I have an issue that I’m trying to troubleshoot. I’ve got a hesitation / Misfire under anything more than light acceleration when under 2500rpm. It will happily rev to redline once you get past 2500rpm. It seems to be the classic bogging down issue that haunts the NB, but I’m having trouble finding the cause.
For context, I bought this car (Mk2 1.8 SE on 150k miles) back in June for the grand sum of £500. Welded up a sill that was stopping it get through the MOT and then got it back onto the road. Then I started dealing with the mechanical engine issues.
Initially I had a jerky acceleration at low rpm, so I changed the HT Leads and Spark plugs. Initially this was a bit better, but then It started misfiring badly.
I bought a good used Coilpack to replace it. While I was changing the Coilpack, I removed the Valve cover, cleaned it, painted it and replaced the gasket with a genuine mazda one. I torqued it all down properly as per the spec to 5.5nm.
While the misfire from startup has now gone, it is bogging down under load under 2500rpm. It’s worse the higher the gear you are in.
First I changed the MAF sensor as that was a suspect cause and a cheap replacement. No joy, made no difference.
I’ve just changed the fuel filter as it well overdue and pretty grotty, hoping that would fix it, but unfortunately it’s still bogging down. That said the engine does seem a little happier when at high revs.
I was advised it could be the 02 Sensor on the exhaust header. This connects right in front of the valve cover. I disconnected it, let the car warm up and tested it. It made no difference and the cars still doing it. So I can assume it’s not at fault.
So next possible issues are:
- Weak Fuel Pump - Replace with a new one?
- Catalytic Convertor - The exhaust from the cat back is very rusted and could do with being replaced.
- A vacuum leak somewhere? one of the metal clips that hold the airbox on has broken, so its cable tied down. Not sure if this would cause an issue?
Is there anything I should look at?
I had a coolant leak that was dripping coolant all over the Crank Position Sensor for a while before I found it and repaired it. Could it possible that this is damaged? The lead to the CPS was also flapping around from a bodged previous repair and was physically rubbing against one of the belts and had almost started wearing through the cable before I noticed and secured it back properly. So I’m not sure if the CPS sensor might be a bit borked.
Other than this I’m not sure where to look. Any advice would be great.