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Going Pig Rich When Under WOT With Turbo

6 May 2010
Chicago-Ft Lauderdale-Georgia Mountains
I have a Mase built 96 turbo that when driving normally has reasonable AFRs but when I go to WOT it goes pig rich 10.0s and stays there making the car undriveable until I back off the throttle a little where it goes to 12s but then within a couple of seconds goes back to 10s and starts bucking until I feather it again which will get me home. When I shut it off and come back the next day it drives normally again and will even take 5 pounds of boost no problem but if I go WOT which is about 9 pounds of boost I get the same issue. I posted in the Atlanta area section looking for a shop that is familiar with AEM FIC. I'm wondering if the MAP sensor has gone bad from the boost?
Normally, I would say it is unlikely that it is the MAP sensor (but, not impossible). MAP sensors, when they fail, normally tend to a fixed output voltage or no output voltage. It would take some kind of weird planetary alignment where the MAP sensor works fine from 0 - 100 kPa and then goes bad when you hit 135 kPa and then sticks there. Also, on an OEM ECU if the MAP sensor goes out of range the ECU will generate an error code. However, the uncertainty is that the AEM FI/C has its own MAP sensor and clamps the voltage signal to the OEM MAP sensor to prevent the signal voltage from going too high when in boost and generating an error code. It might be possible that something has happened to the AEM FI/C; but, I think lower probability.

I am thinking that perhaps you have a fuel pressure regulator problem. As boost increases, the MAP referenced regulator raises the system fuel pressure. If the FPR is sticking or failing to regulate the pressure properly at high boost you may be running high fuel pressure resulting in low AFRs. It should be relatively easy to check. Assuming you are running the same fuel pressures as OEM, do the FPR test as set out in the service manual. If that confirms that fuel pressure under idle is normal, connect the FPR reference line up to one of those Mity Vac style hand held pumps that does both vacuum and pressure. Using the pressure function raise the pressure on the reference line up to your maximum boost level and monitor the fuel pressure while you are doing this. With a normal 1:1 FPR, you should see the fuel pressure rise smoothly 1 psi for each 1 psi that you increase the pressure on the reference line. Gradually release pressure on the reference line and watch for fuel pressure to drop at the same rate as you are releasing pressure on the reference line. If the FPR tests out normal, then time to look someplace else.

The OEM ECU uses the O2 sensors to adjust for fuel mix errors. The O2 sensor feedback signal can correct for a limited amount of error from the fuel pressure regulator. However, the O2 correction has limits and if the fuel pressure gets seriously out of range, it may not be able to control the AFR. That might be why the problem only shows under boost. As an observation, if the AFR goes seriously out of range (10 would be seriously out of range on an OEM ECU) and stays out of range, the OEM ECU would normally be generating an error code. Is your ECU generating any error codes?

As a matter of interest, if you measured an AFR of 10, you must have a wideband O2 sensor. Does the system have an additional bung for the wideband for monitoring only and retains the OEM narrow bands for the OEM ECU? If you are using the wideband for O2 control, do you know that the wideband controller or sensor hasn't checked out, particularly if it is one of the older LSU 4.2 sensors which don't have the durability of the newer LSU 4.9 sensor.
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My best guess on what is wrong is a bad O2 sensor as Old Guy suggested. As you have the wideband, I assume Mase programmed in some type of O2 feedback in the FIC. Personally, I limit O2 feedback to some percentage in case the sensors go bad, but all tuners are different.

O2 sensors are considered wear items that go bad over time, especially when subjected to aftermarket tunes that run the engine rich.

Before spending the time and money taking it to a tuner, I would replace the sensor(s) first. One is only ~$75.
In my system I retain the narrowband o2 sensors for the OE ECU and have a wideband for separate monitoring. Let us know if you do that or use narrowband simulation through the wideband controller.

The MAP sensor/circuit in the FIC is known to fail. I bought a used one that did not function; replaced the sensor, but it still did not work. I had to buy a new one. Still not consistent with your symptoms though.

You can log through the FIC and I have used that to diagnose a problem with a clogged tube to my OE MAP sensor. I advise starting with some data collection.
Unfortunately I’m not that good at diagnosis but I’m sure I can change out the O2 sensors is there a specific part number I should ask for and who sells them other than a Honda dealer?
The 1996 OEM OBDII ECU uses four narrowband O2 sensors; however, only the sensors in front of the cats are used for closed loop fuel control (do you still have cats???). I believe the two front sensors have different part numbers (but, not 100% sure). Do you know whether one of the OEM narrowband sensors was replaced with a wideband and it is sending a synthetic narrow band signal to the ECU or whether a separate bung has been added and the wideband is just providing display? All of this would be a function of how the turbo was plumbed into the exhaust system. If the wideband is the problem then totally different non Honda part number. Probably a Bosch sensor but could be an LSU 4.2 or LSU 4.9 and they are not interchangeable and they have different part numbers depending on cable length (and OEM specific termination) and whether MASE used an application specific sensor rather than one of the general fits all sensors. So, its not necessarily a question of 'a' part number.

If you are uncertain about whether you are running off of the narrowband or wideband sensors and how many sensors you have, it might be best to find an AEM specialist or the shop that did the original turbo install. Don't take it to an Acura dealer. I know from talking to the mechanic at our local dealership that routine maintenance is fine; but, they don't really like doing engine related troubleshooting on modified cars. They are typically working book rates for work and heavily modified cars have too many surprises / unknowns to be discovered and the dealership typically does not have any technical documentation to help them with trouble shooting one-offs.

Is your OEM ECU recording any fault codes? Borrowing a code reader and retrieving any fault codes might give you a clue as to what is going on.
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