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Fuel trims

9

90xj06

NAXJA Forum User
Location
Peabody, MA
So my long term fuel trim is 114 is this good or bad? At what point in either direction should you concern that something is wrong?

I have a new o2 sensor. And recently swapped 746 injectors which didn't change the trim but does seem to make it idle smoother.
 
128 is the target LT Fuel trim according to the MT-2500 manual. Below that is lean. So I would say your running lean long term. Vacuum leak? Or low fuel pressure? or imbalanced flow from injectors, or some cylinders working better than others, compression, plugs???

I think mine was reading 124 the other day with a new O2 sensor. Short term was wanting to run in the 90s at idle. But, BUT!!!!, when in closed loop, the O2 sensor should be allowing it to run slightly rich (The 128 target) to satisfy the Cat converter fuel needs. The fuel trim does not work in open loop.

So the question is why can't the ECU balance it.

I have no idea what is too much on the fuel trim number. Doubt anyone else here does either. You might try the MT-2500 Snap-on forum ,some old timers there and old thread might have the clue?
 
i upgraded my injectors and the numbers didnt change much. its down to 112 now.it has new spark plugs wires and rotor. cap was fairly new. but this isnt the healthiest of motors. its got a fair amount of blow by i presume. when changing spark plugs they looked perfect.

im going to say its ok for now. im guessing nothing would get damaged as long as the o2 sensor is happy? i dont have any misfires or any noticeable loss of performance. aside from the fact that it has a full interior and probably weighs more than the gutted jeep.
 
they are volvo ones but i think they are the correct flow. it didn't really change from the stock injectors. the only thing i can think of is the blow by from the cylinders resulting in less air in the cylinder during combustion.
 
trippled said:
Iirc lower number means removing fuel (o2 detected ehaust as rich). If you put slightly larger injectors in this would be normal.
Click to expand...

Interesting theory at first, except that the fuel trim number IIRC, is an average time the O2 sensor is above and below the stoich value, IIRC what the MT-2500 manual said. I could be wrong!!!! Which of is correct is how the average time is computed, is it the average time of the O2 sensor on one side or the other, or is it the average time the injector is open. Since injector open time is a function of load, I do not see how that would work? I think the Trim is how far above or below stoich it is over time as reported by the O2 sensor???
 
90xj06 said:
they are volvo ones but i think they are the correct flow. it didn't really change from the stock injectors. the only thing i can think of is the blow by from the cylinders resulting in less air in the cylinder during combustion.
Click to expand...

The intake blow by goes right back into the next cylinder, gets recycled. Not sure they idea fits. But an interesting idea. But exhaust blow by would fit your idea!!! Since it has less to no O2!!! Interesting idea!!!!
 
I haven't dealt much with that, but if it's like the old GM fuel trim counts (which it sounds like, since 128 was center trim) any count lower was a reduction in fuel injected. Not really anything to do with where the o2 is at, just a long term deviation to get to stoichiometric. It has nothing to do with the read voltage, but more of a fuel map value if that makes sense. Everyone switched to a percent value later, which is much easier to understand.
 
ill do some investigation and ill let you know my findings. i do seem to have this lovely vomit coming from the ccv hose to airbox.
 
trippled said:
I haven't dealt much with that, but if it's like the old GM fuel trim counts (which it sounds like, since 128 was center trim) any count lower was a reduction in fuel injected. Not really anything to do with where the o2 is at, just a long term deviation to get to stoichiometric. It has nothing to do with the read voltage, but more of a fuel map value if that makes sense. Everyone switched to a percent value later, which is much easier to understand.
Click to expand...

The fuel map is static, non changing, so it has no effect on fuel trims. Snap-on MT-2500 manual clearly says the fuel trim only changes in closed loop, closed loop only happens if the O2 sensor is working, in other words fuel trim is caused by data from the O2 sensor and is an ECU response to the O2 sensor data, in other words fuel trim is an average of the fuel trim rich/lean O2 sensor time domain data. Yes it does create a deviation from the data table, but the working O2 sensor data, interpreted by the ECU and used to correct fuel feed rates, is what the ECU fuel trim data is.
 
Last edited:
90xj06 said:
ill do some investigation and ill let you know my findings. i do seem to have this lovely vomit coming from the ccv hose to airbox.
Click to expand...


Can you bit a more specific? The CCV lines are super critical!!! Far more than we would expect. So is the valve cover vents, and internal valves and valve cover gasket seal.
 
Ecomike said:
The fuel map is static, non changing, so it has no effect on fuel trims. Snap-on MT-2500 manual clearly says the fuel trim only changes in closed loop, closed loop only happens if the O2 sensor is working, in other words fuel trim is caused by data from the O2 sensor and is an ECU response to the O2 sensor data, in other words fuel trim is an average of the fuel trim rich/lean O2 sensor time domain data. Yes it does create a deviation from the data table, but the working O2 sensor data, interpreted by the ECU and used to correct fuel feed rates, is what the ECU fuel trim data is.
Click to expand...

