High Long Term Fuel Trims at Idle

[jacon1965]

I am baffled by my HIgh LTFT at idle. Everything points to a vacuum leak, but I can't find one. I have done a smoke test. Only leak I find is around the butterfly shaft in the TB. I've tried propane and water also around the intake gasket, fuel injectors, TB, and vacuum lines to no avail.

Vacuum
@ idle- 15hg
@2000- rpms 20hg
Drop rpms- rises 3-5hg then settles @15hg

LTFT
Near zero with a load
11-18 @idle (14 avg)

STFT
Always near zero

MPA
40kpa @ idle
70-80 at cruise speed

O2's I used NGK's. Voltage seems to fluctuate normally.

New iridium plugs

119,000mi, seems to run, good bad MPG. Used to get 14-15. now I'm down to 11-12

Over the last year i have replaced every sensor I could think of.

any ideas? Something I'm missing.

Oh and over the years, a big thanks to all and especially to jerry for all the great TJ tips

 

[Trevlaw]

I am baffled by my HIgh LTFT at idle. Everything points to a vacuum leak, but I can't find one. I have done a smoke test. Only leak I find is around the butterfly shaft in the TB. I've tried propane and water also around the intake gasket, fuel injectors, TB, and vacuum lines to no avail.

Vacuum
@ idle- 15hg
@2000- rpms 20hg
Drop rpms- rises 3-5hg then settles @15hg

LTFT
Near zero with a load
11-18 @idle (14 avg)

STFT
Always near zero

MPA
40kpa @ idle
70-80 at cruise speed

O2's I used NGK's. Voltage seems to fluctuate normally.

New iridium plugs

119,000mi, seems to run, good bad MPG. Used to get 14-15. now I'm down to 11-12

Over the last year i have replaced every sensor I could think of.

any ideas? Something I'm missing.

Oh and over the years, a big thanks to all and especially to jerry for all the great TJ tips

I am having the exact same issue and was about to make a post on it. I've replaced or inspected damn near everything after swapping my motor and the high LTFT at idle are the last issue left.

I'll be following

 

[NskLJ]

I am baffled by my HIgh LTFT at idle. Everything points to a vacuum leak, but I can't find one. I have done a smoke test. Only leak I find is around the butterfly shaft in the TB. I've tried propane and water also around the intake gasket, fuel injectors, TB, and vacuum lines to no avail.

Vacuum
@ idle- 15hg
@2000- rpms 20hg
Drop rpms- rises 3-5hg then settles @15hg

LTFT
Near zero with a load
11-18 @idle (14 avg)

STFT
Always near zero

MPA
40kpa @ idle
70-80 at cruise speed

O2's I used NGK's. Voltage seems to fluctuate normally.

New iridium plugs

119,000mi, seems to run, good bad MPG. Used to get 14-15. now I'm down to 11-12

Over the last year i have replaced every sensor I could think of.

any ideas? Something I'm missing.

Oh and over the years, a big thanks to all and especially to jerry for all the great TJ tips

Try plugging the brake booster hose, you may have a leak and can’t hear it inside the vehicle.

 

[jacon1965]

Try plugging the brake booster hose, you may have a leak and can’t hear it inside the vehicle.

Thought I sent a reply to this post. I tried that already. In fact, I removed the booster hose and took readings from that spot on the manifold. Still the same reading.

 

[pagrey]

If you get outside of the normal range of fuel trims you'll get a code. I know that's not super helpful. If you have higher fuel trims at idle that means your injectors, when operated at a low duty cycle, are delivering less fuel than they are when they are pulsing at a medium duty cycle. You could clean them, I think @Irun just sprayed his out and had great luck. My trims are not anywhere near zero, I just replaced my injectors and I'm sure they are all different now. The real question is why does it really matter, I haven't even bothered to check my trim after replacing them. I don't see a point, I know they will be different. Do you notice anything wrong that leads you to investigate your fuel trims? If poor economy is your only symptom I think fuel trims might be a dead end especially if they are close to zero under load where it counts.

 

[jacon1965]

something is eroding my fuel mileage. injectors were cleaned and then later replaced. With the 15hg vacuum and the high LTFT @ idle, I apparently have a vacuum leak, which I cannot find. done 3 smoke test. last one I removed the TB and still no leaks showing anywhere. I also checked each vacuum line port on the manifold with no change (still 15hg). So my thoughts,,,,,,, maybe a valve adjustment (tight valves)?

 

[jacon1965]

something is eroding my fuel mileage. injectors were cleaned and then later replaced. With the 15hg vacuum and the high LTFT @ idle, I apparently have a vacuum leak, which I cannot find. done 3 smoke test. last one I removed the TB and still no leaks showing anywhere. I also checked each vacuum line port on the manifold with no change (still 15hg). So my thoughts,,,,,,, maybe a valve adjustment (tight valves)?

