Engine miss-firing at low throttle.
 

Ahoy,
Thanks for the help on my last post, but I have a new problem with my 87 XJ12. It has developed a terrible miss-fire and stall at low throttle. I suspect the coil or amplifier, but need some advise to determine which. The problem started with the car intermittently just not starting, as in no power to the ignition circuit. I jiggled with the connections around the amplifier and coil and that seemed to fix it for a few days at a time.

Now though despite idling fine it will cough and stall at low speed and low throttle. e.g taking off at lights. It idles perfectly though. If I give it the beans it is no problem and once I am up to speed it is fine. I thought it might have been throttle balance or TPS as I had that issue when I first bought it, but I checked and it is fine. The reason I suspect the coil is that it is definitely getting fuel as it stalls. The other odd thing that it doesn't have a problem if I come to a rolling stop and take off straight away. Only if the car has been idling for 30 seconds or more.

Any ideas???

Thanks

Bruce

Mmm.

The amp is basically a switch that replaced the old contact points, so ON/OFF is very simplistic terms. There is that "shielded wire" to the ECU, but generally if it is damaged, the whole engine is dead.

The fiddling has me thinking in many directions.

The coil LT wires may be broken right at the terminal/s, and old age is now creeping into any wiring under that bonnet.

The TPS may be giving a "fuzzy" signal to the ECU once off idle and into the most commonly used area, say up to about 2000rpm, NOW that is common. An "old style" analogue ohm meter will be needed to test this. Actually this makes more sense now that a JD has passed the lips, and the memory clears a tad. That TPS giving a fuzzy signal to the ECU fuel map will cause exactly what you got. Idles perfectly, and giving it stick is also fine as the latter is a less used area of the resistance carbon. Partial throttle, as in lift off under "normal" conditions, is using the very worn section of the TPS, and stumbling/stalling is what you get.

If the amp or coil/s were at play, you would NOT be able to give it stick, it would simply stall out, and backfire thru the throttle bodies badly. Same would happen with a leaning mixture.

I will keep thinking, but check that TPS for a nice smooth rise in either ohms or volts as it is moved from idle position to WOT. The ones I have had with this issue have terrible readings until about 1/3rd throttle, then it smooths out as designed.