Engine randomly cutting out
Discussion
My '99 450 has developed a very intermittent fault over the last few months, which has typically only happened once every few hundred miles but happened a couple of times on the way to work this morning.
The engine abruptly produces little or no power for a second or two, and then abruptly returns to normal. So far, this has only happened at part throttle when I'm just gently cruising along, but I'm not looking forward to it happening during an overtake! It frequently doesn't happen for hours of driving.
During that one or two second pause, there are no pops or bangs, and the engine doesn't go entirely silent: I'm 90% sure that there was still an exhaust note. It's exactly as if I'd suddenly lifted off the throttle.
If this were an ignition fault, I would naively expect the engine to go silent, and perhaps get a few pops and bangs as unburnt fuel goes into the exhaust. Does that seem reasonable? Also, if it were fuel starvation I'd expect the engine to go quiet.
A possible culprit is the throttle position sensor (TPS), but I don't quite know enough about how the ECU uses that input. If the sensor or its electrical connection were dodgy and momentarily telling the ECU that the throttle's gone to idle, would that give these symptoms? Does the ECU use the TPS as its primary way of knowing how much fuel to squirt in, and then use the lambda sensors to trim and fine-tune the fuelling - is that how it works?
It's going in for a service soon, but any ideas would be gratefully received.
The engine abruptly produces little or no power for a second or two, and then abruptly returns to normal. So far, this has only happened at part throttle when I'm just gently cruising along, but I'm not looking forward to it happening during an overtake! It frequently doesn't happen for hours of driving.
During that one or two second pause, there are no pops or bangs, and the engine doesn't go entirely silent: I'm 90% sure that there was still an exhaust note. It's exactly as if I'd suddenly lifted off the throttle.
If this were an ignition fault, I would naively expect the engine to go silent, and perhaps get a few pops and bangs as unburnt fuel goes into the exhaust. Does that seem reasonable? Also, if it were fuel starvation I'd expect the engine to go quiet.
A possible culprit is the throttle position sensor (TPS), but I don't quite know enough about how the ECU uses that input. If the sensor or its electrical connection were dodgy and momentarily telling the ECU that the throttle's gone to idle, would that give these symptoms? Does the ECU use the TPS as its primary way of knowing how much fuel to squirt in, and then use the lambda sensors to trim and fine-tune the fuelling - is that how it works?
It's going in for a service soon, but any ideas would be gratefully received.
Sounds like the ignition amp. If you search for that phrase on here you should find more about it. Have a read and see if it matches your symptoms.
They are relatively cheap and simple to replace if that is the fault.
Or it could be a HT lead breaking down or not correctly seated. So try unplugging each one in turn and make sure it is connected to the suppressor tightly and the suppressor is then refitted to the plug nice and snug.
Also check the king lead from the dizzy to the coil is OK.
They are relatively cheap and simple to replace if that is the fault.
Or it could be a HT lead breaking down or not correctly seated. So try unplugging each one in turn and make sure it is connected to the suppressor tightly and the suppressor is then refitted to the plug nice and snug.
Also check the king lead from the dizzy to the coil is OK.
All good things to try - so I will do.
Is my assumption invalid about expecting unburnt fuel to pop in the exhaust if it were an ignition problem?
My gut feeling is that it's not a single-cylinder problem like an HT lead, because the engine really does totally stop pulling and decelerates - just like completely lifting off the throttle.
Is my assumption invalid about expecting unburnt fuel to pop in the exhaust if it were an ignition problem?
My gut feeling is that it's not a single-cylinder problem like an HT lead, because the engine really does totally stop pulling and decelerates - just like completely lifting off the throttle.
Check the ignition amp connections, they go brittle. The TPS can cause the symptons if the restistance track has wear points that go open circit. This causes the throttle to look like its closed, so the ECU cuts the fuel. Also you can have the link wire to the ignition trigger in the distributor go open circuit. This will cause the rev counter to drop to zero even though the engine is still turning.
I went out at lunch time to look for loose HT leads etc, and I was able to make the king lead sit a bit more snugly onto the coil. The lead was pulling a bit, so I relieved the strain. Also, one of the HT leads pushed down onto the distributor a bit tighter as well.
I don't know if it's my imagination, but the car felt like it had more urgency on the way home.
However, it did momentarily cut out as I pulled away from a junction, making me look like a right bell-end as I lurched away.
I don't know if it's my imagination, but the car felt like it had more urgency on the way home.
However, it did momentarily cut out as I pulled away from a junction, making me look like a right bell-end as I lurched away.
- chuckle* Mine died six times on the way back from my parents (Epworth in Yorkshire) to my place (Salisbury) after Xmas...
)Phil
When mine did this it was the immobiliser wiring. It did it for years, sometime just cut out for a scecond, other times bringing the car to a standstill, until I was fiddling around one after which car would not start. Took we a while but I eventuallly noticed a plug had fallen off (one side of the plug has black wires looped back into it- dead end if you like).
If it had not come apart completely I would never have found it. Now taped back together never had a problem since.
PM me for location if you are interested since this is immobiliser related. Of course could be a load of other things but this is easy to check! Meta alarm bu the way
If it had not come apart completely I would never have found it. Now taped back together never had a problem since.
PM me for location if you are interested since this is immobiliser related. Of course could be a load of other things but this is easy to check! Meta alarm bu the way
Alexdaredevilz said:
Had similar problem for a few months, fixed the other day
Turned out to be tps out of range, made holes bigger twisted it round a bit, problem solved HTH
Alex
Turned out to be tps out of range, made holes bigger twisted it round a bit, problem solved HTH
Alex
Alex. Can you be a bit more specific on what you did to solve/check the tps. Holing and twisty bits. Thanks. Eddie
I had actually forgotten about this; I was going to add some further information if and when I got to the bottom of the issue.
Well, unfortunately I don't have any useful information. It has been a couple of months since it was serviced by Str8Six, and I've done a fair amount of driving since, and the problem hasn't happened. It just seems to have gone away, despite Str8Six saying they couldn't find anything - but they seem to have fixed it somehow.
Well, unfortunately I don't have any useful information. It has been a couple of months since it was serviced by Str8Six, and I've done a fair amount of driving since, and the problem hasn't happened. It just seems to have gone away, despite Str8Six saying they couldn't find anything - but they seem to have fixed it somehow.
EGB said:
Str8Six said a prayer and it was answered! Good glad it's gone away, hope for good. Also, hope Alex can come back to explain some of the mysteries of the TPS. Symtoms and cure.
Quite a riddle! But there are clues. Engine seems to stall for an instant or two but continues on. What can stall an engine for an instant, allow it to self-start and can be cured with some idle fiddling by a mechanic who is not sure what he did?
The symptoms point to an ignition issue. A loss of fueling does not express itself in the fashion you experienced. The mechanic would have come to the same conclusion and checked out the ignition system..primarily the wires between the coil and the distributor, the amp and the distributor itself.
If the vacuum advance was/is loose or its armature bent slightly..it will either intermittently touch the side of the distributor body or do so under load. That will instantly short the distributor and stall the engine. BUT that, in turn, will cause the armature to retract or move (even slightly) ending the short. As the ignition key is on, and the engine is in gear, and you are moving along at decent clip with a warm engine, the engine (blessed with sparks once more) will happily start again and keep going, leaving you wondering at what hit you.
As the mechanic would have checked the distributor and certainly removed the cap and rotor. He could also have removed the plastic cover under the rotor for a quick spray of cleaner. Any of this jostling could easily have re-aligned the armature...at least for a time.
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