I see the valves are hydraulic. So thats out..... any other ideas? Or I guess I have to live with the crappy mileage

 

[NskLJ]

something is eroding my fuel mileage. injectors were cleaned and then later replaced. With the 15hg vacuum and the high LTFT @ idle, I apparently have a vacuum leak, which I cannot find. done 3 smoke test. last one I removed the TB and still no leaks showing anywhere. I also checked each vacuum line port on the manifold with no change (still 15hg). So my thoughts,,,,,,, maybe a valve adjustment (tight valves)?

You may have a vacuum leak or it could be the computer adjusting for age of the engine and components. You stated you have a leak at the throttle shaft put some propane or brake cleaner there and see if your LFT goes down. Your computer is adjusting for the additional air coming in.

 

[Trevlaw]

You may have a vacuum leak or it could be the computer adjusting for age of the engine and components. You stated you have a leak at the throttle shaft put some propane or brake cleaner there and see if your LFT goes down. Your computer is adjusting for the additional air coming in.

I had the throttle off a little while ago and I don't think the leak he's referring to can be fixed. Mine leaked right around where the shaft on the butterfly valve connects to the TPS sensor, just seemed like a slip fit hole for the shaft to pass through without an o ring or anything.

I'm going to check mine for vacuum leaks soon and see if I can come up with anything. I've recently done a motor swap with another stock 4.0 regarding other issues and I've pretty much replaced or gone through everything

 

[NskLJ]

I had the throttle off a little while ago and I don't think the leak he's referring to can be fixed. Mine leaked right around where the shaft on the butterfly valve connects to the TPS sensor, just seemed like a slip fit hole for the shaft to pass through without an o ring or anything.

I'm going to check mine for vacuum leaks soon and see if I can come up with anything. I've recently done a motor swap with another stock 4.0 regarding other issues and I've pretty much replaced or gone through everything

Most throttle shafts have no seal. They wear with time and will leak usually when closed. I used to install bushing in quadrajet carburetors I had a step drill and bushings. The problem is the screws that hold the butterfly we’re staked so you would have to grind them off remove, retap, reinstall, without damaging the shaft. You could swap another throttle body or just let the computer compensate for the small leak. Might be able to install a bigger aftermarket throttle body but I don’t know if the tolerance would be as good as factory.

 

[NskLJ]

I am baffled by my HIgh LTFT at idle. Everything points to a vacuum leak, but I can't find one. I have done a smoke test. Only leak I find is around the butterfly shaft in the TB. I've tried propane and water also around the intake gasket, fuel injectors, TB, and vacuum lines to no avail.

Vacuum
@ idle- 15hg
@2000- rpms 20hg
Drop rpms- rises 3-5hg then settles @15hg

LTFT
Near zero with a load
11-18 @idle (14 avg)

STFT
Always near zero

MPA
40kpa @ idle
70-80 at cruise speed

O2's I used NGK's. Voltage seems to fluctuate normally.

New iridium plugs

119,000mi, seems to run, good bad MPG. Used to get 14-15. now I'm down to 11-12

Over the last year i have replaced every sensor I could think of.

any ideas? Something I'm missing.

Oh and over the years, a big thanks to all and especially to jerry for all the great TJ tips

What tire size are you running? How are you calculating fuel mileage?

 

[97JadeTJ]

I just had a similar situation… kept getting p0171 codes (lean) high LTFT (30) … thought was a vacuum leak, did smoke, no leak… thought maybe exhaust leak… stuck a shop vac hose in the tail pipe, blowing instead of sucking , soapy water sprayed on the exhaust manifold I found a crack on the back side and a manifold gasket leak… replaced the manifold and gasket and problem solved… no codes and STFT and LTFT are within spec… Good luck

 

[Trevlaw]

Most throttle shafts have no seal. They wear with time and will leak usually when closed. I used to install bushing in quadrajet carburetors I had a step drill and bushings. The problem is the screws that hold the butterfly we’re staked so you would have to grind them off remove, retap, reinstall, without damaging the shaft. You could swap another throttle body or just let the computer compensate for the small leak. Might be able to install a bigger aftermarket throttle body but I don’t know if the tolerance would be as good as factory.

Interesting, I might need to look into that some more. I just figured it always leaked a bit. Wouldn't be terribly difficult to add some type of seal, those screws sound like a PITA though

 

[pagrey]

Vacuum leaks wont kill your mileage, maybe your idle? The Jeep cruising around town doesn't care if the air comes through a hole in the intake manifold or the throttle plate. It's the same air and the 02 sensors compensate completely. If your trims are maxed out you'll have problems and codes, otherwise you've got other things likely going on.

 

[NskLJ]

Interesting, I might need to look into that some more. I just figured it always leaked a bit. Wouldn't be terribly difficult to add some type of seal, those screws sound like a PITA though

There is not much room for a seal, with fuel injection it’s not an issue as the computer will compensate for small leaks. Idle Fuel trim at those numbers won’t be the cause of bad mileage. Throttle shaft leaks on a carburetor will cause erratic idle because after setting the idle mixture for smooth idle when the throttle is opened and close the throttle shaft will cause more or less of a leak. This will change the idle mixture affecting idle quality.