. or the OEM Maf has failed in the range where it meters more air...

i have the ECU refurber and a MAF refurber... i guess i'll be on the phone tomorrow. whats the betting that both say it'll be the part they refurb

hmmmmmmmmm

So if it is over fueling, it's either metering or air flow restricted. If you have ruled out all else, then what's left has to be the cause. I hope you get to the bottom of it, it's been a head scratcher, but I wasn't out of the ball park so I'm pleased with meself

ukadamwest · Jan 23, 2017

ok Doo you can share the crown too

myfirstl322 · Jan 23, 2017

So it's all fixed bar actually fixing it,
I hope laurels are comfier nowadays....

Doo · Jan 23, 2017

ukadamwest said:
ok Doo you can share the crown too

Nah, yer man BTD did all the hard digging. I was only suggesting the (to me) obvious.

Good call though. Buy that man a pint if he's on the ball

ukadamwest · Jan 24, 2017

Just checked the Fuel ECU it's PRC7081

ukadamwest · Jan 24, 2017

BTD, Pete, Doo.. going to have to take the crown back..

Took the seat out swapped over the fuel ECU.. no change.

Tried both ECUs with the OEM MAF and the Sh*tpart with consistent results. Sh*itpart MAF runs terribly on both OEM MAF and a Spare OEM MAF runs the same with both fuel ECUs.

I spoke to the company that overhauls the MAF and was advised that the fuel ECU needs a good signal from the lambda sensors. There us no failsafe on these so a bad O2 sensor will fail open with a rich mixture.. unplugging will make no difference to diagnosis.

Which leads back to my previous thought that the O2 sensors are fouled.

Doo · Jan 24, 2017

ukadamwest said:
BTD, Pete, Doo.. going to have to take the crown back..

Took the seat out swapped over the fuel ECU.. no change.

Tried both ECUs with the OEM MAF and the Sh*tpart with consistent results. Sh*itpart MAF runs terribly on both OEM MAF and a Spare OEM MAF runs the same with both fuel ECUs.

I spoke to the company that overhauls the MAF and was advised that the fuel ECU needs a good signal from the lambda sensors. There us no failsafe on these so a bad O2 sensor will fail open with a rich mixture.. unplugging will make no difference to diagnosis.

Which leads back to my previous thought that the O2 sensors are fouled.

Did not I suggest that back a bit? That would be a big def in the rich running saga, but I understood you had ruled that out...

*places crown back on head*

ukadamwest · Jan 24, 2017

so.. ordered up 2 new O2 sensors and a full set of temp sensors (3) to keep BTD happy

the only pain is the driver side O2 is a bitch to get to as its up on top of the gearbox

Doo · Jan 24, 2017

When I bought George, I replaced pretty much everything! After all, she sat on her arse for 5 years! I've been sat on mine 3 months and I need new legs, feet, arse, willy, ribs....

Doo · Jan 24, 2017

Anyway, the point of my post there was to say,, I removed the pipework so we could get the sensors out and it was a mare! Heat, lots of heat and re-threading. Saved time in the end...

ukadamwest · Jan 24, 2017

I did the sensors when the engine came apart . That's the thing it's been running spit on since then had a misfire one evening and it's been misfiring since.

Doo · Jan 24, 2017

I plumped for gen yoo ine spensive Botchy ones. Did you go for genuine? It's entirely possible a wire was damaged or the sensor itself

ukadamwest · Jan 24, 2017

No wires damaged.. you have seen my engine bay ?

Doo · Jan 24, 2017

Clean!

Twin batteries too

Anyway, it's easy done.... Damage to a wire.

However, it could be the sensors themselves, it happens

ukadamwest · Jan 24, 2017

indeed. well.. i've replaced dizzy cap, HT leads, plugs, rotor arm, fuel pump.. swapped out MAF's and ECU's with no change. new O2 sensors are on the way and to be honest the Bearmach temp sensors are like £4 each give or take, so £12 for the 3 is worthwhile just to rule them out. i have a feeling the O2 sensors may be the issue. purely because one is a right ***ker to reach. so.. being a Range Rover.. of course its going to be that

Doo · Jan 24, 2017

ukadamwest said:
indeed. well.. i've replaced dizzy cap, HT leads, plugs, rotor arm, fuel pump.. swapped out MAF's and ECU's with no change. new O2 sensors are on the way and to be honest the Bearmach temp sensors are like £4 each give or take, so £12 for the 3 is worthwhile just to rule them out. i have a feeling the O2 sensors may be the issue. purely because one is a right ***ker to reach. so.. being a Range Rover.. of course its going to be that

Par for the course mate! I laugh when I hear people say, "It's always the last one you try...!"

WELL, THAT'S BECAUSE YOU FOUND THE FECKIN FAULT!

Course you wouldn't go any further

Classic Misfiring at 2-3000rpm

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