it is very hard to diagnosis any pre OBD2 car, made. OBD1.
no scan tools.
with a scan tool, all this is in view, live.
the outside air, is 20 % oxygen
even a small crack in the exhaust will make the 02 slam to 0v. Even 1% air leaks. the outdoor air is 20 times, more than that. ( like dumping a bucket of water on a burning candle , bam)
the O2 sensor is a switch, it's not linear. (unlike modern wideband sensors are)
so, if the ECU sees this sensor at outside 20% air, it slams to 0v (0 means the sensor sees lots of 02)
the ECU sees that, and stupidly (OBD1 is dumb) and forces the injector huge rich. (in fact to much , corrected in OBD2)
(Chaos, is hard to imagine and explain)
This is all about FUEL TRIM:
so now it's rich, and may(will) misfire. adding more 0xygen for that (caused by cylinder air slugs) the extra 02 , makes it even worse, this is the Achilles heel of all old cars.
Misfires, is like the Prize fighter slugging himself. silly. there must be no air leaks, period. or it will go nuts.
Slug my face mode 2:
now if it goes to "Bosch, white paper, hell mode." (my name) the carbon places a blanket of carbon over the tip. this can now SHIELD it from that oxygen.
this causes the 02 sensor to stick at 1v. (ECU cuts fuel and the blanket now burns off and .... )
the process then repeats, over and over, CHAOS mode. how long , depends on how big that air leak is.
2k RPM? HOT? never.
but sure cold that is the IAC doing its job. to do just that.
the hot idle surge, can be many things, a huge list.
but if engine has perfect compression , only a few things.
misfire,
egr leaking (main)
the 02 CHAOS mentioned above.
leaking injectors
clogged (or partially) injectors.
induction air leaks. of any kind,
even a bad MAF, tap on side gently does RPM change, that be bad. (stock intake,,CAI hacks may do this, most CAI hacks usually beat the MAF to death.)
if car was OBD2
you'd see
1: closed loop dead, it idle
2; 02 going nuts, (plotted live)
3: LTFT , way the H3LL off, not near 0 but say, -40% one minute then +40 next or its DEAD. FUEL TRIM !
OBD1:
if no scan tool , i use a real scope on the CELL+- pins of the 02.
and watch cross counts.
if its not like this , i fix that first.
hot idle.
if it don't swing or is shifted, off center the o2 is bad, or there are exhaust leaks or other causes. there is an error here, see .090 he put the decimal point wrong its 0.9v 9/10s of a volt.
![[Image: O2-dso-real.jpg]](http://www.fixkick.com/sensors/O2%20sensor/O2-dso-real.jpg)
my newer car can swing 6 times a second, at hot idle.
the limit is the speed of the ECU, on old cars, they are slower. (slow 02 sensors waste fuel, they all do that when old)
what you are seeing here, is the ECU hunting STOICH,
Stoich is at 0.45v average. or 14.7:1 AFR ratio.
the ECU must hunt it fast, because the sensor is not analog, it's a switch. lean, rich, lean , rich; switch
this hunting is called CLosed loop. this is because it's a closed loop servo system.
like an auto pilot in an and air craft.
the ECU works real hard, moving fuel trim to keep the AFR at STOICH, by fast hunting the fuel trim. and reading the O2 over and over. in a software loop
the above plot is the results of fuel trim, and the resultant oxygen remaining in the exhaust stream.
the ECU can not measure fuel mix at all , only 02. its an indirect method and does work.
