Howell TBI Jeep Won’t Idle? Fix Unmetered Air Before Tuning - Diagnostic path 1

February 16, 2026

Howell TBI Jeep Won’t Idle? Fix Unmetered Air Before Tuning

The Howell Injection kit is a fantastic addition to the Jeep market. It can take a bad running Jeep and get you on the road. Usually out of the box they work well. Every now and then you get some issues and without any clear path forward you can be stuck in an endless loop of issues.

Before you start diagnostics and repairs

We highly suggest you download the winALDL computer diagnostic tool.

You can also purchase his diagnostic cable that goes from obd1 to usb

If you have issues setting this up refer to out Pre Diagnostic Guide for Howell units.

The problem

If your AMC 258 with a Howell TBI conversion:

won’t idle
hunts or stalls
runs lean → rich → dies
seems impossible to tune

you may have unmetered air entering below the throttle plates.

This is the worst possible leak location because the ECM cannot measure or compensate for it.

With a mechanical vacuum gauge or winALDL this will look like sub 14 psi of manifold vacuum.

Air entering below the throttle blades:

corrupts MAP readings
destabilizes idle control
creates lean stall cycles
makes injector spray look erratic
defeats tuning attempts

Common Howell adapter leak

On some setups this is how the leak forums.

The TBI base sits over the adaptor bolt heads
Adapter bolt heads sit in the airflow path
RTV is used to fill the bolt heads
fuel vapor softens RTV
vacuum pulls it out
A hidden vacuum port forms

Diagnostic Steps

STEP 1 — Perform a smoke test

To perform this you need a Smoke Machine, We use a Solary Automotive Smoke machine. Just a simple one off of Amazon. (We run mineral oil in it. You only need very little in the machine to make good smoke.)

Look for smoke escaping from:

TBI base flange
adapter edges
bolt head areas
EGR unit
Intake gaskets

Smoke below throttle plates = critical leak

STEP 2 — Spray test while idling (can be less accurate)

Spray carb cleaner at the TBI base.

If RPM changes → leak confirmed.

STEP 3 — Inspect adapter

Remove TBI and look for:

bolt heads sitting to high
RTV missing
step gaps between surfaces

Sad to say but if you are getting a code 45 then your intake gasket is most likely fuel saturated and has failed.

STEP 4 — Fix permanently

In some cases RTV has worked just fine. We have seen that RTV has failed more times then it has lasted.

Before you go any farther. Install a new gasket between manifold and adaptor with the 4 bolts. VERY IMPORTANT! On the base gasket (intake side) only seal this with Permatex 29132 Sealer. It is a Solvent based sealer that withstands fuel much better then others.

Torque the 4 bolts to 120-180 INCH lbs

Now that that is correct we will proceed with our shops final type fix for the vacuum issue that we faced.

On the bolt heads we could not get RTV or 29132 sealer to work. it kept failing and sucking vacuum through the cracks.

That being said our Nucellar option was to use JB weld HighHeat Epoxy. This essentially makes a completed mating surface with the aluminum. The down side is you will have to chip it out if you ever have to remove the bolts for the adaptor.

If you choose to use the epoxy, clean the heads with brake clean and a wire brush.

Mix the epoxy and be through about filling every air bubble of every crevice. We followed the initial forming of the epoxy with a very flat metal block and a hammer with a few light taps to compact the epoxy into the bolt head.

Now let that fully harden overnight. Come back the next day.

Now that its hard you can use a course FLAT file. Gently run the file over the top and plane the epoxy to the adaptor face.

Once you are happy and can see the round outline of the hole the bolt head is in. finish off with buffing the surface with a hard durablock or machinist block wrapped with 320 grit sand paper. should leave a uniform sealing surface.

Now you can re install the TBI unit.

With a fresh TBI gasket (DO NOT PUT ANY RTV ON IT)

Install it dry and torque the 3 bolts to

60 Inch LBS

then

144 Inch LBS