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So, The Beast is my loyal steed, it just recently something irritating has started to happen.

In short, when I decelerate, the revs drop away entirely and the engine cuts out - but not every time.

There's no spluttering or owt from the engine, the revs just drop off. I can avoid it by gunning the accelerator a bit, when it then typically idles ok.

Please tell me this is an easy fix!

I think your loyal steed may need to see a metallic vet.

Sounds like something fairly serious has gone wrong.
Engine cutting?

Electrical?
I mean it need it for spark plugs etc and chips.

By decelerate do you mean take you foot off the throttle or braking? By the way you describe I'm guessing you mean not applying the throttle.

How does the engine run on idle after you start it? Will the car start again immediately after it stalls? Old or new car?

My guess is you ain't getting the right amount of fuel at idle. With absolutely no car maintenance experience to back my suggestion, I'm guessing either the Engine Control Unit or fuel injector.


Answers from google: https://www.yourmechanic.com/services/when-i-slow-down-and-stop-the-car-engine-shuts-off-inspection

In an older car I had, a Citroen saxo vtr (oh back in my youth again) I had the exact same problem it was the ECU which at 1st they recalibrated ( by them i mean a mechanic) which sorted it for a while and then it started again and I had to get it replaced. My car when idle also began dropping revs and stalling only sorted by applying the accelerator.

It's quite old, 1999 Mazda 626.

She idles fine on start up. This is happening on deceleration.

I'm not a mechanic, but I'm cheap!

First off, it might be helpful to know what make/model of car it is. Personally, with no info, my bet is maybe the throttle position sensor. I say this because I had this problem some 15 years ago and that's what fixed it. Could be a lot of things though.

Maybe call a garage and not go to dakka? (I'm reminded of the "I've been coughing blood all night, better check dakka" meme that Ouze or someone made years ago)

But where's the fun in that :p

Reckon I'll get a quote from a couple of garages for fuel pump replacement.

 Mad Doc Grotsnik wrote:
This is happening on deceleration.

You'd need to be specific here Doc - does deceleration mean reduced throttle input, no throttle input or application of brakes?

Given that it looks like you've got a fuel flow issue you haven't got much choice but to get it to a garage.

Actually, for a more serious response, now that I'm thinking about it:

Check your transmission fluid level. It's free and easy.

Is your check engine light on? Apparently Mazda doesn't have a key dance you can do to get the error codes and you have to wire a harness to the OBD jack (!?). That's gonna make it harder to self-diagnose if so.

So the following instructions may be hard to follow if you have no mechanical experience at all but I will give it a go:

Engine off while you do this:

1. There is a big box on the right side of the engine (facing engine bay from front of car) that houses the air filter.

2. Follow this tube to the back of the engine and you will see where its held to the engine by a hose clamp. This is the throttle body.

3. Remove the tube from the throttle body and you will see a (usually) copper looking butterfly valve in there that if someone was to push the accelerator pedal will open.

4. Shine a flashlight in there and look at the butterfly itself as well as inside when it is open.

5. If it is black then I would start by cleaning the entire thing with carburetor/intake cleaner (comes in a can with a little red straw to make it easier to direct the flow).

6. Let the car sit with the valve open for a bit while this evaporates, and then reconnect the tube and try to start the car.

7. It may be difficult to start and may take a few minutes/run like poop until the carb cleaner clears out of the cylinders but it will clear up after a few seconds of running and maybe a couple of quick revs on the engine.

I have had a few cars that would do this and this was always the fix. Its caused by the PCV valve on the valve cover allowing oil to go by and be re burned in the engine via a tube that connects the valve to the intake tube that you remove from the throttle body.

Failing this, it may be a bad idle air control valve, which is a bigger problem with Hondas than it is with Mazdas but lots of 4 cylinder engines have the same issue as they age.

Good luck and I take no responsibility for anything that happens but its a very straight forward process


Automatically Appended Next Post:
To expand that explanation: The vapor that is pulled from the crank case coats the inside of the throttle body with a film of oil. This film prevents the fuel/air mix from being measured and adjusted accordingly on deceleration. When in normal idle the engine "computer" can adjust accordingly, and while driving there is enough fuel and air to overcome the effect. When decelerating the engine is switching between the part of the throttle body that controls idle and the part that controls normal driving operation under power. In this transition the idle can't adjust quickly enough to compensate for the additional fuel required to keep the engine running before it dies.

