Looking For Help With A 91 Code

scld1354 · May 21, 2014

The car is a bone stock 88 GT in CA. (The car is a CA car, and has a MAF.) I need to smog the car, but have a 91 KOER code (and also a 91 CM code). I tested the o2 sensors, and both tested good. The wiring between the o2 sensors and the ECU tested good. The o2 ground tested good. As the 91 indicates always lean on the driver's side, I checked everything that was specific to that side of the engine: I had one loose lower intake manifold bolt on that side. Most of the header bolts were a little loose, as well. The collector bolts were both tight.

I cleared the codes, warmed the car up, and still got the 91. I also checked the A/C vacuum circuit, the C/C vacuum circuit, CANP vacuum circuit, FPR vacuum circuit, and TAB/TAD/EVR vacuum circuits. Everything tested fine.

I'm out of ideas. I don't know what to test next. Help please......

madspeed · May 21, 2014

Are you getting the code 91 during KOEO or KOER testing after you cleared the code?

scld1354 · May 21, 2014

After clearing the code, KOEO is 11, but the continuous memory (CM) code is a 91. KOER is 91, also.

madspeed · May 21, 2014

vacuum leak still somewhere on the drivers side bank

madspeed · May 21, 2014

Switch O2 sensors from sides....... if code 41 shows up in instead of a 91, that O2 sensor is defective.

scld1354 · May 21, 2014

I did a ton of research into this before I posted. I didn't want to overlook something as simple as a bod o2 sensor. I read through all of Jrichkler's posts having to deal with this, and went through his process to test the o2 sensors. I tested both just to make sure I had reliable readings, and both o2 sensors tested good.

I cant find any other vacuum connections that only affect the driver's side. I also used starter fluid to test the lower intake manifold gaskets, and I didn't notice any change in idle. I would imagine if the TB or upper intake gasket is bad, I would get a lean code on both sides.

madspeed · May 21, 2014

If you're sure they are both good,

THEN...

madspeed said:
vacuum leak still somewhere on the drivers side bank

scld1354 · May 21, 2014

I would really like to find the problem, instead of just throwing parts at it. Does the 91 give any indication of how big the vacuum leak is? Does it mean it's only a little vacuum leak? Could there be a tiny split in the lower intake manifold gasket that wont really respond to the starter fluid?

madspeed · May 21, 2014

no, no, yes

jrichker · May 21, 2014

Finding vacuum leaks

Revised 04-Aug-2011 to add pintle cap, PCV grommet & power brake check valve grommet to checklist.

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and every hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as starting fluid, propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

The PCV grommet and the power brake booster check valve grommet are two places that often get overlooked when checking for vacuum leaks. The rubber grommets get hard and lose their ability to seal properly. The PVC grommet is difficult to see if it is correctly seated and fitting snugly.

Fuel injector O rings can get old and hard. When they do, they are prone to leaking once the engine warms up. This can be difficult to troubleshoot, since it is almost impossible to get to the injectors to squirt oil into the fuel injector mounting bosses. If the plastic caps on the fuel injectors (pintle caps) are missing, the O rings will slide off the injectors and fall into the intake manifold.

Fuel injector seal kits with 2 O rings and a pintle cap (Borg-Warner P/N 274081) are available at Pep Boys auto parts. Cost is about $3-$4 per kit. The following are listed at the Borg-Warner site ( http://www.borg-warner.com ) as being resellers of Borg-Warner parts:
http://www.partsplus.com/ or http://www.autovalue.com/ or http://www.pepboys.com/ or http://www.federatedautoparts.com/

Most of the links above have store locators for find a store in your area.

Use motor oil on the O rings when you re-assemble them & everything will slide into place. The gasoline will wash away any excess oil that gets in the wrong places and it will burn up in the combustion chamber. Heat the pintle caps in boiling water to soften them to make them easier to install.

Diagram courtesy of Tmoss & Stang&2birds

Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.

Mustang5L5 · May 21, 2014

Swap the o2 sensors from one side to the other. If the code 91 remains, you have eliminated the sensor as a cause

v8only · May 21, 2014

Like he mentioned, swap the o2 sensors...I don't care how sure you are that they're good, if you haven't swapped them yet, then do it.

If it's not that, it's a vacuum leak, or an exhaust leak before the o2 sensor

scld1354 · May 22, 2014

Swapped the o2s like everyone suggested..... Still getting a 91. I'll take a closer look at the brake booster and PCV grommets, but wouldn't those cause the entire system to run lean???

It looks like the intake manifolds have to come off...

madspeed · May 22, 2014

I say you have a leak in there on the drivers side. Those item would likely cause both sides to go lean. Have you checked the injectors on the drivers side? Maybe one is gunked up?

Mustang5L5 · May 22, 2014

Are you positive you do not have exhaust exhaust leaks on that side?

Grab a can of carb cleaner, and spray the engine on that side. See if the idle rises indicating a vacuum leak.

scld1354 · May 23, 2014

update - I am also experiencing engine vibration. I thought it may be a bad balancer, so before I checked the injectors and around the intake manifold gasket (I was going to use motor oil, like jrichkler suggested), I unplugged the coil and spun the motor over for about 20 seconds. I was showing my dad how the balancer was a little wobbly, and spun it for about another 20 seconds. Anyway, I think it flooded, and I couldn't get it started after that. The balancer is on its way (thanks Amazon!). Once that is replaced, I'll get back to testing the injector o-rings and intake gasket.

Mustang5L5 · May 23, 2014

Perform a cylinder balance test. See if you have a dead or dying cylinder on that side.

It's part of the check engine diagnostics

scld1354 · May 23, 2014

I've already done the cylinder balance test. It checked out fine.

jrichker · May 23, 2014

Time to do the voltage and resistance checks as described below...

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 20-Jan-2014 to add code 43 to test collection

Code 41 is a RH side sensor, as viewed from the driver's seat.
Code 91 is the LH side sensor, as viewed from the driver's seat.

Code 172 is the RH side sensor, as viewed from the driver's seat.
Code 176 is the LH side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned 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

Testing the O2 sensors 87-93 5.0 Mustangs
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.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (L\RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

Testing the O2 sensors 94-95 5.0 Mustangs
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 red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 29

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

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

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports, Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

scld1354 · May 23, 2014

I did that test. That's the test I referred to in the first post. Each sensor was fluctuating between .10 and .78 volts.

Looking For Help With A 91 Code

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