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Idle vacuum drop with 2jz-ge aftermarket intake manifold

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we installed aftermarket intake on 2jz-ge, and we noticed that it idles at 80kpa and the afr dropped to 10:1, which is what the minimum aem gauge can read,

we checked for a vacuum leak but didn't find any. Is this normal? And is there any way to increase the vacuum with an aftermarket intake?

Was the manifold change the ONLY change you made?

Usually a lower intake manifold vacuum is due to an air leak, which will usually result in a higher idle rpm and, depending on the tuning method, a leaner mixture. I'd start with checking there weren't any hoses I missed, or perhaps unused open tap holes or nipples I missed.

If I didn't find anything I'd take the manifold off and check the flange that mates to the head was actually flat - most are fine, but sometimes fabricated manifolds aren't as well made as would be expected.

It was only the manifold, but we re-did the piping and vacuum lines, I told my friend to do a smoke test on his car, and he said they could not find any leak; I'm not sure if they did not do it correctly or only from the intercooler side.

Two reasons I can think of might be why we have this issue.

We are using a 90mm throttle, and I believe the stock one is 65mm. Maybe that's too big to build a vacuum, or maybe the vacuum line is kinked somewhere

Well, that's at least two things...

From the description of the symptoms, I'd expect it to be a messed up tune*, the engine is running too rich and to develop sufficient power to maintain the idle, more air has to be drawn into the engine, reducing the vacuum because that meant the throttle opening needed to be greater.

*You may be wise to double check the ignition side, as even a light mis-fire will shoe a lean condition that the ECU will try and compensate for by over-fuelling. When you pull the spark plugs, if there is a mis-fire, you may see one, or several, that are showing signs of rich running and even, in extreme cases, they could be shiny with unburned fuel.

When we first installed the intake, the idle was around 1500 rpm and 10:1 afr. We lowered the idle to 750rpm and pulled 10% fuel at a time until we got 14.7 afr, the ve was half the previous value, and there is now a weird transition between idle and rolling. It goes from 14.7 to immediately 18+ afr before going back to 14.7 while cruising, and I'm not talking about transient enrichment I mean, once the load increases, it leans out by the same about we pulled.

I will double-check the distributor, and I don't think its misfiring, but we are running really tight gap, and we did change the spark plug since, the ECU is ecumaster, by the way

Vacuum exist in the manifold, between the intake valves and the throttle plate. To lower the vacuum that much, you would need a crazy big plenum. So yeah, it looks like "unwanted" air is entering the manifold.

Have you check the mechanical stop of the throttle plate? It's not uncommon to see those not adjusted properly when they arrive, if it's the model I'm thinking, with the 90mm throttle body.

Another quick test you can do is pinch vacuum line (or block them off) and see if it helps with the vacuum. Don't forget to check the brake booster hose!

OK, back to fundementals.

Idle speed primarily depends on the pressure from the energy released by the combustion of the mass of air/fuel in the cylinder and the ignition point, to develop an average torque to the crankshaft.

The amount of air entering the cylinder is approximately proportional to the absolute pressure in the manifold. As such, if the absolute pressure is higher (vacuum lower), more air is entering the cylinder. If the idle speed is the same with higher abs' pressure, either the mixture burn and/or the ignition point is going to need to be compromised to reduce the torque produced.

You are reporting an overly rich mixture, which would reduce the burn efficiency, so may be the first thing to check.

Plenum size, per se, shouldn't affect steady speed/rpm mixtures but may be expected to make more difference with any transitions.

You don't mention if the engine has an electronic throttle, or uses TPS. In either case, if it's a parameter the ECU monitors you will need to account for/correct it in the software because changing the throttle body size will make most difference in airflow at idle and light throttle blade angles. It should be a simple calibration issue.

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