Need help: rough idle and crappy city MPG

[pet575]

I changed my intake manifold back in the spring and ever since I put it back together it has not idled the same. After that repair my highway MPG and city MPG got better but always the rough idle and hesitation from a start or a slow roll.

Well now that hesitation has gotten worse and the rough idle has as well. On cold startup, the vehicle nearly dies before recovering and revving to 1,000 RPM or so and then settling back down to abround a rough 4-500 MPH. Even worse is now my city MPH has gone down somewhere to the 8-9 range in recent months when it used to be in teh 14-15 range.

I used to get 18+ MPG on a tank of mixed driving. I could get 21 or 22 if I was nearly all highway. Now I'm lucky to get more than 16.5 if I'm doing 75% highway but I'll still get about 19-20 if I'm all highway.

Can someone help me out on where to start/what to clean/what to check to see if I can track down something that is causing this? I put in all new plugs back when I did this repair and checked out all of the coil packs-replacing a few due to damage from coolant.

I've got some confusion on what this vehicle does/doesn't have as far as cleanable throttle body parts, MAF, etc.

Thanks!

 

[FIND]

Sounds like you have a vacuum leak on your UIM gasket.

 

[cross6]

Start with a cold engine, crank it up, let it find it's idle, or have an assistant keep it idling if it wants to stall.

Take a can of brake cleaner and stick the straw down around where the intake manifold meets the heads. Do short quick squirts.

Listen for the engine rpm change. OR have an assistant check the tach.

Squirt around the back of the manifold a little, too, around the PCV valve/hose.

If the engine reacts to your squirts of brake cleaner then you found the leak.


If you have access to a scan tool check out the short and long term fuel trim. If they're in the double digits then you probably for sure have a vac leak.

 

[pet575]

Thanks guys. Follow-up questions:

1. The idle issue has been present since the first time I restarted after my intake manifold change. But the low city mileage thing didn't start until a couple of months after. I'm trying to figure out how I could have any type of manifold gasket leak if I know I installed that thing absolutely correctly.

2. Plus, wouldn't I have seen that drop in MPG right away?

3. Would the colder temps of winter just be making it worse since cold air is more dense? Maybe it was there all along but I didn't know it in the warmer temps?

4. Also, the rough idle/near dying is only for one second on cold startup and then it rights itself. Every so often you can feel it idling a bit rough, but not every single time. The bigger concern is the hesitation from a stop or a slow roll then need to accelerate again. Biggest place I notice it is in my parking garage when I coast slowly around a turn at the end of the garage and go back the other way. When I try to accelerate again, THAT is usually where I see the biggest hesitation.

5. I've been wondering if I somehow inadvertently damaged the EGR valve or a PCV when moving all of the stuff around for that intake manifold change.

6. If I do have a leak at the manifold, why did I see such a significant MPG improvement inititially and for a few months after?

 

[tower]

Leaks can get worse over time or temp changes, you may indeed have something other then a leak but what you are describing sure sounds like a leak.

I would check for a leak first as it is easy to do and does not cost anything. Not sure a PCV or EGR would give you the problems you are having but a IAC would so you may want to check that too.

 

[pet575]

Should I clean or replace the IAC? Also, I have a lot of confusion on where the IAC is on my make/year. I think I've read a few posts stating that I don't have one? If I have one where is it located?

 

[wildchild2]

i just had same problem, there is vacum leak,mine was the 5/8 diameter 90% thin hose elbow that runs over to the passenger side valve cover. elbow is located bottom right side of t/body,comes out front, turns left.you must take plastic cover off top of engine to see it. hope it helps

 

[swshawaii]

Scan for possible trouble codes so you can eliminate any more guessing. GL

 

[tower]

Should I clean or replace the IAC? Also, I have a lot of confusion on where the IAC is on my make/year. I think I've read a few posts stating that I don't have one? If I have one where is it located?

What motor do you have? But yes you have one, it is near the TB it is cylinder shaped and has a electrical connection.

 

[pet575]

I have the 4.6L V8. Thanks everyone!

 

[FIND]

Go ahead and start with what cross6 recommended. Spray a little carb cleaner around and note if your idle changes any of the places you spray it. Hit your vacuum hoses, around your Throttle Body, and around your UIM.