Codes I'm getting...

[SHOYAY]

So I'm getting codes again as I'm actually driving the car now.
The 2 codes that keep coming back
172
176
And also got...
542
Changed both O2 sensors, no known vacuum leaks(have 22hg) at idle.
Have good fuel pressure 35psi idle, reacts correctly with regulator unplug.
Can it have a pcm problem? Former owner had pcm rebuilt. If possible to be bad, can I use my 94mtx pcm?

 

[rubydist]

No, you cannot use the mtx pcm in the atx.

In addition to bad oxy sensors and vacuum leaks, you have these potential issues: dirty injectors, clogged fuel filter, sick fuel pump, sick pcm, sick mafs, sick tps, sick iat.

The 542 code is set any time the engine stalls, so that can be ignored on your car.

 

[SHOYAY]

No, you cannot use the mtx pcm in the atx.

In addition to bad oxy sensors and vacuum leaks, you have these potential issues: dirty injectors, clogged fuel filter, sick fuel pump, sick pcm, sick mafs, sick tps, sick iat.

The 542 code is set any time the engine stalls, so that can be ignored on your car.

It's a 92mtx... my parts car is a 94mtx
New fuel filter, fuel pump seems fine via fuel pressure readings, cleaned maf, changed tps,

 

[sperold]

Get another PCM by placing an ad for a 92 on this forum. It my not be the problem, but it is good to have. Rebuilt components are suspicious, in my experience, when problems arise.

There are huge differences from year to year in these cars, even though they all look alike. You have probably noticed that you have a 2 speed rad fan in the 94 and a single speed in the 92, and that alone involves wiring changes.

Keep driving it, and trouble shoot regularly like you are. For replacement parts, look for names like Denso which are original equipment for many components, and of course, refer to RockAuto. I have found these early units are very fussy on O2 sensors and once had a set that turned on the light above 3000 rpm but shut it off when i dropped below that RPM. Yet the car ran great and gave good gas mileage.

Move your wiring harness around a bit, but gently, in the engine compartment, and see if anything changes.

 

[rubydist]

yes, you can switch the 92 mtx pcm with the 94 mtx pcm - as long as neither is a CA emission spec model, or as long as both are.

 

[SHOYAY]

yes, you can switch the 92 mtx pcm with the 94 mtx pcm - as long as neither is a CA emission spec model, or as long as both are.

92 is ca emissions egr deleted and 94 is not ca emissions

 

[SHOYAY]

Get another PCM by placing an ad for a 92 on this forum. It my not be the problem, but it is good to have. Rebuilt components are suspicious, in my experience, when problems arise.

There are huge differences from year to year in these cars, even though they all look alike. You have probably noticed that you have a 2 speed rad fan in the 94 and a single speed in the 92, and that alone involves wiring changes.

Keep driving it, and trouble shoot regularly like you are. For replacement parts, look for names like Denso which are original equipment for many components, and of course, refer to RockAuto. I have found these early units are very fussy on O2 sensors and once had a set that turned on the light above 3000 rpm but shut it off when i dropped below that RPM. Yet the car ran great and gave good gas mileage.

Move your wiring harness around a bit, but gently, in the engine compartment, and see if anything changes.

The o2s were good known ones from the stock y pipe put into my catless system. I'll try the wire wiggle

 

[NoSlo]

Or the engine is actually running lean? Or has learned a rich strategy but goes lean sometimes in the power curve? Have you only used an OEM paper air filter, and replaced it at the scheduled 30k miles or 30 months?

How did you clean the MAF sensor? Remove with security Torx from the housing, and spray the fine filaments with CRC electrical contact cleaner in a can (Harbor Freight has it cheaper). Inspect the filaments, that they aren't broken. Reinstall just as carefully.



Also, throttle position sensor you've swapped for free. Besides seeing that TPS is measuring around .95 volts at idle position (adjusted by twisting the sensor on the loosened screws), with a multimeter probing the TPS output wire while the car is in "run" key position but not running, you can also examine that the voltage slowly and steadily increases as you very slowly move the throttle by hand, with no gaps or blips in the value.

After any of these management sensors are changed, again, you need to clear the PCM memory with 10 minute battery disconnection, do idle relearn, and let the engine re-learn trim corrections by doing a complete drive cycle.

Here's a link to a free multimeter coupon at Harbor Freight, also (timing-light-era mechanics often had an analog "engine analyzer" with a big needle voltmeter; I just donated mine to Goodwill).

