mercmark

Hi Guys,
Is it possible that a poor or dying battery could cause a misfire at idle? Reason I ask is my 92 Saff 4x4 has a very periodic “miss” on idle that seems to get more frequent as you switch on more electrical equipment. The battery is a good few years old and does loose charge if left for a few days. That said the car runs fine on the road.

Also, where does the rev counter take it’s signal from; is it coil or CPS (Crank Position Sensor) Strange thing is if the car has been stood for a few days, on start up, the rev counter needle flicks up and down, whilst the boost solenoid clicks away . It does it for 20-30 seconds or so, then all’s well.

I may be putting 2 and 2 together and making 3, but could it be a sign that the CPS is going bad? Another reason for making me think this is at the weekend whilst driving the car, as I pulled away from a junction, the power was cut at between 3500-4000 rpm, almost like a rev limiter coming in. Did it a couple of times, then all was well. I’ve read that if a CPS or phase sensor is going down, the ECU won’t allow the engine to rev past 3500rpm’ish to prevent potential damage.

So many questions, hope someone can help!

Cheers,
Mark.

rsnissan

iTrader: (9)

Battery "shouldnt" make a difference once running providing the alternator is working properly IF there is enough power to start he car. Rev counter is green wire from the COIL.

If CPS was out that would cause a missfire and possibly problems starting (more attempts than normal)

Best bet is volt meter on the battery and monitor what the alternator is doing when the car is running.

Check the gaps on the phase sensor in the dizzy and the gap on the CPS also checking for corroded wiring/breaks/crap on the sensor etc.

tabetha

Cps out will not cause a misfire, unless you have a lug on the pulley missing ?
The fact that it starts is proof that the cpos is working as it primes first thing in the morning if it has cps signal.
This is a electrical problem, the voltmetre, sorry tacho is VERY sensitive to bad connections, but they do go tits up as well.
I would start by cleaning all the earth points that you can, particularly the ECU earth points on the plenum cover, running a seperate earth to this point straight from the battery is a good idea as well.
There is no harm in checking the alternator output, but it's sounds ok as it charging enough to keep battery topped up, if you suspect the voltage is going low when driving switch on the wipers, DC motors are very voltage sensitive, and these will slow down and stop well before the engine cuts due to no power(if electrical problem), next your tacho would stop then a few miles later your ignition/engine.
As said the thin green is the tacho wire, it sounds very much like bad earth/s.
The phase and cps gap can be wildly out and you would be unlikely to get problems, certainly not of this nature, more of an does it go or not thing.
tabetha

mercmark

Thanks for the suggestions and advice.

I actually may have in advertently found the root of the problem. When the engine was running, I could just hear a faint high pitch wheezing noise. Whilst wiping away some excess WD40 on the plenum, the noise disappeared as I ran the rag over the top of it. Turned out that one of the Torx bolts on the plenum was loose, and causing it to suck in air.

Still did what you said Tabetha, cleaned the engine earth points and ran a separate earth strap direct from the battery negative to the plenum, and it’s made a world of difference in so much as revs are much, much less affected by electrical load, and the idle valve seems much more dynamic and quicker to react now, even the starter motor seems more energetic!

I also removed, cleaned, tested and re-gaped phase sensor, so all’s well there

Finally removed all the plugs, and found that gap was a bit small, and to be honest could do with replacing anyway, as looking a bit tired.

It's idling much, much better now and I think once plugs are changed, all will be fine!

Thanks again!
Mark.

tabetha

Make sure the main earth lead onto the bellhousing under bolt head is spotlessly clean, other wise it will actually draw earth anywhere it can, and the fave place is the ecu earth points on plenum, then the wires will actualy smoke and melt, and it kills your ecu, had that a few weeks back, with someone on here, as he had forgotten to tighten the bolt down!!
tabetha

mercmark

Cheers for that Tabetha,
Will do that this evening. Thinking about it I'm pretty sure many of the probs I've had have been at least partly down to poor earths. All the electrical bits seem to have found a new lease of life!

Cheers,
Mark.

tabetha

Most electrical "odd" problems are down to bad earths, but I suppose for the age of the car it's not bad!!
tabetha