Rough idle

[zanewindle59]

Gday, I was wondering would anyone know what would be causing rough idle to the point where my 1999 au falcon sounds like a rotary. All leads are getting spark but one cylinder isn’t firing even after I replaced that spark plug for that cylinder. Any help appreciated. Cheers


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[PG2]

You need to change the coil pack.

Also, welcome to Ford Forums.

[zanewindle59]

Hey man cheers for that


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[Uncle_Ken]

There are a few posts about AU coil packs, use the search bar to find more info on the preferred types.
UK

[zanewindle59]

So I went down to the local mechanic and he’s said if the lead is getting spark which it is then it’s not the coil pack. Any other suggestions


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[aussiblue]

More likely a leaking intake manifold gasket.

But on the issue of spark; how much spark do you have? When you check for spark, bear in mind Ford specify a minimum 6mm spark plug jump test; i.e. the spark at number 1 (or any other lead) must jump at least 6mm (in a healthy system it should actually easily jump about 15mm or more) ? Easiest to test with a tool similar to this https://www.supercheapauto.com.au/p/...ter&lang=en_AU or https://www.supercheapauto.com.au/p/...ter&lang=en_AU If the spark can't jump more than 6mm the car often won't start at all let alone run correctly. It should also be blue rather than yellow (yellow is often an indication that the spark is weak). These efi engines need a very strong spark for a very short time that is timed for the short time injector spray. You will often find car that won't start or run as the spark falls just short of the 6mm jump test.

However, in sound to me more like a major vacuum leak at the intake manifold gasket. Check for vacuum leaks particularly at the inlet manifold at the bottom of number 1 and 6 intakes near the head both when the engine is hot and cold (as the expansion and contraction of the head and manifold with heating and cooling will change the extend of the gap and leak).

Use a length of windscreen washer sized hose with one end in or close to the ear to listen for the no mistaking tell-tale sucking noise of a vacuum leak while poking around under the manifold at idle. Note that the old pre-computerised fuel injection trick of using/spraying engine start sprays, benzene , water, oil etc around the manifold will most often not work to find these leaks as the ECU rapidly adjust for the change in air in the mixture and it's hard to pick up any change in the engine noise or rpms using this old method. The manifolds typically leak at number 1 and or 6 as the nuts on the manifold retaining studs there loosen over time and the bottom parts of the gasket then burns out. Because a small part of the gasket has burnt out, simply re-tightening the nuts won't fix it and you will need to replace the gasket remembering to torque to specification in a circular patter from the center of the manifold outward.

You can reuse the thin metal gasket on the upper variable length plenum intake (after giving it the thinnest even coating of Hylomar gasket goo you can) that you have to remove first when replacing the main intake manifold gasket.

Also check the PVC valve for vacuum leaks but a leaking intake manifold gasket is the most common cause.