Help!! Tearing my Hair Out Here.............

[The Aardvark]

This is a follow up to a couple of earlier threads with regards to the idling. In the last couple of days I’ve had 2 extremes –

1) Last Friday revs dropped when slowing down for a junction / parking etc. Dropped almost to zero and would stall unless I caught it, or moved off in a lower gear and ‘bumped’ it back to life.
2) Yesterday engine was revving high. While waiting for a car in front to turn right, revs were approx 1600 rpm, and at one point moving between 1400 – 1800 rpm approx as though my foot was on the accelerator.

Since the turn of the year, car has been to local specialist 4 times and had lamdas checked and replaced, ECU reset twice, stepper motor replaced, thermistor replaced which was out of range, and most recently a corroded earth on the wiring loom on the engine block cleaned and checked. Was also serviced 2 weeks ago and therefore am assuming that all leads and plugs were checked.
Just really want to get out there in the good weather and enjoy the car, but this issue is seriously damaging my confidence as to what it will do next. As a side thought, it only generally does it when the engine is up to running temp (approx 90 degrees in the current climate) and I’m finding it difficult as its an intermittent issue but the recent extremes are very hard to pinpoint!!

Does anyone have any ideas??!

Any help VERY much appreciated……….

[tvrbob]

There's one thing that many people miss when dealing with this issue. The stepper motor controls flow by moving the valve plug in and out of a valve seat. We spend much time worrying about the carbon deposits on the plug but very seldom do people mention cleaning the seat. I have cleaned the seat from time to time and have found that there are thick carbon deposits there too.

Not easy to access but worth the effort.

[blitzracing]

Any air leaks in the stepper thread, plenum base, or ANY rubber pipework, will make the tickover unstable, as will any sort of misfire. The variable RPM you are experiancing is a side effect of the ECU trying to keep the RPM correct when either mixture (like airleak) or ignition problems is making the tickover RPM drop faster than it should be, so the ECU overcompensates. When its ticking over when warm, disconnect the stepper and tickover should remain stable as long as its warm. This will eliminate stepper and its control circuitry (it wont like a cold start however.). Do a search on this forum for such problems !!

Also

http://www.g33.co.uk/fuel_injection.htm

About half way down the page.

Mark

[jayzcycles]

hi,i have had the same problem for years and cant find a cure..looking at the coil i found its leaking a bit of oil.im ordering one today if that makes a dif i will let you know.cheers .jay.

[The Aardvark]

Hi Jay

Thanks - had the AA out a couple of weeks ago and he mentioned that the coil looked like the original (its a 99 V reg) and may have started to give a weaker spark - so could be worth changing - please keep me posted!!

Thanks also Mark - bit above my head but will certainly refer upwards!!

And will also look at the stepper motor chamber for carbon deposits - I live in hope that this will one day get sorted.......

[tvrlee]

Hi, had a similar problem on mine, after looking at all the sensors etc i found that it was the AFM connection that was faulty, only found it by accident, car was running bad so i opened the bonnet and put my hand on the afm to lean into the engine bay and it must f moved something as the engine note changed instantly, so i cleaned up the connection and it now runs with no problem, i did double check it by pulling the connector off slightly, engine got lumpy and erratic straight away.

Made sure the connection stays on with a couple of cable ties!

Hope this helps

tvrlee

[The Aardvark]

Thanks for all suggestions so far - gives me something to work on, would just be nice to have the confidence that its not going to do something erratic.

With regards to cleaning the stepper chamber, I'm thinking cotton wool bud soaked in petrol or WD - or any other suggestions?

[tvrbob]

The Aardvark said:

With regards to cleaning the stepper chamber, I'm thinking cotton wool bud soaked in petrol or WD - or any other suggestions?

I use a lint free cloth wrapped around a piece of wood dowel and soaked in petrol. Comes out completely black so I assume it's doing some good.

[blitzracing]

Mr Muscle oven cleaner is fantastic for oil and carbon removal. It does darken the alloy at bit however.

Mark

[taylormj4]

jayzcycles said:

hi,i have had the same problem for years and cant find a cure..looking at the coil i found its leaking a bit of oil.im ordering one today if that makes a dif i will let you know.cheers .jay.

Yeah, Coils tend to begin to fail when the engine bay temperatures rise. A common failure is failure of the insulation between the high and low voltage sides of the coil or the coil windings to earth. The insulation level reduces with heat, so if it's on it's last legs it will fail initially when the engine bay is hot. If oil is leaking, this is definitely a sign of failing insulation, so a change would be advisable.

[The Aardvark]

taylormj4 said:

jayzcycles said:

hi,i have had the same problem for years and cant find a cure..looking at the coil i found its leaking a bit of oil.im ordering one today if that makes a dif i will let you know.cheers .jay.

Yeah, Coils tend to begin to fail when the engine bay temperatures rise. A common failure is failure of the insulation between the high and low voltage sides of the coil or the coil windings to earth. The insulation level reduces with heat, so if it's on it's last legs it will fail initially when the engine bay is hot. If oil is leaking, this is definitely a sign of failing insulation, so a change would be advisable.

can a faulty coil possibly cause revs to go high? driving back from work yesterday, and strangely while car was getting up to 80-90 degrees, again revs were approx 1500+ for a while. when AA had a look at the car a couple of weeks back, he mentioned that the coil looked original and could be something worth looking at with regards to the stalling issue (which yesterday wasnt present, revs dropped low at one point but did recover), but wasnt sure whether the coil could cause high revs??!
just trying to check all possibles........

[pringli]

No definitely not, this has all the hallmarks of an air leak problem as mentioned previously with ecu overcompensating, sorry cannot be more specific these are a bit of a bd without having someone else on hand to donate parts.

[The Aardvark]

think thats half the problem - intermittent problem that never seems to occur when in the hands of a TVR mechanic!
he says that everything else has been checked, and judging from input on here am working on the theory then that it is an 'air leak' issue and hence the fluctuations in revs. not much of a mechanic myself, but i'm presuming that a change in air flow would upset the air/fuel mixture and hence engine stability? is this something that would be more obvious when warm - would automatic choke when warming up help to keep it stable, as that is what it seems like?
one final thought - does this engine have a road speed sensor? reason i ask is that the other half has a golf that kept dropping engine revs to zero a couple of years ago while driving before firing back to life. took 4 trips to dealer before that was spotted, and described as faulty because it had become 'cloudy' and couldnt 'see' the speed of the flywheel and hence cut the fuelling (may not be quite accurate, but that was the general description)