6G Celicas Forums

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

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.

PHOTOS:

From the driver's side looking down at the headers, which are obviously not stock - not that sensor hanging out down there.


And a bit closer up:


Back on the firewall, it looks like this. Note the sensor connector zip-tied up. There are actually three sensor plugs in that harness that are not plugged in:


Another angle of that area. The sensor plugs are a little out of frame toward the top:


Toward the firewall on the passenger's side:


Airbox - not that the battery is apparently backwards, so the ground is stretched taut. Looks like it should be slack. Also looks like there's some sort of hose elbow missing between the airbox and some duct that comes from beneath the fusebox:


Lastly, under the alternator and towards the passenger's side of the motor, there is (by the oil filter) a sensor. Is that a knock sensor? If so, there's a tiny strip of yellow wire that looks like it's on its last legs connecting it to something I cannot yet see. Could this be why I'm getting the knock code or have I misidentified that sensor? This is the area - if you had X-Ray vision, you'd see the wire I was referring to. :lol:


Any help is appreciated, guys. I know Imprezas, not Celicas. Tell me what's wrong with these pictures so I can go about getting it corrected.

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

EDIT:

You say you've got a 94? Your intake manifold is that of a later model- which leads me to believe that some of those plugs are probably OBDI and have nowhere to go on the later-model OBDII 7A-FE.

Regardless, I'll snap some shots for you.

This post has been edited by 95CelicaST: Jul 30, 2009 - 1:16 PM

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.


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

Ah. That makes me feel a bit better. I think. :lol: TIA for the pics. Like I said, it runs very well and drives normally. Just throws codes.

Why are you selling the STI?


After looking through my photobucket I found some pictures of my old Celica's intake manifold - and I retract my previous statement. I suppose I haven't looked at a Celica engine bay for a while, and I thought you had the later model setup going on, but it is the 94-95 intake manifold.

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.

i commented above certain pictures.

I was bored with the STi, honestly. Looking for something a bit more grown up. Not sure what, yet, so the Celica beater it is. For now.



Okay. So, if the EGR is malfunctioning, that's not really all that bad. I understand what a knock sensor is, I was more wondering why it would be unplugged. :lol: Any idea where that sensor is located? There are three knock sensor codes the ECU can throw. Mine threw 51 if that helps at all. I'll replace it ASAP once I know where it is.

knock sensor is behind the intake manifold

http://www.toyodiy.com/parts/p_U_1994_TOYO...2.html?hl=89615

It looks like someone swapped a engine in there and didn't do a very good job.... i doubt anything is really wrong if the car is running fine, but the mass amount of plugs hanging there is more than likely the source of your CEL codes.

This post has been edited by samir0189: Jul 30, 2009 - 2:03 PM

I looked up Code 51. not a knock sensor code. Copy & paste says:

51 ............... (2) Problem In One Of 3 Circuits Monitored By ECM

more research is necessary.

ECM uses Park/Neutral (P/N) position switch signal to
determine whether transmission is in Park or Neutral, or in some other
gear. ECM uses the output from A/C switch to determine whether or not
A/C is operating, so that it can increase idle speed if necessary. ECM
detects idle condition through "IDL" terminal of throttle position
sensor.
DTC 51 is set when throttle position switch is off 3 seconds
or more after engine starts. P/N position switch is off with shift
position in either Reverse, Drive or Low (A/T). A/C switch on. Check
throttle position sensor "IDL" circuit, accelerator pedal and cable
adjustment, P/N position switch circuit, A/C switch circuit or for
faulty ECM.

This is what my cluster looks like when the car is off but the ignition is turned to "on":



Everything but the CEL goes off when it's running. I checked the oil several times. It's clean (fresh change) and full.




Sorry: 52 and 71 are the codes. My mistake.

Thanks everyone for the responses so far. God I love the internet sometimes.

the wiring does go back towards the engine, but thte plug clips onto the radiator. i know i've owned 3 STs.

the grey and brown unplugged sensors are for EGR.

there is no "elbow" the hole in the box is supposed to go over the thing coming from under/near the fusebox.

ok code 52:

DTC 52 is set when ECM detects an open or short in the knock
sensor circuit with engine speed of 1200-6000 RPM. Check for open or
short in knock sensor circuit, a loose or faulty knock sensor or
faulty ECM.

Ah, so the airbox itself is not the correct size/shape, or the hosework on the opposite side isn't. Like someone else said, probably not the correct airbox to the car, but it seems to get the job done.

Thanks for the information on the knock sensor.

So here's where I'm at:

I need to know how to "bypass" that EGR circuit so the code doesn't trip anymore, since it's obviously not that big of a deal.
I need to know the specs on uninstalling/installing a knock sensor - which I know I can find at toyotapartsbarn for a halfway decent price.

Is there anything else you guys think I should consider? And can anyone walk me through a bypass (what/where exactly to perform the bypass more than how )? I'm going to throw some fuel injector cleaner in the tank on the way home and top off with some 93 octane for a bit of insurance (and to calculate mileage to see if the gauge is lying to me).

This post has been edited by boony: Jul 30, 2009 - 3:03 PM

Also, bump for whether this picture of the cluster (engine off, key to ON) means anything else needs replacing:

UPDATE:

So the knock code has not returned. Just the EGR code. As I cannot find the EGR on the car, I can only guess that whatever existing bypass was there has failed. I have to find it. Any suggestions?

Also, I'm getting 29.8mpg in mixed driving. Does that sound right to you guys?

Seems a bit low... My last mixed driving fill up was 35MPG. I took those pictures, but they're probably of little use due to me not having a OBDI set up.

Post'em anyway as I'd like to know where to look for the EGR in case I want to replace it.

you have no EGR system. now I am pretty sure the grey plug is the one that needs to be shorted. just get a twist-tie or small paperclip and short the two pins together.

my mpg is, obviously, between 25-31mpg.

your oil sending unit is fine, assuming that can symbol goes out after starting the engine.