Cleaning this film out allows the air to flow as intended and should fix the issue

Have you tried prayers and incense? Holy oils? Appease that machine spirit!

Back in the day I had an old crown vic station wagon with a loose idle. There was a little screw that controlled how fast the engine ran, but it would shake itself loose after a bit. Had to tighten it up every few weeks.

If you want any useful help you need to be *very* specific about what's happening. As Henry said, is it caused by lifting the throttle? Is it the application of brakes that's causing the problem? Or does the engine just slow down with the car itself until it eventually cuts out?

Also is your car manual or auto? Can you put it in neutral and will that stop it cutting out?

I drove a mate's car that would cut out as you came to a stop, you could put your foot on the accelerator a bit as you were coming to a stop and it wouldn't cut out, turned out the throttle body needed cleaning out. Another car I had used to stutter when you'd lift off the throttle and it was a tuning issue (carburetted car), a different car again I had would cut out easily when lifting and the engine was cold and it turned out to be a vacuum leak in the intake manifold (though she was also a dog to start).

 Mad Doc Grotsnik wrote:
It's quite old, 1999...

That's not old, she's just a young pup!

Pray to the Omnissiah, for your vehicle's machine spirit has become weary.

(Sorry I don't have any really helpful advice, I just had to slip that in there )

AllSeeingSkink wrote:
If you want any useful help you need to be *very* specific about what's happening. As Henry said, is it caused by lifting the throttle? Is it the application of brakes that's causing the problem? Or does the engine just slow down with the car itself until it eventually cuts out?

Also is your car manual or auto? Can you put it in neutral and will that stop it cutting out?

I drove a mate's car that would cut out as you came to a stop, you could put your foot on the accelerator a bit as you were coming to a stop and it wouldn't cut out, turned out the throttle body needed cleaning out. Another car I had used to stutter when you'd lift off the throttle and it was a tuning issue (carburetted car), a different car again I had would cut out easily when lifting and the engine was cold and it turned out to be a vacuum leak in the intake manifold (though she was also a dog to start).

 Mad Doc Grotsnik wrote:
It's quite old, 1999...

That's not old, she's just a young pup!

Your mate's cars prob sounds identical.

Provided I can kick some extra revs in, nothing untoward happens.

Thankfully it doesn't make her dangerous or difficult to drive. Just mildly irritating.

Without seeing the car, and assuming it is a 1999 model (not a car that was sat around for a few years unregistered) so a 6th gen, and a petrol I4, I would take a guess at one of the following:

Lambda (O2) seonsor (cheap and likely)
Throttle position sensor (cheap but less likely)
Dirty throttle body (cheap to clean or replace, if they are easy to get at, but could be a sign of another issue)
ECU (cheap if you can find a scrapyard one to fit.)
Wireing fault (could be cheap, could not be)

I would take it to be looked at. I would guess the lambda sensor, which are about £20 and easy to fit, but I can't be sure.

I had a similar problem on my old 98 Megane; ended up having to call out a breakdown van. The guy initially thought it was the ECU, but turned out to be the hose connecting the inlet manifold to the ECU.

Essentially a vacuum leak like AllSeeingSkink was describing.

Basically, as in a lot of cars around that time, the engine had originally been designed for a carburettor, but they'd modified it to use fuel injection to meet newer emissions regulations, etc. In order to adjust the injection mapping for different driving conditions the ECU needs a sensor in the inlet manifold, but rather than mounting the sensor directly on the engine they'd used a little rubber hose to go from the inlet to a chassis mounted sensor. The rubber had aged and cracked, which meant that it was giving the wrong pressure reading, resulting in the wrong fuel mixture and cut-out. Didn't affect start-up because the automatic choke setting overrides the fuel mixture.

Obviously could still be the ECU, inlet manifold leak or some other random thing, but worth checking the easy (and cheap) things first.

Agree with some of the above posters. it is must likely the mass flow sensor on your air intake gunked up and not reading correctly, or not working at all, causing the engine to run lean.

 daedalus wrote:
Maybe call a garage and not go to dakka? (I'm reminded of the "I've been coughing blood all night, better check dakka" meme that Ouze or someone made years ago)

In addition to medical advice, the OT is also good for making life-shaping decisions on how to educate your special needs children as well as removing scale from bathtubs.

I am continually amazed by the kind of thing people think this forum is a good resource for.

