Hunter Gray

I have a 1999 Xj sport 4.0 I'm stumped on what's wrong with it I've replaced the intake and exhaust gasket after I did the head gasket. I put it back together and it ran well then took the intake off again to do freeze plugs and put it back on and now it is starting up to 4,000 rpm I haven't waited very long to see if it will go down because just nervous with it running so high for an extended time. I didn't notice that where the dowel goes is broken just wondering if that would be enough to cause it or even a crack somewhere else. Also I put it in reverse and drive and it just does a burnout so any help would be great thank you

jordan96xj

4000rpm is a lot of air getting in. (not just a small crack). Look for something substantial like:

1. The butterfly valve in the throttle body is actually stuck open for some reason.
2. The IAC on the throttle body is completely open for some reason
3. The intake manifold is not properly seated (this is quite likely, and comes up often on the forums and is usually not detected until the person gets in there and loosens things up to try seating again, being hung up on a dowel is often cited, but not the only cause).

Hunter Gray

The butterfly valve stays closed the whole time and should the IAC be fully extended or retracted. the dowel holes are lined up as well have seen that on these forums.

jordan96xj

The IAC will extend or retract as needed (commanded by the computer) to allow the required amount of air through the throttle body to achieve the computers desired idle RPM (typically around 1000rpm at startup and 750-850 after warming up for a few moments). If it is extended, it is -closing- off the air, and if it is -retracted- it is allowing more air in. So if for some reason it was stuck in the retracted position, it might be allowing the air in (or if it is for some reason being commanded by the computer to be fully retracted). It is a a small electro-magnet that functions somewhat like a relay, but through a range of positions, instead of just on/off.

I would probably get access to the throttle body, and pull the IAC off, and cover the air hole it controls. Then if at startup, the engine is still going up to high RPMs, I would know for sure that the air is coming from somewhere else down below the throttle body (or at least below the IAC and butterfly).

But one thing that we can know for sure, is that your engine is getting a lot of air. It wouldn't be able to achieve 4000rpm without it.

Hunter Gray

On the bottom right is the intake if you zoom in there is a crack I just found sure looks like the possible cause

CCKen

Yep.

Hunter Gray

So it would be new intake manifold? Are there new ones available? Haven't been able to find any

s346k

if that is indeed the cause - go to the junkyard. i got my 99 intake manifold for $35. and don't he-man the bolts next time haha.

typically your situation is caused by poorly seating the intake manifold to the cylinder head.