P0303 Engine code trouble shooting

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: NB (Mk 2.5)
  2. I’m based near: Guildford
  3. I’m looking for technical help or recommendations on:

I have just bought a 2003 NB. I got it for a good deal as it has a P0303 engine code error. I did a bit of research before buying it to check it wasn’t too serious and gathered it should be a relatively easy fix. It had new spark plugs and coils before I bought it so I assumed it was the injectors.

A friend and I recently swapped the injectors between cylinders and it was still throwing up the same code for the 3rd cylinder (aka what we thought was the faulty injector works fine in another cylinder). We checked and there’s good spark and the coil is working fine.

Any ideas as to what it might be???

We’re currently thinking it the injector harness/loom?

It runs very smoothly for only 3/4 cylinders but definitely still has the misfire (backed by the code).

Any help would be greatly appreciated.

2

Check also ALL the connectors on that loom, tarnish can be a problem. If so, then Servisol Super 10 is your friend here.

Other possibilities include a rub-through of the insulation where the loom is tight against something metallic.

3

Have you carried out a compression test?

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4

Not yet, just replaced the injector harness/wiring loom and still have the problem. Think that is my next bet.

5

Just done a compression test and cylinder 3 was literally 0psi. The others were all 160/175 max compression so they’re fine. Any ideas of what it could be? Maybe valves? There’s no white smoke or anything that could suggest piston ring or gasket etc.

6

With compression that low, I’d suspect valves first.

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