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#1 10-06-2008 23:36:28

Registered: 09-27-2008
Posts: 107

Re: Grossly over-rich fuel/air mixture - No success yet

'96 XJ, 4.0L, 205K miles. The problem onset was absolutely sudden. It
set in all at once and doesn't seem to have worsened or varied since.
Here's a description.

The engine isn't hard to start and seems to idle pretty normally for the
first 30-60 seconds of running. If you just let it sit and idle it will
then begin to hover at a cycle of near stalling about two times per
second. The exhaust coming out is clearly black and reeks of unburned

When you drive and accellerate hard there's a visible dark cloud of
exhaust you can see in the rear view mirror. The gas mileage I'm
currently getting is approximately 5mpg. It's only a little better on the
highway. Otherwise driveability doesn't seem to be all that bad. It
doesn't stumble, hesitate or miss at all. It's just guzzling prodigious
amounts of gasoline.

When you are coming to a complete stop the engine almost stalls at first.
Then it recovers. In another few seconds it begins to cycle at a near
stall again as described above.

The check engine light is on and comes back on in a few minutes after I
clear it with my code scanner. The two codes the PCM reports are P0172
and P0171 - mixture too rich and mixture too lean. I don't know why it
throws both codes when the problem is clearly that the mixture is too

Here's what I've checked/done so far, based on the info I got from helpful
people the last time I posted about this:

1.) Throttle Position Sensor

The engine promptly almost died from an idle when I unplugged this.
It returned to running as above as soon as I plugged it back in. It
doesn't seem like it's malfunctioning, I believe at the moment.

2.) O2 Sensors

I replaced the one downstream from the catlytic converter because I
think it may have been the factory original. The upstream one is
about two years old. The present replacement for the downstream one
made no diffference in the problem.

3.) Engine Temperature Sensor

I haven't checked this but the temperature gauge in the dash is
working normally.

4.) MAP Sensor

I tried replacing this and it made no difference. I put the original
one back in service.

5.) Fuel Pressure Regulator

The fuel pressure regulator is part of the fuel pump assembly buried
in the gas tank so I have no way of evaluating it easily.

Could the fuel pressure be way too high for some reason and that is
causing the amount of fuel being delivered by the injectors to be way

6.) Catlytic Converter

Since the problem began suddenly but I have never heard anything that
sounds like rattling/clunking broken up catlyst material inside the
catlytic converter body I'm not sure what else could be wrong here
that could explain the situation.

7.) Idle Air Control Motor

If this were malfunctioning I would not expect to see gross fuel
overconsumption problem at anything except at an idle, correct?

8.) PCM

Without another one of these to swap in I don't know how else to
verify this component. Aside from the problem as described
everything else with the system's sensors and operation seems to be
functioning normally. I have never gotten any other trouble codes
besides the ones already mentioned.

Should I be looking to try and come up with some kind of more
sophisticated console that I can attach to my OBDII connector than the
simple code scanner I have now? Do you think there's any chance that
there's more clues the PCM could give me about what's going on if I had a
more detailed analyzer to interrogate it with? Does anyone out there have
a PC laptop based one they use that they like and could recommend?

Does anyone else have any other ideas about what could be causing my
troubles? I'm getting kind of discouraged. Thanks again for everyone's


10-06-2008 23:36:28


Re: Grossly over-rich fuel/air mixture - No success yet

#2 10-06-2008 23:40:15

Registered: 09-27-2008
Posts: 107

Re: Grossly over-rich fuel/air mixture - No success yet

> An upstream O2 sensor stuck reading a lean condition could cause your
> problems also. The computer would think you are always lean and will
> dump fuel like crazy.
> Testing fuel pressure is a very good idea also. Also test to see if
> it bleeds down when shut off, you might have leaky injecter.

I checked the fuel pressure today. It is a steady 48psi regardless of
engine speed. Also, it holds pressure even when the engine is shut down.

> If you have a good high resistance digital VOM you can test the O2, MAP
> and the TPS sensors easy enough. You just need to figure out the right
> wires at the sensors. The TPS is easy to test with the engine off.
> One wire is the 5 volt feed, one is groung and the other is the output,
> which is the one you are looking for. The map wires are the same way
> just test them when reving it. The O2 has heater wires and such. You
> are looking for the 0-1 volt output wire. A thin paperclip can be used
> to backprobe connectors if you are careful and makes it easy.
> If you are going into closed loop mode odds are your coolant sensor
> isn't that far off. If it was it would stay in open loop.

That is a good point. I like it!! I think I am going to try checking
things out with the voltmeter next. Perhaps I am too hasty in trusting
the upstream O2 sensor. That would be nice to find that's my problem.
The gas guzzling and choking fumes really blow.


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