Mid-throttle splutter
My 1995 Vauxhall Cavalier 1.8 singlepoint injection manual with 121,000 miles has a fault on mid-throttle. About two months ago, I started buying supermarket petrol because it's cheaper. After about five fill-ups, the car now stutters on part-throttle and misfires at around 2000rpm when stationary. I tried to get some Wynn’s Dry Fuel from Halfords, but they no longer stock it, so instead I bought some Redex one-shot injector cleaner, which did nothing. The last two fill-ups were made from a fuel station and a supermarket, respectively. I then tried the Dry Fuel and there was a slight improvement, but then it returned to how it was.
I bought a new throttle position sensor off ebay and fitted it – do these need to be primed? At the same time, I replaced the distributor cap and rotor arm with new Bosch genuine parts. At first there was no change, but after revving a few times the misfire went away, but now has returned. I then did a cylinder isolation test and there was very little difference apart from number 3 cylinder seeming much stronger than the others, with a noticeable difference in idle.
The plugs are NGK BKR6EK doubleelectrodes and have done 10,000 miles. There is now an upgrade – BKR6EIX – are they worth the additional cost? The leads are much older, but are mostly wrapped in insulation tape to avoid stray sparking.
It's still shunting on mid-throttle, yet starts first time and pulls OK. Although sometimes it feels as if the brakes are on. Geoff Martin I would expect the problem to associated with fuel rather than the ignition system. For this reason, I would not expect that replacing the spark plugs with BKR6EIX items would improve the situation.
Although there should be no reason why different supermarket petrol would be an inferior quality, it may well be the case that you have picked up a contaminant in the fuel that is the cause of your problems. For this reason, I would replace the fuel filter, which is located in the fuel line near to the fuel tank, to ensure that no contaminants remain in the system.
The throttle potentiometer does not need to be primed when fitted. If this was the cause of your problems, it would be very likely that the EML would illuminate. Although you do not mention an engine management light being visible, it would be worth checking the vehicle for any codes present. Your vehicle should have a 10-pin data link connection under the bonnet, which requires dedicated equipment to access the codes. Many good independent garages should have this equipment.
As you mention a slight imbalance found during your isolation test of the cylinders, I would also consider carrying out a compression test to ensure that the problem is not due to poor compression on one or more cylinders.