So, I know these "System Too Rich" Check Engine codes are a rabbit hole, but just hoping for any insight from you far more enlightened folks on my '98 XJ8 VDP.

So far I've had the following work done in effort to rectify:
- Replaced Air Mass Sensor
- Replaced Breather Hose to right valve cover
- Installed new air filter
- Replaced Purge Valve (this rectified code P0441)
- Replaced Bank 2 Sensor 1 Oxygen Sensor (this rectified code P0151)

Had a few weeks of beautiful Check Engine Light-free driving. Then today, bam, Check Engine again with Restricted Performance and P0172 & P0175 codes again...

Thanks for any insight!

 

The Mass Air Fuel sensor should always be the first thing to look at for a dual bank
rich condition.
Past that, the other things I would check are the temp sensors (air and coolant), leaking
injectors, and fuel pressure regulator.

 

Clean the MAF (check connector); are you getting temp readings on your OBD scanner; clean throttle bore and plate; clean part load breather (carefully, do a search); run a can of good fuel system cleaner through a tank (your injectors are dirty.

 

This is just my experience.

When I owned my 98’ XJR, my car threw the same codes. It has 283k miles on it today. I had done a lot of investigating and tests but came up empty handed. I would drive the car hard, perform a hard reset and quite a few times it was happy with that. Other times that wouldn’t work. Then one day with nothing more to lose, with the engine running, I had my scanner on live data showing rich fuel trims, I merely lifted the ECM a few inches upwards and all of a sudden the fuel trims were returning to normal and those codes have yet to return in at least 2 years. So with that being said, it’s likely an ECM causing the codes but, you still have to investigate and test before taking my ‘repair’ as the cure. Even if you acquire another AJ26S ECM, it too can have those problems. Cleaning the fan (gently) underneath those modules may also net some positive results.

 

So if I am actually "getting the picture"....the ECM just had some poor
connections? and that caused those codes?

 

To answer your question Randy, no.

One of the many things I did prior to ‘fixing it’ was to disconnect, inspect, reconnect modules, sensors and grounds. All the male/female pins I saw looked great. I opened all the relays and cleaned off the contacts if it wasn’t too bad and swapped the relays with less important ones with front and rear fog lamp relays. I have opened up the ECM before and saw/smelled nothing burnt or corroded. I’m sure there’s some scientific reason why it ’fixed’ the ECM but, I’ll just say I was lucky.

 

A few other things to check:

1. A stuck-open coolant thermostat can lead to rich running because the ECM continues to apply cold-start fuel enrichment even after the engine should be warm;

2. As Randy suggested, a failed Engine Coolant Temperature Sensor can cause the same problem;

3. A leaking diaphragm in the Fuel Pressure Regulator can allow unmetered fuel into the intake manifold via the vacuum hose, leading to rich running; a quick test is to disconnect the vacuum hose, start the engine and inspect for fuel coming out of the vacuum hose fitting on the FPR;

4. And ditto on Jhartz' suggestion that your fuel injectors are probably sticky with deposits and not closing completely and are also probably squirting a stream of fuel that doesn't burn completely like properly atomized fuel does.

Finally, did you replace your MAFS with an OEM Denso, or with an aftermarket brand? I have not had much success with the aftermarket brands.

Cheers,

Don

 

My 2001 XJ8 had those DTCs and I tried a few things.
I swapped a MAFS from another good running AJ27 X308. (better but still rich)
I swapped intake manifold/injector set from a spare used engine. (no change)
Finally got a WALKER MAFS from Rock Auto and the trims went to normal.

I tried the WALKER MAFS in another car and the trims were not in spec. (don't know why)

The WALKER MAFS only worked in the 2001 XJ8 and was slightly out of ranges for other XJ8s? (don't know why)

Sometimes you go with what you got and move on.

 

One of the aftermarket MAFS I tried on an X350 was a Walker, and it was out of spec. Off the top of my head, other aftermarket brands I have not had luck with are Standard Motor Products and Delphi.

Cheers,

Don

 

Aaand just like that, the Check Engine light and Restricted Performance have disappeared once again, with everything seemingly normal (knock on wood). This is probably a dumb question, but sense you mentioned the Engine Coolant Temperature Sensor causing this problem, I'll ask. I've been getting the startup beeps that indicate problems with the AC system, and the code is for the external temp thermometer, which only works intermittently. My mechanic tried to fix that but couldn't get it to work consistently despite ordering a new part. That external temp thermometer can't cause this, can it?

Thanks everyone for the awesome input!

 

I believe the external temperature gauge is driven by the exterior Ambient Temperature Sensor, which is mounted in the front bumper cover near the radiator. As far as I know, its signal is not referenced by the ECM to make fueling determinations. The sensor that has a direct impact on fuel mixture is the Engine Coolant Temperature Sensor. Are you sure this isn't the sensor that was causing your rich running?

Cheers,

Don

 

In my 2001 xjr I took out the AITS from the TB , it was filthy. Cleaned it up and it cured it.
cheers

 

Hi Julia700,

Are you perhaps thinking of the Intake Air Temperature Sensor 2 (IATS 2), which is mounted at the rear of the A Bank intercooler? The IATS 1 is mounted in the MAFS housing. These are completely different from the exterior Ambient Temperature Sensor that drives the Climate Control Panel exterior temperature display. There is also an interior Ambient Air Temperature Sensor with a little aspirator fan mounted in one of the interior knee bolsters.

Are you saying that you had the same rich running and Restricted Performance as mattuar85 and cleaning the IATS 2 cured the problem?

Cheers,

Don

 

I'm pretty sure the AMBIENT AIR TEMP SENSOR is on the SCP network and will not affect the CAN bus engine running.