 

[SHOYAY]

Or the engine is actually running lean? Or has learned a rich strategy but goes lean sometimes in the power curve? Have you only used an OEM paper air filter, and replaced it at the scheduled 30k miles or 30 months?

How did you clean the MAF sensor? Remove with security Torx from the housing, and spray the fine filaments with CRC electrical contact cleaner in a can (Harbor Freight has it cheaper). Inspect the filaments, that they aren't broken. Reinstall just as carefully.



Also, throttle position sensor you've swapped for free. Besides seeing that TPS is measuring around .95 volts at idle position (adjusted by twisting the sensor on the loosened screws), with a multimeter probing the TPS output wire while the car is in "run" key position but not running, you can also examine that the voltage slowly and steadily increases as you very slowly move the throttle by hand, with no gaps or blips in the value.

After any of these management sensors are changed, again, you need to clear the PCM memory with 10 minute battery disconnection, do idle relearn, and let the engine re-learn trim corrections by doing a complete drive cycle.

Here's a link to a free multimeter coupon at Harbor Freight, also (timing-light-era mechanics often had an analog "engine analyzer" with a big needle voltmeter; I just donated mine to Goodwill).

Only OEM style filter. Maf was taken off and cleaned, looked good, also switched with known good one. I didn't probe tps, but did switch with known good one and neither had any adjustment possible in screw holes.

 

[FastCAD]

So I'm getting codes again as I'm actually driving the car now.
The 2 codes that keep coming back
172
176
And also got...
542
Changed both O2 sensors, no known vacuum leaks(have 22hg) at idle.
Have good fuel pressure 35psi idle, reacts correctly with regulator unplug.
Can it have a pcm problem? Former owner had pcm rebuilt. If possible to be bad, can I use my 94mtx pcm?

Did you delete your old codes after your last service?

Glad your up and running.
Joe

 

[SHOYAY]

Did you delete your old codes after your last service?

Glad your up and running.
Joe

Yup, battery was unhooked.
Thanks, she's a little noisy but drives ok.

 

[FastCAD]

Yup, battery was unhooked.
Thanks, she's a little noisy but drives ok.

You should use a paper clip at the diagnostic plug method.
When the KOEO diagnostic cycle gets to the part where the test queries the cylinders, pull the clip out.
You can tell when the cylinders are being queried you can hear the drop in idle.

This method will guarantee the codes will be erased. Sometimes just unhooking the negative battery cable
does not totally drain the power to the ECM because the computer has a power backup supply.

If you do not have details on the above self test procedure I can provide written details with a diagram.

 

[SHOYAY]

You should use a paper clip at the diagnostic plug method.
When the KOEO diagnostic cycle gets to the part where the test queries the cylinders, pull the clip out.
You can tell when the cylinders are being queried you can hear the drop in idle.

This method will guarantee the codes will be erased. Sometimes just unhooking the negative battery cable
does not totally drain the power to the ECM because the computer has a power backup supply.

If you do not have details on the above self test procedure I can provide written details with a diagram.

I don't have details on that procedure.

 

[NoSlo]

If the battery is disconnected long enough (with the headlights turned on) to lose the learned idle and fuel trim, then it is also disconnected long enough to clear the memory codes.

If you want to clear stored memory codes without also resetting the learned management, then you can disconnect the diagnostic link jumper while the codes are flashing. To be sure, turn off the car, plug the jumper back in, and see Key-On-Engine-Off codes blink "111", for both the first sensor diagnostic test and the second memory code readout.

Instead of a paperclip, I crimped spade terminals on four inches of insulated wire to make a jumper. I also painted touch up paint on the connector pin so I don't have to remember which pin to jumper.

 

[FastCAD]

I don't have details on that procedure.

Correction.
The procedure I described above to erase all codes is as described (but) is for the K-ey O-n E-ngine R-unning.
Warm the car up first to get the O2 sensors hot then shut the engine down and install the jumper wire and restart the car. The self test will start flashing the check engine light. When the test gets to the cylinder check, disconnect the jumper.

Drive the car for ten miles. Put the car through all of its task's (a/c on, braking, turning, accelerating etc.). At first it will run poorly. It will relearn the codes and it is interesting because you can tell by driving what is going on as the computer relearns its settings and you know that you have clear newly recorded codes.

