Assistance sought in diagnosing 'stumbling off boost'

[Uncle Arthur]

Hi all,

Just after some general ideas/thoughts.....

The car has developed a stumble off boost - at around 2500-3000 at cruise - be it in 3rd/4th/5th, the car will stumble - like it's coughing.

Watching the stoichmeter this morning, it looks as though it's running way rich when it happens. On boost - all is good and it goes like a rocket - but then again when I'm on boost the O2 signal is intercepted and the fueling held at rich.

Anyone got any ideas what it might be?

My first thought is the O2 sensor, but after that, I'm also thinking it could be the mixture screw setting on the (RX7) AFM.

What about fuel filter? I don't know when it was last changed.....

Injectors?

Any assistance will be most welcomed, before I take it it Mazda (because it really doesn't seem to be a boost issue)

[Fatty]

so you already have an o2 clamp intercepting the o2 sensor? is that what you're saying?

coz all i was gonna suggest is to get an o2 clamp


when my car was \"stumbling\" as you've described, it was the leads. from memory it was at similar rpms as well, about 2500 - 3000.

of course it could be the short nose crank, get boagsy to check it for ya.

[NMX516]

Jules broke it!!!

UA I used to have that sort of problem in my turbo'd Pulsar. It was caused by the wire/plug from the airflow meter being bumped loose at the computer. I would start by checking that everything is connected properly, both at the sensor end and the ECU end.

[Uncle Arthur]

so you already have an o2 clamp intercepting the o2 sensor? is that what you're saying?

coz all i was gonna suggest is to get an o2 clamp


when my car was "stumbling" as you've described, it was the leads. from memory it was at similar rpms as well, about 2500 - 3000.

of course it could be the short nose crank, get boagsy to check it for ya.

I don't have a short nose crank ;) - but I do have fuel

I'm also now thinking it might be leads - it happens under light application of throttle under load, and goes away as the revs increase - and I replaced the damn leads not that long ago.....

[AJ]

try OEM this time mikey, instead of them thar fancy coloured ones

[Woo]

What sort of boost do you run UA and what/if any BOV do you have?

[Uncle Arthur]

try OEM this time mikey, instead of them thar fancy coloured ones

But I'm using plain black Top Gear leads from Repco.....

[Boags]

Does it stumble under light excelleration, or when you are cruising at those revs?

If you plant foot to get the boost up before 2500rpm do you get the stumble?

It does sound a bit like leads, that sucks seeing as they are new.

Boags

[Uncle Arthur]

Does it stumble under light excelleration, or when you are cruising at those revs?

If you plant foot to get the boost up before 2500rpm do you get the stumble?

It does sound a bit like leads, that sucks seeing as they are new.

Boags

Light acceleration - and no - planting it means no (or not noticeable) stumble - hence why I think it is the leads. It's the thing that makes the most sense, and would also account for it reading rich when stumbling - lots of fuel and uneven spark.....

[Boags]

Light acceleration - and no - planting it means no (or not noticeable) stumble - hence why I think it is the leads. It's the thing that makes the most sense, and would also account for it reading rich when stumbling - lots of fuel and uneven spark.....

Yeah, agreed. How many Ks have they done?

[Uncle Arthur]

Light acceleration - and no - planting it means no (or not noticeable) stumble - hence why I think it is the leads. It's the thing that makes the most sense, and would also account for it reading rich when stumbling - lots of fuel and uneven spark.....

Yeah, agreed. How many Ks have they done?

Not enough - with the mileage I'm doing these days less than 7500

[JBT]

Jules broke it I think UA.

[jules]

Oh stop it you heathens! Max enjoyed her stay with me. I was like a priest at a boys school. Very well behaved

My thoughts when it did it was leads, loose wiring ('cause it does it over bumps), or fueling.

It would do this on constant throttle often as well Mikey, ie: cruising with throttle 1/3rd of the way down in that rev range. It is not a boost issue
A bit like a supermodel with a zit.

Jules

[Uncle Arthur]

Oh stop it you heathens! Max enjoyed her stay with me. I was like a priest at a boys school. Very well behaved

Rest assured Jules - I didn't for a moment think you broke her.....

My thoughts when it did it was leads, loose wiring ('cause it does it over bumps), or fueling.

It would do this on constant throttle often as well Mikey, ie: cruising with throttle 1/3rd of the way down in that rev range. It is not a boost issue
A bit like a supermodel with a zit.

Jules

Can you tell me when you first noticed it happening? There'd often been a slight stumble over bumps (which I thought might be due to my having not enough slack in the throttle cable), but this is something completely new.....

I've thought TPS position, I'll look at all the connections and make sure they're firm, but any other suggestions are welcomed.......

I'm not keen on buying brand new leads just for the sake of it, so is there a key thing to look for as a sign of failure? There is no visible sign of external arcing under idle.

[JBT]

....There'd often been a slight stumble over bumps....

Sounds like a loose connection or an intermittent short circuit. But where? Start at the battery end, check all connections and no loose cables there.