Misfire/Rough idle and hissing sound?

EvilDreams · 14 Jan 2012 at 11:44

Hi,

My crappy 1999 Mazda 323f Auto has recently started idling rough when warm. It will drop to about 4-500 revs in drive, and then everything will kind of struggle and jump to 1000 revs, and lurch forward if I am not on the brakes. This is annoying, if not dangerous.

You can hear the misfire (at least I think that's what it is) through the exhaust....I have checked all the leads and they appear to be fine (but not the plugs yet as see further on). I cleaned out the MAF sensor in the airbox as recommended elsewhere but that hasn't helped.

Then, I noticed that there is a hissing sound coming from the back of the engine (in front of the firewall, behind the engine itself) but it is hard to pinpoint it, and I can't really see any obviously cracked vacuum hoses back there. When I shut off the engine the hissing continues a second or two after the engine is off, then fades out.

Since I do not have my own smoke machine, does anyone have any tips as to how I can locate the source of this apparent vacuum leak?

TIA

Ricochet J · 14 Jan 2012 at 11:56

I had something similar. The symptoms on my Focus were:
Rough idling
Stalling when the car comes to a stop
Hissing noise

It turned out to be a split air/breather hose. The part was £10 and took 30 minutes to fit.

EvilDreams · 14 Jan 2012 at 11:58

Thanks, the main problem seems to be finding the source of the leak, though. I kind of need to get beyond "there's a hissing noise coming from the back of the engine" and that's where I am currently stuck.

bledd · 14 Jan 2012 at 12:00

http://www.youtube.com/watch?v=9CPqbaSgcok

EvilDreams · 14 Jan 2012 at 12:09

^ Thanks....need to get me some carb cleaner and a fire extinguisher then.

The Edge · 14 Jan 2012 at 20:45

Definately an air leak. WD 40 is a good subsitute for carb cleaner and less flammable. You have to absolutely drown it though!!

If it's hissing its a big leak, and the hose may have come off rather than split.

EvilDreams · 14 Jan 2012 at 23:33

WD40, that sounds safer. I shall pick some up tomorrow morning.

It's a steady, non-stop hiss btw.

Amoeba · 15 Jan 2012 at 01:07

Snake in the engine bay.

dalin80 · 15 Jan 2012 at 14:24

Had a few mazdas with those issues where it was the inlet manifold gasket going hard and cracking.

MatteH Oxford · 15 Jan 2012 at 18:08

Eric the car guy is spot on, I like how he explains quite technical issues & topics in plain English.

He gets his voltage drop with his voltage loss mixed up, apart from that he comes recommended