Misfiring on very light throttle
Discussion
Before I go and book the car in for a check, does anyone know what might cause a misfire while under a very light throttle, i.e. just pottering along at 40 mph? I can feel is missing, occasionally there is a bigger misfire. It seems to disappear when accelerating.
Also, when I fire up the car from cold and I touch the accelerator slightly the revs DROP, as soon as the engine has warmed up a little though, this stops happening. Any ideas folks, or is it one of a million possibilities?
Also, when I fire up the car from cold and I touch the accelerator slightly the revs DROP, as soon as the engine has warmed up a little though, this stops happening. Any ideas folks, or is it one of a million possibilities?
Simple check - have a look under the bonnet in the dark while the engine is running to check that there is no sparking to earth.
Timing hasn't been changed has it? - if it slightly advanced it would suffer when you just touch accelerator (I would think it hasn't, no-one touches timing without good reason)
Otherwise see my trouble on Ignition Module thread
TonyP
Timing hasn't been changed has it? - if it slightly advanced it would suffer when you just touch accelerator (I would think it hasn't, no-one touches timing without good reason)
Otherwise see my trouble on Ignition Module thread
TonyP
Could be the throttle pot, but you can't just reposition these to reuse a different bit of the track - they need to be calibrated to give the correct reading at tickover. Try disconnecting the vac advance line (and blocking the port on the throttle body) - does that help? If you had an inlet manifold air leak this would show up under light throttle, would affect idle too though I would have thought. If you had an exhaust leak this would throw the lambda sensors out and lead to overfuelling under light loads. If you have a duff water temp sender it can lead to overfuelling. If the fuel pressure regulator is failing or has a leak in the vac line this would lead to overfuelling. Basically lots of things to check and a good mechanic would soon get there by elimination but trying to solve this yourself by guesswork could be a long a frustrating job. This is something I'd hand over to the experts unless you're really keen to tackle it yourself.
Ooooerrr...
OK, there is a whiff of petrol occasionally, sometimes when I come to a standstill, maybe the pump is faulty or maybe as suggested its too much fuel being dumped into the pots, so there's one thing that might be amiss. Also I think that the exhaust might be blowing a very small amount after it bottomed out during some spirited driving one day. I did briefly have the idling going nuts, it would stick at 1800 rpm, then after restarting the engine it would almost stall (200-300 rpm), although this problem has since gone away after I cleaned the stepper (maybe co-incidence?). I spoke at length to Mark Adams when the problems first started about 3 weeks ago with the idling and slight misfire problems, he did say that it sounds like I should get it on a rolling road to determine whats up with it, but like most people I suppose I am fishing for an easy solution before having to spend lots of money again
Thanks for the advice guys.
OK, there is a whiff of petrol occasionally, sometimes when I come to a standstill, maybe the pump is faulty or maybe as suggested its too much fuel being dumped into the pots, so there's one thing that might be amiss. Also I think that the exhaust might be blowing a very small amount after it bottomed out during some spirited driving one day. I did briefly have the idling going nuts, it would stick at 1800 rpm, then after restarting the engine it would almost stall (200-300 rpm), although this problem has since gone away after I cleaned the stepper (maybe co-incidence?). I spoke at length to Mark Adams when the problems first started about 3 weeks ago with the idling and slight misfire problems, he did say that it sounds like I should get it on a rolling road to determine whats up with it, but like most people I suppose I am fishing for an easy solution before having to spend lots of money again
Thanks for the advice guys.
Keith, it sounds as though I have a similar problem to you.
My 1992 4L Griff seems to have a similar problem to yours. About 2 years ago it very occasionally cut out without warning and this has been accompanied by a strong smell of petrol. After putting in a reconditioned ECU and rewiring some of the loom, replacing the HT leads and various other bits and pieces I thought Fernies had cracked it. It's just started intermittently hesitating and dying on a light throttle but no smell of petrol this time so far.
Basically pulling out of junctions on a light throttle causes the engine to almost die, this is recoverable by revving like mad at which point it revs perfectly, but once it starts misbehaving it doesn't stop all the way home and you have to rev like mad out of every junction. It almost feels as if it is barely running and indeed it cut out with a misfire and a puff of smoke from the exhaust manifolds on one occasion.
The problem happened the last 3 times I took the car out, including when I tried to drive it to Fernies for investigation. In the end the RAC trailered it down there and of course Fernies have had it a week and can't find anything wrong because it's running perfectly!!
If anyone has any ideas I'd be really grateful - my wife is now scared to take it out on her own and it's a jointly owned car. I really need to sort this out because frankly even I'm edgy about using it and it's taking the fun out of having it.
