350i Misfires under light Throttle - Fine under full!!

[keynsham]

I have just returned from the TVR club season opener meet. I did 165 miles to get there with no issue but no motorways. On the way home, I came down on the motorway and discovered a misfire.

The car starts and runs fine and accelerates Ok too. It is when you are cruising and just lightly press the throttle that it hesitates and misfires a little. If I put my foot down, the issue goes away.

I checked AI and it says with great confidence the following:

"The most likely cause on a TVR 350i (Rover V8 + flapper‑EFI) that misfires/hesitates on light throttle but pulls cleanly on heavy throttle is a weak or failing ignition component, especially the ignition amplifier (module) or coil.

That pattern — bad at light load, fine at heavy load — is a classic Rover V8 giveaway.

The ignition amplifier on the side of the distributor is the #1 suspect, followed closely by the coil and HT leads.
Light‑throttle misfire is almost never fuel on a flapper‑EFI Rover V8 — it’s almost always ignition breakdown.


Most likely causes (ranked)

Ignition amplifier (Lucas AB14 module)
Heat‑soaks and fails intermittently
Classic symptom: light‑throttle misfire, OK when floored
Often worse when warm
Cheap to replace or relocate to a heatsink"

Does that sound likely? I am supposed to be off to the Spa Classic in ten days so I need to get it properly sorted out. Any help would be greatly appreciated!

[mrzigazaga]

A multi-meter is your best bet, could be a failed coolant temperature sender putting the fuel mixture out which would cause a mis-fire at low revs, or a dirty or worn part of the track in the AFM..the old SD1 site seems to be down since the passing of its creator, some info on the SD1 forum, i was trying to link it for the data around values but couldnt...being methodical is key, dont change parts unless they really need it.

Good hunting...if i find anything relevant i will post.

Cheers

[Wedg1e]

Last time I had similar symptoms it was a broken wire to the throttle pot., but wear to the carbon track in the pot can cause the same sort of issue.

[BlueWedgy]

mrzigazaga said:

A multi-meter is your best bet, could be a failed coolant temperature sender putting the fuel mixture out which would cause a mis-fire at low revs, or a dirty or worn part of the track in the AFM..the old SD1 site seems to be down since the passing of its creator, some info on the SD1 forum, i was trying to link it for the data around values but couldnt...being methodical is key, dont change parts unless they really need it.

Good hunting...if i find anything relevant i will post.

Cheers

Info can be found here:
https://web.archive.org/web/20251108221319/http://...

[KKson]

Number 1 suspect for me would be the ignition module on the dissy. I've had three fail on me, in 10 years. As others have said, also check wiring to connector to coolant temperature sensor. In the past I've also had the coil break down and also HT King lead break down, where it rested on the rocker cover. Both only misbehaved when warm. Rule is, be VERY systematic when fault finding. Change one component only, and then test it. The coil and HT breakdown were visible in the dark.