Yep, you can have a problem with a sensor etc. without throwing a code!!!! funny, I've checked almost everything, Fuel Pressure, ohmed plug wires, checked ignition module, pickup coil, replaced plugs,
ohmed injectors, blah,blah, blah!!!! Now my 87,stocker runs GREAT, starts almost instantly, pulls fantastic....but, there always seemed like
something was (what's the right word) not there!! well, I just got my scanning program up and running.. with alot of help, Craig Moates, Morley, Ray Qualye,byebyel98, and alot of others..daughter included. And guess what?? the scanner shows a CODE 34.. MAF sensor low..I have NOT had a SES, or CES light flashing, or had a code
for so long, I cannot remember!! The scanners etc. are cheap..compared to hours and parts. Good Luck..Glad I can now scann!!!

 

Yes, codes and SES only get set when something is completely fubar.

Things can be working incorrectly for a long time before a code gets set.

And there are also plenty of faults for which there is no self-test.

 

Not so!!, I was at one time throwing an EGR code, EGR was fine..EGR switch was the culprit!!!! So, the EGR was not completely fubar, but the switch, whether sticking or shot was the problem.

 

When your ECM throws a code it isn't telling you that the part is bad, it means something in that sensor's system is not right. For example a code 33 MAF voltage high is very rarely a MAF sensor at fault but rather the power relay for it. And as you found out a code 32 isn't always the EGR valve...in fact a code 23 means that the diagnostic switch isn't sending the signal to the ECM when it should OR it is sending it when it shouldn't....meaning the feedback switch isn't seeing hot exhaust gas when it should or is seeing it when it shouldn't

 

I thought the computer only showed active codes. When the condition goes away, so does the SES light, but the code stays in the computer.

I had an SES light come on, went back off. Pulled the code at the gas station, was EGR. Went under the hood found vacuum to it loose.

Unhooked the battery to clear the code. Fixing the EGR (or it 'fixing' itself) will put the light out but not clear the code.

So maybe the light came on when you didn't notice, for a very short period of time. How can a scanner find code 34 if it never threw a code?

Am I off base here?

 

I'm a bit skeptical about this too, CC. As you said the light is for active codes. This 34 must be a history code. Also if the 34 is stored in the ECM (as a history code) I believe it should be displayed when shorting ALDL A to B. I thought there was no difference here than when using the scanner, but I'll have to verify this.

Rick,
I believe one of the first TS steps in all the FSM charts is to clear the code and see if it returns. Try this and see what happens. Also, before clearing, while connected with your scanner and engine off, try putting the ALDU1 switch in the bottom position. This is the same as the A to B paperclip trick. See if the light flashes the 34 after the 12.

 

Right on the money!

 

I think there are soft codes and hard codes. The computer remembers soft codes and hard codes set the SES light. When I unhook the tan/black wire to set my timing and hook it back up after, I get no SES light. If I run a scan the code will be there. An intermittent failure may be stored without setting the light. (like an occasional sticking MAF relay) A hard failure always will. My 2 cents.

 

Thanks guys, Ray, I'm gonna have fun with this, and I will try what you suggested...but, one thing is, what, about 3 weeks ago I replaced both the battery and alternator, haven't driven it that much lately, but I do know it hasn't thrown a code....I did have that miss that led me to doing alot of trouble shooting, that's when I found a bad pigtail to an injector..even that hadn't thrown a code.Getting into my FSM, I just think it's great to be able to watch these funtions.You know my idle is smooth, real smooth. One thing I'm reading in the FSM trouble shooting says to run engine above 2300 RPM for 1 minute or until Code 34 sets. Does Code Set? Guess that's just part of the TS, but, I don't usually run my motor in that RPM range, sure I get a little spirited, but even cruising down the FW, I'm usually below those RPMs. You know what? God only knows when or if the TB has ever been cleaned.

 

My SES light comes on when running with the EST wire unhooked, light goes away when I hook it up and restart. The smog guy should check for this. It sets the light only when the condition is occuring, and stores a code.

 

you have my interest tell me more about this scanner. were do you get them ? how much do they cost?

 

You should visit the scan and tune section. Basically I'm using an old laptop-like so many guys do. The scanning software, I'm using TunerPro RT, recommended by Craig Moates is free. You need a cable, which connects to the ALDL, available from Moates, I'm using his ALDU1+CABL1. The laptop I'm using is and older Toshiba-which ever laptop you select, IT HAS TO have a USB port. I also have a power inverter, some of the batteries in these old laptops are damned near as expensive as the laptops. Take a look in scan and tune. BTW, it's fantastic watching it scan, especially on a screen like a laptop..compared to the tiny 3 inch screens on scanners you can buy..and they cost a heckava lot more.

 

You don't need a scanner to pull codes, just a paperclip.

 

Very true, but a paper clip just gives a code-if thrown, whereas a scanner shows what things are doing, including BLMs, TPS readings,
MAF sensor readings etc. it really lets you see things functioning. Other than time-my investment is $180. I consider that a cheap investment that I expect to save me some headaches.

 

A lot of times a sensor can be going bad and it will affect the operation of the engine yet not set any code. The 88 FSM specifically states this on page 6E-7 where it recommends use of a scan tool. "A sensor may shift in value but not set a code." The O2 sensor is one of the most critical sensors where this can happen.

As an O2 sensor ages, the tip can get contaminated causing it to react to oxygen changes in the exhaust at a rate much slower than it did when new. This can also affect its average reported range due to a less accurate reading of the mix. With a correct fuel/air ratio it should rapidly transition between 0.10v - 0.90v. With age or contamination, the range it reports can slip to 0.10v - 0.70v or worse. Since the ECM uses the voltage values to determine fuel mix, as the maximum reported volatge gets lower, the ECM starts making fuel mix changes based upon erroneous input from the O2 sensor. If the range drops too much or shows an open, then the ECM sets a code, but when the O2 readings are within normal operating range tolerances, it cannot tell if the O2 sensor is off a bit or the mix needs adjusting. With a scan tool you can see exactly what the ECM is seeing and make your own decision. Even if it is operating in the appropriate range which will not set a code, if the O2 sensor is transitioning too slowly, that is a sign of a sensor problem. One which you can only see with a good scan tool.

With OBD2 systems and the bi-directional scan tools, you can actually turn off the DTCs and regulate the operation of specific relays and sensors to enhance troubleshooting capability.

I love my Ease Scan Tools. I can scan and record in charts everything that can be scanned during hours of operation.