Hesitation when setting off

JustAnotherSaver

As i come to set off in a morning or after work & i'm in 1st gear and occasionally 2nd, there is noticeable lag with the car, as though it thinks about getting a move on for a while before deciding to actually do it.

Spark plugs were changed a couple month ago for genuine ones & the throttle body is cleaned each year.

Weirdly, i'm actually getting the best MPG from this car than i ever have done in my 12 years of ownership. I used to hit about 33-36. Now i'm hitting 38-39 and on the last fill up i touched 41. I'd never hit 40 before, never mind 41. I'd previously done everything i could to hit 40 but when i hit an overall 41 on the last tankfull i didn't drive particularly economically at all. I also found tonight when i checked before setting off from work that one of my rear tyres was down at 16psi instead of 28psi. No idea how long it's been like that for. I also noticed that the oil was barely scraping minimum on the dipstick. Looks like i've got a bit of a leak around the engine.

The engine light comes on once every 1 to 2 months. More often than not it says:

P0140 - (8) o2 Sensor 2 Incorrect Signal - not present.

Any idea what this sudden lag is all about?

Norman_Castle

If its only when the engine is cold it might be the enrichment system isn't working as it should.

JustAnotherSaver

Norman_Castle said:

If its only when the engine is cold it might be the enrichment system isn't working as it should.

What's that when it's at home?

Yeah i only notice from cold. If i've been driving and i come to a set of reds and then come to set off for example, i don't notice it.

Although i do notice that if i'm climbing the revs in say 3rd & 4th then if i get say somewhere around the 4-5k region it pulls me back a little.

I remember that happening to me a while ago & i forget what the issue was. It could've been that that required a new ECU IIRC.

[Deleted User]

https://forums.moneysavingexpert.com/discussion/6086522/car-coughing-hesitation-when-driving/p1

same car?

AdrianC

O2 sensor 2 is probably a post-cat sensor, which only tells the ECU that the cat is working.

Norman_Castle

JustAnotherSaver said:

Norman_Castle said:

If its only when the engine is cold it might be the [cold start] enrichment system isn't working as it should.

What's that when it's at home?

Modern version of the choke. The ECU adjusts the fuelling based on sensor readings.

facade

JustAnotherSaver said:

Weirdly, i'm actually getting the best MPG from this car than i ever have done in my 12 years of ownership. I used to hit about 33-36. Now i'm hitting 38-39 and on the last fill up i touched 41. I'd never hit 40 before, never mind 41. I'd previously done everything i could to hit 40 but when i hit an overall 41 on the last tankfull i didn't drive particularly economically at all.

Pre-cat sensor reading rich, and leaning it out all the time?

Unusual as they normally read lean, and increase fuel consumption, but not impossible.

Get the emissions checked, if the lambda is above 1.00, I'd swap the pre-cat sensor as a start.

daveyjp

Is the type, model, age, engine of car you drive a state secret?

Goudy

Modern fuel injected petrol cars will use different fueling strategies depending on cold or hot running.
Hot running and the O2 sensors will trim the fueling back and forth from lean to rich depending on the heat of the exhaust gases passing over the O2.
Hot exhaust is due to a lean mixture so it adds fuel (high volt produced by the O2 sensor of around 0.9v).
Cooler exhaust is rich so it leans out the mixture. (low volt reading of around 0.1v) 
It will do this around once a second and it's aim it to get a trim average to around 0.45v.
This is called Closed Loop.

From cold start it doesn't use the above system to fuel, but uses a preset warm up fueling perimeters within the ECU.
It ignores the O2 sensor and tries to warm up the engine so it can switch to this closed loop as soon as possible.
This is called Open Loop.
These days ECU's don't wait too long before entering Closed Loop as they preheat the O2 sensors so they can quickly trip into the more efficient fueling strategy. 

This is often where you find the problems after a cold start. It starts ok, but once on the move it starts acting up.
The O2 sensors heater element or circuit might be broken or slow to heat and the ECU just can't get the fueling right to trip into Closed Loop so now you get a Open Loop "Fault" fueling strategy which is all lumpy and causes stutters and hesitations.

You'd think the ECU would start to complain about the issue and it will usually, but only after it detects the fault three or more times in a drive cycle when quite often the driver has stopped the engine and restarted it within a drive cycle, Restarting causes it to directly trip into Closed Loop, hence there's no more issue (until the next cold start).

As Adrian suggested the O2 senor 2 is the post cat sensor and only monitors the cats performance and most modern cars won't use them for fueling though some will (some Toyotas and Chryslers). I suspect the trouble is showing up with the code and there is a problem with the O2 heater circuit which will heat both pre and post cat sensors, but only the rear has complained as yet.

Another issue could be a small vacuum leak that the Open Loop just can't cope with at start up but the O2 can trim out a bit in Closed Loop, probably the best place to start.

JustAnotherSaver

blueskinnedbeast said:

https://forums.moneysavingexpert.com/discussion/6086522/car-coughing-hesitation-when-driving/p1

same car?

Nope. I'd have bumped the thread if it was.

Norman_Castle said:

JustAnotherSaver said:

Norman_Castle said:

If its only when the engine is cold it might be the [cold start] enrichment system isn't working as it should.

What's that when it's at home?

Modern version of the choke. The ECU adjusts the fuelling based on sensor readings.

Thanks. It feels like it's getting worse.

That said, i'm going to look at a car for sale tomorrow so all being well the current car will be replaced. That isn't a cert though.

daveyjp said:

Is the type, model, age, engine of car you drive a state secret?

Nope. Wasn't aware that something like this would be model specific aside from say being specific to being a petrol or diesel.

Rather than give 10 pages of info you may not need, i prefer to tell you what's happening. Anything you need to know, you can ask.

And it's free

Type? Not sure what you mean by type. Hatchback could be an answer.

Beyond that it's a 2001 1.8 Astra petrol.

greyteam1959

daveyjp said:

Is the type, model, age, engine of car you drive a state secret?

Looks a bit like it.
Basic info that would assist it giving an answer to the problem.