How to diagnose an issue with your car’s fuel line

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In the past, I was trying to diagnose an issue with fuel line where vehicle in hand used to run poorly on petrol whereas very well on CNG. Problem as it may seem simple, but it is not so simple where I can assure that local technicians won’t be able to resolve it as this is a costly repair, just because of fact that it requires through and through diagnosis, proper simple equipment, and later-in, end an expensive purchase. Neither these expertise are common in market nor every shop has these simple equipments. End result is that technicians will make you replace expensive working fuel pump, fuel filters, pressure regulators over and over again and still your problem will remains intact. Not satisfied you will keep on spending and reassigning the job to another technician.


Poor and less power on petrol and more power in CNG, Vehicle only runs on CNG but refuses to run on petrol, Petrol over spills from fuel tank even before its filled completely, Vehicle gives a very bad average on Petrol etc, poor starting etc.


Advice first: Visit the manufacturers workshop as they will charge more but that would be your best chance to resolve the issue once for all and money rightly spent.

Advice Second: follow the following steps below.

1)  fuel-pressure-gauge    Get hold of Pressure gauge and measure the fuel pressure by clicking the ignition(“look at your manufacturers booklet for more details and right pressure readings”). If problem is diagnosed with the pressure go to  fuel pump, take the wiring clip out and measure the Voltage which goes to the fuel pump by clicking the ignition. If fuel pump does not receives the Voltage at all then change the fuel pump relay, EFI relay and re-verify the good grounding signal.  Voltage on fuel pump grip should be equal to the one you have on battery terminals at that instant.

2)      If Voltage at pump side and battery side at that instance are same but then you need to run your vehicle on CNG. Go to the battery and measure the Voltage while Alternator is charging it. Perform the tests on Alternator through and through as at times, due to a dead diodes in alternator, it’s not producing correct Voltage. Low voltages also causes issue with fuel pumps and does not let it create a pressure needed to let the vehicle run properly on petrol.

3)      By default, replace the plugs, take the injectors out and clean them with good carbonator cleaning. Later, you can take it for proper testing on fuel injector machines near your vicinity. But don’t emphasis on injector testing on machines as first you need to diagnose the issue.

4)      Does your vehicle spills petrol while refilling, if not then proceed ahead.

5)      If you have not found your culprit then take the fuel pump line off from pump side and take it off through engine side. Try to blow air through it either via pressure pump or try to suck it through compressor. Keep a pen and paper on hand and keep on writing the results. Let the fuel in line spills and after if air flows without any obstacle then get hold of good pressure regulator from scrap yard, take the fuel assembly out, replace the regulator along with the fuel filter(genuine or Japanese ) and this should resolve any issue you have with fuel line and if problem still intact then get ready for a next step.

6)      Take the fuel pump out, test it and replace if necessary.

7)      If problem is not with fuel pressure then go to the engine area, start the car and run it on petrol. Take the wiring clip off from every injector one by one and put it back before going onto next injector. Do the same with ignition coils once though with injectors test.  If engine stalls on any injector or coil then you have found your culprit, don’t be quick in replacing your injector as wiring should be the first to be suspected, test with OHM METER for continuity. If all ok then pay attention to suspected injector or coil.

8)      Re-verify that Advancer emulator is connected properly, wirings are done fine so that you know once spark plug timings are advanced on CNG then does it retrieves back to petrol settings once switched onto the petrol. Rest details regarding how to do it are lengthy.

9)      Tappet settings can also be suspected.

10)   Last of all, if your vehicle also spills patrols during refilling then problem points charcoal-canistertowards charcoal canister. Bad vacuum condition might be a culprit which does not lets your pump work properly as charcoal canister maintains a correct pressure at fuel tank and relieves it in engine as per its own principal.  Remove the charcoal canister, its valves, clean the purge valve using carburetor cleaner. Take the canister to the tire shop and ask him to pump air fir every hole with pressure. further, you can dig on this on internet how to resolve the issue.

Don’t let the technician work by fluke, Hope this will help.

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1 Comment
  1. Talha Malik says

    Correction on point 7, which is if engine does not stalls while once you have removed the injector or ignition coil, then that’s your culprit.

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