The above pin jumper wire diagram seems to be correct. It should be between the STI and the fuel pump pin.

 

[NoSlo]

Correction.
The procedure I described above to erase all codes is as described (but) is for the K-ey O-n E-ngine R-unning.
Warm the car up first to get the O2 sensors hot then shut the engine down and install the jumper wire and restart the car. The self test will start flashing the check engine light. When the test gets to the cylinder check, disconnect the jumper.

Drive the car for ten miles. Put the car through all of its task's (a/c on, braking, turning, accelerating etc.). At first it will run poorly. It will relearn the codes and it is interesting because you can tell by driving what is going on as the computer relearns its settings and you know that you have clear newly recorded codes.

The above pin jumper wire diagram seems to be correct. It should be between the STI and the fuel pump pin.

1. None of the driving etc is required to clear codes. You just disconnect while engine-off codes are being blinked, nothing more. Source: done this many times.

2. If you clear the codes as described, the car will not run poorly, because you have not made the computer forget its learned engine management trim.

Only disconnecting the battery for many minutes will do that. And when you do reconnect the battery, the first start-up should follow the idle re-learn procedure, otherwise one may have persistent idle symptoms. Which, as a reminder, is: To start the car without touching the gas, let the car idle for a minute without touching anything, and then turn on AC, press brakes, turn wheel to produce loads at idle. Then I also like to let it idle in reverse to ensure it learns to stay alive while in gear. Then you can finally touch the gas when you go for a drive.

Symptoms and codes may not be re-set for quite a while, as EGR and canister purge doesn't kick in until hot highway speeds, 10 minutes in or more. I have a flaky O2 sensor on my car now that has only set a code after two hours of freeway driving.

 

[FastCAD]

1. None of the driving etc is required to clear codes. You just disconnect while engine-off codes are being blinked, nothing more. Source: done this many times.

2. If you clear the codes as described, the car will not run poorly, because you have not made the computer forget its learned engine management trim.

Only disconnecting the battery for many minutes will do that. And when you do reconnect the battery, the first start-up should follow the idle re-learn procedure, otherwise one may have persistent idle symptoms. Which, as a reminder, is: To start the car without touching the gas, let the car idle for a minute without touching anything, and then turn on AC, press brakes, turn wheel to produce loads at idle. Then I also like to let it idle in reverse to ensure it learns to stay alive while in gear. Then you can finally touch the gas when you go for a drive.

Symptoms and codes may not be re-set for quite a while, as EGR and canister purge doesn't kick in until hot highway speeds, 10 minutes in or more. I have a flaky O2 sensor on my car now that has only set a code after two hours of freeway driving.

Makes sense but I don't recall Shoyay saying that his SHO ran poorly after his service or any driving to reset the pcm settings. He did say that he had the battery disconnected for some time. Maybe he will confirm the results.
Either way many SHO owners or new owners have been caught out by not clearing "old" codes which cause confusion when you get a CEL. At least starting out with a clean slate gives you an accurate account as to what is really going on.

You mentioned the purge canister. If the canister is clogged or not connected properly could that contribute to Shoyay's loud knocking sound?

Joe

 

[SHOYAY]

Makes sense but I don't recall Shoyay saying that his SHO ran poorly after his service or any driving to reset the pcm settings. He did say that he had the battery disconnected for some time. Maybe he will confirm the results.
Either way many SHO owners or new owners have been caught out by not clearing "old" codes which cause confusion when you get a CEL. At least starting out with a clean slate gives you an accurate account as to what is really going on.

You mentioned the purge canister. If the canister is clogged or not connected properly could that contribute to Shoyay's loud knocking sound?

Joe

I've gotten the codes since long battery unhook.
Chance of it being canister is slim to none. The sound is definitely coming from inside the engine.

 

[FastCAD]

I've gotten the codes since long battery unhook.
Chance of it being canister is slim to none. The sound is definitely coming from inside the engine.

The long battery disconnect will not delete old codes. I was suggesting to remove the old codes to get an accurate read on what has or has not changed.
You're confirming that after the long battery disconnect that your pcm went through it's learning process?

 

[SHOYAY]

The long battery disconnect will not delete old codes. I was suggesting to remove the old codes to get an accurate read on what has or has not changed.
You're confirming that after the long battery disconnect that your pcm went through it's learning process?

Yes, battery was unhooked for a week while I was doing things to it