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extremely high idle after header install!

H

Highmiles

NAXJA Forum User
Location
New Hampshire
I have a '99 and I just finished installing an APN header and Dynomax cat back (and Brown Dog motor mounts) this weekend. I've just started the XJ for testing and have an extremely high idle - 5,000 - 6,000 RPM, like I was flooring the gas except my foot is off the pedal. It'll run at 5K, but I get worried and shut it down after 20 seconds or so.

I've checked the throttle lines and return spring and everything apprears correct. The butterfly in the throttle body remains closed. All my sensor connections and vaccuum lines have been reconnected and are snug.

I followed Go-Jeep's instructions and the FSM - the install went without a hitch (did not need to clearance #6 intake as others had to). Only modification was to use a FelPro gasket instead of the APN one.

Everything ran normally before the install - just had to fix exhaust manifold crack and figured to replace the exhaust and motor mounts while I was in there.

Any suggestions are appreciated.
 
Make sure your intake manifold is on the pegs and is seated against the head. Watched a buddy of mine do it and it did EXACTLY as you described. He first checked for vacuum leaks and we noticed the intake was crooked in the process. ;) And 20 seconds is a bit long for it running at that high RPM. Shut it down if it does it when you start!
 
I'd look into vac as well.
I always likw to reset computer after mods.
Disco batt, turn key to start, short mains together, reconnect, turn key to run, lights on, lights off, key off, start up
 
xjtrailrider said:
You most likely have a vacuum leak at the intake manifold gasket. Try tightening the bolts again.
Click to expand...

X2
 
I snugged everything down again this morning and visually inspected the intake manifold - it looked snug against the head, but then again I can't see underneath. The high RPM issue is still there.

I wondering if I really do have an interference with the APN head and the intake? The intake seemed to slide into place OK and I was able to torque all the bolts (and did use the correct washers).

My thought is the headers are pushing up on the intake enough to cause a gap between the bottom of the intake manifold and the head.

Does this make sense? Has this happend to anyone else who has an APN header with new-style intake manifold interference?

PS - the high RPM run was probably all of 5 seconds - it just feels like 20 when you are thinking "Holy Sh*t - this isn't good!"
 
Did you hook up the down pipe before tightening the header down? If so that may have pulled it out a little, I always pry the down pipe out of the way and attach it last. It will not take much of a leak to cause a issue.
 
Highmiles said:
(and did use the correct washers).

PS - the high RPM run was probably all of 5 seconds - it just feels like 20 when you are thinking "Holy Sh*t - this isn't good!"
Click to expand...
The washers, were they supplied or the belleville spring washers?
My Pacesetter header came with new washers due to thicker flange.
The washers were not the spring type and had a step to them to compensate for the thickness difference.
If you got step washers one could've turned putting thick end on ex. man letting in. man have a gap but still tight and torquable

5k RPM isn't too bad I've seen 7k and that's a big HOLY $HIT, like sitting on a grenade. I feel ya.
 
I re-used the original spring washers. I'm pretty certain I orientated the correct way - I'll feel pretty stupid if that turns out to be the reason. I'm going to re-seat the intake manifold tomorrow and will let you know what I found out.

I connected the header outlet to the exhaust pipe last - after torquing down the intake and exhaust manifolds.

Making that connection was a bear! I actually had to use 5 inch bolts to "pull" the flanges together and then cut off the excess. Maybe that strain pulled the manifolds away?
 
I also used High Temp RTV Super Copper on gasket.
TPS didn't somehow get hooked up backwards?
Use an unlit propane torch near manifold, if vac leak 5k idle will raise a little and tell you approx. if and where leak is.
 
I'll try that propane trick this evening, but I do worry about raising the RPM any higher than it is. I don't think this rig ever saw 5K, and never immediately on a cold start up (I'm wincing for the bearings just thinking about it).

I did put a very thin bead of Permatex Copper High Temp around the gasket ports (as per Go-Jeep's site).

I'm mostly pissed at myself because I was so carefull taking everything apart - not looking forward to another tear down even though it will only take half the time because all bolts are new.

The prideful part of me kinda hopes it is a bad sensor since it will vindicate my workmanship!
 
The intake manifold can be a b*&*& to set properly. I cracked mine the first time i replaced the header, and had to get a new one. I also assume that you are torquing everything in sequnce as per the FSM, as doing it in the improper sequence could cause the header to not seal properly. At least that is what the FSM told me.
 
I can't believe no one has pointed this out yet. You cannot use an APN header in conjunction with the 99+ intake without clearancing the #6 cylinder tubes. What you have is a major, major vacuum leak because the intake isn't seated against the head. You have to take the intake back off and grind a bit of material off of each the intake and exhaust on the #6 runner. Search on NAXJA here...you will find a thread that talks about the interference and tells exactly where to grind. Good luck!
 
Yes - I did see that clearance issue regarding the #6 intake and exhaust header. I couple of guys said they had no issues, so I thought I was lucky - guess not.

When I mounted the intake it seemed to slide right into place -felt nothing hitting.

What I need is a 3 day weekend so I can take it apart and check everything again - gets dark too early!
 
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