Most likely culprit? TPS or pedal?

[Bowser]

At the risk of firing the parts cannon - which of the above parts would likely cause surging on very light, constant throttle?
ie, cruising in 5th at 60mph
I've done the basics, leads, coil, plugs and also changed MAP sensor and cleaned throttle body.
I wonder if the sensor in the pedal is at fault, although can't see much evidence that these generally fail.
Thanks

 

[STEVE.M]

At the risk of firing the parts cannon - which of the above parts would likely cause surging on very light, constant throttle?
ie, cruising in 5th at 60mph
I've done the basics, leads, coil, plugs and also changed MAP sensor and cleaned throttle body.
I wonder if the sensor in the pedal is at fault, although can't see much evidence that these generally fail.
Thanks

I`d be looking for an air leak?

 

[Bowser]

Good idea.
Inlet manifold and throttle body gaskets spring to mind.
Any tips on finding a leak other than spraying carb cleaner around the engine?

 

[STEVE.M]

Good idea.
Inlet manifold and throttle body gaskets spring to mind.
Any tips on finding a leak other than spraying carb cleaner around the engine?

Air leaks other than gaskets/O rings could be caused by leaking vacuum hoses for carbon canister, acoustic valve, or less likely as you would probably experience a poor braking problem, the brake servo pipe.
The MAP or air temp sensor at the other end of the inlet manifold can be a loose fit too.
@bloke any ideas?

 

[bloke]

air leak should show up on idle, will almost try and switch from low to high revs. i'd guess at lambda possibly?

 

[Bowser]

Lambas replaced with NGKs.
It actually idles really well, and starts first time.
Only issue i have is this slight surging at constant low rpm throttle.
(And the fact it failed on fast idle emmissions...Retest on Weds, and i've even resorted to a can of Cataclean)

The air temp sensor is indeed a loose fit, but after investigating my loose MAP sensor and snapping it off, i decided to leave him alone.
French plastic really doesn't like cold weather.
I will check for air leaks tomorrow.

 

[STEVE.M]

@Bowser
A little information as to how long you have had the car, how familiar you are with it etc would go a long way.
Does it hunt about on tickover, normally a steady750/800 rpm when hot?

 

[Bowser]

Yes its a relatively steady 800 rpm on hot tick over. Maybe a hint of hunting.. But that's more audible than actually having a discernible affect on tacho needle.
Even cold, its a fairly steady 1000 rpm.

I've had the car 3 1/2 years, and have replaced brakes, suspensions, driveshafts and numerous other parts, So i know my way around it.

This surging problem seems to have got slightly worse since i left the car for from November - Feb, without being used.
However, she still drives smoothly (no probs pulling away) and revs happily - its just constant slight throttle that it doesn't like.

 

[Bowser]

Acoustic valve you say....? Hmm.. i remember removing that a year or so back, but i don't recall blocking of the pipe..

 

[STEVE.M]

@Bowser
Good that you know your car and the time that is has been in your ownership.

As it has stood for 3 months, fill up with fresh fuel, nail it for a couple of hundred miles, and fingers crossed it will sort itself.

 

[Bowser]

I've done all that - but hoping your're onto something with the air leak.
Possibly any inlet air leak could affect emissions..and my failure. MOT tester said it was running slightly lean.

 

[STEVE.M]

Acoustic valve you say....? Hmm.. i remember removing that a year or so back, but i don't recall blocking of the pipe..

And the top of the carbon canister has a habit of popping off.
Cap the take off here.
Acoustic valve hose take off point.

 

[Bowser]

Yep, will block off tomorrow - i've got a good feeling about this lol
Not touched my carbon canister yet

 

[STEVE.M]

Yep, will block off tomorrow - i've got a good feeling about this lol
Not touched my carbon canister yet

You could cap the carbon canister in the short term, just to see if you loose the surging.

 

[Bowser]

Acoustic valve vacuum was already blocked. (Would have been surprised if i hadn't really)

Will cap carbon canister as per your picture. Just need the feeling to come back to my hands.. fook its cold.

 

[Bowser]

Been out today, and no change.
Couldn't find any leaks, using the carb cleaner method.
Even stuck some Red ex in (im getting desperate)

 

[STEVE.M]

Been out today, and no change.
Couldn't find any leaks, using the carb cleaner method.
Even stuck some Red ex in (im getting desperate)

tbh, take your pick, ignoring the bits like EGR valves that are not applicable.

https://www.autoserviceprofessional.com/article/92420/The-dreaded-engine-surge

The power steering pressure switch is there to raise tickover on full lock to prevent the engine stalling.

Why did you change the lamba`s? not something many people do for the sake of it, and @bloke is very often on the money.

Where are you based?

 

[Bowser]

Interesting article, if slightly depressing.
I tested the injectors today, and they all measured 13.5 ohms. Im not sure that is conclusive however.

I changed the Lambas because the car failed on Fast Idle emissions.
Based in Suffolk 15 mins south of Bury St Edmunds.

 

[STEVE.M]

Interesting article, if slightly depressing.
I tested the injectors today, and they all measured 13.5 ohms. Im not sure that is conclusive however.

I changed the Lambas because the car failed on Fast Idle emissions.
Based in Suffolk 15 mins south of Bury St Edmunds.

I can only suggest you have a word with Engine Dynamics near Colchester, who have the experience & kit to get to the bottom of it.
Better than spending more money on parts that don`t fix the problem.