Slight hesitation from closed throttle

[chrisg]

Morning All !

Since doing the belts over the winter/ spring I've now done ~ 1500 miles on the turbo. I've noticed since doing it that when first started up (particularly if cold) it will fire up and then miss just once/ slight hiccup then idle fine.

Similarly, if driving along (in any gear, more noticeable at slow speed) if u come off the throttle completely then boot it hard it will hesitate/ stutter for a fraction then it's fine. Regarding all other aspects of running, tickover, full or part throttle under load it pulls like a train and boosts well.

It's just from closed throttle.

As I'm currently using it as my daily driver I'm reluctant to start fiddling.


Thought I'd ask if any has any thoughts in addition to/ instead of:

1. Throttle position sensor - either faulty or needs adjusting

2. Slight boost leak, particularly from the J-boot - caused by pulling apart to do the belts

3. Slight boost leak from any other pipes whether IC or small boost pipes.

Any suggestions or recommendations gratefully received !

Yours

Chris

 

[Lowtimer]

My thoughts:
- Small induction leak, could be anywhere really
- AFM a bit sticky or dirty-tracked, down at the low flow end of its range

 

[edh]

My experience of TPC problems was either a jerkiness coming off throttle ( "off" signal too early) or a raised idle ( no "off" signal) I know it has a variable resistance as well, so it might just be the cause. I have an old one kicking around somewhere if you want to try it and see if the fault persists.

Induction leak - could be sucking in unmetered air I suppose (Is this a problem on boost as well? Does the car reach the same peak boost levels?) I would expect an induction leak to be more noticeable elsewhere.

I also wonder about AFM - possibly causing a very short bit of overfuelling? First step might just be to whip off and clean the connector.

 

[chrisg]

Good thoughts ! thanks for this will pull the AFM off at the weekend and try cleaning it up/ checking its plugged in securely.

Yours

Chris

 

[edh]

If you're popping in to the Bedford trackday I could bring the TPS there ( Alex and possibly Ross are coming as well)

 

[zcacogp]

ORIGINAL: chrisg
Good thoughts ! thanks for this will pull the AFM off at the weekend and try cleaning it up/ checking its plugged in securely.

Have to say, AFM not doing what it should was my first thought too. Fairly easy to check the plug, and not that hard to take apart and clean the tracks on either.


Oli.

 

[924nutter]

Two things it isn't is a boost leak in either of the places you mentioned. If you look at your boost gauge from closed throttle you will notice it is something below 1 bar. In Porsche speak 1 bar on the gauge is normal atmospheric pressure, anything below means the engine is sucking harder the turbine can blow so you simply can't have a pressure leak.
The resistance tracks in the body of the MAF meter (term used advisedly as that is the Porsche part name) do get dirty and the contacts make intermittent contact with the track. Secondly if it is anything like the barn door type on normally aspirated cars the bushes or bearings wear with age and the flap momentarily sticks when snapping open the butterfly until manifold depression overcomes the "stiction".
Having done my belts last october I don't understand why you removed the J pipe, and without a hint of irony or sarcasm it would be great if you could tell members if you have found a new wrinkle or some kind of helpful info.

 

[944 man]

Im going to guess AFM too, having just replaced mine and sorted out the poor tickover and general roughness issues.

 

[chrisg]

Quick answer, I didn't remove the J pipe but did remove the AFM and air box and the two intercooler pipes thereby gaining full access to the belt casings.

Chris

 

[robwright]

Hesitation is with AFM for me mate []

 

[944 man]

What sort of MPG figures are you achieving Chris?

 

[robwright]

Chris I am still rebuilding my engine for upgrade so any parts you want to borrow AFM etc are yours if you want them to substitute.

 

[chrisg]

Rob

most kind of you, will probably take you up on the AFM borrow if that's OK ? Will text you about picking it up sometime.

Thanks,

Chris

 

[Hilux]

ORIGINAL: chrisg

Good thoughts ! thanks for this will pull the AFM off at the weekend and try cleaning it up/ checking its plugged in securely.

Yours

Chris

Those AFM`s are used on many cars of the era and low idle issues are common knowledge.

I had something silmilar on my old 944T but cleaning it got rid of my issue but IIRC I researched it and apparently what normally happens with that type of traverse contact (Motronic??) AFM is the wiper arm creates tracks in the contact plate.

The fix was to move the contact plate so it is fresh and flat where the wiper traverses and it `should` eradicate poor contact. This assumes the contact plate is adjustable but if you google I am sure you`ll find what I found.

Be careful they are very sensitive to change/damage.