Thread: Engine bogs down intermittently at idle - poor MPG

04 GT N/A 65K "no CEL"

For several months now I've noticed the car bog down to 300 RPM then pop back up to normal - sort of a hesitation or hiccup - at idle only when I was coming off the freeway to a stop. Didn't happen often and only 1 or 2 "hiccups". Idle never went high, always bogged low.

I changed the wires and plugs recently and added a HAI as shown in some posts on here with PVC and couplers from Home Depot. After these changes, the car ran better but problem still persisted.

In the last week, the same problem is becoming more frequent and now it's happening at any time, not just coming off the highway. And sometimes the idle will go down a dozen times in a row. It hasn't died, no problems starting, and no power issues that I can tell. When I go WOT everything seems fine.

Then only other thing I've noticed and I can't put my finger on the timing is that I'm getting pretty lousy gas mileage. 18MPG in the city with A/C running. I don't drive it hard. I drove it out of town a couple weekends ago and got about 26 on the HWY. 93 octane gas. Tires at 35psi. It does seem that I noticed the drop in MPG about the time I installed the HAI although that was also the time that I started paying closer attention to it - so not sure...

I don't know if this is one problem or two but I wanted to provide all the info I have.

I ordered an O2 sensor and fuel filter from rockauto and should have that installed when it arrives.

What else should I be looking at?

Thanks in advance.

Well, being N/A the 93 won't help you any, but your call there. Being an 04 I'd pull the entire throttle body off and clean it until it looks new, and then also clean the MAF with approved MAF cleaner (do NOT use brake or carb cleaner on the MAF). Reinstall and see what you get. I'd bet the TB is all gunked up.

Bill, thanks.

With a screwdriver I tapped on and around the MAF and no change. When you mentioned oil though I remembered that I did oil the filter some time after I installed it - I wasn't sure if it needed it off the shelf - I tried to go very lightly on the oil. So:

1) I checked the TB - still clean.
2) I noticed some of the coupling bands were loose - not sure how I missed this yesterday - but I tightened everything back up.
3) I cleaned the MAF again with MAF cleaner - I will drive today and see if that changes anything. Otherwise, I will unplug and test as you suggested.
4) I've attached a picture - can you tell me if it is ok for the MAF to be closer to the filter as it is or should it be up closer to the TB? I wasn't sure about this when I installed.
5) Should I oil this filter or not - I've read differing opinions. Spectre filter.

I'll post back updates once I have them.

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Update:

Drove city and hwy after cleaning MAF and tightening up coupler bands.

Car fired noticeably quicker. Throttle response and acceleration noticeably better. RPMs didn't pop up and down, but it hasn't been doing it all the time so I'll keep an eye on it. I started out with the AC off. Drove about 3/4 of my drive with it off. Then at a stop light turned it on. When I turned on the AC the RPM fluttered for a bit then seemed to settle out before I came to the next stop. Didn't see much fluctuation after that though so maybe it was just adjusting to the cleaner MAF when load came on it. Although the AC seems to be straining it...I don't know, maybe I'm making it up in my head.

Cleaning the MAF certainly made a difference. I had no idea filter oil would affect it that much and that fast. Is there a dry cone filter?

So I filled up the tank and will see what happens. Keeping an eye on it bogging down.

BTW - MPG for my last 4 tanks including this last one:

6/29 - 18.6 city
7/1 - 26.5 hwy (I think I oiled the filter right before this trip)
7/15 - 17.5 city
7/25 - 16.3 city

2) I noticed some of the coupling bands were loose - not sure how I missed this yesterday - but I tightened everything back up.

I'd bet you were getting unmetered air in after the MAF and it was messing it up. Cleaning the MAF probably straightened the rest of it out. I love cheap fixes, lol..

FordMan - you were right the MAF needed cleaning. Wondering if I shouldn't just go with a dry flow cone for the future.

