UK Car Repair: renault 19 16v, blown head gasket, crank sensor

Question
the car starts but dosnt run and it back fires out of the air filter. i have changed the plugs leads dizzy cap arm, battery. i have also checked the timing and it seems to be okay. can you help please?

Answer
That sounds to me like a timing or valve issue. Backfiring into the intake tract is caused by the intake valve still being open when the cylinder is trying to fire. This can be caused by a bent valve, badly worn valve seat, misadjusted cam timing “can happen on some vehicles without warning”, improper ignition timing “can also happen and be unverified without the car running”, and or an excessively worn set of lifter/tappets that are becoming stuck in the lifter bores and keep the intake valve open all the time.

It will need a compression test both dry and wet to get a good idea on what exactly is going on with the car. Once the compression is tested you will have a much clearer picture as far as if you have a mechanical problem or an engine control systems problem.
If you do not already own a compression tester or have a friend with one I suggest you take it to a shop and allow them to fix it. It could be as simple as a cam sensor or crank sensor and as complicated as getting the head machined and refitted. This can also be caused by a partially blown head gasket that is igniting the fuel mixture in an adjacent cylinder during its intake stroke. The possibilities are endless, but the compression test will pare it down to a good short list.

Regards,
John