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Low Ignition Coil HT Lead Resistance. Engine Misfiring

Hi, I have a 1988 Alfa Romeo 75 (Milano) 3.0 V6 and the engine is misfiring. It sometimes cuts out at idle and has no power on acceleration. It sometimes backfires under full throttle.
Pulling out the spark plugs one by one, cylinder 1 has no effect on the engine (but spark is firing).
All fuel injectors are firing, brand new spark plugs, new fuel pump & filter.

I am running out of ideas. Testing the resistance of all the spark plug HT leads, they all have a resistance of 25k, however the lead from ignition coil to distributor has a resistance of just 1k. It is also a little corroded inside (stripped it to make better contact at coil end).
Could this be the cause of the misfire and poor running? If not, any other ideas?

Thanks for your help!

You should a compression test and pay close attention especially to the Number One cylinder. A cylinder that is dropping can cause something like this and if compression is down that’s a big deal; or possibly even a much bigger deal all depending.

How many miles on this thing?


I’ll do the test today and get back to you. It’s got 75,000 miles. If all seems good, you got any other ideas?

How did the old #1 spark plug look, compared to the others?
I agree if #1 is getting fuel and spark all that’s left is compression.
Does this engine have adjustable valve lash?
If it’s never been checked a burned exhaust valve is possible.
Could also be a blown head gasket or wiped cam lobe.
Other more $erious possibilities involve piston or ring damage.

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Just done a compression test:
1 - 210 psi
2 - 190 psi
3 - 200 psi
4 - 200 psi
5 - 195 psi
6 - 195 psi

I’m out of ideas. There’s a hole in the exhaust cat which drips water, if that’s a clue for anyone

Backfiring when throttle is opened can be caused by not enough fuel, You say the injectors are all firing, now it is time to find out if #1 is not plugged and has a good spray pattern.

Oldtimer brings up a very good point

How are you verifying that the injectors are working properly?

You could measure the resistance of all 6. I don’t know your specs, but if they’re all the same, then at least you know it’s not open circuit. If it’s significantly lower or higher than the other 5, that could be a problem

A noid lite kit would let you know the injectors are being properly controlled

You could also perform an injector balance test, but that requires special tools. I’d check resistance first, though, as it should only require access to the connectors and a decent multimeter, which you hopefully already have

One more thing . . . #1 could be firing to ground. If so, this is typically due to bad plug wires. Read hole or tear in the sheathing. You can often hear the clicking, and sometimes even see the arcing. It helps if it’s dark outside

I have removed all the fuel injectors and checked the spray pattern one by one. All seems good. Resistance measures at 2.9ohms across all injectors, 4.5 on the cold start injector.

I have measured resistance across all spark plug leads. All equal.

db4690, please would explain further? When I take the spark plug out and plug it into the lead, I can see a spark. I can also hear a spark when I lift it out of the cylinder just slightly. This to me proves it’s supplying a spark. What do you think?

Thanks for all your help.

The compression is great so no issues there. You might check the inside of the distributor cap for cracks or carbon tracking.

You also might consider a new set of plug wires; especially seeing as how the coil wire was corroded. I will point out that it’s quite possible for plug wire resistance to be fine and yet the wire will still not function correctly.

You could try swapping a spark plug and wire around as a test method to see if Number One makes a difference in the idle.
It is also quite possible (but rare…) for a new spark plug to be faulty. I ran into this a number of times years ago with Champion plugs. That is why I haven’t used a Champion in many decades.


Doesn’t the fact that I can hear a spark from each plug when I pull it out of the cylinder slightly mean that all sparks and leads are fine? I’ll have a go at swapping the leads round.
The ignition coil primary resistance is slightly out of tolerance too (tolerance 0.7-1.2 ohms, we get 1.3 ohms). Could this be a cause? Seconded reistance is fine.
I’m also going to try taking the cat off the exhaust today. Research shows it could be a plugged cat causing the issue. Our exhaust rattles loudly when the car is close to stalling. It also has a big hole in it.

Thanks for the help

Also, this is inside the distributor cap. No tracks, but quite a build up. I have scrapped most of it off. Resistance between each contact and end of ht lead is good

The ignition coil and catalytic converter are not going to cause a misfire on a single cylinder

That would not be time well spent, in my opinion

But if you want to investigate the cat some more, you can use a back pressure gauge. Sure beats removing it. But as I said, I don’t think that would be a step in the right direction

Please remind me . . . just how old is that ignition wire set?

I presume from 1988, like the car. Before I replace them, how can I check that they are the problem? A new set is over £100.


Here’s how I would approach it . . .

Replace the ignition wires, cap and rotor . . . as preventive maintenance. This is a no-brainer, as they’re nearly 30 years old

If your misfire problem is resolve, congratulations. You’ve killed 2 birds with 1 stone

If not, well, scratch that off your list of possible suspects

A word to the wise. Get reputable parts, not store-brand.

By the way, can you show us a picture of the distributor cap “towers” . . . the other side of the cap, if you will, particularly for #1 wire

Cylinder 1 is the furthest left

Just noticed these black rings around each of the towers

Thanks for the pictures

On some designs, it’s virtually impossible to remove old ignition wires without compromising them in some way, no matter how careful you are. It’s just a fact of life. That’s why I recommend replacing plugs and wires as a set, along with the cap and rotor, if the vehicle is so equipped, as yours is

Many misfires are simply caused by old secondary ignition components. If you flex all the wires, you might see a burn-through, or cracked insulation. It would be quite typical

The black rings could be cracks, or they might simply be residue from the old wire boots

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The electrodes inside the distributor cap are quite corroded and burned & indicate the cap & ign rotor definitely need to be replaced. Resistance is just one parameter a spark plug wire has to pass. The more common failure mode for wires is cracking in the insulation. The wires need to contain the 20,000 volt spark signal within the wire and all the way to the tip of the spark plug. If there’s a crack in the insulation part or all of the spark will slip out that way. One test you could do is test for spark using a spare spark plug, holding it against an engine ground during cranking. Does number one look different than the others? I think you’d spend your time more efficiently by just replacing the wires, cap, and rotor, then you’ve have a clean start knowing all that was ok. If that didn’t fix it, I’d be inclined if it were my car to take it to a shop who can do an ignition system test on it using specialized shop equipment. You don’t have to let them fix it, but they can tell you what’s wrong if anything with the ignition system. If it isn’t the ignition, and you’ve already tested the compression which is very good, then you’re pretty much left with

  • fuel problem (injector could spray ok, and be getting the pulse, but the input fuel pressure might be wrong)
  • ignition timing
  • valve timing

A quick test idea: Are there any visible sparks seen in the engine compartment when you run it at night, as dark of place as is available.

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Reassembled fuel rail and intake manifold yesterday. Car eventually started and now cylinder 3 is misfiring, 1 is fine (based on pull out spark plug and listen to rev drop test).
New distributor cap and rotor arriving Friday so I’ll update then

Be careful, this can result in damage. There is another discussion here about someone who did that and spent a lot of time repairing the ignition system.