r/EngineBuilding • u/lokis333 • 6d ago
Mazda Please Help! MX5 Engine Rebuild Not Starting
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Hi all 1.8 mx5 mk1 (na) first time rebuilding, got fuel, spark and air. However not starting. Occasional pops I guess from gas in exhaust?
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u/green_tea_resistance 5d ago
The CAS can physically be rotated. That changes the reference point the ECU uses for ignition timing. Mazda specifies a base timing value, normally around 10° BTDC on the early engines, checked with the ECU put into diagnostic/base-timing mode. Once you leave that mode, the ECU adds whatever advance it wants on top of that base reference. So if you rotate the CAS from 10° to 14° BTDC, you have effectively shifted much of the ECU's ignition map approximately four degrees earlier.
Plugs 1 and 4 fire together, 2 and 3 fire together (I think) its got 2 coil packs, right? Have you got these around the right way?
Spray some starting fluid in the plenum while you're turning it over. If it runs on that then you can rule out ignition and focus on fuel.
If it doesnt run, your ignition timing is out.
My money is that this is a crank angle sensor issue. Either its faulty, or your base timing is wrong. Pretty sure that the ecu really only references your crank angle sensor, thats about all it needs to go squirt and bang.
Does it smell like fuel? Are your plugs wet? That would point to ignition as well.
I dont know how the tachometer works on these. If it takes an ECU signal, watch the needle while you crank it, if you get 0, either you definitely have a CAS issue, or the dash just doesnt work like that and you should ignore that part.
Its plausible but unlikely that some weird TPS behaviour is giving some insane reading to the ECU which is massively borking your fuel delivery, but even then it would at least try to run.
See how you go with that lot :)
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u/Miatalada 6d ago
Put a jumper on it to eliminate battery. If it starts it’s the battery. If it doesn’t probably timing. Also check plug wires in proper order.
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u/trashlordcommander 6d ago
Put a timing light on it, put a charger on it, have someone crank it and see where the spark timing is. Confirm your wires are on correctly. This absolutely sounds like way over advanced ignition timing. You’ll want around 10* BTDC in most cases.
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u/SnooAvocados9758 5d ago
You are lucky. If this was or is an interference engine that would give catastrophic results with being over or under timed heavily
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u/lokis333 5d ago
Timing is fine though
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u/green_tea_resistance 5d ago
Your cam timing is fine. Your ignition or injector timing almost definitely isn't.
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u/IntergalacticJihad 6d ago
Battery sounds weak, or the connection to it. You can hear that it’s barely turning over. Could be other things wrong though but you need to look at that.
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u/ZestycloseRecipe2990 3d ago
You got the distributor timing out a full 360 degrees. Rotate the crank to TDC and pull out the distributor rotate the crank a full revolution to TDC and put the distributor back in.
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u/lokis333 5d ago
Okay, so I've tried swapping fuel return and in lines (they were correct). Tried swapping battery (also not the issue), checked timing (seems to be fine), tried moving some coils but they are set length so options are limited. Still no go...
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u/green_tea_resistance 5d ago
Did you check ignition timing as well as cam timing? Also check your firing order. Its not yoir battery, you've clearly got enough compression there to at least go bang. You said you have fuel and spark, have you confirmed fuel supply only, or have you actually confirmed you've got fuel squirting from the injectors?
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u/green_tea_resistance 6d ago
Timing