Engine won't start after rebuild.

[merlin's S4]

I've spent all evening trying to figure this out with no joy. I've just rebuilt my AAN engine with new rods bearings and rings and fitted a modified 7a head. The engine turns over ok and I see oil pressure when cranking but when I try to start it, it just pops and back fires and doesn't give any hint that it will fire up. I've checked the timing twice and have got the flywheel mark spot on with the cam pulley. I also fitted a new hall sender when the new head went on.

Please help!!

[Billman]

What flywheel are you using? Do you see the revs rising during cranck. Maybe is the distance flywheel pin to sensor.

[merlin's S4]

Still using the original dual mass flywheel. I've not noticed the rev counter, i've been too busy looking at the oil pressure

[Billman]

Check engine and head grounds. Also check that you connected the crankshaft position sensor and the RPM not mixed up.

After that check the coil connectors, see that you connected injectors!

What ECU are you running?

[merlin's S4]

Engine management is Motronic. Injectors, hall sender and crank sensors are all connected correctly. Coils should be fine as they were perfect before the work started. I've taken the plugs out and they're covered in petrol but surely something must be firing to cause the backfire?

[Jamo]

you haveuel going in but little or no spark, sounds like the hall sensor window is out of alignment with the crank sensor

I had this when i had too much fuel on vems trying to start the car, have you change your stock ecu map or injectors / fpr?

[20vQC]

backfiring? mmm...then you have spark right? take a little look to the coils mate, check the firing order is fine. If no spark then do what Jamo says too! Take a look over the cam pulley sensor if looks fine, then take another look to the VR sensor (flywheel pin) and if you have a spare one, replace it just to see if it is the problem. cheers!

[merlin's S4]

Thanks for the suggestions! Looks like I'll have a nice list of things to check when I go to the workshop on Sunday

you haveuel going in but little or no spark, sounds like the hall sensor window is out of alignment with the crank sensor.

I've been thinking about this and I'm really hoping that the hall sensor is in the correct position with this being a modified 7a head!

Ecu map is the same MRC one that I've had for years and I've not changed the injectors or FPR. Hall sensor is new but I could change it back to the old one to see if anything changes.

[Jamo]

If ecu is the same then i'm leaning to timing window, wiser minds than mine can tell you how to check this

[pixsaudisex2]

And of course make sure all your plugs are the right way around on the coils an pos

[driftin jo]

I don't think it will be the hall sender form the cam as this is only used by the ecu to determine what stroke the engine is on i.e. compression or exhaust . It would not need to have very accurate poisoning as it doesn't actually time the spark. Once the engine is running it can even be disconnected as it is only used by the ecu for starting purposes, all ignition timing is done flywheel.
I am thinking you have your coil/ power output stage wires in the wrong order.

[Tonks4x4]

And of course make sure all your plugs are the right way around on the coils an pos

I agree, I have had these the wrong way round before and it coughs and backfires when running.

Some of the coilpack plugs have 2 black connectors instead of a brown one and a black one.

[Jamo]

I don't think it will be the hall sender form the cam as this is only used by the ecu to determine what stroke the engine is on i.e. compression or exhaust . It would not need to have very accurate poisoning as it doesn't actually time the spark. Once the engine is running it can even be disconnected as it is only used by the ecu for starting purposes, all ignition timing is done flywheel.
I am thinking you have your coil/ power output stage wires in the wrong order.

Not so, if the cam sync and the crank are out of sync the engine won't start. The ecu has to see the cam sync within a window of the crank, it isn't that senstive but if they aren't inline they don't start.

After market ecu guys know this more than the stock ecu guys as it's auditrigger is pain in this regards. Also if you retime your cams you have to watch out for this ( incases such as Bram's car you have to install the dizzy to get the window in alignment )

If the engine is running and it's out of sync it will run from crank sensor alone

[driftin jo]

Not so, if the cam sync and the crank are out of sync the engine won't start. The ecu has to see the cam sync within a window of the crank, it isn't that senstive but if they aren't inline they don't start.

After market ecu guys know this more than the stock ecu guys as it's auditrigger is pain in this regards. Also if you retime your cams you have to watch out for this ( incases such as Bram's car you have to install the dizzy to get the window in alignment )

If the engine is running and it's out of sync it will run from crank sensor alone

you seem to be missing the fact that if it was out of sync as you say the ECU would not give the signal to fire the sparks at all and this engine is clearly sparking.

[Jamo]

you seem to be missing the fact that if it was out of sync as you say the ECU would not give the signal to fire the sparks at all and this engine is clearly sparking.

Who says it's 100% out of sync every cycle?, can't see how you can eliminate it as it's one of the primary changes done on the engine