leaking throttle body, lean code

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Jan 14, 2026 | 09:30 AM

Michael Lizotte

Thread Starter

Mountain Pass

Joined: Jul 2019

Posts: 135

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leaking throttle body, lean code

How common is a leaking lower gasket on the intake manifold assembly? I sprayed the left side (ds) and it changes the idle, so i think that's were the leak is. Will need to smoke test it to make sure. Truck runs great, but plugs on drivers side have a lot of carbon.
2001, 4.2, 230,000 miles f150xlt

Last edited by Michael Lizotte; Jan 14, 2026 at 01:00 PM.

Jan 14, 2026 | 11:29 AM

BareBonesXL

Fleet Mechanic

Joined: Jun 2023

Posts: 1,352

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From: WA

What is a "ds"? And a "lower" throttle body? There's only one throttle body on the front of the intake manifold.

There are known leak spots on the 4.2 intake system.

Jan 14, 2026 | 11:56 AM

Michael Lizotte

Thread Starter

Mountain Pass

Joined: Jul 2019

Posts: 135

Likes: 2

Quote:

Originally Posted by BareBonesXL

What is a "ds"? And a "lower" throttle body? There's only one throttle body on the front of the intake manifold.

There are known leak spots on the 4.2 intake system.

Drivers Side. Parts show a lower and upper gasket set. Would you recommend replacing the intake with a after market one too?

Jan 14, 2026 | 12:09 PM

BareBonesXL

Fleet Mechanic

Joined: Jun 2023

Posts: 1,352

Likes: 373

From: WA

I think that you're talking about the intake manifold assembly. 2001 has the plastic upper and middle manifold, which work just fine, not like the early 4.2 manifolds. The most common leak cause is the isolator bolt grommets hardening. The isolator grommets are what press down on the manifold, causing the gaskets to seal.

Anyway, the fix is to replace the grommets, or just replace the bolts which already have the grommets attached. While you have it apart might as well replace the upper gaskets, and check the IMRC bushings. Unlikely that you need to mess with the lower manifold. Watch out for torque specs, some are in inch-pounds.

https://www.rockauto.com/en/catalog/...tor+bolt,14336

Here's a few videos.

isolator bolt videos

https://www.youtube.com/results?sear...isolator+bolts

Last edited by BareBonesXL; Jan 14, 2026 at 12:14 PM.