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?
Mine does this seemingly every time I venture out, but that generally means a very good run so its a long time relatively between incidents, mainly when accelerating away from traffic light's, and the engine dies and instantly comes back in with a bang, head rocking back and forth tyres screeching and bystanders thinking look at this dick ..throttle applied for pulling away, not noticed the rev counter dropping, and to be honest it happens that fast I doubt I would .... only does it the once mind, not repeatedly during the drive, bloody weird and frustrating but perversely once it happened I tend to then think I can enjoy the rest of the drive.
Would like some pointers myself.
Would like some pointers myself.
rigga said:
Mine does this seemingly every time I venture out, but that generally means a very good run so its a long time relatively between incidents, mainly when accelerating away from traffic light's, and the engine dies and instantly comes back in with a bang, head rocking back and forth tyres screeching and bystanders thinking look at this dick ..throttle applied for pulling away, not noticed the rev counter dropping, and to be honest it happens that fast I doubt I would .... only does it the once mind, not repeatedly during the drive, bloody weird and frustrating but perversely once it happened I tend to then think I can enjoy the rest of the drive.
Would like some pointers myself.
That could be the rev limiter kicking in if accelerating briskly. Normally set to just over 6000rpm (6250?) Would like some pointers myself.
I had something similar which happened every time the cooling fans kicked in on my Griffith.
I traced it to a problem with the earth point on the drivers side of the transmission tunnel, it wasn't fully tight. Took a bit of tracking down and I'd cleaned and tightened most of the others as well as adding an additional earth lead from the starter moter to the chassis before I found this one.
Might not be exactly the same as your issue but I would check all the earths are clean and tight as they can cause strange problems.
I traced it to a problem with the earth point on the drivers side of the transmission tunnel, it wasn't fully tight. Took a bit of tracking down and I'd cleaned and tightened most of the others as well as adding an additional earth lead from the starter moter to the chassis before I found this one.
Might not be exactly the same as your issue but I would check all the earths are clean and tight as they can cause strange problems.
I had this fault not long after I purchased my car when I was still running the Lucas if it does it when under constant throttle like mine did the car would cut out until you moved the throttle i.e coast to the side of the rad idle perfect and you could pull away again until you foot was in exactly the same throttle position for me it was the throttle potentiometer I checked for a linear sweep twice and it showed no problem but when it got warm it was a different matter very annoying and very hard to simulate off the car 
get or borrow a fault code reader
get or borrow a fault code reader 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
Gassing Station | Chimaera | Top of Page | What's New | My Stuff