• Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech

Running Lean, Don't understand why?

  • Thread starter Thread starter ram360
  • Start date Start date Aug 5, 2005

ram360

Founding Member
Oct 19, 2002
2,162
1
59
Pennsylvania
Aug 5, 2005
#1
  • Aug 5, 2005
  • #1
I decided to pull some codes today to figure out why my check engine light is on. I only pulled the KOEO codes for now and got the following 22, 29, 41, 91. Two of them are saying the engine is running lean. I'm really not sure what I should do to correct this. I only put 80 miles on it (new motor) and it runs fine. I'm thinking about upping the fp but not sure. Right now Its at like 38 or 40 psi. the fuel pump is brand new 225lhp in tank, 24lb injectors w/ calibrated maf. I'm a bit suprised by this b/c I thought it would be on the rich side if anything w/ bigger injectors and fuel pump. The fp is constant the whole time I'm driving so I'm out of ideas.
 

jrichker

StangNet's favorite TOOL
In Remembrance. Thank you for your contributions
Mar 10, 2000
27,512
2,813
234
Dublin GA
Aug 5, 2005
#2
  • Aug 5, 2005
  • #2
Code 22 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture and changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it.

Baro or MAP test using frequency meter - run the test key on engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Code 29 - Vehicle Speed Sensor (VSS) is an electronic sender mounted on the speedo pickup gear on the trans. It works the cruse control for both 5 speed and auto trans cars. The VSS is used to tell the computer to speed up the idle as you slow to a stop. This helps keep the engine from stalling when you slow down for a stop sign or stop light.
Check to see if the electrical connector is plugged into it. Clean the connector & contacts with non flammable brake parts cleaner prior to replacing the sensor, as that may fix the problem. The sensor cost is under $30 and it is easy to replace.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Fix the MAP sensor problem first and then be sure you don't have any vacuum leaks.
 

ram360

Founding Member
Oct 19, 2002
2,162
1
59
Pennsylvania
Aug 5, 2005
#3
  • Aug 5, 2005
  • #3
Ok, thanks for the info. I'll get the map sensor taken care of first.


also forgot to mention my o2 sensors are new only put about 200 miles on them. I do have long tube headers so that kinda moves them downstream a bit not sure if that has anything to do w/ it either.

The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum.
Click to expand...

I never knew this. My car is Mass air and I have the map sensor connected to the intake manifold vacuum. So your saying I shouldn't do this on a mass air car? The map sensor should have no vacuum hose connected to it when it's a mass air car or am I reading this wrong?
 

Euphoric306

New Member
Apr 5, 2004
905
0
0
Aug 6, 2005
#4
  • Aug 6, 2005
  • #4
that seems to be correct
 

ram360

Founding Member
Oct 19, 2002
2,162
1
59
Pennsylvania
Aug 7, 2005
#5
  • Aug 7, 2005
  • #5
I ended up swapping the map sensor for a bap sensor. I had both but I thought they were both map sensors. But obviously they weren't. Problem fixed and no more check engine light. Thanks !
 

ram360

Founding Member
Oct 19, 2002
2,162
1
59
Pennsylvania
Aug 10, 2005
#6
  • Aug 10, 2005
  • #6
I drove the car for maybe 3 miles and then that damn check engine light came back on. If I shut the car off and start it up again the check engine light won't turn on until I drive it a few miles. Before it would turn on as soon as it started up. I cleared the codes drove the car a few miles and pulled the KOEO codes again. Looks like the BAP sensor solved some probs but I'm still getting code 91. Which worries me since the last thing I want is my new engine running lean. The other codes which still showed up are 29 (which I'm going to replace. I tired cleaning the connections but had no luck fixing the prob) I also still have code 33 but I don't have the egr hooked up so that's not a suprise.
 

ram360

Founding Member
Oct 19, 2002
2,162
1
59
Pennsylvania
Aug 10, 2005
#7
  • Aug 10, 2005
  • #7
I also pulled the plugs and they look good. Like I mentioned before I put in new bosch 02's and they have less than 200 miles on them. I'm not sure if having the long tube headers is effecting something w/ how they read. I know they are mounted way down in the collectors vs the stock location in the h-pipe (which is much closer to things). Could it be that they don't get as hot being down there and that's effecting things? I also looked over everything for possible vacuum leaks. I found one very small one but that was it. Is there anyway I can test to make sure I have the correct vacuum w/ a gauge or something?
 
You must log in or register to reply here.

Similar threads

B
Electrical Why does my fox body keep frying eecs
  • brandyn200
  • Jun 20, 2026
  • Fox 5.0 Mustang Tech
Replies
8
Views
510
Fox 5.0 Mustang Tech Jun 25, 2026
AeroCoupe
S
thread no longer needed
  • skidad
  • Oct 17, 2026
  • 1996 - 2004 SN95 Mustang -General/Talk-
Replies
0
Views
140
1996 - 2004 SN95 Mustang -General/Talk- Oct 17, 2026
skidad
S
T
Fuel Looking for a little guidance
  • Tufgriz
  • Sep 11, 2025
  • Fox 5.0 Mustang Tech
Replies
9
Views
476
Fox 5.0 Mustang Tech Sep 12, 2025
Noobz347
T
  • Locked
Digital Tuning A9l issue or something else
  • Tufgriz
  • Sep 11, 2025
  • Fox 5.0 Mustang Tech
Replies
2
Views
359
Fox 5.0 Mustang Tech Sep 11, 2025
Noobz347
Engine 1990 LX 5.0L EFI Mustang Fuel Management Problems - HELP!
  • 5.0LXStanger
  • May 25, 2025
  • Fox 5.0 Mustang Tech
  • 2 3
Replies
45
Views
3K
Fox 5.0 Mustang Tech Jul 11, 2025
5.0LXStanger
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?