Inconsistent ignition sparking

Just a quick one to see if anyone has any ideas.

1997 3.9v8 all running well then all of a sudden stalled and wouldn't restart. Finally started but with a nasty misfire throughout the rev range, hot or cold. Checked everything like coolant temp sens, maf readings, tps, timing...when checking the timing i was getting a very inconsistent strobe down from the light. Swapping the timing light over all the ht leads im getting varying levels or inconsistency. Iv'e tried new cap and arm, coil and 3 different ignition amplifires that i know are good all with no change! Now i'm left wondering if the it could be the magnetic pick up/ trigger wheel in the dizzy but from my knowledge all that does is send speed signals in pulse form to the coil that the ecu reads for the injection system.

I suggest the inconsistent strobe might be a clue. My first thought was ignition amplifier but you say you have tried substitutes with no change. A duff coil could also cause the problem you describe but you say you changed that - did you use an ordinary coil or a "high performance" one - only use a standard coil.

Check very carefully the wires to and from the ignition amplifier and on the coil - corroded/damaged wires and/or connectors could easily be the culprit. I would also check and clean the earth points - good clean earth is essential for good running.

With regard to pick up in the dizzy, you can check this with a multi meter. Have you had the distributor out? or done anything internally to it? Is the pick up air gap set correctly?

My only other thought is the HT leads themselves. Have a look under the bonnet when its dark and see if there is any arcing.(engine running of course!)

The coil was a proper genuine bosch one, The dizzy has a tab by the amp thats got a good earth running to it. All connections between coil and dizzy look good, even ran another lead from mp to coil that i had with no change. I work on car wiring daily at work and have a proper set of ratchet crimpers. Dizzy has been out but nothing not taken apart, air gap looks to be good but i don't have any plastic feelers to check it. Ht leads are only a few month old magnecores, on the worsed lead i swapped one out for a bosch one i bought and it had the exact same strobing problem.

Im guessing you'd check the pick up with a meter using continuity?

any play in the dizzy shaft?

No play, its nice and clean in there form what i can see too and springs back to place nicely an all. If i put the timing light on No1 ht lead i get a flash flash flash nothin flash flash flash flash nothin nothin flash flash etc kind of thing going on.

Would the pick up in the dizzy cause this? It's as basic as ignition systems get really and the pick up is the only thing i haven't got a spare of to swap over. Everything else has been swapped over and didn't make a difference.

P.s i do sometimes play with my shaft

dirtydiscov8 said:

Dizzy has been out but nothing not taken apart, air gap looks to be good but i don't have any plastic feelers to check it.

Im guessing you'd check the pick up with a meter using continuity?

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Air gap can be checked/set using a piece of paper. Standard 80gsm printer paper is ideal, fold a small piece so it is twice as thick then use that as your feeler gauge, should give you 0.2mm. Pick up is checked for resistance IIRC put meter across the two pin plug on side of dizzy.

I've ordered a new pick up anyway it was cheap which probably means it's also crap but worth trying. I've swapped out all the other ignition components so i hope it is that.

Like the tip of folding paper, don't have much luck with finding plastic feeler gauges

i just set my air gap with metal feeler guages, set it tight so i knew it was touching both sides. works fine

the other day i was rebuilding the dizzy. car was fine but needed the dizzy cleaning out and resealing.

anyway not the first time ive done it so nothing new to me.

built it all back together and ran like ****.

very lumpy, couldnt figure it out, and odd enough i started getting irregular flash on HT1 with the strobe. was ****ing about with the timing etc, thinking why the **** is this engine not running right when it was an hour ago. sparl plug testers gave a good strong flash on all 8.

slept on it, spoke to a wise old man (dad) and proceeded to fit a new set of plugs (others were fitted in january) and pulled the dizzy cap again to scrape the contacts back to good shiny metal.

ran sweet and did 250 miles the day after.

so tell me what that was all about.

so:

check your mechanical advance weights. set that air gap (cant remember what gauge i used but whatever it said in haynes and just set it up tight) and check the cap and rotor are decent.

check all leads for resistance with a multimeter, i think good leads are 5k ohms, ****e leads are around 15k ohm and obviously ****ed leads are more than that.

i had a rusty connection giving me a misfire once, checked the leads and found a bad'un, cleaned it and sorted.

new set of plugs NGK BPR6ES, and set gap to 0.8mm. VERY OFTEN i get problems and new plugs fixes it. mine LOVES plugs. change every 6 months usually.

check youre getting a nice pulse at the coil, and then set about getting the timing right.

see how that goes

mine was missing on 5 and 7 btw, no apparent reason!

all that oil getting past the rings noisy

So What you're saying there Noisy is that although you had an irregular strobe on Ht1 the change or plugs cured it? Did after the plug change the irregular strobe stop?

Everything else on my ignition system is in good order and cap and arm are genuine lucas, ht's magnecore. I'll put some new plugs on it for the cost they are and then see if the old slag will fire on all 8 properly!

Yep, Friday night car went silly, couldn't fix for love nor money.

Saturday morn car still dog ruff so fitted new plugs and then checked timing on ht1 and I had a good flash (so to speak) and it ran sweet again.

I did also clean the cap contacts of course.

So yep sounds very similar to my fault.

Pull your plugs and see which are wet!

fett said:

all that oil getting past the rings noisy

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Yeah I'm suprised it's not hydraulicing it mate

PMSL :hysterically_laughi

Just been outside fitting new dizzy pick up and some new spark plugs. Still got the irregular flash and running rough. Im bored of this now, time for it go to a garage for the first time!

Be carefull on what garage- most know less than you i expect

I live about 10 mins from the garage ashcroft use for all the fitting work and the bloke used to run a modified V8 90. Should be in good hands there!

How did you get on mate? I have 1994 hotwire 3.9efi and that also has an inconsistant spark I cannot even get mine to cough it just turns over and doesen't even try to start.

It's been in the garage for 3 weeks now and the bloke doesn't seem to have done much. He's convinced its water ingress somewhere and is letting it "Dry out" when its sunny. Ahhh well i feel sorry for the local petrol station not getting all my money everytime i use it!

Has he said what's suffering with the water ingress?

Sorry to hear of the probs. It's very easy to end up throwing time and money at it with this sort of problem. Had my fare share, but got lucky when one of mine had similar symptoms and a brand new complete dizzy sorted it

Hope ya get it sorted