OBD I driving me crazy - JeepForum.com
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post #1 of 31 Old 09-24-2020, 09:41 AM Thread Starter
konoug2
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OBD I driving me crazy

I am new to the Jeep family as I bought my 1995 YJ with a 2.5L about a year ago and it is the first jeep I've owned. Ever since I got it, it has had an intermittent CEL with code 51 for running lean. The code comes on at random times but does seem to be worse when its cold. After warming up if I turn it off and back on it will go away but then still come on at random while driving. From past experience I figured it was the o2 sensor that was going bad and worked better as it warmed up so I changed that and put a new Denso one in. Needless to say that did not fix the problem. A few months down the road and I had to do a head gasket on it so while doing that I replace the TPS, MAP, TEMP sensor and TEMP sender, new plugs, new wires and new fuel filter. When doing the head gasket I had the head resurfaced and ended up having to have the exhaust valve seats done also because the machinist said it looked like it had been driven with a CEL and that destroyed the seats. I am trying to get this figured out before I destroy them again.

Not only have I not gotten rid of the original CEL now it has gotten worse. With the same behavior of being worse when cold and intermittent (sometimes will go days with no light other times will come on 15 times in a 10 mile trip).
Now I get codes: 21, 22, 24 ( for some reason shows up twice in sequence of codes), 25 and 51.

I don't have a lot of experience with the OBD I system and it is driving me crazy as I can not figure out what is causing all this. I have a lot of experience with OBD II where it can give me more information.

Through all of this the Jeep runs great and seems to have no change in performance with or without the light on. Any help or information from someone with more knowledge on these would be great.

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post #2 of 31 Old 09-24-2020, 02:26 PM
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Well these aren’t answers but issues with codes that seem to go away after driving a while are generally symptoms that could indicate a vacuum leak. Doing the head gasket involves things that could ‘cure’ that. Unless it’s a line, boot, whatever.

The second generality with intermittent issues like you described are electrical- either a bad connection or a bad connector, or a failing component.

The third generality: were ALL the seats burned? Or just some? OBD1 doesn’t study loads and signal like OBD2 so a couple plugged or gummy injectors could introduce that lean code but not tell you exactly why.

Fuel pressure is another question I thought of

Those should keep you busy for a while.

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Cranking IS turning over
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post #3 of 31 Old 09-24-2020, 03:33 PM Thread Starter
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I had thought of low fuel pressure due to a weak or clogged line or injector. I have been doing seafoam fuel treatment to try and clean the system and that is also why I did the fuel filter at the same time to try and rule out something.

All the exhaust seats were bad, some were a little worse than others.

As far as vacuum leaks I can't find any at this point. The idle is good and low, I don't hear any sucking and I have even sealed some of the boots that were a little worn. Not sure what else I could do to try and look for leaks?

I was just testing my sensors and everything looked good except my MAP voltage on the center wire was 4.25 instead of closer to 5 and my ohms on the ground were around 1.2 instead of closer to 0.00 that the test stated. Could this be a result of a weak ground? If the MAP is not reading right would that throw off all the other sensors?
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post #4 of 31 Old 09-24-2020, 03:39 PM
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A head gasket was that for a coolant leak? be advised coolant getting into exhaust is NOT good for o2 sensors - lets think positive to start.

You changed the ECT sensor on thermostat (PCM) Correct?


21 O2S Stays at Center Neither rich or lean condition detected from the oxygen sensor input.
22 ECT Sensor Voltage Too High ECT sensor input above maximum acceptable voltage.
24 Throttle Position Sensor Voltage too hi or low (We love OBDI)
25 Idle Air Control Motor Circuits A shorted condition detected in one or more of the idle air control motor circuits.
51 O2S Signal Stays (Lean) Oxygen sensor signal input indicates lean air.

A WAG I would look at fuel pressure AND as suggested vacuum leaks; A gauge and the carb spray trick shoould be fast easy place to start.

31PSI with regulator getting Vac. 38-42 with vac line removed.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #5 of 31 Old 09-24-2020, 03:56 PM
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Did u reset after the o2 sensor change? As in both battery cables removed and touching and grounded out for a few minutes to reset ANY stored data?

I'm not sure why. But having the same code flash twice in a row is something I never heard of. I may be leaning towards electrical issue causing all of these CEL codes if the jeep is running just fine with zero change.

Perhaps not necessarily an issue with the computer itself. But the loom that runs to the specific sensors related to your codes. Then again, if the computer were operating normally, you should experience some sort of performance difference. A bog, misfire, backfire, sluggishness... at least sometimes/sporadically. Have you checked your grounds are clean and nice and tidy for that specific circuit/set of circuits?

