1998 Mustang here in the shop to walk through on this topic. The car was said to be overheating, cooling fan not working, overflowing coolant, and at one time steam rolling from under the hood. A/C not cold and stopped working suddenly. An intermittent miss or cutout at steady road speed and Check Engine on.
The coolant was overfilled and because there was no room in the tank for expansion, it would push coolant out the overflow at times. There were no other coolant leaks and the cooling fan worked as designed. A lot of people think an electric cooling fan isn't working because the fan doesn't come on until the coolant temperature reaches around 220F and they don't wait that long. Nothing was wrong with the cooling system. No codes or scan tool needed for that diagnosis.
The A/C system was empty and the obvious leak was at the high side service valve which had sprayed refrigerant oil (and refrigerant) all over under the hood. Replacing the service valve, evacuating and charging the system seemed the fix. No other obvious leaks. No codes or scan needed for that diagnosis.
Now the "misfire". The car is running well and no obvious problem and they did say intermittent. This is when it might help to look at codes and scan data for clues.
I have stored memory codes:
P0136 which is a circuit fault code for the B1S2 (post cat) sensor
P1130 which indicates the fuel adapts have peaked for B1S1 (fuel control) sensor
P1131 which means B1S1 is showing lean fuel
P1000 which means someone has cleared codes and not all the monitors have run since that time
P1260 indicates a security system lockout, engine disabled.
Now, a cat monitor sensor isn't going to affect the engine performance under normal circumstances. The engine lockout is obviously not a current problem because the car starts and runs. The P1130 and P1131 could very well be related to performance problems. The P1000 could be important but only because it lets me know that some of the information that was previously stored is now missing. That would be information like other codes and other test results such as misfire history which I'm going to take a look at next.
I like to use OBDII Mode 6 / 53 Misfire Monitor to look at the misfire test results. The misfire monitor has stored misfire values for 1,2,3 and #4 cylinders and the exhaust does smell a little rich.
The conditions weren't met for the car to set cylinder misfire CODES but the "Value" counts are misfires seen during the monitor test.
There were no logged misfires on 5,6,7 or #8 cylinders. Noticeable is that all the misfires are logged on bank 1, the bank with the O2 codes for the fuel monitor. Since I can smell a rich exhaust but the code is for lean exhaust I am thinking the sensor is reporting lean and the computer is driving the system rich to compensate. The sensor isn't recognizing the richer mix. Theory.
Let's look.
At idle the right (bank 1) tail pipe is spitting out black carbon and has a rich smell. The sensor for bank 1 is showing lean at the same time.
The sensor could have failed and stuck lean (low voltage) but they also can foul from a too rich mixture so it could be the sensor failed and drove the bank rich or it could be the bank was rich and fouled the sensor. Either way it very likely has a bad 02 sensor at B1S1. Lets look.
Oh this looks fun.
Meanwhile days later.....
No it wasn't so bad to get out. Just looked it.
Let's see what the new one does.
If the sensor starts switching normally we are done with this part. If the sensor, which should now be working, starts showing rich then we know that the rich condition fouled the old sensor and we'll have to find the problem before it does the same to this one. In preparation we'll need to clear the KAM Keep Alive Memory, which will also clear the codes. If you clear the codes only it won't clear the KAM. Think of the KAM as learned programming. The KAM has learned to run bank 1 rich so we have to clear that out and let it start over. If we don't do that then the bank will still run rich until KAM updates the strategy which won't be now.
Signal now looks good.
Its a '98 with 122,000 miles on original cats. I doubt they work very well but we have that B1S2 code left over right? Bank 1 had the problems and that rich mixture going through the converter could have caused problems further downstream. What we'll do there is drive this enough to get through some monitors, look for pending codes and then see what the cat monitor sensors are doing.
Sometimes a bad cat will start working after you give it a good fuel mix and that rear sensor on bank 1 may be fouled right now like the front sensor was. Drive and test again after.
....................
The B1S2 sensor did begin showing activity but wasn't working properly so I replaced with new.
If you want to get an idea of how well a cat is working you look at the fuel monitor sensor (which is exhaust before it goes into the converter) versus the cat monitor sensor (which is the exhaust after the converter). Generally speaking, the more your cat sensor mirrors the fuel sensor signal the less efficient your converter is. These are actually working enough to keep from setting a code.
So, diagnostics is not code reading and code reading is not diagnostics. Diagnostics is about knowing how a system works and about how to find why it isn't.
I think we deserve a pat on the back for this job well done!
Thanks for reading!
Kenny@ggauto.repair
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