misfiring Classic

I've been having problems with my 3.5efi misfiring, always at low speed. Initially suspecting ignition system problems I've so far replaced the ign. module (now remote via link lead) coil, spark plugs & leads. The replacement dizzy cap & rotor arm (in green Lucas boxes) were of such poor quality that I cleaned & reused my existing 'made in UK' items.
The car runs fine for the majority of the time, even when I floor it through the kick-down, but will occasionally loose power & 'stumble' a low speed, picking up again when throttle is opened. In a worse case scenario, stopping & giving the old girl a good revving will restore all power. Some journeys can be completed without a hint of trouble, even cruising at the legal limit.

2 suggestions have been made by friends, firstly the usage of supermarket fuel could be a contributory factor (I have always pumped E5 protection grade since this E10 rubbish came onto the market) & to that end I put 40 litres of BP Ultimate in the tank yesterday.

The second suggestion was to spray a suitable aerosol cleaner into the AFM with the engine running in order to cure a possible sticking flap when the air flow is low, as it would be at low RPM. To this end I considered removing the air cleaner trumpet & filter & spraying into the empty filter casing.

Any advice or comments would be appreciated guys, many thanks.

I don't know that engine but a sticking butterfly or blocked idle valve / bypass sounds plausible.

Have you checked what's actually happening? Let it idle and misfire for a couple of mins then pull the plugs, are they all the same? How do they look?

Is it actually misfiring or do the revs just dip then pick up again?
Check the wiring connection of the harness at the AFM.

The flapper doesn't have the stepper motor idle valve of the later ones.

kermit_rr said:

Have you checked what's actually happening? Let it idle and misfire for a couple of mins then pull the plugs, are they all the same? How do they look?

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Generally the same appearance.

Ratae said:

Is it actually misfiring or do the revs just dip then pick up again?
Check the wiring connection of the harness at the AFM.

The flapper doesn't have the stepper motor idle valve of the later ones.

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Definitely a miss-fire/loss of power, sometimes accompanied by a slight 'pop' from under the bonnet.

What's got me is that the problem never occurs when the engine is above 1500 RPM, cruising at the legal limit or even through-the-kickdown hard acceleration .. which is surely when both the fuel & ign. system is under the greatest demand

Hoping the better quality fuel will improve the situation, no problems so far but a bit early to tell yet.

norseman said:

Definitely a miss-fire/loss of power, sometimes accompanied by a slight 'pop' from under the bonnet.

What's got me is that the problem never occurs when the engine is above 1500 RPM, cruising at the legal limit or even through-the-kickdown hard acceleration .. which is surely when both the fuel & ign. system is under the greatest demand

Hoping the better quality fuel will improve the situation, no problems so far but a bit early to tell yet.

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Weak mixture flashback?
Unmetered air entering the system after the airflow meter?

You may have a leaking injector, running it rich at low speed, but not causing an issue higher up. I had that on my 3.9

It had lumpy idle issues, but also had a hell of a time getting any useful idle after a hot restart, like if sat for 5 mins then restarted.

Thanks for the replies Ratae & Allan W
I will investigate further bearing your pointers in mind.