Misfire Part Deux
Discussion
Following on from the previous "Misfire" thread my 400se has started to do this intermittently.
Took the car our for a run this morning and had been going for about 40 miles of easy driving. Long straight, drop to third, foot flat to the floor and all ok until approx 3800rpm when the acceleration stops and it starts to back fire*. Eased of the throttle and changed up to fifth. For the next mile or so kept in fifth, around 60 mph, and very light throttle changes produced very minor stutters. This then clears itself.
Continue on journey taking it easy and no problems. Hit the motorway and try some experiments:
drop to third, accelerate without putting pedal to the floor no problem.
another go drop to third pedal to the floor and at 4500rpm+ acceleration stops and backfire/pops&bangs change upto 5th and problem goes away.
Anyone any ideas ?
Took the car our for a run this morning and had been going for about 40 miles of easy driving. Long straight, drop to third, foot flat to the floor and all ok until approx 3800rpm when the acceleration stops and it starts to back fire*. Eased of the throttle and changed up to fifth. For the next mile or so kept in fifth, around 60 mph, and very light throttle changes produced very minor stutters. This then clears itself.
Continue on journey taking it easy and no problems. Hit the motorway and try some experiments:
drop to third, accelerate without putting pedal to the floor no problem.
another go drop to third pedal to the floor and at 4500rpm+ acceleration stops and backfire/pops&bangs change upto 5th and problem goes away.
Anyone any ideas ?
- Note that the "backfire" may not indicate anything in this situation as my car tends to pop and bang like a goodun if you lift of the throttle at high rpm
I think you're having fuel starvation.
Combinations of high throttle and high RPM place the fuel pump under the most stress. When you floor it, the injectors open for about 20ms or so. At light throttle, this will be about 5ms (off the top of me head, so pls dont shoot me). So you can see that when you put the boot in, the flow demand increases 4 fold. As the revs climb, that 20ms period happens more frequently, so the total fue flow rate goes higher still. Add to this the rising rail pressure demanded by the regulator, and if the fuel pump has an issue, this is where it'll show.
I had this and it turned out that the fuel pump electrics were bad. poor fuse contact was making the fuel pump cut out-in-out-in-out very quickly due to vibrations from the engine etc. So the fuel pump was only working on average about half the time, i.e only generating half the required flow rate.
As soon as I accelerated or hit high RPM, the engine was using fuel faster than the pump could provide it, and hence the rail pressure dropped, and the car went lean.
Sorry for the long post, but it seems probable.
Good luck and keep us posted!
Ed.
Combinations of high throttle and high RPM place the fuel pump under the most stress. When you floor it, the injectors open for about 20ms or so. At light throttle, this will be about 5ms (off the top of me head, so pls dont shoot me). So you can see that when you put the boot in, the flow demand increases 4 fold. As the revs climb, that 20ms period happens more frequently, so the total fue flow rate goes higher still. Add to this the rising rail pressure demanded by the regulator, and if the fuel pump has an issue, this is where it'll show.
I had this and it turned out that the fuel pump electrics were bad. poor fuse contact was making the fuel pump cut out-in-out-in-out very quickly due to vibrations from the engine etc. So the fuel pump was only working on average about half the time, i.e only generating half the required flow rate.
As soon as I accelerated or hit high RPM, the engine was using fuel faster than the pump could provide it, and hence the rail pressure dropped, and the car went lean.
Sorry for the long post, but it seems probable.
Good luck and keep us posted!
Ed.
You've got a fuel supply or ignition fault. I don't think it's possible to narrow it down further than that on the symptoms given so far. If that was me I'd put a passive strobe in line with one of the HT leads and confirm that the spark is still strong at high revs and full throttle, put a fuel pressure gauge on and check the same. Don't be tempted to start swapping things just because somebody says they had the same symptoms and X fixed it, it's a very expensive approach until you know what the problem is.
Thanks for all the advice, will have to have a think about what to do next.
As an aside re this mornings run How bloody cold was it for almost August.
Having done about thirty miles I noticed the temp gauge was reading slightly higher than normal. Kept an eye on it and spotted it rising and falling: eventually dawned on me that it was the thermostat opening and closing
.
KJR
As an aside re this mornings run How bloody cold was it for almost August.
Having done about thirty miles I noticed the temp gauge was reading slightly higher than normal. Kept an eye on it and spotted it rising and falling: eventually dawned on me that it was the thermostat opening and closing
.KJR
Gassing Station | Wedges | Top of Page | What's New | My Stuff