Thanks,
Jonathan
My 1992 4L Griff seems to have a similar problem to yours. About 2 years ago it very occasionally cut out without warning and this has been accompanied by a strong smell of petrol. After putting in a reconditioned ECU and rewiring some of the loom, replacing the HT leads and various other bits and pieces I thought Fernies had cracked it. It's just started intermittently hesitating and dying on a light throttle but no smell of petrol this time so far.
Basically pulling out of junctions on a light throttle causes the engine to almost die, this is recoverable by revving like mad at which point it revs perfectly, but once it starts misbehaving it doesn't stop all the way home and you have to rev like mad out of every junction. It almost feels as if it is barely running and indeed it cut out with a misfire and a puff of smoke from the exhaust manifolds on one occasion.
The problem happened the last 3 times I took the car out, including when I tried to drive it to Fernies for investigation. In the end the RAC trailered it down there and of course Fernies have had it a week and can't find anything wrong because it's running perfectly!!
If anyone has any ideas I'd be really grateful - my wife is now scared to take it out on her own and it's a jointly owned car. I really need to sort this out because frankly even I'm edgy about using it and it's taking the fun out of having it.
Thanks,
Jonathan
Jonathan, your problem sounds worse than mine, I hope you find the cause soon. I imagine all that revving attracts a bit of attention!
I took mine out again yesterday and the misfire seems to be intermittent on a light/feathered throttle (although it is there most of the time), when I accelerate it pulls well but there does seem to be a bit of a flat spot before the power really comes in. The misfire is worse when the engine is at 90 degrees than when at 80. Sometimes the problem seems to go away, but not for long, so maybe there's a duff sensor cauing the problem. It does seem to pop and bang on the overrun more than it used to, which would would point to overfuelling I am guessing.
Hopefully its not a major problem (fingers crossed) and after some rather expensive house refurb projects recently I can't afford a big car related bill at the moment, so the car will have to soldier on as it is for the time being. I used to spanner on my old TVRs but the modern ones seem quite complicated (electronics I mean) and I'm reticent to fiddle with anything. I think it really is a job for an expert, as was suggested earlier in the thread.
>> Edited by KeithS on Monday 16th June 13:29
I took mine out again yesterday and the misfire seems to be intermittent on a light/feathered throttle (although it is there most of the time), when I accelerate it pulls well but there does seem to be a bit of a flat spot before the power really comes in. The misfire is worse when the engine is at 90 degrees than when at 80. Sometimes the problem seems to go away, but not for long, so maybe there's a duff sensor cauing the problem. It does seem to pop and bang on the overrun more than it used to, which would would point to overfuelling I am guessing.
Hopefully its not a major problem (fingers crossed) and after some rather expensive house refurb projects recently I can't afford a big car related bill at the moment, so the car will have to soldier on as it is for the time being. I used to spanner on my old TVRs but the modern ones seem quite complicated (electronics I mean) and I'm reticent to fiddle with anything. I think it really is a job for an expert, as was suggested earlier in the thread.
>> Edited by KeithS on Monday 16th June 13:29
Hi,
Just got the car back from Fernies and I think they've cured it - replaced the ignition amp, rotor arm and distributor cap and all went well on the 40 mile run home last night.
They also did a clutch master cylinder replacement, MOT and brake fluid top up. Finally they did an oil change and filter.
That little lot was £300 labour and I was wondering if that seemed reasonable? I was hoping for a bill of around £300 total (they had said earlier in the week that it was around £200 before they did the electrical work) but in the end with labour it was £535.
To be honest I've just given up my job and am about to put myself through a very expensive training course for a new career so maybe I am just being very sensitive to cash at present! Fernies have looked after my cars for the 8 years I've owned TVR's and I've recommended them to others. I'm really just curious if I was to use a specialist independent whether there's be much of a saving (while cash is tight)
Thanks,
Saxon
Just got the car back from Fernies and I think they've cured it - replaced the ignition amp, rotor arm and distributor cap and all went well on the 40 mile run home last night.
They also did a clutch master cylinder replacement, MOT and brake fluid top up. Finally they did an oil change and filter.
That little lot was £300 labour and I was wondering if that seemed reasonable? I was hoping for a bill of around £300 total (they had said earlier in the week that it was around £200 before they did the electrical work) but in the end with labour it was £535.
To be honest I've just given up my job and am about to put myself through a very expensive training course for a new career so maybe I am just being very sensitive to cash at present! Fernies have looked after my cars for the 8 years I've owned TVR's and I've recommended them to others. I'm really just curious if I was to use a specialist independent whether there's be much of a saving (while cash is tight)
Thanks,
Saxon
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