Misfire at low revs

[NWTony]

New development in my ongoing battle with all things TVR and electrical...

I've noticed a misfire when accelerating from low revs in third gear. Starting about 1800 and partial throttle it judders and stutters until about 3000ish, but after that pulls fine. It happens when the engine is hot and cold, but I'd say more noticeably when cold. Car idles fine (bit low when cold, but 900 when warmed up) and at more appropriate revs it seems fine.

Checked the plugs, all seem OK, similar condition, no sign of fouling. No sign of water ingress under the cover either.

Coil pack maybe? Throttle bodies?

Help please

Oh, T350 with 4.0L Sagaris engine.

[phoenixz]

I'm going through the exact same thing, was coming up with theories like:

throttle pots need re-aligned
throttle cable is loose/tight
clutch is slipping
oxygen sensor is faulty
etc...

after talking to a few people another Sag owner was down at Str8Six and very kindly asked for me.

Straight away the response was "it's a common fault. one of the throttle sensors needs replacing. about £100 part"

The Sag has been tucked away all winter so i havn't gotten around to seeing if that will fix it, but will let you know the results when i do

[NWTony]

quick boost before it moves off the front page...

Does everyone think it is throttle pot sensors, is there any way you can tell which one?

[Ant.]

NWTony said:

quick boost before it moves off the front page...

Does everyone think it is throttle pot sensors, is there any way you can tell which one?

Get it plugged into the diag software, get them checked. But also consider plug leads and coil pack.
When were the bodies last checked for balance?

[Targarama]

NWTony said:

quick boost before it moves off the front page...

Does everyone think it is throttle pot sensors, is there any way you can tell which one?

Potentially yes (anyone see what I did there?). Mine had this once or twice.

Get it checked by a specialist even if you know which throttle pot it is then you still need the tools/skills to adjust it correctly.

[chris watton]

I had the same symptoms with my first Tamora with less than 20k on the clock. Dom at TVR Power diagnosed worn throttle pots (they are a Ford part IIRC) - had those changed and the problem went away. Assuming that is the problem you're having..

[shep1001]

If it happens under load could be the HT leads. Poor throttle balance or failing cil pack can also cause it.

Plug it in the diagnostics an watch the throttle position sensor reading they should start about 15.2% and go up to about 90% no more than say 1.5% difference between the two as you depress the accelerator.

Edited by shep1001 on Wednesday 12th February 15:35

[NWTony]

Thanks to all for the advice, I'll take a look to see how the throttles look before buying anything. The leads and plugs look fine as I've already had them off. I did consider the coil pack as well but couldn't get another one to try.

I'll have a fiddle and a play this weekend, see if anything shows up.

Thanks again.

[natben]

I had a similar issue,
I replaced both throttle pots, the one at the bulkhead was a pig to get on/of as there was no room to get a screwdriver in(cerbera SP6).
I changed the HT leads
I changed the Coil pack
I tried to reset the throttle bodys and messed it up so the car was running terribly.
I removed the throttle body's, all six and had them refurbished.
I fitted powers throttle pot gaskets
I put it all back together but I still could not get it balanced correctly ( I am not good at this sort of thing)
I had the car trailered to TVR Power and Dom set it all up perfectly and it has been fine ever since.

If It happens again I would just take it to TVR Power and get them to fix it, I recon I could have saved a fortune and a lot of time if I had went down this route in the first place.
In my opinion unless you understand what you are doing leave balancing the bodies to the experts as the tollerances are minimal between running well and running rough.

[shep1001]

NWTony said:

Thanks to all for the advice, I'll take a look to see how the throttles look before buying anything. The leads and plugs look fine as I've already had them off. I did consider the coil pack as well but couldn't get another one to try.

I'll have a fiddle and a play this weekend, see if anything shows up.

Thanks again.

The leads might look ok but the heat off the engine knackers them and the insulation breaks down or the core gets damaged. Not easy to check without replacing but don't discount them

[flying]

Totally agree with Shep having had various misfires on my car including coil pack, the loom to the injectors the last one being one of the leads as he said looking good is not always the case another tip I had with the coil pack is to check the back of them over time and with the heat what ever they use to seal the back with can crack and take in moisture touch wood never had problems with the throttle pots.