2grfe running rich with negative fuel trims - MR2 Owners Club Message Board
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post #1 of 14 (permalink) Old August 21st, 2019, 13:28 Thread Starter
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2grfe running rich with negative fuel trims

Those of you who know me know it's been a long running saga with my mr2. Life, kids, work all keeps getting in the way. I've fixed most of the issues I've had and I think I'm left with just the one now (famous last words...).

I'm getting an ecu error code thrown up saying both banks are running rich. My ltft and stft are both around - 20. Are the issues related?

Cheers guys!

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post #2 of 14 (permalink) Old August 21st, 2019, 14:16
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Nick, if I recall correctly, you are running a supercharger with a NA ECU but what injector size are you running. If you are running a supercharger-sized injector (500cc?) with a NA ECU that expects 360cc then you might expect to see something like this.

If this is not the case, what else might cause this? A grossly undersized MAF tube, for example a 2.5-inch MAF tube instead of the "standard" 3-inch tube would cause something like this, by overestimating the airflow. If you look in your Torque App you should be able to see a live number for MAF g/s. At idle this should be about 3 to 5 depending on the temp and the idle RPM.

Next thing would be fuel pressure. If you are running elevated FP, say 70 or 80psi, instead of the standard 43.5, then you might see something like this.

As a test I suggest unplug the MAF - yes the car will idle and drive quite happily but with a 3000 RPM rev limit - disconnect the ECU for a few minutes, to clear fuel trims, then see what happens to the reported AFR. This may give some clues about what is going on.

Last edited by merryfrankster; August 21st, 2019 at 14:33.
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post #3 of 14 (permalink) Old August 21st, 2019, 16:29 Thread Starter
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Hey Frank. Correct, I've got a TVS1320 fitted, running 5-plug N/A tune.

Injectors, I'm 99% certain I'm using the standard 2GR-FE ones (brown??). I know when I bought the supercharger it came with some brand new supercharger-sized injectors (blue??) but when I asked around at the time it was advisable not to fit them without a re-tune of the ECU. I'd have to go look at my car to check and search my back history but i'm pretty confident on that one.

MAF tube - now that may be a culprit. I bought it from some guy on TwoBrutal who made a load up for us UK guys. I'll measure that tomorrow as well as taking a look at the MAF g/s.

Fuel pressure I checked today and was just over 40psi, so that should rule that out.

Unplug the MAF - cheers, will try that tomorrow.

Thanks for the suggestions, will see how I get on!
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post #4 of 14 (permalink) Old August 22nd, 2019, 06:21 Thread Starter
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Here's a shot of my maf g/s, 6.5 on average with a warmed up engine.

The air intake measures 3" outside to outside, is that the correct way of measuring it?

Injectors are brown as blue ones are still in the box.

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post #5 of 14 (permalink) Old August 22nd, 2019, 07:41
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Three inch Outside Diameter (O.D.) is the correct size. Tubing you almost always measure the OD. Hoses you measure the ID.

I see a potential issue in the photo - the MAF is too close to the filter/tube entrance. This is known to cause issues with the MAF reading.

On my intake I have several inches of straight tube in front of the MAF. See pic. This is 3-inch tube. This geometry works well with the stock MAF calibration. This is the kit from eBay that I cut to fit:
https://www.ebay.com/itm/FOR-07-08-T...R/232177042003

To make something similar you might try to shorten the silicone elbow that attaches to your TB by chopping off a few inches, then use whatever length you cut off as a coupler and add a few inches of straight tube before the filter. You can get 3-inch tube at most performance shops.

Another solution that seems to work well is Marc's new intake tube with the modified MAF calibration. This requires that you purchase the new tube and send your ECU in for re-flash.

Another possibility is to use a stock airbox with MAF housing and intake tube from the vehicle that matches the calibration in your ECU.

