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New Exhaust Manifold = Lower Idle?

1K views 13 replies 6 participants last post by  Hawk37 
#1 ·
I spent all day Saturday swapping out my cracked stock 4.0 manifold to an APN header. (the last bolt in the head broke off, that was fun) I finished the install, with one 7/16 bolt instead of a 3/8's. I've driven it about 150 miles now and I've noticed that after it warms up the idle rpm is below the first tick mark on the tac, I'd guess 400 rpm. It's supposed to be around 600 rpm isn't it?

Is that bad? What would be causing this?
 
#2 ·
no thats good because you have now properly tightened all the manifold bolts including ones that are used for the intake manifold. Now that you have eliminated a vacuum leak, readjust your idle to ~700 rpm and you are good to go. You should also now have a more stable idle, since intake manifold leaks change depending on temp, causing idle to flucuate.
 
#3 ·
If you have a 4.0 and are running it as a FI engine with the computer and all the sensors, then there is no way to adjust the idle (the computer controls the idle) - you have a problem. If this is the case, can you smell gasoline on the exhaust?

However, if you're running the 4.0 with a carb, then you can most likely adjust the rpm and may not have a problem.
 
#4 ·
Per my Haynes manual, should be around 750 rpm's. When I had my Rugged Ridge in, my idle was lower. I put in a cheapo one that looks more like the OEM, and the idle is back up to 750.
 
#5 ·
Mine idles just below the 500 mark. Has since I bought 40,000 miles ago (@211,000 now). I wouldn't have it any other way. Where it's controlled by the computer if you don't' give it enough fuel to pull out without killing it the computer copensates for it as much as it can within reason. I can pull out "relatively fast" without touching the gas pedal at all now that i'm used to it. It makes it to where I can idle up pretty steep grades in first gear in 4Lo or 2Lo without touching the gas at all. My friend don't belive what they see. If you press in the clutch while climbing this way it'll jump up to the rpm level that it would be at if there was no drivetrain resistance then go back to around 500 after about half a second. How is idling a little lower than factory specs a bad thing if it's steady and the computer enjoys it?
 
#6 ·
Alfons:

I'm definitely running MPFI not a carb. The computer is setting the idle. I'm planning the 02 sensor now that I don't have a leaking manifold. Would that do it?

The exhaust isn't very strong, at least not that I've noticed it.

50rc:

Did you get rid of the rugged ridge just because of the low idle?

Jesse 87:

I'm not opposed to the lower idle; I just want to know if it is going to cause any problems. You just made me a lot less worried about it.
 
#8 ·
Alfons:

I'm definitely running MPFI not a carb. The computer is setting the idle. I'm planning the 02 sensor now that I don't have a leaking manifold. Would that do it?

The exhaust isn't very strong, at least not that I've noticed it.

50rc:

Did you get rid of the rugged ridge just because of the low idle?

Jesse 87:

I'm not opposed to the lower idle; I just want to know if it is going to cause any problems. You just made me a lot less worried about it.
If you're running without the oxygen sensor, you'll be running rich - this would affect your idle. Toasted exhaust valves will also give you the same condition & each condition should give you a slight gasoline smell to the exhaust. Add the sensor and see how that helps and then I would plug a vacuum gauge to the intake and see what that tells me, like idle vacuum and how the needle reacts during idle.
 
#7 ·
Got rid of it because it cracked after about 25,000 miles.
 
#9 ·
If I am understanding you have normal idle while its cold but low idle after it goes into closed loop and warms up ?
 
#10 ·
Yes Que89YJ that's correct. It's a little high while warming up 700-900, but when I'm sitting at a stop light 15 minutes later it's idling around 400.

Alfons:
I am running the old o2 sensor, but as far as I know its still the stock one. I'd be surprised if its not bad. Does the computer look at the o2 in closed loop?
 
#13 ·
Yes Que89YJ that's correct. It's a little high while warming up 700-900, but when I'm sitting at a stop light 15 minutes later it's idling around 400.

Alfons:
I am running the old o2 sensor, but as far as I know its still the stock one. I'd be surprised if its not bad. Does the computer look at the o2 in closed loop?
In open loop the ECU doesn't look at the O2 sensor, but as soon as it switches to closed loop, the O2 sensor is the prime driver for injection calculations.
 
#11 ·
Clean the throttle body and the IAC. If the IAC,TPS, and Throttle body are clean(Do a search on the forum for cleaning the IAC) are clean and the idle is still low then You need to replace a bad sensor. IAC first, temp, then map.
 
#14 ·
So, I cleaned up the IAC. That seems to have solved the low idle. Haven't gotten around to the O2 sensor yet. At $50 a pop I want to be sure that none of the residue burning off the new manifold ruins a new sensor. My mpg went up from 14 before the new manifold, to 16 now. My wallet likes that.

Thanks for your help guys!
 
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