I must've misinterpreted your other post. But yes were basically saying the same thing. I fully understand how fuel trims work. Work on cars everyday.
 
Ecomike said:
Can you bit a more specific? The CCV lines are super critical!!! Far more than we would expect. So is the valve cover vents, and internal valves and valve cover gasket seal.
Click to expand...

the ccv system has a new vent hose and vacuum hose. its been pretty cold here recently and it has a emulsified oil coming through the air box to valve cover hose. and that's leaking on the air filter.

im not sure if a head gasket could leak into an oil passage and pressurize the crank case and not burn oil.
 
That's a common problem. There's a few band aid fixes that help if you search. I ended up swapping a later valve cover on and adding an air compressor oil/water separator in line to help keep it out of the intake. The other thing you can check is the rear vent elbow on the valve cover. It's a metered orifice that can plug up. But if you pull it off, it will most likely break. Can be found at the parts store tho.
 
i feel like it fell off. it looks like i just have a straight hose going to the back cover. i do remember changing maybe 5 or so years ago.
 
:cheers:

My background is chemical & environmental (EPA) process engineering, not a full time mechanic, but have done enough and been in the repair industry enough to be dangerous :twak: at times, LOL. Worked on electronics junk since I was about 8 YO (Old WWII tube radios, TVs and todays computers, software and hardware...) But I always question even what I think I know, LOL, so don't take my debates the wrong way :)

I just always try to make sense out it, if it sounds wrong, I always want to know what I am missing or try to figure out what others missed.

Still not 100% sure I have a complete handle on the fuel trim story (just recently got my used MT-2500 and was reading up on this month, so I am still new to what fuel trim really is), and the MAP versions are the only ones I have really researched. I have worked on rigs with MAF sensors, but never had to get deep into them. But I know the text book college theory side from a chemical engineering-physics point of view and process control (hardware and software) point of view. That does not mean I right!!!

trippled said:
I must've misinterpreted your other post. But yes were basically saying the same thing. I fully understand how fuel trims work. Work on cars everyday.
Click to expand...
 
90xj06 said:
im not sure if a head gasket could leak into an oil passage and pressurize the crank case and not burn oil.
Click to expand...

Head gaskets can have many different odd types of leaks. I had one that only leaked between two adjacent cylinders into each other. It could also be piston blow by during acceleration (low vacuum) causing excess crankcase pressure that exceeds the CCV vacuum line (the small on on the rear of the VC) volumetric flow rate, and thus causes the crank case to vent into the intake.

Normally the air flows from the air filter box to the crank case via the valve cover CCV large hose, then the high vacuum at the small CCV lines sucks out the blow by gasses with a mix of filtered fresh air and CC blow by gasses. If the blow by gas volume is too large it vents to the air filter box.

Is the emulsion under the valve cover too or just the large CCV hose????

You could have a cracked head leaking a tiny bit of coolant if it is only the large CCV line that shows the emulsion??? You need to see if it is on the head and or VC itself!!!!
 
90xj06 said:
i feel like it fell off. it looks like i just have a straight hose going to the back cover. i do remember changing maybe 5 or so years ago.
Click to expand...

That metering orific in the rear CCV line trippled mentions also affects Idle speed and IAC control of idle!!!!!

If it is too large, ie missing it can cause idle speed wander and high idle.

I learned from trial error years ago that Renix is super sensitive to any VC gasket leaks, and VC built in valve issues (clogged rear type valves built into the cover) and poor seals or missing orifice on the CCV line connections.

In my early days I fine tuned my 87 with a larger orifice (solved the oil on the air filter problem) till I found and fixed many other VC seal and CCV hose seal issues. My 89 CCV (old) seemed OK but when I replaced it all the remaining minor once in blue moon idle issues vanished.

Rock auto and I think Ebay now has bargains on the entire CCV hose assy. ALL of IT!!!
 
I should add that if it is leaking on the air filter, you have combustion gas going the wrong way in that front hose, or a small coolant leak?

Causes?

Poor seals on the VC gasket to the head.

Poor seals on the CCV hoses

Clogged or restricted orifice or rear CCV line (sludge does build up in the small CCV lines and they hard to clean)

Too much Piston blow by.

Coolant leak?

90xj06 said:
the ccv system has a new vent hose and vacuum hose. its been pretty cold here recently and it has a emulsified oil coming through the air box to valve cover hose. and that's leaking on the air filter.

im not sure if a head gasket could leak into an oil passage and pressurize the crank case and not burn oil.
Click to expand...
 
i recently changed the valve cover gasket with a premium gasket. didn't have any emulation under the valve cover only in the large ccv hose and just a little trace on the dip stick and that's it. maybe come spring ill pull the head off. i have a good head from a 93 that i wanted to put on it. but i may just order up a rebuilt one instead. but if i can get this issue fixed maybe i can just swap heads.
 
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