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Ford Falcon: 4.0L Inline-6 Plastic Intake Manifold Cracking (AU Series)

4.0L Inline-6 Plastic Intake Manifold Cracking (AU Series) is a common engine fault on the Ford Falcon on the 1998–2003 cars, and we rate it major. Putting it right costs $400 to $900 at New Zealand independent workshop rates. Below is what goes wrong, the symptoms to notice on a test drive, where to look, and what to ask the seller before you hand over any money. It is one of 36 documented faults on the Ford Falcon.

Vehicle
Ford Falcon
Engine
4.0L
Years
1998–2003
Fuel
petrol
System
Engine

This page covers the Ford Falcon as a model. Some faults only affect particular engines or build years — enter a plate below to narrow the list to one car.

All Ford Falcon faults36 documented faults on this model

What you are looking at

No photograph available

We do not hold a verified photograph of the Intake manifold yet. We would rather show none than show a picture of a different part.

What goes wrong

The AU Falcon introduced a composite plastic intake manifold on the revised 4.0L inline-six. The manifold cracks at port runners and along the plenum body from heat cycling and material degradation over time. Cracks cause significant unmetered air entry, upsetting fuel trim and causing rough idle, lean misfires, and in some cases vacuum system failures affecting the brake booster. A well-documented and widely reported AU-generation specific defect.

What you would notice

  • Rough or uneven idle
  • Lean misfire codes P0171 or P0174 stored in ECU
  • Hissing air noise audible from engine bay at idle
  • Hesitation or stumble under light throttle
  • Hard starting when engine is at operating temperature

How to check it

Where to look. Top of engine, plastic intake manifold and plenum assembly spanning all six cylinders

With engine running at idle, listen for hissing from the intake manifold area. Spray a small amount of carburetor cleaner carefully along manifold seams and port runners — a change in idle speed indicates a vacuum leak at that point. Visually inspect the manifold plastic for cracks, particularly at port runners and plenum body joins.

Parts involved: Intake manifold.

What it costs to put right

$400 to $900 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

Ask the seller

  • “Has the intake manifold been replaced or repaired?”
  • “Any rough idle or hesitation complaints?”
  • “Any lean-related fault codes in the ECU?”

Now check the Ford Falcon you are actually looking at

Everything above is true of the model, and it is free. What it does not tell you is whether THIS car has been looked after: its odometer history and whether the reading has ever gone backwards, whether there is still money owing on it, whether it has been reported stolen, how many owners it has had and what it is really worth today. That comes from the official records for that plate. Enter the plate for the free details, or get the full history and a market valuation from $13.49.

Not sure what you would be paying for? See what a full KickTyres check includes.

Where this comes from

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.

Ford Falcon buyer questions

Is 4.0l inline-6 plastic intake manifold cracking (au series) serious on a Ford Falcon?

We rate it major and common on the Ford Falcon. The AU Falcon introduced a composite plastic intake manifold on the revised 4.0L inline-six. The manifold cracks at port runners and along the plenum body from heat cycling and material degradation over time. Cracks cause significant unmetered air entry, upsetting fuel trim and causing rough idle, lean misfires, and in some cases vacuum system failures affecting the brake booster. A well-documented and widely reported AU-generation specific defect.

How much does it cost to fix on a Ford Falcon?

$400 to $900 at New Zealand independent workshop rates. That figure is for this job on its own. A car showing the symptoms on this page is usually worth either walking away from or negotiating down by at least the repair figure.

Which Ford Falcon years are affected?

Our research records this fault on the 1998–2003 Ford Falcon. Build dates move around, especially on imported cars, so treat the range as the years to look hardest at rather than a guarantee either way. The official records tell you the exact build and first registration date for the car in front of you.

How do I check for it before I buy?

With engine running at idle, listen for hissing from the intake manifold area. Spray a small amount of carburetor cleaner carefully along manifold seams and port runners — a change in idle speed indicates a vacuum leak at that point. Visually inspect the manifold plastic for cracks, particularly at port runners and plenum body joins. Look here: Top of engine, plastic intake manifold and plenum assembly spanning all six cylinders