Car Problems, Engine cutting out - Clutch and power steering

[MarJay]

Hello,
I hope someone on here can give me some insight into an issue I have been experiencing with my car recently.

Its a 1992 Mazda MX3 1.8 V6.

At relatively low speeds (10-20mph) the engine sometimes cuts out. Now this always seems to happen just as I don't want it to, for example when braking hard or putting in steering inputs. I was wondering if it was something to do with the power steering pump as if I flick the wheel from left to right at a standstill the revs jump a bit...

However, I also have noticed that when I undo the fuel cap to refuel the car, there is a loud sucking noise as if air is unable to enter the fuel tank until I open it. Are car tanks supposed to have breather pipes in the same way that bike ones do?

I have also noticed in the owners manual for the car, that the clutch fluid is the same as the brake fluid (IE from the same reservoir), and so is it possible that heavy braking could affect the clutch in some way? The clutch has to be all the way out (I mean right to the floor) in order to disengage, so I wonder if this could be related too.

Any inputs appreciated, and I am also hoping that Robby can reply to this thread as he seems to be rather good at all this complex car stuff!!

Cheers,

Mark.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[finpos]

The good/bad news is it's probably none of the things you mention.

The tank thing is quite normal - a car of that age probably won't be open vented in order to meet emissions controls.

The clutch is probably on it's last legs by now and the hydraulics may be out of self-adjustment, so that's possibly why the pedal is going to the floor.

The revs are probably jumping because the power steering pump is signalling the ecu to put out more power when it's put under load.

So what could it be? my GUESS is that you have an old fashioned air flow meter (AFM) in your car, which is of a flappy-paddle type - i.e. it's plate held in they airflow by a spring. 13 years later the spring is not too clever any more, at low revs it's marginal and will be affected by physical movement of the car, so the AFM keep signalling the ecu that there's no air flow (a lie) and that makes the ecu shut down the engine.

They can sometimes be 'adjusted'. Google for AFM or MAF, see what you get.

fin.

[Ichy]

Totally agree, If its a similar type of air flow as old BMW's then you can rotate the activation switch slightly to compensate. Uses slightly more fuel but keeps tickover constant.

[MarJay]

well it seems to happen when the clutch is down and I brake hard enough to make the front suspension compress...

so it could well be that, thanks!

Any further input appreciated tho!!
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Kickstart]

Hi

Clutch down means that the road is no longer acting as a dirty great flywheel to keep the engine spinning so more likely to stall. Turning the steering will affect the idle speed as it is putting a load on the engine.

I would very much doubt that Mazda has an AFM. Too new for one of those (and the problem is that the spring will weaken with age and so richen the mixture, or some pilchard adjusts the AFM to compensate for a worn fuel pump and really screws things up). Probably uses a MAF sensor (ie, probably a hot wire sensor where the incoming air cools a piece of wire and the mass of the incoming air is calculated by how much current is required to keep the wire up to temperature).

Personal guess it that the idle control valve is sticking. Basically this is a small passage to bypass the throttle which is opened / closed by a small servo motor controlled by the ECU. As they age they stick. Find it and clean it out with carb cleaner.

All petrol tanks are vented. If they were not then the tank would be crushed as fuel is used. However they normally go through a carbon cannister vapour trap to stop fumes getting into the air. Chances are this is working as it would be quite capable of crushing the tank if it was blocked (even a low pressure mechanical pump on a carbed car can do this).

Unless the brake fluid level is VERY low then the shared reservior is not going to cause a problem. Normally the feel for the clutch is above the bottom of the reservior. This way if you run out of fluid then you loose the clutch first before you loose the brakes.

All the best

Keith
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[Robby]

First culprit to me is battery. My old golf used to stall in traffic when the battery was knackered, but it could still turn the engine over to restart it. a new battery sorted that.

I'd be cleaning the idle control valve as keith said, and also clean the MAS (air mass sensor) - to do this you need the car to be stone cold. Do not touch the ignition. Pop the bonnet, find the MAF, spray some carb cleaner onto the wire. Do not use brake cleaner, its too strong and the MAF is generally a fragile thing. Worth cleaning out the throttle body too, seeing as you'll be in that area to do the idle control valve.

With the MAS and idle control valve sorted the car should be able to chuck in the correct mixture. If its still messing around it may be worth testing or just replacing the lambda probe.

Also worth checking the batteries water levels and trickle charging it overnight to see if that helps.


To give a little explanation to my thinking. MASs get old, and they get dirty. This is the thing that tells the ECU how dense the incoming air is, while it uses the throttle to gauge how much of the stuff is coming in. Dense cold air cools the hot wire on the MAS, so dense air=lower resistance=more fuel. A dirty wire will have a higher resistance than a clean one, so the mixture will be leaner.
Combine this lean mix with a sudden change in inlet manifold pressure - heavy braking draws more air into the servo from the inlet manifold- add a slow idle control valve, and you have a fair chance of stalling.

Overfilling oil also kills MASs. Excess oil vapour and droplets are vented into the airbox, and they get baked onto the hot wire.

Is the tickover low and uneven?

[MarJay]

[Robby]

Clean out the idle control valve and throttle body first in that case then, sounds like it just isn't getting enough air.

[finpos]

I'd check in a manual what the idle speed is really supposed to be - 200-400 sounds to me very low for any engine (700-900 would be more normal). Could be just that the idle base speed needs adjusting, but having said that, you'd probably just be adjusting out a problem somewhere else.

fin.

[finpos]

To satisfy my own sense of disbelief, I looked up your idle speed - it should be 650-710. Now all you have to do is find out why it's so low...

fin.