Vacuum tubes - help me understand/find a leak

langdang

Guys,
I've a basic understanding of the mechanical/electrical/combustion process taking place in a standard car. But one thing I don't understand is vacuum?

Unfortunately it appears I have a vacuum problem at the moment.

-car idles rough if started and left alone, and will eventually cut out
-car is a dog to drive around town - stop at lights, clutch in and it hovers around 1200rpm for a few seconds then nosedives way below normal idle.
had to try and "catch it" myself with minute amounts of throttle, but usually leaves me looking like an unlikely boy racer (revving away going nowhere)

-even on a good day you can see the needle bobbing around at idle
-leads and plugs are good.
-distinct hissing noise coming from the inlet manifold side of the engine, but can't track down the leak.

Any tips? I've compensated for the bad idle by adjusting the screw on the throttle body to raise the idle until I get this fixed.

Can anyone give me advice to track this down?
Engine is a mazda 626 as in this video

https://www.youtube.com/watch?v=prHkn9E-ElI

edit - not sure how to make the video embed - heres the linkhttp://www.youtube.com/watch?v=prHkn9E-ElI

biggus

If there is air entering the engine by a means other than through the normal ways , this will cause the car to run lean. ie too much air for the amount of fuel being delivered. This will cause rough idle and lack of power.

One way to find a leak is to spray some engine start fluid or other flammable aerosol around the suspected area and see if idle improves or rises (FIRE hazzard doing this so have an extinguisher handy).

So in the video if you sprayed around where he took off the pipe revs would rise dramatically, so if the revs rise you've found the leak. Could be pipes, inlet manifold gasket etc.

alternatively you're Airflow meter could be faulty or dirty.

langdang

Only got around to this today:

Got busy with my can of Easy Start, all the tubes seem fine BUT- when I start spraying near the top of the inlet manifold then I hear a definite change in the revs.

See here (not my photo, not my manifold but very similar)

Not quite accurate enough with the spray to figure out exactly if it is where the fuel lines meet the manifold, or the manifold gasket itself.
I presume it's the manifold gasket rather than a leak around the injectors?
Spraying near 1,3,4 gives a change in revs, 2 seems ok.
I expected a rise in revs, what I got was a chugging for a few seconds, then it recovered...

Any tips folks? Didn't want to go fiddling with injectors without a responsible adult present. I could take it to a garage obviously, but I'd like to learn a bit more about engines while I still have a basic old school engine to practice on(1991cc petrol mazda 4cyl, DOHC, no hydraulic valves or anything fancy...should be plenty more miles in it yet!)
Sooner or later we'll all have big plastic sealed boxes in there, and won't even be able to adjust the idle without a laptop!!

beachlife

Where are you located ? Why don't you bring it to a garage and get a smoke test done? There are a couple of places in dublin that I know do it.

Alanstrainor

Youtube video fixed.

langdang

Cheers Alan.

Beachlife, yup will probably go that road. Didn't know there was a practical way of doing that. In Limerick most of the time.

How big a job is it to put a new gasket on? (Not that I'd be doing it?) There's good space to work in that engine bay, enough for a 2.5 V6 in other models.
Is there any sticking-plaster type solution? Cars not worth much, but my next one won't be either... so if I could knock a year or two more out of this then I'd be happy (NCT is in 6 months)

stealthyspeeder

beachlife wrote: »

Where are you located ? Why don't you bring it to a garage and get a smoke test done? There are a couple of places in dublin that I know do it.

Dont want to hijack the thread, but could you tell me where in Dublin does this? I suspect I have a boost leak in my car

Muckie

If you wanna go down the "sticker plaster route". Which alot
of people do if the car isnt worth a lot. There's a kind of paste
in a tube(i know not a great name for it). But most motor factors
or even Halfords might have. Take it out of its package. Break and roll
it the place on the hole. Hardens quite like steel. Could get you out.

beachlife

langdang wrote: »

Cheers Alan.

Beachlife, yup will probably go that road. Didn't know there was a practical way of doing that. In Limerick most of the time.

How big a job is it to put a new gasket on? (Not that I'd be doing it?) There's good space to work in that engine bay, enough for a 2.5 V6 in other models.
Is there any sticking-plaster type solution? Cars not worth much, but my next one won't be either... so if I could knock a year or two more out of this then I'd be happy (NCT is in 6 months)

I don't know much about limerick as I'm up in dublin.But find a garage that specializes in diagnostics and they should pinpoint the problem for less than 50 euro,Probably a few small hoses and old gaskets rather than the main manifold leak.

stealthyspeeder

beachlife wrote: »

I don't know much about limerick as I'm up in dublin.But find a garage that specializes in diagnostics and they should pinpoint the problem for less than 50 euro,Probably a few small hoses and old gaskets rather than the main manifold leak.

Can you name any in Dublin which do this please?

JHMEG

I never understood them myself.