Check engine light...worse idle?

Effieman · Nov 7, 2018

Once again the check engine light came on in the truck. Forget what exact code was, but it said O2 sensor slow switching I think. First time EVER had 'pending codes'. One bank was lean according to that one.
But, as soon as the light came on, the idle got worse! The truck would rock from side to side from it, and would get close to only 500 rpm. Shouldn't that NOT be the case?
After doing nothing, the next day (only 2 'starts' from the light coming on), the light goes off!! Shouldn't that also be 'wrong'?

Dirtman · Nov 7, 2018

Not to sound condescending but you need to post the actual codes not what you "think the definition was". Not only does the specific code matter, but all the other codes NOT shown for a specific malfunction. Get it scanned, write down the actual codes and report back.

Pending codes mean there is a problem but it either has not been through a long enough cycle to clear, or the issue is intermittent and not reporting enough to become a stored code. When this happens the CEL will come on as the issue occurs and go off when it stops until it happens enough times in a drive cycle to become a stored code and then the CEL will remain on until the problem is fixed and you clear the code.

Idle dropping like that could be a ton of things, the computer could be switching to open loop mode when the CEL comes on because the 02 sensor is failing which shouldn't really cause anything more than crappy gas mileage unless open loop is cutting the injector pulse way too much. but you need the codes to pinpoint the issue.

Effieman · Nov 7, 2018

Hooked the reader up today and it still has an O2 sensor code, and only 1 pending code for 2 side lean. The reader says the light should be on, but it isn't. And I checked for the light when first turning the key on, so it's not burned out.
I know what you're saying about the codes, but I thought that once a code set the light on, it was suppose to stay on for 40 or so 'drive cycles'. I know when I've had codes in the past the light will stay on for weeks if I don't reset it. I'm to the point now where if the code is O2 related I just clear it, as it likes to come on several times a year, no matter how much I drive it.
After doing more reading, I'm wondering if it's just the PCM, since I've had weird drivability issues when there is no codes.

Dirtman · Nov 7, 2018

What have you done to actually diagnose or fix the issue besides just constantly clearing codes? Did you replace or test the o2 sensor? It sounds like your jumping to blame the ECU before even using the information it's giving you to perform a proper diagnostic or repair. If the o2 sensor is not reading properly it can cause the lean condition. You still haven't posted the codes. Also the computer doesn't throw a code for every issue that can happen, and it can also have driveability issues before it reaches the threshold of setting a code.

And again pending or intermittent codes do not turn the CEL on. If a circuit malfunctions the CEL will come on, if the condition does not repeat the CEL will go off and leave a pending code which can then be cleared in just a few minutes depending on the parameters of the code. They dont all clear with the same set cycle. Some circuits are tested 100 times a minute and the programming states something like 3 bad tests out of 100 delivers a pending code. 100 more good tests clears it so that code could come and go in 2 minutes.

The scanner doesn't know if the CEL should be on unless it knows the code setting parameters for that specific ECU. Unless you have a $3000+ dollar scanner that thing is just giving you generic code info.

Bottom line. Fix the problem the computer is reporting to you. You have a malfunctioning o2 sensor/circuit issue of some kind...

Effieman · Nov 8, 2018

Finally found my Chilton manual for the Ranger (1991-1999). Sure enough it states “These codes are contained in memory for 40 warm-up cycles.” So, I got a better memory than I thought.
Of course it also says (like you mentioned) that the light will go out, but the code will stay in memory if it should “...self-correct...” This was the first time I've ever had it self-correct before I cleared the code myself. Never knew it would do that.
As for the idle...I always thought there was basically 2 fuel tables. One 'basic' for when the light is on, and a 'complex' one that uses all the sensors when the light is off. The truck has always ran fine before when the light has been on, so I was just curious. I always figured idle problems/dead spots were related to the fuel tables, as long as everything else is functioning like it should.
And the $60 scanner I got from Harbor Freight tells you if the CEL if on or off. It's the very first line on the menu as soon as you go to the OBBII/???? option. The AutoTap software I had back around 2010 also told you if the light was on or off, and it was less than $300 and required you to enter the VIN.
As far as the O2 issue; I've ignored most of the codes (O2 stuff) over the last 10+ years. Of course that is based on my experience with this particular truck. I've had the wiring checked several times and it's always been fine. Never had any issues until I replaced the original sensors, after 100K miles. Gone through 2 sets of sensors since then, even though they always tested good too. What else is there besides wiring, sensors and the computer?

