89 gt rough idle and stumbling - Muscular Mustangs
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post #1 of 6 Old 03-25-2011, 07:19 PM Thread Starter
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89 gt rough idle and stumbling

I have an 89 mustang gt and the problem I'm having is as follows:

Rough idle--searches for idle from 600-1000 rpm.
Stumbles under acceleration. Feels like its missing.
When shifting to Reverse (AOD) it will stall out. Stalls out even coming to a stop.

There only mod to this car is the Throttle body (BBK 70mm) that a previous owner installed.

Error codes came up as KOEO 34, 63, 66 and KOER 21, 41, 91. Cylinder Balance Test shows 3 on first run, 7 on second run, 5 on third.

Checked compression and its 170 for cyl. 1-4, cyl 5 has 180, cyl 6 185, and 175 for both cyl 7 and 8. Fuel pressure is 30 with vac, 40 without.
Checked for vacuum leaks and could not find any. Installed new cap, rotor, wires, and plugs as well as a new IAC. I've cleaned the MAF with MAF cleaner and set TPS to .98.

I don't want to be just randomly buying new sensors and stuff. Where should I start? Any help would be appreciated
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post #2 of 6 Old 03-26-2011, 06:15 PM
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Re: 89 gt rough idle and stumbling

I hate problems like this because many times the trouble codes send you running in circles and in the end you wind up dumping part after part into it. I'm no expert but I'll help as much as I can.

Looking at your error codes 41 is lean, and 91 is lean. What that tells me is that your O2 sensors are reading lean which in turn sends a signal to the engine to dump more fuel into the cylinders which is likely why your car is stumbling. The question is why the sensors are reading a lean air/fuel mixture. It is possible that the O2 sensors are worn but considering you're getting code 66, which is for a low MAF sensor voltage, I'd suspect that is where the trouble lies. Check out the diagram below and see what readings you get for those connectors. It will give you an idea whether the MAF isn't getting voltage or is sending back lower than usual voltage or if there is a short between the MAF and computer.

http://carbdford.com/tech/Diagrams/mafwiringtests.gif

Another thing to consider is that code 34 is a faulty reading at the EGR position sensor (that white plastic thing on the EGR valve). That could be due to the sensor being bad but it also can be caused by the EGR valve not opening at the correct time because of low vacuum. The lean codes you're getting are also possibly from a vaccum leak. Both of those together may point to a possible vacuum leak being your problem. An old trick is to run the engine and spray carb cleaner around the intake manifold and vacuum hoses, if the engine rpms change you've found your leak. Just be sure to have a fire extinguisher around because carb cleaner can be very flammable.

BTW Did you run the diagnostic tests while the engine was cold or hot? Code 21 may be a false reading due to the engine being cold.
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post #3 of 6 Old 03-29-2011, 08:14 PM Thread Starter
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Re: 89 gt rough idle and stumbling

Tested and cleaned MAF again. KOEO codes are now gone. I ran the engine for 10 mins before I started pulling codes. I'm still getting codes 41 + 91 though.
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post #4 of 6 Old 03-30-2011, 05:00 AM
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Re: 89 gt rough idle and stumbling

Well I've been reading up on these two codes and it seems like most eventually narrow it down to either worn O2 sensors or a problem with the O2 sensor wiring. Honestly though it sounds like it could be many things. Here is a quote I found from a member of Stangnet about testing the O2 sensor circuit.

Quote:
Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

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. 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.

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.

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.

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.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driverís side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.
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post #5 of 6 Old 03-30-2011, 05:04 AM
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Mustang engine wiring diagram

One other thing to look for, make sure this terminal (#19) is grounded to the back of the heads or intake. Its the O2 sensor ground and the wire should be orange.
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File Type: gif mustangwiringharnessengine.gif (53.6 KB, 4316 views)
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post #6 of 6 Old 06-15-2012, 11:25 PM
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Re: 89 gt rough idle and stumbling

These problems are way too hard to fix some codes leads to another and at the end of the day you'll just end up fixing it wrong.
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