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[Jonnycab]

On the WJ 4.0 inlet manifold, there is a spare/unused vacuum connection. Anyone know why it is there ?....& should it be open or plugged.

When I bought the Jeep in June 2012, the vacuum connection wasn't plugged & was obviously sucking in air (you could hear it). The first thing I did was plug it up, as I assumed air going into the manifold at this point would be unmetered & affect economy. Was I right in doing so ?....

Seeing as the 4.0 WJ has no MAF sensor, does it make any difference at all whether the hole is plugged or unplugged ?

I'm assuming the IAT sensor will give false readings to the PCM if air is entering the manifold from somewhere else apart from the air inlet trunking ?

Some sound advice would be very much appreciated  

Cheers,  Jon  

[Lukemagnum]

Most inlet manifolds have a spare tappings on them, likely to be there for test purposes or have you checked for a loose pipe locally that's fallen off?

It's best to have it capped off, as its only small it's unlikely to have any effect on the running of the engine (if anything it may give a lumpy tick over) but it's a pathway for crap getting into the engine as its unfiltered.

You sure you don't have a MAF sensor? Most engine management systems since the early 90's use them. It will be a rigid tube fitted between the filter and throttle and probably contain the IAT sensor as well.

[Jonnycab]

Thanks for the reply. There is no unconnected vacuum pipe & the sticker/diagram under the bonnet states that it is an unused vacuum port. Can't understand why it wasn't plugged in the first place though, but I'll keep it plugged because as you say, it could suck in all sorts of debris if left open.

Also there is definately no MAF sensor on the 4.0 WJ.

Thanks again