New Member, New Problems - PICS ADDED POST 3, 1994 7A-FE with a couple of trouble codes Options

[boony]

So I'm getting code 51 (Knock Sensor) and 72 (EGR system) codes on my 1994 ST auto. I just got the car extra cheap as a beater and frankly know little about it. I know it is not stock.

The car runs (but is slow to me, but that's the nature of it I think). The engine runs and idles smoothly. The AC works, etc. These codes come on at random while driving and stay on until cleared (this has happened twice in my 100 miles of owning it). Gas mileage seems to be superb. The engine temp gauge on the dash barely goes above 1/4 of the way up, and it stays where it is after it reaches that position.

So, 6g gurus, what do you suggest. I will attempt to post some engine bay pics early this afternoon. This will soon be my only car and I need it to run well for at least another 50k miles. It has 190k on it, new belts (including timing), new seals (including rear main), and new gaskets.

This post has been edited by boony: Jul 30, 2009 - 1:07 PM

[stephen_lee]

Code 72 is probably just the thermistor bad on your egr system. you can bypass that by just shorting out the two pins for said sensor. Code 51 is probably a bad knock sensor. you'll need to replace it. But, first i would check if its even plugged in. its on back of the block, under the intake manifold.

[boony]

Code 72 is probably just the thermistor bad on your egr system. you can bypass that by just shorting out the two pins for said sensor. Code 51 is probably a bad knock sensor. you'll need to replace it. But, first i would check if its even plugged in. its on back of the block, under the intake manifold.

What are the consequences of bypassing that thermostat? I will be (eventually) driving the car over distances in excess of 100 miles and don't wish to be stranded.

Also, why would the knock sensor be unplugged? In an OBD1 car, does the sensor throw a code if it detects knock or only if it is not functioning?

These codes are not thrown immediately at startup and instead require driving around for some random distance. Would they really take that long if the thermostat wasn't functioning or the sensor was unplugged? Forgive me. I have basic car knowledge, but I'm going to ask often for clarification. I appreciate your response very much.

When I got on lunch I'll take some shots of the same areas so you can see how they're supposed to be. (Roughly 45 mins)

Thank you! And hooray for the bugeye wagon. Just sold my 06 STi - or am about to rather.

[stephen_lee]

Code 72 is probably just the thermistor bad on your egr system. you can bypass that by just shorting out the two pins for said sensor. Code 51 is probably a bad knock sensor. you'll need to replace it. But, first i would check if its even plugged in. its on back of the block, under the intake manifold.

What are the consequences of bypassing that thermostat? I will be (eventually) driving the car over distances in excess of 100 miles and don't wish to be stranded.

Also, why would the knock sensor be unplugged? In an OBD1 car, does the sensor throw a code if it detects knock or only if it is not functioning?

These codes are not thrown immediately at startup and instead require driving around for some random distance. Would they really take that long if the thermostat wasn't functioning or the sensor was unplugged? Forgive me. I have basic car knowledge, but I'm going to ask often for clarification. I appreciate your response very much.

When I got on lunch I'll take some shots of the same areas so you can see how they're supposed to be. (Roughly 45 mins)

Thank you! And hooray for the bugeye wagon. Just sold my 06 STi - or am about to rather.

All that sensor does is let the ECU know if the EGR is functioning. The ecu looks for a resistance lower than a specified value while it is operating the EGR system. Now, when you short it, there is little to no resistance, so the ECUs requirements are satisfied. And to answer your question bluntly, no, it will not cause you any issues. The worst that may happen is a SLIGHT decrease in economy (mpg).

The knock sensor is a piezo-electric.. microphone basically, and the ecu 'listens' for knock through it, and when it 'hears' knocking, it will retard the ignition timing. It uses it as an input to control your ignition timing, so it will not let you know of your engine knocking, it only adjusts the timing by its input. Now if the sensor is not there the ECU is missing a very important input it uses to control spark timing, and tries its best to guess using other sensor inputs, without the safeguard of protecting your engine from knocking & pinging. So w/o this sensor, you seriously risk damaging your engine. So use supreme gasoline and get that sensor taken care of.

The sensor may have been unplugged accidentally, or just came loose, or it can actually be a bad sensor.