As I stated in my above post that I went to Lahore to Islamabad and then back. My Santro was driven on cng as well as petrol. Mileage was good. I was driving at a constant 120kmph and near Gujranwala, suddenly my car's speed dropped to 100 kmph on petrol and the car started shaking/trembling badly. I stopped and had a look inside the bonnet but everything seemed fine. Car was working fine on cng. Anyhow, Jaisay taisay karte I reached back Lahore at around 100kph(car wouldnt go above 100). Had my mechanic checked my car. He did the following:
1. Checked the petrol pressure
2. Checked the Sensor.
3. Did the tuning of the car
After doing the tuning, the car started missing extremely badly and wouldnt pick the accelerator when pressed(i.e things got much worse instead of getting better). He then checked the fuel injectors, they were giving proper current. Then the Spark plugs, Spark Plug number 1 was giving considerably lesser spark than the others. He swapped the spark plugs but no success. After that, we went to some shop in Model Town, Lahore, connected the computer type thingy, and asked us to replace the Sensor. We got a new sensor for Rs. 2500, car got a little better but still it was much better before I visited the mechanic.
Anyways, he then went to a shop to check the pressure of the spark plugs using some pressure gauge thingy. Pressure came out to be fine. We werent able to figure out what the actual problem was there with the car. We then went to a Shop Near Ex-Hyundai Township Motors Showroom and got our car checked. He told us that there could be 2 problems: One there could be problem with the cars HEAD VALVE or with the Fuel pump. Head Valve costs around Rs. 15000-20000 alone 
Anyways, we checked our fuel pump, it was creating some problems. I got it replaced with the KABLI fuel pump worth Rs.1500. The car got a little better again 
then he checked the fuel filter at the back, it was jammed badly and we had it replaced with the Genuine one for Rs. 2080.
Started the car,and it got exactly or a little better when I hadn't had my car checked from the mechanic. After spending 2500+2080+1500+1000 rupees the problem is still there. I have got my car checked from everywhere but no luck. My car works perfectly fine on CNG, no missing whatsoever. Mechanic says k agar Head Valve ki problem hoti to car CNG pe bhi masla karti, but it doesn't.
Any help u people can do?
