Engine abruptly loses all power for a second or two
Discussion
I have a weird intermittent problem that happens really quite rarely, and apparently randomly.
The engine abruptly loses all power for a short moment, perhaps only half a second or perhaps a second or two, and I have had it last for up to 3 or 4 seconds. During this time, there is still an exhaust note - the engine is still running perfectly smoothly, but it's as if something has told the ECU that the driver has suddenly lifted right off the throttle. And then, equally abruptly, the power suddenly comes back with a jolt.
It happens perhaps once every hour or so of driving, although sometimes the car will seem to go for a few weeks without doing it and then might do it twice on one journey. There seems to be no pattern to when it happens; it is totally unpredictable. If it were to happen mid-way through an overtake it could be a brown-trouser moment, and likewise if the power is suddenly removed and restored going round a wet roundabout I could end up pointing the wrong way. Thankfully so far it has only ever happened when I'm going straight, and not when I'm really relying on engine power.
The only thing I can think of is something to do with the throttle position sensor. Could that cause the ECU to believe that the driver has lifted off, and would that give these symptoms? Does the ECU use the TPS to help decide how much fuel to use, or is it driven entirely by the lambdas? It's tempting to think that there might be a loose connection, but the problem doesn't happen after a bump in the road, and why would a loose connection last for several seconds and then go back to normal for hours?
Anything else?
The engine abruptly loses all power for a short moment, perhaps only half a second or perhaps a second or two, and I have had it last for up to 3 or 4 seconds. During this time, there is still an exhaust note - the engine is still running perfectly smoothly, but it's as if something has told the ECU that the driver has suddenly lifted right off the throttle. And then, equally abruptly, the power suddenly comes back with a jolt.
It happens perhaps once every hour or so of driving, although sometimes the car will seem to go for a few weeks without doing it and then might do it twice on one journey. There seems to be no pattern to when it happens; it is totally unpredictable. If it were to happen mid-way through an overtake it could be a brown-trouser moment, and likewise if the power is suddenly removed and restored going round a wet roundabout I could end up pointing the wrong way. Thankfully so far it has only ever happened when I'm going straight, and not when I'm really relying on engine power.
The only thing I can think of is something to do with the throttle position sensor. Could that cause the ECU to believe that the driver has lifted off, and would that give these symptoms? Does the ECU use the TPS to help decide how much fuel to use, or is it driven entirely by the lambdas? It's tempting to think that there might be a loose connection, but the problem doesn't happen after a bump in the road, and why would a loose connection last for several seconds and then go back to normal for hours?
Anything else?
Alas, I've never had the presence of mind to look at the rev counter. I will try to remember next time it happens. Would that indicate an ignition issue? My gut feeling is that the engine is still firing normally when this happens, because the exhaust note sounds normal.
Other ideas I've had: fuel pump? What would happen if it were to cut out momentarily? And if it were the fuel pump, could that actually indicate an immobiliser issue?
Other ideas I've had: fuel pump? What would happen if it were to cut out momentarily? And if it were the fuel pump, could that actually indicate an immobiliser issue?
Some very helpful suggestions and ideas - thanks folks!
The coil was replaced a couple of years ago after I broke down near Paris. The replacement is a decent Bosch one, so it shouldn't be that, although I can't rule it out. The ignition amp is an interesting idea as well. I do have a new set of HT leads to go on, although I doubt that'll be it.
Constant throttle - hmmm, yes that might be true. It has certainly happened while cruising along the M6 toll, which is as smooth a road as you'll find so I don't think it's caused by vibration or jolting. Mind you, it happened a few days ago a moment after I'd changed gear. Next time it happens I'll try to remember to lift my foot right off the throttle - probably a good idea anyway to avoid the jolt as the power comes back.
I do keep coming back to the key fact that when it happens, the engine certainly doesn't go silent - it sounds exactly the way it does when you lift off the throttle, i.e. there's still a normal engine note.
Sardonicus, when you refer to the throttle pot, I assume that's the same thing that I referred to as the throttle* position sensor - we're just using different words for the same thing? Where is it - somewhere near the assembly of springs at the end of the throttle cable?
* Very nearly ended up with the "hottie position sensor" due to iPad autocorrect. Now THAT is a feature I want on my next car!
The coil was replaced a couple of years ago after I broke down near Paris. The replacement is a decent Bosch one, so it shouldn't be that, although I can't rule it out. The ignition amp is an interesting idea as well. I do have a new set of HT leads to go on, although I doubt that'll be it.
Constant throttle - hmmm, yes that might be true. It has certainly happened while cruising along the M6 toll, which is as smooth a road as you'll find so I don't think it's caused by vibration or jolting. Mind you, it happened a few days ago a moment after I'd changed gear. Next time it happens I'll try to remember to lift my foot right off the throttle - probably a good idea anyway to avoid the jolt as the power comes back.
