Engine Fault Code Help...

dz01 · Jul 19, 2014

In my quest to fix my floating idle issues, I finally decided to check codes vs. continue to replace parts. To my surprise I did pull codes even thought my CEL wasn't on.

Here's what I found:

Key on engine off- Code 67 and Code 66 in continuous memory (VAF/Mass air flow sensor fault, below minimum voltage). So I got 67, 10, 66.

Key on engine running- Code 41 (HEGO HO2S sensor voltage low/system lean) and Code 91 (which is the same as Code 41)

Code 41 and 91 is odd since I recently (within 300 miles) replaced both O2's with motorcraft sensors. That was the first thing I did to address my floating idle issue.

I also should note I am not running cats. When I did, I still had idle problems so nothing changed.

Where should I go from here??

modulistic · Jul 20, 2014

things that can cause lean codes:
vac leaks
dead or weak cylinders (no fuel or spark to a cyl will just push raw air into the exhaust)
exhaust leaks can pull fresh air into the exhaust

your symptoms sound like a vac leak.

jrich, if there is a large vac leak, would it pull less air through the maf and throw a low voltage code?

modulistic · Jul 20, 2014

i would start by taking the car to a desolate road, running hard all the way to 4th gear, shutting car off after letting off the gas, coasting to a stop then looking at each plug. ive solved dozens of problems that way, im constantly looking at my plugs. its like having a window to the combustion chamber.

dz01 · Jul 20, 2014

Thanks! funny, but I do suspect a vacuum leak. My car is so loud it's really hard to pin point. I've been hitting some spots with carb cleaner (motor cold). I'm going to focus on this first. It's cheap and probably my issue.

I hope not dead or weak cylinders. Car seems to run strong. I bang some gears out today and it runs like it should.

jrichker · Jul 21, 2014

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 134,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

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

Revised 29-Sep-2013 to add back in a clogged crossover tube as cause for code 41

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.

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 (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 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 43

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.

Code 67 –
Revised 2 Nov 2012 to add definition of the NSS functions for both 5 speed and auto transmissions

Cause of problem:
clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code may prevent you from running the Key On Engine On tests.

External evidence from other sources claims that a code 67 can cause an idle surge condition. Do try to find and fix any issues with the switch and wiring if you get a code 67.

What the NSS (Neutral Safety Switch) does:
5 speed transmission: It has no connection with the starter, and the engine can be cranked without it being connected.
Auto transmission: It is the safety interlock that prevents the starter from cranking the engine with the transmission in gear.
What it does for both 5 speed and auto transmission cars:
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running computer diagnostic tests. It also checks to see that the transmission is in Neutral (5 speed and auto transmission) and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

The following is for 5 speed cars only.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!

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 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.

dz01 · Jul 21, 2014

This is great, thank you very much jrichker! This is awesome. I did find the idle/stall checklist a few months ago which is what I've been going by. In no particular order I've replaced the IAC, TSP (and correctly set it), both O2s, temperature sensor, Air Charge Sensor, cleaned the TB and MAF, pulled my cats off (unrelated to this issue), started checking the obvious vacuum lines and most recently pulled the codes with a reader I just bought which is where I am today (my car has 65K on it if that matters). I either address the codes I pulled and/or check every vacuum line. Your guidance helps however it does pose some questions:

1. doesn't it seem odd both O2s would crap out at the same time? Particularly since I just replaced them with motorcraft sensors. Leads me to believe it is a vacuum leak. Doesn't seem like it would be worth my time to test and chase O2s at this point.

2. Code 67- I'm not tracking with your comments about clutch in. Does this mean when I run codes with car running someone has to have the clutch pushed in while i read codes (or vice versa)? I ran the codes with car running in neutral with e-brake on. The instruction book did not indicate I had to keep the clutch in. My AC was definitely off, but the vent was probably on low. I can try this again with clutch in if that's what you mean. Or I can replace the neutral safety switch if that's the next obvious step.

3. Vacuum leaks. Sounds like I really haven't ruled this out yet. I'll go after the low hanging fruit, but lower intake gasket sounds like a whole different beast.... I would say it does sound like I'm sucking air somewhere, but I don't have much to go by.

Thanks again. Any clarity would be appreciated.

jrichker · Jul 22, 2014

Troubleshooting problems over the Internet is mostly done by picking out the likely suspects and then doing the rest by process of elimination. Unfortunately, your problem requires more elimination, so there are more things to look at...

Pushing the clutch pedal down when dumping codes is recommended to make sure you don't get the code 67. That includes dumping the codes with the engine off or running.

The O2 sensor code test path was posted so that you had the information to test the O2 sensor harness and sensor output. That is a somewhat simpler process than spending time peering around in the engine compartment looking for small cracks and flaws in the vacuum plumbing.

The lower intake gasket is only suspect if the lower intake manifold has been removed.

dz01 · Jul 22, 2014

Thanks for clarifying. I'll post back once I solve this.

jrichker · Jul 22, 2014

dz01 said:
Thanks for clarifying. I'll post back once I solve this.

We are looking forward to the posting of your results...

dz01 · Jul 24, 2014

Few updates/observations after a careful vacuum wire check (for the 5th time)...

- I noticed the new PCV grommet I installed not too long ago was not seated flat and was torn (said made in China in bold letters). I reinstalled the original one which isn't the best, but better than nothing for right now. I'll get a new one tomorrow.

- The PCV hose from the valve to the fitting under the intake seems fine, but the hose from the other side of the fitting under the intake to the front of the underside of the intake was rock solid. Not cracked, but definitely not pliable. Seems reinforced so maybe this is normal?