The thing that is very perplexing here is that the typical MAF tube issue is a lean fuel trim. You are getting a rich fuel trim. So there could be something else going on too. It is something affecting both banks equally. Misfires cause a lean fuel trim. Exhaust leaks cause a lean fuel trim. [Vacuum leaks also cause a lean fuel trim.] I can't think of anything aside from what we've discussed already that would cause your engine to run rich, unless both AFR sensors are somehow off by the same amount or there's a measurement issue in the ecu. Are your ECU grounds connected properly on the engine? There are two or three multi-wire grounds that attach to the cylinder heads above the transmission, and one or maybe two each on the valve covers. They look like eye terminals with a 6mm hole and multiple wires attached. Do you have any trouble codes besides the lean codes? Anything that would indicate a sensor malfunction?
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Last edited by merryfrankster; August 22nd, 2019 at 07:54.
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post #6 of 14 (permalink) Old August 22nd, 2019, 07:59
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Originally Posted by merryfrankster View Post
I see a potential issue in the photo - the MAF is too close to the filter/tube entrance. This is known to cause issues with the MAF reading.
In my testing a shorter length of pipe produced higher readings (very close to the stock airbox actually), while a longer pipe produced lower readings. However, both of those were without a filter installed. Adding the filter dropped the reading significantly, and if I recall correctly made the length somewhat irrelevant.

All that said, 6.5g/s at idle sounds really high to me. Like about double what I have usually seen. And a high MAF reading would cause negative fuel trims.


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post #7 of 14 (permalink) Old August 22nd, 2019, 14:09
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I agree with Alex that the MAF reading is high. Possibly there's an issue with the MAF sensor itself or its wiring. Was the engine wiring harness extended to reach the MAF in that position? It might be worthwhile to try replacing the MAF sensor. The repair manual lists some diagnostic procedures for the MAF sensor and its wiring. I'm attaching the relevant section.
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Last edited by merryfrankster; August 22nd, 2019 at 14:14.
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post #8 of 14 (permalink) Old August 23rd, 2019, 05:19 Thread Starter
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Cheers guys. I'm away for a week now on the holiday, but I'll measure up for an extension tube for the induction pipe when back home. I've got a spare maf as well, so will firstly swap that over to see if there is any change.

Earthing wires, good question tbh. Yes, but not sure how many. Another point for me to check.

I'll let you know how I get on!

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post #9 of 14 (permalink) Old August 28th, 2019, 17:53
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Yes, these issues are related. Your engine appears to be running rich and your short and long term fuel trims demonstrate that the computer is trying to "lean" the mixture to compensate. I don't know what all of the issues are that could contribute to this, but I would start by checking the fuel pressure. You may be experiencing problems with your fuel pressure regulator. Also, while checking that pressure, turn your engine off and see how long it takes fuel pressure to return to zero. If it does so within one or two minutes, you probably have one or more leaky injectors. Leaky injectors could cause a rich condition by allowing fuel into the engine when they're supposed to be closed off.
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post #10 of 14 (permalink) Old August 30th, 2019, 06:15
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I’m willing to bet you’ve got a MAF/MAF tune issue.

Marc, seems like a good platform for a RAV4 ECU boosted tune....


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post #11 of 14 (permalink) Old August 30th, 2019, 09:43
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Also, while checking that pressure, turn your engine off and see how long it takes fuel pressure to return to zero. If it does so within one or two minutes, you probably have one or more leaky injectors. Leaky injectors could cause a rich condition by allowing fuel into the engine when they're supposed to be closed off.
That, or it just means he has one of the common aftermarket fuel pressure regulators.


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post #12 of 14 (permalink) Old September 7th, 2019, 10:45 Thread Starter
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Update! I've added a 100mm long tube now in between the air filter and the MAF, and my STFT's have dropped from -20 to between +1 and -2, which one would assume is within normal tolerances? my MAF g/s is now around the 4.5 mark.

So this simple change does appear to have fixed my over-fuelling. It certainly smells less rich now.

Just need to get it MOT'ed this month and see what it's like on the road... It's been a while, lol.

Thank for the help guys!
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post #13 of 14 (permalink) Old September 7th, 2019, 14:58
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Thank for the help guys!
We are counting on you for some high quality videos with some NOISE!
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post #14 of 14 (permalink) Old September 7th, 2019, 16:04 Thread Starter
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We are counting on you for some high quality videos with some NOISE!
I'll get my go-pro up and running!

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