Running Rich

[chewy012]

Hey guys,

I can't figure this one out. The truck keeps throwin' the CEL for running rich. '92 4.0 2wd 5-speed.

After my hour commute this morning, I did the flood clear test (foot to the floor and crank). No fire, so injectors ain't leaking. From what i've seen on my scanner the O2 sensor is working, fluctuates between 0 and 1 volt, when CEL is on, it reads a steady .9 to .95.

Any ideas on what would cause this? Fuel tank cap? Fuel vapor recycle system (or what ever it's called)?

Any help is appreciated

Sent from my XT1254 using Tapatalk

 

[RonD]

The high voltage from O2 sensor, .9, means you are running Rich not just a code issue.

Has the MPG gone down alot?

No on the EVAP system.

Check the Fuel Pressure Regulator(FPR), it has the Fuel Return hose and a Vacuum hose attached, check vacuum hose for signs of fuel.
If FPR diaphragm starts to leak fuel can be sucked in to intake via vacuum hose while engine is running.

Also unplug the MAF sensor after engine is warmed up see if O2 voltage drops closer to .45
Without MAF data computer uses fuel mix presets in memory based on RPM and throttle position

EDIT:
Does the '92 4.0l just have the one O2 sensor on the "Y" pipe?
If so could be O2 sensor failing, this would most likely cause pinging/knocking on acceleration as engine is actually running too Lean

My '94 has two, one on each bank, and if there are two are both showing Rich?

 

[AllanD]

'90- 92 have a single HEGO

First year for dual HEGO on a 4.0 is 1993

 

[chewy012]

Yes, it's just got the single o2. I don't have pinging or knocking.

When I unplugged the MAF it didn't seem to affect the o2 sensor reading, it either stayed at high voltage or continued to fluctuate (I tried it a few times) but plugging and unplugging you could hear the engine make an adjustment if that makes sense.

One thought I'm having is, my AC condenser is looking fairly blocked, bent fins, pea gravel sorta stuff. It's been real hit here and on the real hot days I've been experiencing vapor lock.

Do you think the engine may be trying to dump fuel to help cool?

Sent from my XT1254 using Tapatalk

 

[chewy012]

Also, after today's hot commute home, my IAT was reading 135F which seems pretty damn hot to me.

Sent from my XT1254 using Tapatalk

 

[RonD]

IAT is on the upper intake so 135degF with outside air temp above 90degF seems very low, 60degF above outside temp would be more normal, at high RPMs, even higher at idle with less air flow

But does bring up another point, high outside temp means air is not as heavy, i.e. hot air rises, so cooler air is heavier
Air/fuel ratio of 14.7:1 is a WEIGHT ratio, 14.7 pounds of air needs 1 pound of gasoline

Since you are running Rich it could be the IAT sensor is reporting cooler air temp so computer is adding too much fuel.

Look here: http://www.aa1car.com/library/air_temp_sensor.gif

test voltage or ohms after engine has cooled down to outside temp, so cold, see if that reflects actual outside air temp

IAT sensor gets 5 volts from computer on 1 wire and sends back "test" voltage on the other wire, you can use a sewing pin to pierce the wire to read voltage, key would need to be on but engine off.
So if it is 85degF outside and IAT shows above 3volts(65degF) then replace it


MAF sensor also measures the "weight" of the air, Ford uses a "hot wire" MAF so the faster the wire cools off the more air and the "heavier" the air, this also compensates for elevation as air gets "thinner"/"lighter" as you get higher and higher elevations(mountains)

 

[chewy012]

Oh nice, thanks for the chart! And yeah that makes sense, I'll give that a test a day or two.

Do you have any thoughts on this potential vapor lock type condition I seem to have?

When it's hot outside, running for awhile (1 hr commute), if I let off the throttle, while in gear, the engine dies. If I can throw into neutral it'll keep running. I know at closed throttle and high rpm and high vss the injectors shut off, so when I throw it into neutral it prevents them from shutting off.

Sometimes if it's hot enough the ol' neutral trick doesn't work, but disconnecting the vacuum to the fpr and plugging open vac port solves the problem.

Do you think this could be related to the IAT misreading? Pending test results confirm that...

Sent from my XT1254 using Tapatalk

 

[chewy012]

Alright so after the test of IAT... current outside temp is 75 -> resistance on the IAT was 17.9k ohms. According you chart Ron, I'm getting a new IAT as that correlates to about ~100 degrees or a 30 degree inaccuracy.

Sent from my XT1254 using Tapatalk

 

[RonD]

If there was no residual heat left in upper intake then yes I would replace IAT.

Fuel injection can vapor lock it is just not as common as it was in the old carb days.

Removing the vacuum hose from FPR slows down the fuel from returning to the gas tank, and will add a few more PSI to the system, maybe 5psi, FPR still opens at 45psi on it's own, the vacuum setup helps prevent major swings in fuel pressure as it is directly related to engine load.

But more interesting is the "when I take my foot of the throttle"
FPR valve is pulled open with higher vacuum, the point of the vacuum hose, at idle fuel demand is low and vacuum is high, so more fuel goes back to the tank.
Now when you take your foot of the throttle in gear the vacuum sky rockets, engine braking causes very very high vacuum in intake, that's where the braking comes from.
So if fuel pressure was low it could drop to 0psi with high vacuum at FPR and engine would stall
Shifting into Neutral, not using engine braking, would lessen vacuum but if pressure was low already idle vacuum could still stall out engine.
So you could be looking at a flow issue, dirty in tank screen, or filter, it might not show up with static fuel pressure test, but over a long drive the flow may start to drop with higher demand and that drops the pressure, and if it is low enough then the opening of the FPR fully drops it to 0
Removing the vacuum hose prevents the sudden drop in pressure when vacuum goes high

 

[chewy012]

Yeah I checked the IAT after an all night sit, just snagged a new one.


Yeah that's what I was thinking. Fuel filter is 9 months old, and I run clean/additive about ever 2-3 full tanks. Hopefully the in tank screen isn't clogged, may just have to live with it and take it slow on the hot days

Sent from my XT1254 using Tapatalk

 

[RonD]

Could just be pump is tired as well, have you ever checked fuel pressure at the rail?
35-40psi is spec, engine running with FPR
Without FPR vacuum attached should put it close to 45psi

 

[chewy012]

Haven't checked rail with a gauge. Would pump make a different sound if it was weak? I hear it when I turn the key one and it's a strong sound. Fuel rail I've basically just witnessed a strong spray of fuel when you push on the schrader

Sent from my XT1254 using Tapatalk

 

[RonD]

Don't think you can "eyeball" or "hear" fuel pressure close enough to make a judgement.

Auto parts stores often have loaners and you can just use it in the parking lot.
Tire pressure gauge also works but it should be a screw on type of fitting

 

[chewy012]

May be early to say, but I think the new IAT fixed the rich mixture condition. Thanks Ron

Sent from my XT1254 using Tapatalk

 

[RonD]

Thanks for the update, good work