Lumpy Mazda 6

Bought an high-mileage 05 Mazda 6 lately as a bangernomics machine to last me for a year or maybe two. It drove well for a week after purchase, then starting to get lumpy. It drives fine at high revs, and idles smoothly out of gear, but shudders and loses power driving at around 2000 rpm. It seems to be getting worse.

I plugged it into a mate's OBD diag tool. It showed no codes and a specific mazda 6 scan diagnostic came back clean on all systems.

The car appears to have been serviced just before I bought it (new NCT, clean oil, air filter is spotless) and my feeling is that the dealer did something to temporarily mask the issue for the NCT and sale. But what?

I'm thinking MAF, 02 sensor, coil, degraded plugs, catalytic - but would have expected the diag machine to return a code if any of these were wrong. Any ideas? I don't want to start throwing money at it randomly.

Polo_Cluvie

Id try unplugging the maf and see if that makes any difference. If it gets worse then maf is not your issue. If it was lambda surely diag would pick up. Id be inclined to run her nearly empty and fire in a full bottle of dipetane ive seen that stuff work wonders. If it is cat not the end if the world you can get a spurious one for around a 100 euro fitted

Thanks - I'll try the MAF. Might spray it with contact cleaner as well while it's out. Was looking at universal cats - would be a shame to change if the original is fine, and no way of telling until it's off.

Now that you mention it - dipetane is probably what the dealer used to get it through the test!

Polo_Cluvie

Ive been thinking about it and the signs do point to the cat as the majority of other faults would show on the diag. Its step by step. Another thing you could try is cleaning the throttle body. Worst case scenario is head gasket but lets not get ahead of ourselves

jelutong

If it's Diesel the suction control valve might be the problem.

Ah don't be mentioning the HG! I'll chance the MAF and a dose of dipetane, but as you say it is probably the cat. Forgot to mention that there is a horrible smell off the exhaust.

@jelutong

@jelutong It's a 1.8 petrol.