Ignition problem??

[JonathanT]

So my '99 450 has recently developed a couple of odd symptoms:

A slight hesitation on constant throttle: When accelerating hard (happens to be constant throttle) the car will start pulling just fine but then briefly lose power before pulling again. This loss is for a short time, maybe half a second or so? It will usually drive fine after that.

Misfire on constant throttle around 2500rpm: When accelerating gently (happened to be in third) and revs reach approx. 2500rpm then engine misfires badly and rev counter bounces around. It wasn’t possible to accelerate out of this – ie applying more throttle did not increase revs and engine stayed misfiring. However, changing gear would clear the problem and normal driving was possible. I managed to recreate this on one journey about 5-6 times before I got home.

I'm not sure if the two are actually the same, in that my car has accelerated quickly through the misfire. It's not happened much, so hard to spot a pattern. Could be that things are nice and hot before it does this.

Is this my ignition amp on its way out? I'm confused that it seems to be in a narrow rev range.

[bobfather]

First point of call would be the plug extenders. I eliminated those symptoms from mine by simply removing the extenders and adding plug socks to protect the HT leads where they come close to the exhaust headers. If you take this route it's worth getting some new leads, Genuine Lucas is all you need and they are long enough to cope with the extra reach.

[Steve_D]

When you say the tacho 'bounces around' do you mean it is reflecting the change in revs or is it dying then coming back?
If the latter then yes I would suspect the amp.
The tacho signal comes from the coil so if that is dropping sparks the the amp is a good call plus amps more often fail hot rather than cold.

Steve

[blitzracing]

A high bouncing tacho means the coil primary is not discharging in the correctly, as this spikes the tacho with extra pulses. The rate the coil discharges depends on the resistance in the HT system, so if you have a short circuit somewhere in the leads or the coil is breaking down it ill do this. If the tacho drops suddenly, this is a loss of trigger from the amp.

[JonathanT]

Steve_D said:

When you say the tacho 'bounces around' do you mean it is reflecting the change in revs or is it dying then coming back?

It's not done it enough times for me to spot a pattern yet - but it does feel like someone's disconnected and reconnected the tacho. The engine revs are not rising at all when this happens.

[Steve_D]

Do you have a heat gun? If so you can perhaps identify a cylinder or two that are not performing, Point it at each manifold in turn and look for one running colder than the others.
If you find one then Blitz had the answer with lead, spark plug or dizzy cap.
If the misfire is still there but no definitive temp difference then the answer is more likely coil or amp.

Steve