I do keep coming back to the key fact that when it happens, the engine certainly doesn't go silent - it sounds exactly the way it does when you lift off the throttle, i.e. there's still a normal engine note.
Sardonicus, when you refer to the throttle pot, I assume that's the same thing that I referred to as the throttle* position sensor - we're just using different words for the same thing? Where is it - somewhere near the assembly of springs at the end of the throttle cable?
* Very nearly ended up with the "hottie position sensor" due to iPad autocorrect. Now THAT is a feature I want on my next car!
Edited by Dr Mike Oxgreen on Friday 12th June 06:34
As it happens I do have a set of bog-standard Lucas HT leads waiting to go on, to replace the Magnecores that are currently fitted. Will add the AFM to the list of potential culprits; I suppose if it were to suddenly say "there's no air coming through" then the ECU would respond by cutting the fuel right back, which might give these symptoms. I'll try things in increasing order of price, so I suspect the AFM might be some way down the list!
Just a tentative update: Has the problem gone away?
Having finally got around to fitting my bog-standard Lucas HT leads a week or two ago, I haven't had a further instance of this 'dead throttle syndrome' in a good few hours of driving. In the previous few weeks it was doing it at least once every half-hour or so, so it's beginning to look like the problem has gone away. I can't think why a new set of HT leads would have cured it, or maybe I've simply disturbed a wiring connector in the process of doing it.
One thing I can now say: Before I changed the HT leads, the last time the problem occurred I did see that the RPM gauge stayed steady, i.e. it didn't drop to zero.
The other problem I have, namely a misfire when stationary, has also been greatly improved by the new HT leads. I can hear that it's still missing very slightly when I touch the throttle after standing stationary, but it clears much more quickly and doesn't really cause an issue. It also doesn't misfire if the fans have kicked in, so I'm sure it's due to airflow and cooling. Replacing the ignition amp is next on my 'to-do' list - it must sit in a pool of hot air at the top of the engine bay.
Another improvement that the HT leads seem to have made is that the throttle response seems smoother and less snatchy. Particularly when gently re-applying power after lifting off the throttle, and particularly at lower speeds, it was previously quite difficult to do it without the engine snatching and making the car lurch a bit - but now it is much easier and much less prone to snatching. I've no idea why that would be, but I'm quite pleased about it.
As expected, there appears to be no difference in overall power and performance.
I do need to sort out the routing of the cables though: at the moment they loop up before plugging into the distributor and I've a feeling they're rubbing on the bonnet, which isn't good.
But overall, I'd recommend anyone to replace old HT leads with a bog standard set of Lucas ones for about £25, especially if your car is fitted with fancy ones. As ChimpOnGas has said in the past, perhaps we should consider all these ignition components as consumables and replace them every 30,000 miles.
Having finally got around to fitting my bog-standard Lucas HT leads a week or two ago, I haven't had a further instance of this 'dead throttle syndrome' in a good few hours of driving. In the previous few weeks it was doing it at least once every half-hour or so, so it's beginning to look like the problem has gone away. I can't think why a new set of HT leads would have cured it, or maybe I've simply disturbed a wiring connector in the process of doing it.
One thing I can now say: Before I changed the HT leads, the last time the problem occurred I did see that the RPM gauge stayed steady, i.e. it didn't drop to zero.
The other problem I have, namely a misfire when stationary, has also been greatly improved by the new HT leads. I can hear that it's still missing very slightly when I touch the throttle after standing stationary, but it clears much more quickly and doesn't really cause an issue. It also doesn't misfire if the fans have kicked in, so I'm sure it's due to airflow and cooling. Replacing the ignition amp is next on my 'to-do' list - it must sit in a pool of hot air at the top of the engine bay.
Another improvement that the HT leads seem to have made is that the throttle response seems smoother and less snatchy. Particularly when gently re-applying power after lifting off the throttle, and particularly at lower speeds, it was previously quite difficult to do it without the engine snatching and making the car lurch a bit - but now it is much easier and much less prone to snatching. I've no idea why that would be, but I'm quite pleased about it.
As expected, there appears to be no difference in overall power and performance.
I do need to sort out the routing of the cables though: at the moment they loop up before plugging into the distributor and I've a feeling they're rubbing on the bonnet, which isn't good.
But overall, I'd recommend anyone to replace old HT leads with a bog standard set of Lucas ones for about £25, especially if your car is fitted with fancy ones. As ChimpOnGas has said in the past, perhaps we should consider all these ignition components as consumables and replace them every 30,000 miles.
Edited by Dr Mike Oxgreen on Friday 10th July 10:37
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