Misfires at constant speed while tachometer jumps around

[SouthSwedeSpider]

Same thing has happened twice lately; after driving around for a whole day, say about 100 miles at both high and low speed, with stops in between, the car has started to misfire on my way home. This happens at constant speed at revs around 2500-3000 rpm. The car does not stop, but there is a clear misfiring. As soon as I push the throttle it goes away and the car runs normal. The funny thing is that when it misfires, the tachometer (rev counter) jumps around, mainly up into the 3000-4000 area without any change in speed. I dont know what comes first, is it likely that a fault in the tachometer can cause the misfiring, or is it more likely the other way around e.g. that the misfiring causes my well functioning tachometer to go berserk? And why doesn't it occur all the time (well I'm glad it doesn't, but everything that can't be explained bothers me)?

I have checked the cables in the distributor, they look absolutely perfect, and the connections at the coil seem fine as well. For long, I have had trouble with a rough idle, see http://www.fiatspider.com/f08/viewtopic.php?f=5&t=14287. I have not yet been able to find the problem, could this be an extension of the same problem? Something to do with the ignition coil or distributor maybe, or a short in the tachometer? As always, all thoughts are welcome.

[racydave]

Put a ohm meter on the wires in the dizzy. Best to disconnect them, then apply vacuum to the advance unit. There must be a break in the wires. The readings should show a open at some point.

[lanciahf]

Happened to me once, Turned out to be a loose pick up inside the distributor. Hope yours is that easy.

[Fiatmonkey]

If the Tach is getting involved and any misfire problem then it is almost always your coil. Check all the connections to the coild and check the OHM on the coil. It is either the wires (usually the ground) are making bad contact or the coil is just finally giving up the ghost.

Coils will normally last until the nuclear war and only Cher and cockroaches are left, but that doesn't mean they will still put out the same power. Just like Cher they need to be prettied up every now and then. Sadly, though coils don't have Coil-plastic-surgeons so you just have to buy a new one.

But start with the coil ground.

Monkey Rescue adopt a monkey today!

[SouthSwedeSpider]

Thanks for the advice so far, gonna buy myself a multimeter today. Appears to be an essential tool in any Fiat owners toolbox. Anyone want to trade one for my complete Cher collection...?

[SouthSwedeSpider]

There has been no tachometer jumping the last days, but I haven't driven any longer distances so I don't know if the problems gone. The car still idles rough and there is popping through the exhaust at idle every 2-3 second.

Got myself a multimeter, tested the coil resistance, all values are within spec. Voltage at coil is 11,5 V with ignition on, engine off, fuel pump running, everything else off. Is that OK or too low? By the way, how is the coil grounded? I couldn't find any ground cable so I assumed it is grounded through the mounting bracket. Am I right there?

Resistance in the magnetic pickup is around 800 Ohm, also within spec. I also ran the car with the tachometer disconnected, no change. The throttle plate switch is correctly adjusted.

The search continues, how do I best check if the fuel mixture is too rich/lean?

Could a worn exhaust camshaft cause the rough idle (before my time, the car was driven with way too low valve clearance at the exhaust side for a prolonged time). Head and valves are done since then, but I use the old cams.

Not much to check after that, otherwise I just have to live with it.

[azruss]

sounds like an air leak to me

[SouthSwedeSpider]

That was my first thought, but I just haven't found any, neither before nor after the cylinders. All air tubes are new, intake plenum gasket as well as intake and exhaust gaskets. Any more places air could leak in?

[manoa matt]

The symptoms sound like a failing ignition control module. Check the small black box mounted behind the coil. A $25 part at the local auto parts store. Yes, the coil grounds through the mounting bracket, then to the body. Make sure where metal touches metal its clean.

[SouthSwedeSpider]

If the ignition control module is fried, would the car run at all? It's mainly the idling that is rough, when pushing the throttle the car usually runs just fine (except from the hickups bescribed above). Could bad grounds in general cause this problem, by lowering the input voltage to the coil? When the car is idling, the turn signal flashes very slow, especially when the fan is on, even if I have headlight relays installed. Not to mention the slow wipers... Too low output from the alternator

[124ADDHE]

Sometimes the pickup or amplifier will work fine cold and then give problems when up to temp....

[seabeelt]

Sounds like you need to run through your elecrical connections (a PITA) but necessary. This will improve many functions. Search the electrical section on brown wire surgery and cleaning the grounding pods.

[timinator]

Yes, the coil grounds through the mounting bracket, then to the body. Make sure where metal touches metal its clean.

The coil does not require any ground to the car. It is grounded thru the ignition which collapses the magnetic field in the coil resulting in the stored energy to jump the gap at the spark plug. Fiat uses an oil filled coil mounted vertically in the engine bay. Any oil filled coil manufacturer recommends that the coil not be mounted vertical since the oil is used to cool the coil, and should not be mounted closely to a high heat source. Oil filled coils degrade over time and will not produce enough power to jump the spark plug gap at high load. High load can be an overly lean mixture caused by high engine operating temperatures. Fiat tachs are almost unique in the way they work using a pulse wave instead of a square wave. These tachs are not precise and can be effected in unusual ways by outside sources. Guy Croft on his website states that the ignition system sold by Allison's Automotive is the best he has seen. Might be worth your time to look into obtaining one. For sure don't waste your time grounding your coil.

Tim

[Sunracer]

Did you remove the distributor cap and check the condition of the wires to the pick up? Also ditto the comment about the pick up malfunctioning when hot and testing ok when cold. I had one that tested fine but at idle, the moment the fan kicked on the the motor would stop.

[SouthSwedeSpider]

Cables inside distributor are OK, but I discovered that the pickup air gap was 0.6 mm instead of the specified 0.3-0.4 mm. After adjustment it runs a little smoother, still not perfect but good enough for the time being. I will continue to overhaul the ground points, alredy done most of'em in the engine bay, but still have the ones in the trunk to fix (or is it only one back there?)