Help with slight misfire !

[TVRSJW]

Ok car ran great (new plugs and lead etc last year and done about 6-7K ) no shunting and very smooth for last 6-7K miles , then last night went for a run out and on very light throttle in any gear it “flutters / hesitates but it’s very very slight , needle on rev counter does not even move !! as soon as you accelerate it’s fine and pulls well , but.....it pops and bangs a lot more now On decel but is fully decatted) But more than normal

So today done following

Plugged into rover gauge no fault codes and all looked ok (to me anyway don’t know how to read the the trims etc )

Cleaned stepper motor
New plugs BPR6ES checked gap
New leads
New quality Doctor distributor Rotor arm (old one the brass arm was loose )
Cap

And general check on everything pipes etc

Test drive after each alteration end result is it feels a bit better but still not as smooth as it was !?!

(Always fill same garage with super unleaded so no change there either )

Any ideas / help ? (Pic of Rover gauge screen when it was at operating temp if it helps ?)

[Loubaruch]

How old is the petrol in the tank?

I used to have the same symptoms in a TR3 when it had not been used for a few months, a refill always cured it.

[indigochim]

Do you have plug extenders fitted? These where the cause of misfire on mine when I bought it. I could see it arcing to the block when it was dark.

[TVRSJW]

No I use a tanks worth most weeks so very fresh !

No extenders were junked last year when I changed the plugs back then and it ran very smooth till now !

[pete275]

Could be the ignition amp module on its way out, mine started with this 2 years ago.

[Belle427]

I’d check the dizzy cap fitment too, the aftermarket stuff is complete junk.

[QBee]

Ignition amp, I agree is a strong possibility, and not an expensive swap.
It's a shame you swapped so many other components, as that makes it harder to decide what is wrong, with so many fake new parts out there.

But that sounds to me like a slight misfire.

Easy to test.
Get a nice long cable tie.
Bonnet up.
Start the engine and let it idle for 30 seconds to a minute (long enough to warm up and not so long that it is full of heat).
Quickly go round the engine, touching the end of the cable tie lightly onto all eight exhaust manifold pipes in turn, one inch from the cylinder head.
The misfire will be the one that doesn't melt the cable tie.
Alternatively, if you have a £10 infra red thermometer that will give you accurate readings.

[TVRSJW]

Get thanks

Yes I test drive after each component fitted so I would know what was at fault , all parts are from known sources like the NGK plugs from motor factor with direct account with NGK etc I’m careful to buy parts that are known supply , the rotor was from Distributor doctor also known for good quality etc

I have a new coil (always had new spare) and I have an IGN Amp (Quality used from my old Chim and some dialetic grease) again known to work so will fit them And see ???

It’s had new injectors in the last five years ,only thing then would be Fuel filter ? Or some kind of vac or intake leak but can’t see anything obvious and not touch anything to induce an issue

After that I’m stumped ?

[QBee]

Are you still using plug extenders? How new are they?

[DVR V8]

I had the same problem many years ago after a new rotor arm was fitted by a mechanic. I kept the old parts and put the original lucas rotor arm back in and ran perfect. If you can try to get an original lucas one. Quite hard to find now as the copies have a rivet in them.

[TVRSJW]

No not using the Beru extenders , and the rotor I fitted was a quality part from Distributor Doctor , the one I took out was with the rivet Type and was loose but new bonded rotor arm never made any difference (the one I took out was worse condition)

Will put in new coil and amp and temp ECU sender and if don’t cure then off the specialist I think !

[Spunagain]

Hi
Does the problem happen just at low throttle openings? it might be the throttle position sensor. I got low speed shunting issues with my Chimaera and the fix for me was an ACT replacement. If you have a multi-meter with ohms setting you should be able to see if the TPS resistance changes smoothly. You may need to make a little cable with a couple of connectors to securely connect the TPS to your meter.

[TVRSJW]

Yes starting to think it’s the throttle pot (as above and all new parts fitted bow to include ECU temp sender) still not cured

I had an ACT TPS fitted about 10K ago (16 months ago) and car has ran great till now called my TVR specialist but he is stacked out with work so can’t get in at present

I’m also told a test meter can pass a faulty TPS and really needs an oscilloscope to test ?

[Belle427]

When you operate the throttle throughout its range does the reading on Rovergauge move smoothly?
A multimeter can be used, it needs to do as above and show no bouncing or erratic operation.

[TVRSJW]

So if I look at throttle position % on rover gauge screen “bar goes green” look to that was smoothness ? Will also test with multimeter

If you rev it (stationary) between 1600-2200rpm “ish” at a constant to hold revs you can hear it just very slightly missing like little hiccup buts it random

[lancepar]

I found this site useful when resetting the TPS. Might be worth a look.

http://www.v8engines.com/carbs-2.htm#setting-up

[Belle427]

Here too midway down the page.
http://g33.co.uk/pages/technical-fuel-injection.ht...

[blitzracing]

You should get a fault code if the TPS goes open circuit when the AFM is reading airflow and I see the MIL light is not on , and I dont think it will cause just a misfire. Ideally you need an HT tester so you can check the peak HT voltage on all the plug and king leads, but the next best thing is to have a look at the HT lines in the dark and see if you can see any ionisation or tracking. RoverGauge looks healthy far as I can see, but it worth taking a look at the short term trim cycle at the misfire point. An HT type misfire should shown up as more "adding fuel" but only if its bad enough for the lambdas to pick up . Also get your strobe out and see if the timing is stable and consistent at the point of misfire. You may need to add an extra white point on the pully as reference. Try the strobe on each plug lead to check it fires consistently.

[Steve_D]

Back in my youth this would have been called 'An off-load misfire' and something to be ignored.

Steve

[TVRSJW]

Ok so far....

No faults code checked twice
ECU reset - didn’t cure
New plugs - didn’t cure
New leads - didn’t cure
New cap and bonded quality rotor arm - did t cure
New ECU temp sender - didn’t cure
Stepper cleaned and checked - didn’t cure
Fitted coil/amp - helped a lot smooth things out but not 100% as it was before the issue , it’s still not happy and....it’s decatted so pops and bangs but the pops and bangs are on overtime now !

Took it out yesterday and today and noticed that when I boot it it accelerates and with foot at constant accelerating it all of sudden feels like a “turbo” has kicked in and pulls harder much harder but I’ve not moved my foot and throttle did it 3 times today ????