As of Monday this week my car started making a faint but noticeable growl from about 3krpm upwards. Recently changed alternator and PAS belts so thought it was failing bearing somewhere and too tight belt. Took it all apart again and spun everything I could to listen for grumbly bearings and nothing. Ruled that out. Engine sounds a little cammy/tinny from the front but its done that for ages so dont think thats the problem. Can’t really hear the growl in in the engine bay, only when I’m in the car driving and it sounds to be coming from near the passenger footwell area so possible seals around the cat failed/failing. I thought cracked manifold but no exhaust smell in the cabin so dont think its that.
Now she’s dropped MPG from 31/32 and 29/30 on normal/hard driving to 27 on normal driving. I know the cold weather wont help but this combined with the growl is making me think exhaust problem and a small hole somewhere - which would correlate with poor MPG. With the lower MPG I also thought failed O2 sensor but my MOT in November put Lambda at 1.03 and right on the pass limit so she’s running lean if anything which confuses me with the reduced MPG. Lean should equal higher MPG. (am I right with high lambda = lean, lower = rich?)
Also noticed my exhaust has been burbling and popping more which I’d have thought was unburnt fuel in exhaust but lambda says otherwise if I’ve got my lean/rich values the right way round.
I’m going to try and get underneath this weekend and have a look for leaks but any other ideas on what to look for?
Hi Guys, could you let us know which car you have please, just pop a detail into your sig. Also a location in your profile is useful
Might be worth a look at the front wheel bearings, they sometimes growl when on the way out, and can cause a bit of friction…
forgot to say its definitely not wheel bearings. that crossed my mind early on but disengage the cluth at 70 on motorway and noise goes away as revs drop. Think it has to be engine/exhaust related - transmission is still being turned by the wheels when I push the clutch when I’m moving and you get the noise in neutral when parked up.
Your high lambda reading could be caused by an exhaust blow, some holes actually suck air in as exhaust gasses pass over them showing a lean mixture (extra oxygen). Quite a rare phononomen on the MX-5 in my experience though.