Tesco 99 and misfire. Are they connected?

[stevemac]

I have just run on three consecutive tanks of Tesco 99 after regular use of 95 RON/Optimax and the performance difference (2000 GTS) is marked J although there was a misfire/eng warn light problem along the way. Anyone had similar misfire problems when changing fuel grades?

During the first tank full of Tesco 99 there was a slight misfire in the 1000 to 2000 rpm range to start with but it cleared up by the end of the tank. Two tank fulls trouble free later` when going back to 95 ( ¼ tank after nearly running out – misfiring due to no fuel) ) and then to Optimax ( full tank) the misfire came back for a while at motorway speeds plus engine warn light. Once the Optimax was used up I went back to Tesco 99 and now there is no eng warn light although there was a misfire to start with.

All is well again now and the performance improvement across range has remained …

Is this misfire/eng warn light intermittency all due to the PCM adjusting for fuel grades?

[island boy hsv]

I switch between both grades of fuel very regularly because of the intermittent supplies to the Isle of Wight. I have never had any problems with my GTS illuminating the engine check light. I would take it along to some one who can read the DTC that has been stored in the PCM and find out what is causing it.

By the sounds the engine warning light might not be directly connected to the chenge in fuel.

>> Edited by island boy hsv on Sunday 19th February 18:12

[stevemac]

did you get any misfire when going from 95 to 99?

[BigNige]

Went from 95 to 99 when I had the Wortec upgrades done and had no misfires or problems.

There's a Tesco up the road from me and I mostly buy their 99grade stuff, never had any issues.

[A57 HSV]

Never did any "scientific" tests but my S/C HSV went as well on Tesco 99 as it did on Optimax. I used Tesco 99 for several weeks when Optimax was hard to find & the 99 was 89.9p.
However, evo (or was it Autocar!) did a test on fuel & it seems that it is quite common to get a dodgy batch. Best to buy from a busy station.
I often use Tesco 99 in my Saab Aero with no probs. to date.

[stevemac]

It seemed worst when going from 95 to either tesco 99 or Optimax. First time had misfire a few times at idle (stopping at traffic lights)when first changing from 95 to Tesco 99, this cleared up when accelerating away and didn't return towards the end of that tank full, no problems at high speeds at all. Then later, after two more trouble free tanks of tesco 99 when going from 95 to Optimax had misfire at motorway speeds which cleared up again after a while.

I thought it may be caused by the PCM/anti-knock adjusting to the new grade.

Anyway, sticking to Tesco 99 now from a busy station in Camberly so hopefully no water vapour probs as have been reported in this type of fuel.

So what next? I need to pull up the fault codes on the dash to see why the eng warn light comes on intermittently.... I think there was thread on how to do this somewhere.

[island boy hsv]

I should have put it in my previous post I don't get any miss fires with changing fuel.

To read DTC codes from LS1.com.au

"DIAGNOSTIC MODE OPERATION
To enter the instrument cluster diagnostic mode, press the trip computer switch MODE button down together with the "UP" button while turning the ignition from off to on. Further pressing the MODE button will step through the various instrument cluster information displays.
When the Diagnostic Mode Display is reached, pressing the "UP" button increments the DTC number and related information displayed, while the "DN" button can be used to decrement the DTC number and related information. To exi the instrument t cluster diagnostic mode, press both the "UP" button and the "DN" button together.
A DTC is shown on the Diagnostic Mode Display as ‘t NN XY’ , where:
t = diagnostic trouble code (DTC),
NN = the actual DTC number,
X = the history DTC status (H for active and – for inactive), and
Y = the current DTC status (C for active and – for inactive).

A DTC may also be cleared from the instrument cluster using the trip computer switches while in the diagnostic
mode. With the DTC displayed in the instrument cluster window, press both the "UP" button and the "DN" button together
for more than 2 seconds. Pressing these buttons for less than 2 seconds will exit the instrument cluster diagnostic
mode."

The list of codes is

www.dd.id.au/LS1/Dash/Dash_DTCs.html

It will help you localise the problem to a certain area but wont get the exect problem every time.

Let me know how you get on.

[stevemac]

Thanks for that, great info...

so, I got a t19 HC and according to the listing this is a "PCM fault logged"

all the rest were either "--" or "nu" (Not Used?)

Q. is there anyway of getting to the PCM fault log via the dash buttons?

[marctucker]

Hi,
Nosing around and saw this thread.
I tried Tesco 99 once in my Rover V8, it seamed to run worse and with less power/torque than using 95 or 97 octane.
My understanding of the octane rating test is that it does not use anything directly related to car performance? So my question is this, is Tesco fuel over spec'ed or maybe badly kept?

[stevemac]

Overall I think the Tesco 99 gives the best performance. Maybe it's a timing issue on yours, or perhaps a duff batch.

However, my issue is that from the first time I used it there has been an intermittent misfire, now mostly at low revs & tickover and the eng warn light has started to come on during or just after the mis-fire. it's now settled down to coming on after driving about a mile in stop/start (low rev) conditions following starting first thing in the morning.

Mis fire comes back now and then when revs drop right down.

If I put my foot down it picks up revs and then takes off as I would expect with 99 RON.

I figured it is the "PCM fault logged" that has caused the eng warn light although i see form the link above the "t 19 HC" DTC may not activiate the eng warn icon. Anyway,I just dont know what the fault cause to the PCM is yet.

My thoughts are that the PCM/ECU is relearning the 99 characteristic and hasn't got the tickover/low rev timing bit right yet. There is nothing wrong from mid to high rev range. Perhaps the eng warn light is linked to the exhaust sensor picking up unburnt fuel or whatever from the mis fire.

Think I may reset the PCM and start from there. Perehaps I should get the DTC codes read somewhere first.

Oh the joys of HSV motoring

[stevemac]

Misfire was nothing to do with Tesco 99.

Turned out to be a faulty O2 sensor, running OK in start up open loop but overfuelling/misfire in closed loop until eng warn light comes on (goes to open loop fuelling)when picks up OK.

Have fitted new HT leads (AC 748CC), spark plugs (AC 41-932, K&N filter and O2 sensor.

[ringram]

O2 = junk, tune them out

[gmccrin]

Does anyone know of the LS1 or LS2 are E85 (Boi-ethanol) approved. Boiethanol is not being sold by Morrisons (First Station up and running is in Norwich) who plan to run it out across the country. Ethanol is about 120 octane and E85 rated vehicels can run on it without any further modifications.

[mose]

I ran a few tanks of 99 when I first got the car. The M3 didntmind it at all, but the Monaro didnt like it, car felt very lumpy and lost performance. Optimax has given the best results, failing that a few tanks of Texaco super.