- Another forum recommended making sure the orange ground wire for the O2s is clean and tight on the back of the drivers side head. I noticed the orange wire (that leads into the abyss of wires) and the silver ground that goes from the same bolt to the firewall were both loose enough to completely spin around the bolt. Everything looked clean so I tightened up the bolt.

- The 90 degree elbow that comes out of the charcoal canister and then connect to the hose that goes up past the distributor and under the intake is cracked and old (just the elbow). Need to replace this, but seems like it's a different size on both ends. This is right near the fan so might affect idle if sucking air.

- On a side note I did notice all the vacuum lines seem either new or are in great shape other than what I mentioned.

I haven't gone for a test drive yet, but figured I through these things out. If the loose bolt on the back of the head could throw code 41 and 91 and affect my idle maybe I'm in the clear. Seems odd this bolt is loose so the previous owner might have been chasing the same gremlin.

Any thoughts?

jrichker · Jul 25, 2014

The orange wire is the O2 sensor heater ground, and it could cause the 41/91 code if it was loose. You are on the right track.

dz01 · Jul 25, 2014

Code 41 and 91 still there, but I did forget to ask if I need to pull the battery cable, drive for x miles or erase codes, etc.? ODBII I would just reset and call it a day.

Idle hanging still present. Just for kicks, I pulled the IAC plug while the car was running. Hanging idle completely gone. I did put a brand new motorcraft IAC on, but took it off since it made no difference. Seems odd I have zero idle float with the IAC unplugged. I did notice idle was much lower unplugged and a little rough. What does this mean? No vacuum leak, IAC problem, TB problem???

jrichker · Jul 26, 2014

Disconnecting the test jumper or scanner while the codes are dumping clears all the codes.

Disconnecting the battery clears the codes and erases the stored settings so that the computer re-learns them all over again. This is recommended anytime a sensor is changed.

Disconnect the battery, drive the car for 30 minutes of mixed highway and city driving and dump the codes again.

The IAC allows more air into the intake by bypassing the throttle butterfly. The idle speed should decrease when it is disconnected.

Fix ALL the codes and then set the base idle speed.

From the Surging Idle Checklist setting the base idle.

Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it.

When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.

An engine that whose idle speed cannot be set at 600 RPM with the IAC disconnected has mechanical problems. Vacuum leaks are the #1 suspect in this case. A vacuum gauge will help pinpoint both vacuum leaks and improperly adjusted valves. A sticking valve or one adjusted too tight will cause low vacuum and a 5"-8" sweep every time the bad cylinder comes up on compression stroke. An extreme cam can make the 600 RPM set point difficult to set. Contact your cam supplier or manufacturer to get information on idle speed and quality.

dz01 · Jul 28, 2014

Disconnected battery to reset codes. Drove around and rescanned. Engine off, no codes. Engine running, pulled 41 and 91 again. Car actually seemed to run worse. Idle hang was definitely more appeared for some reason.

I'm going back to looking for vacuum leaks.

dz01 · Aug 11, 2014

Update- borrowed my buddies Snap-on smoke machine. Hooked it up last night and quickly found smoke pouring out of the back of the engine. Turns out with the help of a small mirror, I detected a crack in the intake plentum. The "ear" where the last bolt is located is cracked and smoke pouring out of that area. Maybe from heat/age or maybe someone tightened bolt down too hard when they replaced gasket. My car has low miles and it's hard to tell if plentum has ever been off before.

I do have a new trickflow intake and plentum, 70mm BBK TB and MAF, 30 lb injectors that came with the car (guy probably knew about the leak). I'm pretty light on time due to a new baby coming so I may just replace the plentum with a stock one for now. Anyone have one in nice condition for sale???

What kind of HP gains can I see with the trickflow and BBK set up? The 30 lb injectors are probably overkill and not needed with this setup.

jrichker · Aug 11, 2014

It's a good thing you had access to a smoke machine. Otherwise you could have been searching for a long time.

dz01 · Aug 11, 2014

yeah, worked great!

dz01 · Aug 12, 2014

I would recommend to anyone searching for leaks to rent a smoke machine. I know some parts stores have loaner tools. Not sure if this is a tool on the list, but worth checking.

Worse case it would be worth it to pay a shop to smoke the lines and point you in the right direction. Using carb cleaner or oil is great if you have a general idea of what the problem is. Ford with their infinite vacuum line strategy makes vacuum leaks a difficult probably to find.

I'll report back once fixed.

dz01 · Aug 18, 2014

Here are the pictures of the plenum. Looks like someone over tightened the bolts. Front has a crack too. Should be fun getting this back on with my AC line is the worst spot...

Few questions:

- I noticed the PCV grommet is loose in the hole in the back of the intake. This grommet is brand new. Thought this was odd. I would think this would fit better?

- Back bolt on passenger side had oil on it when I pulled it out. I'm assuming this is normal although all other bolts were clean?

- Should I pull the spacer out and replace the gasket between the lower intake while it's apart?

- Anything else I should do while I have easy access to the top of the motor?

Thanks!

dz01 · Aug 21, 2014

If anyone is tracking the status of my fix here's an update...

- It appears when the previous owner installed an intake spacer that he overtightened the plenum. Torque specs are 12-18 foot pounds which isn't very much. The spacer would also explain why my strut tower brace won't fit and why my AC line is pinned against my cervini hood when shut.

Picking up new gaskets tonight and will report back once back together and running.

Engine Fault Code Help...

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