I’ve been looking at idle heat soak on the AJ133, particularly whether stagnant under-hood air is a meaningful contributor, rather than just accepting rising IAT at idle as unavoidable.
I ran a simple A/B idle test, keeping the setup deliberately boring and repeatable.

Test setup

• Engine: AJ133 5.0 NA
  
• Condition: stationary idle, hood closed
  
• Duration: 15 minutes idle, plus normal shutdown window
  
• Sampling: OBD Fusion, 1 Hz
• Metric used: Δ(IAT − OAT) (to normalize ambient)
• Two runs, back-to-back:
• Baseline – no added airflow
• Forced under-hood airflow enabled (~40 CFM fan extraction)

No tuning changes, no coolant mods, no changes in operating procedure. Airflow was the only variable.

Why Δ(IAT − OAT)

Raw IAT alone moves with ambient. Using Δ(IAT−OAT) isolates how much additional heat the intake charge is picking up from the under-hood environment.

Results

• Baseline - Stock Motor:
  
  Δ(IAT−OAT) rises quickly and stabilizes around ~9–11°F by ~10 minutes of idle.

​​​​​​​

• With Supplements Airflow: (Extraction Fan)
  
  Δ(IAT−OAT) remains ~0–4°F throughout the idle period.
  
• Averaged across the idle cycle, forced airflow prevented about ~6–7°F of intake heat accumulation.
  

I also logged the shutdown window. ΔT still rises after key-off (expected once intake flow stops), but it starts from a much lower baseline with airflow present, indicating less retained internal energy available to redistribute.

Discussion

This suggests idle heat soak on the AJ133 is largely a heat-evacuation problem, not a heat-generation problem.
Even modest forced airflow appears sufficient to break up stagnant air and reduce heat transfer into the intake system under low-flow conditions.

This doesn’t affect highway operation or peak power, and it doesn’t change coolant temperature or ECU logic. The benefit shows up where airflow is weakest:

• idle
• stop-and-go traffic
• hot restarts
  

Which also happens to be where ECUs tend to be most conservative.

I’m posting this primarily as data for discussion. I’d be interested in how others interpret the mechanism, or whether similar measurements have been made on AJ133 or related Jaguar platforms.

Happy to share plots if helpful.

 

In hot summer texas this heat reduction might help aging plastic. but by how much and WHY bother?
If the heat soak concerned me I would install an XKR hood. Better heat evacuation after shut down and better handling at high speeds.
just my .02.
wj

 

Fair question. I’ve been logging idle runs with and without forced under-hood airflow and normalizing to ambient. Fan-off, closed-hood idle heat soak plateaus around ~9–11°F (IAT–OAT) after ~10 minutes. With modest airflow (~40 CFM), that rise is held to ~3–4°F under otherwise identical conditions.



An XKR hood helps at speed, but idle heat soak forms when vehicle speed and natural convection are near zero — that’s the regime I’m addressing.

 

Doesn't heat rise? Aren't there two semi open areas on the hood? I assume, heat would escape and that everything combined = lower underhood temps .
wj

 

wymjym -
Good observation — hot air rises, and heat radiates in all directions. On the XK, however, there’s a perimeter gasket on the underside of the bonnet that largely seals the engine bay and limits passive airflow out the sides. Also, the NA cars don’t have the supercharger hood vents.

In practice, when the car is running but not moving, the engine compartment behaves much like a convection oven: heat is generated continuously, but air exchange is weak. Creating a defined exit path draws hot air out and pulls cooler air in around the engine, which breaks up stagnant pockets and collapses local thermal boundary layers. That, in turn, reduces how much heat is transferred into the intake air.

 

I was referring to my first statement that if I was concerned I would use an XKR hood to reduce the heat.
it's all about me

WJ