Dirtman · Nov 8, 2018

The two "fuel tables" are called closed loop and open loop. When you first start the truck it runs in open loop. Open loop ignores o2 sensors and runs on essentially a factory determined program. Its generally tuned very rich. Once the o2 sensors heat up the computer switches to closed loop and determines the injector pulse width directly from feedback from the sensors.

If something goes wrong with the sensors the computer will also switch to open loop mode. BUT... not always. Every time the CEL comes on does not mean the computer automatically goes into open loop. It will only do so if the CEL is related to a fuel issue that the computer cannot compensate for. The computer can throw a lean code but still be compensating while in closed loop mode. Your scanner may tell you which mode the system is in if it has live data function.

Onto the o2 sensor issue. I have seen numerous times where people replace o2 sensors and they fail within a few months. There are so many garbage brands or brands that simply dont work well on certain vehicles. Its always best to go with oem sensors. But there is some info that may be valuable to narrow down the issue, again if your scanner supports live data check your fuel trims (not confused with fuel tables i explained above). There is a short term and long term, any numbers + or - 15 or so is a sign of something wrong. You also want to look at the bank 1 and 2 upstream or #1 o2 sensor voltages (the second or downstrean o2 sensors dont really do anything, just ignore those) the voltages should be constantly going up and down. If one or both is a steady voltage there is definitely an issue.

If the issue is only on one bank a idiot proof test to rule out a bad o2 sensor is to just switch bank one and two and see if the problem changes also. If not its a circuit issue and not a sensor issue. Then if you absolutely rule out a bad wire, connection, pin etc would I consider the ECU being the problem.

Effieman · Nov 9, 2018

I had some Bosch sensors, and the Echlin(?) ones from NAPA. Pretty sure the NAPA ones are the ones that I would pay 1.5 times the price for at the local Ford dealer, considering that's where the dealer gets his parts. I have switched the sensors before, still getting the bank 2 lean code, but still taking months before it would show again. That's when I first had the wiring checked, as my neighbors had problems with mice, and I found a nest under the intake. But that was around '05 or '06. Had it checked again within the last few years just because; still good.
I've checked both short and long term fuel trims, at idle and while driving (Autotap software could record while driving). Always what they should be.
For $150 (including core) from Rock Auto, a new computer is cheaper than troubleshooting a problem over and over. And, as far as I know, I don't need to program the new one like some newer models, so should be a quick, easy swap. Let it idle for 10 (20?) minutes, drive a little, then wait a few months to see what happens.

Dirtman · Nov 9, 2018

1998 may or may not have the PATS system. They started switching in 97-98. If it is PATS make sure the computer comes with 2 new blank cut paired keys or your in for a trip to the dealer or a very good locksmith for programming.

Effieman · Nov 10, 2018

From your comment, I'm guessing the PATS is the newer HUGE key with the chip in them. My keys are 'normal'. My friends kid had a Explorer with the 'chipped' keys, and seem to always have trouble with it not working (wouldn't start).
Thanks for all the info/help you have given here.

Dirtman · Nov 10, 2018

I wouldn't say they are "huge" the left key is a PATs key, right is non-chipped.

Check engine light...worse idle?

Effieman

Member

Dirtman

Former Middleweight Moss Fighting Champion

Effieman

Member

Dirtman

Former Middleweight Moss Fighting Champion

Effieman

Member

Dirtman

Former Middleweight Moss Fighting Champion

Effieman

Member

Dirtman

Former Middleweight Moss Fighting Champion

Effieman

Member

Dirtman

Former Middleweight Moss Fighting Champion

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