Followed Bill's instructions and disconnected MAF - car runs even better with it disconnected. Wondering if I shouldn't replace MAF?

Reconnected MAF this morning and put about 30 miles of city driving on it. Although not as bad as before, RPM still dance a little when coming to a stop but I wouldn't say it is the same bogging down I was experiencing. Could this be computer sorting itself out after the disconnecting and reconnecting yesterday?

Also, w/ MAF disconnected yesterday the car drove much better in the low end range - accelerating from stop to say 35 MPH. Today it seems to work harder in that range with the MAF reconnected compared to it disconnected.

is the maf in the same location as it was when it had the stock intake?
if you moved the maf closer or further away from the stock location that will mess up the tune and cause the issue you're having

QwikGT - I wondered about that - and no, I moved the MAF further away from TB towards filter. I can change this and move closer to stock and see what that does.

Just got back from another hour of test driving and the RPM issue still exists same as before. More pronounced when AC on, car warm, in gear, coming to a stop. Struggles to find it's idle.

I received my O2 sensor and fuel filter today but want to sort this out before I throw other changes in the mix. I'll keep you posted.

So instead of screwing around with trying to re-position the MAF - I just put the stock box back on this morning - and cleaned the MAF again for good measure - and took it for a drive. This resolved the struggling in low RPM I was describing - in other words, a lot more acceleration with a lot less accelerator. Seems like this would address my MPG and general sluggishness issue I described. Stands to reason as those two issues seemed to appear about the time I installed the HAI.

But the RPM jumping/bogging down still persists. Same conditions: at idle, after deceleration, transmission engaged, under load of AC.

I'm going to install the fuel filter first, test, then install O2 sensor, test. Just to isolate them so if one of them works, I'll know which one. (not sure if these will address issue but I have them so I'm going to use them). Anything I else I should try? Does anyone think I'm still looking at a bad MAF?

Update:

I received my new MAF two days ago and installed it. Have put about 50 miles on it. RPM surge issue still exists. Car feels and runs better in some ways for sure, but original issue persists.

I'm aware the issue only exists under load of A/C compressor. Where else should I be looking?

Originally Posted by QwikGT

is the maf in the same location as it was when it had the stock intake?
if you moved the maf closer or further away from the stock location that will mess up the tune and cause the issue you're having

Curious as to how distance from TB/engine would be the key factor.

AFAIK, the tubing before and after would be the primary influences on accuracy.

Having the MAF right after the filter means it might not be seeing the most laminar airflow that it could be. However a similar issues applies to having a bend before the MAF as well.

As for the stock intake being better than a HAI, I would agree. The stock setup is essentially a quite "CAI".

Unsure on what specifically is causing your issues.

I find if the fuel trims are out of wack, it will struggle to find an idle in similar situations as you've described. Its very likely these are off as you had the HAI installed, and the PCM adapted to that.

Since you've reinstalled the stock air box, I would suggest disconnecting the battery in hopes of resetting the fuel trims. Some say you should connect the neg/pos battery cables as apparently this causes a reset of the trims/trans adapt settings.

Originally Posted by matt5112

Since you've reinstalled the stock air box, I would suggest disconnecting the battery in hopes of resetting the fuel trims. Some say you should connect the neg/pos battery cables as apparently this causes a reset of the trims/trans adapt settings.

I'll try this. Thanks.

Any chance this could be a vacuum leak?

Yup.

Originally Posted by matt5112

Yup.

Sorry I'm a little new to some of this - any tips on a vacuum diagram - the only one that is obvious to me is the brake booster. I've searched on the forums and can't exactly find one. Is there a tree I should follow from?

should be on your hood, on the emissions sticker.

Problem resolved - at least it seems. While looking for a vacuum leak, I noticed a nut missing on the crossover manifold right before the O2 sensor. After replacing, all seems good.

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Thanks for everyone's help. This forum has been a lifesaver ($$ saver) more than once.