mis fire at 2500 revs Help !

[mike various]

I'm after a few thoughts or idea's about a problem i've got with my car.
Its a 3.9 with hot wire fuel injection system.
The car has an intermittent misfire at approx 2200 / 2500 rpm.
Ive done the following checks:-
It doesn't seem to do it when cold it happens as the car warms up and sometimes it there and sometimes it runs perfect.
Replaced dizzy with known good one no difference,
Replaced coil and put original back on no difference,
changed fuel pressure regulator nothing,
changed dizzy cap / rotor arm no change
changed ecu no difference
new fuel pump, with inline filters no difference

and before zig ask's i've checked the wiring and changed the coolant temperature sensor new one, no difference.
The car supposedly has a mark adams chip in the ecu but it runs very rich you can smell the fuel with it up to temperature.
I didn't check when i had the replacement ecu in the car so i can't comment on the fuel smell.

It's driving me mad and doesn't make the driving experience very nice !!

Any idea's ?

Cheers

MIke

[mrzigazaga]

Not too sure about the Hot-wire....Sounds like something like a sensor failing..ATS...Lambda????

[smash]

The ignition amp is remote on these isn't it? If it is then swap in a known good ignition amp.

Is your vac advance tube clear - can you suck air through it?

[mike various]

Hi i think it's the hot wire it's definitely not the flapper system.
It's the 14 cux system from memory.
No lambda sensor or air temp unless it's in the air flow meter.

Mike

[mike various]

The ignition amp is on the side of the dizzy and i've swapped that with a known good one still misfires.
Will check the vaccum pipe tomorrow.
It definitely seems temperature related

[kev b]

Just a wild guess, try looking under the bonnet at night whilst making it misfire, then look for sparks jumping from the plug leads.

May just be a dodgy HT lead.

[Jack Valiant]

Plug in RoverGauge or a ECUMate and run the diagnostics if it's a 14CUX

Chris

[jimburr]

Dont rule out the dizzy, had a similar thing on the 350SE. did all the same apart from the FPR and pump,changed the cap and rotor and it wouldnt even start(ornerments now).now does it miss under load? and ok when light footed? the amp came on the new secondhand dizzy(rangerover) so not ruling that out. mine runs a little rich and goes like stink now.

[leorest]

Does the hotwire system have a throttle pot? If so that might be worth swapping out for a known good one.

[ElvisWedgely]

I know it's the most obvious, but no one has mentioned the spark plugs. Have you tried changing them to a set of new ones of the correct type?

Tony. TCB.

[mrzigazaga]

How old are the plug leads...And was the king lead changed as well...A lot of the time with our cars the standard plug set does not contain a long enough coil lead and the old one often gets left on...As was the case with mine, And i had a mis-fire at around 2500-3000rpm but wasn't all the time...Sometimes the car would cut out for a second or two as I'm putting me foot down and then would jump start itself again...Changed the plug leads and the coil lead and happy days....

[mike various]

Thanks for the idea's.

There is a lead on electrical harness that allows it to be plugged into the ecu via a lap top but i have no programme data to link it to.

I'll try the leads off my other car i think they are magnecour or something like that.

The plugs were brand new and have done about 300 miles but i'll get another set. What is the correct grade of plug ?

Thx

Mike

[maston]

No one has mentioned the ecu multiplug pin terminals ?
Check that the gap between the pins in the plug is not too big or corroded etc.
Just an idea as I had all sorts of poblems with miss fires etc. and changed this and that (scatter gun approach that we all do) and in the end a new multiplug with new terminals wired in did the track.

[mrzigazaga]

I can't see it being plugs as they will either run lean/Rich or not at all...Never heard of spark plugs causing an intermittent mis-fire?..But could point to coil...Amp..Leads...CTS...Bad earths...Connectors...Even a loose battery terminal...Personally i don't believe in a "Scatter gun" approach as it is hit and miss and doesn't always prove productive...Sometimes new parts can be faulty, especially if they are compromised with cheap parts...In my eyes cheap parts = False economy..

Rover gauge would be a good investment for anyone with a 14cux...

[smash]

Yes scatter gun approach not the best idea - trust me on that one!

[adam quantrill]

mike various said:

and before zig ask's i've checked the wiring and changed the coolant temperature sensor new one, no difference.

Just to be absolutely sure did you check the CTS resistance also from the ECU end? While someone wiggles the loom? Because an intermittent change in resistance here will cause problems.

Also you didn't mention the throttle pot - check this for smooth voltage across the range as you rotate the throttle slowly, no sudden jumps.

[mrzigazaga]

Bobby Shaftoe said:

mike various said:

The car supposedly has a mark adams chip in the ecu but it runs very rich you can smell the fuel with it up to temperature.

MIke

Some of the MA chips are 'best guess' maps by post so it could feasibly be running too rich.

mrzigazaga said:

Never heard of spark plugs causing an intermittent mis-fire?..

They can do, i've worked on a couple of type-R hondas that misfired irregularly below 3000rpm but ran spot on above that. Both were traced to over wide plug gaps and/or weak ignition coil.

Ah of course .."Plug gap"..A lot of the time people do not check the gap of new plugs...Im guilty also in the past of this...I now use the iridium plugs and whenever i buy them and check the gap its spot on...I have noticed with the BPR7 and 5ES that there is an inconsistency in the gapping....Well worth checking as its a simple cheap fix and we all love them...Also the overfuelling could be contributed to by a weak spark not burning the fuel properly......

[ElvisWedgely]

What ever the fault is, it's almost certainly electrical. As most components have already been eliminated, my money is on the spark plugs as mentioned earlier. Spark plugs can break down in performance at high temperatures and give the effect of an intermittent misfire. The ceramics can fail and short out. Now may be a good time to eliminate the plugs by fitting a new set of the correct type and gap.

Tony. TCB.