~YJOTM MAY '16, JULY '19~


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post #6 of 31 Old 09-24-2020, 06:58 PM Thread Starter
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I had the battery out of the Jeep for a week while I was doing the head gasket. I had to do the head gasket because I was losing coolant into the cylinders but I hadn’t driven it much between the new o2 sensor and head gasket.

On the drive home today there was new developments. I could not get the light off it just kept coming right back on and twice when I restarted the Jeep it back fired which it’s never done before. The other thing that was odd is when I restarted it it was idling around 1200. Now with that happening I’m kind of leaning towards a vacuum leak so tomorrow I will get a can of carb spray or something and start checking all the lines.
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post #7 of 31 Old 09-24-2020, 10:42 PM
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Good call.

~YJOTM MAY '16, JULY '19~


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post #8 of 31 Old 09-25-2020, 05:48 AM
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Backfire: exhaust bang, or out the throttle body? I suspect exhaust. When fuel doesn’t fire in the cylinder it can blow up in the exhaust. The lean code is going to wanna be dumping fuel into the cylinder. So basically popping in the exhaust is saying a cylinder or cylinders got fuel but didn’t fire
If it was backfires out the intake side there’s a whole new group of potentials.

MAP sensor. Yes, I’d be looking at that. The O2, distributor pickup, CPS, and the MAP are your critical engine management feeds. (They’re all critical, yes, but EFI can run out of parameter in a very harmful way with the MAP or O2 sensors failing. A carburetor engine enough “out” to create anything close the bad things efi can do probably wouldn’t even start, or stall out if it did)
Find out why your neg(-) ohms are wonky.

As jtec said there’s a couple things to hunt down. But the thing that sticks out to me is the Code 25. That’s a certain particular condition whereas the others are not specifically indicating anything besides a symptom and thereby may not be a sensor failure.

[size=“3”]Shackles & D-rings are different things.
Cranking IS turning over
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post #9 of 31 Old 09-25-2020, 06:16 AM
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What are the symptoms of an ECU with bad capacitors? I'd add that to the list of possible causes.

https://www.jeepforum.com/forum/f12/...rs-ecu-492597/


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post #10 of 31 Old 09-25-2020, 06:46 AM
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All those sensors have common power and ground. If I am not mistaken they all have a signal wire to the ECM. So check power and ground. Your getting a CEL and its starts with no problem so its most likely not the capicitor issue.
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post #11 of 31 Old 09-25-2020, 09:38 AM Thread Starter
konoug2
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Quote:
Originally Posted by fishadventure View Post
Backfire: exhaust bang, or out the throttle body? I suspect exhaust. When fuel doesn’t fire in the cylinder it can blow up in the exhaust. The lean code is going to wanna be dumping fuel into the cylinder. So basically popping in the exhaust is saying a cylinder or cylinders got fuel but didn’t fire
If it was backfires out the intake side there’s a whole new group of potentials.

MAP sensor. Yes, I’d be looking at that. The O2, distributor pickup, CPS, and the MAP are your critical engine management feeds. (They’re all critical, yes, but EFI can run out of parameter in a very harmful way with the MAP or O2 sensors failing. A carburetor engine enough “out” to create anything close the bad things efi can do probably wouldn’t even start, or stall out if it did)
Find out why your neg(-) ohms are wonky.

As jtec said there’s a couple things to hunt down. But the thing that sticks out to me is the Code 25. That’s a certain particular condition whereas the others are not specifically indicating anything besides a symptom and thereby may not be a sensor failure.


The backfire is out of the exhaust and from your description it sounds like you hit that cause right on the head. The 2 times it did it one was pretty loud but the other was smaller and more of a popping just like you said.
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post #12 of 31 Old 09-25-2020, 10:56 AM Thread Starter
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Well i did found an air leak due to a bad oring on the IAC valve and replaced that and tried it again to verify that the leak was sealed and it was. Unfortunately, that was not causing the check engine light because when I started the jeep back up the light came back on like it has been. All other vacuum lines and sealing surfaces seemed to be good. I guess I will start tearing into the wire harness?
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post #13 of 31 Old 09-25-2020, 12:22 PM Thread Starter
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Another update, I pulled the wiring harness out of the loom and inspected it and didn't find anything that looked bad. I also checked every wire from the IAC valve, TPS, MAP and o2 to the PCM plug with a multi meter and all the wires checked out good. I am really getting to a loss on what else to look at.
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post #14 of 31 Old 09-25-2020, 04:10 PM
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A cracked exhaust manifold will allow air into the exhaust stream,
the O2 sensor will see this as a lean condition and the ECU will add fuel.

Now the engine is running rich and could backfire out the exhaust,
as the crack grows the problem increases.

Is you exhaust in good shape?

Whats over that next ridge?
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post #15 of 31 Old 09-25-2020, 04:42 PM
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you found and repaired a vac leak - disconnected battery? CEL returned - what codes are showing now?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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