The car suffers from a hesitation when first pulling away after a cold start, and occasionaly when changing up a gear. It misfires and feels like it will stall, but never does. Once the engine has a slight warmth, the problem disappears and the car runs perfectly. This takes about a mile, perhaps less, and it revs freely right from the word go, the problem is only apparent as one transfers load to the engine. The symptom is very much like the way a cold carb fed engine would perform with insufficient choke.
This problem first started when I replaced the OEM Denso plugs with NGKs:
Denso chart recommendation: K16R-U gapped at 0.8mm (that is straight from their website. Autodata contradicts this and says to use K16R-U11 with
1.1mm gap. The car was fitted with K16R-U when I acquired it.)NGK recommendation: BKR5EY-11 gapped at 1.1mm
So, since they appeared to cause the prob, I removed the NGKs and got a new set of Densos, gapped them at 0.8mm, fitted them and the problem disappeared. For about 2k miles. Now, it has returned, yet the plugs are still like brand new and the gaps are still correct. Deposits are the normal light grey, with no fouling or evidence of poor combustion.
Could it be that the plugs are a red herring and the car has some other problem? Checks I have done so far are:
- Checked for intake air leaks using soapy water. Found none.
- Measured coil resistances, all four are between 21 and 23K
- Checked readout from CTS via OBD scan. Reads normally, i.e. around ambient temp when cold, rising to 80-90deg warmed up.
- Checked intake air temp readout as above. Reads ambient temp with a cold engine.
- Checked for any stored DTCs. None present.
- Measured TPS voltage: 0.54V closed, rising to 3.87V at WOT. No dead spots.
Any other ideas, please? It's a very small issue but doesn't seem right for an EFi car.