 Ouze wrote:
removing scale from bathtubs.

Oh, well, that's easy. All you need is a 5 gallon bucket of cocoa powder and then... hey, wait!

 Mad Doc Grotsnik wrote:
But where's the fun in that :p

Reckon I'll get a quote from a couple of garages for fuel pump replacement.

No no no.

Get it looked at, find the problem, then get a quote. Don't get quotes for something unless you are sure that is the problem.

IronWarLeg wrote:
So the following instructions may be hard to follow if you have no mechanical experience at all but I will give it a go:

Engine off while you do this:

1. There is a big box on the right side of the engine (facing engine bay from front of car) that houses the air filter.

2. Follow this tube to the back of the engine and you will see where its held to the engine by a hose clamp. This is the throttle body.

3. Remove the tube from the throttle body and you will see a (usually) copper looking butterfly valve in there that if someone was to push the accelerator pedal will open.

4. Shine a flashlight in there and look at the butterfly itself as well as inside when it is open.

5. If it is black then I would start by cleaning the entire thing with carburetor/intake cleaner (comes in a can with a little red straw to make it easier to direct the flow).

6. Let the car sit with the valve open for a bit while this evaporates, and then reconnect the tube and try to start the car.

7. It may be difficult to start and may take a few minutes/run like poop until the carb cleaner clears out of the cylinders but it will clear up after a few seconds of running and maybe a couple of quick revs on the engine.

I have had a few cars that would do this and this was always the fix. Its caused by the PCV valve on the valve cover allowing oil to go by and be re burned in the engine via a tube that connects the valve to the intake tube that you remove from the throttle body.

Failing this, it may be a bad idle air control valve, which is a bigger problem with Hondas than it is with Mazdas but lots of 4 cylinder engines have the same issue as they age.

Good luck and I take no responsibility for anything that happens but its a very straight forward process


Automatically Appended Next Post:
To expand that explanation: The vapor that is pulled from the crank case coats the inside of the throttle body with a film of oil. This film prevents the fuel/air mix from being measured and adjusted accordingly on deceleration. When in normal idle the engine "computer" can adjust accordingly, and while driving there is enough fuel and air to overcome the effect. When decelerating the engine is switching between the part of the throttle body that controls idle and the part that controls normal driving operation under power. In this transition the idle can't adjust quickly enough to compensate for the additional fuel required to keep the engine running before it dies.

Cleaning this film out allows the air to flow as intended and should fix the issue

Good advice about the carb cleaner, also try to squirt some down the little drilling's on the side of throttle body (idle bypass holes, that bleed air behind throttle butterfly) and check EGR valves not stuck. (used to foul the fuel/air ratio and lower nitrous oxide emissions on overun) .

As a certified ork mechanic I say at findin the biggest hamma you got and wack the engine proper good-like. After a few knocks she'll come round, if that don't work give it a harder jobbin til it does.

Is there white smoke coming out of the exhaust pipe?

pismakron wrote:
Is there white smoke coming out of the exhaust pipe?

There maybe if stanman gets to wack it with a big hammer

 loki old fart wrote:

pismakron wrote:
Is there white smoke coming out of the exhaust pipe?

There maybe if stanman gets to wack it with a big hammer

“percussive maintenance” is the technical term.

As someone who’s worked computer tech support, when the magic smoke leaves the box, it stops working. Computers run on the stuff; once it escapes, it will no longer function. Now cars produce non-magic smoke as part of their normal operation, so seeing that escape the vehicle is fine. But once the magic smoke leaves, you are going to need a lot of holy incense to replace it.

Hitting it with a hammer: £0.50
Knowing where and how hard to hit it: £399.50

 Nevelon wrote:

 loki old fart wrote:

pismakron wrote:
Is there white smoke coming out of the exhaust pipe?

There maybe if stanman gets to wack it with a big hammer

“percussive maintenance” is the technical term.

As someone who’s worked computer tech support, when the magic smoke leaves the box, it stops working. Computers run on the stuff; once it escapes, it will no longer function. Now cars produce non-magic smoke as part of their normal operation, so seeing that escape the vehicle is fine. But once the magic smoke leaves, you are going to need a lot of holy incense to replace it.

That's magic blue smoke. Cars have lots of that in too that can't escape. Only the white smoke is allowed to escape as that's not magic. Blue smoke is what is inside integrated circuits, which is what makes them workz