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95heepyj 08-09-2012 09:50 AM

Check engine light drivng me nuts!
The drivability of my jeeps seems to be fine (1995 yj 2.5) but it keeps throwing a code and setting the check engine light, the code was number 51, which was saying oxygen system, during this issue I have replaced my exhaust manifold and exhaust along with a fresh O2 sensor. I tried looking for an vacuum leak with no sucess but I have not ruled that out... any ideas?

Rebel_Ryders_Offroad 08-09-2012 10:42 AM

The code probably never cleared the computer after you change everything. Disconnect the battery for a half hour or do then see if it throws the code.

roadrunner58 08-09-2012 11:25 AM

Code 51: Oxygen sensor stuck at lean position (Bob Lincoln wrote: may be tripped by a bad MAP sensor system causing a rich condition, and the O2 sensor trying to compensate. The O2 sensor may still be good. The MAP assembly consists of two pieces, the valve and the vacuum transducer (round plastic unit with cylinder on top and both electrical and vacuum connections) - If you get hot rough idle and stalling, especially on deceleration, accompanied by flooded engine and difficulty restarting, that can be a bad MAP sensor causing the O2 sensor to try to compensate. If you get poor cold driveability, stumbling and bucking, and acceptable warm driving with poor gas mileage (a drop of 10 mpg or more), that is usually the O2 sensor. [Webmaster note: MAP sensors seem to die regularly.]

95heepyj 08-10-2012 07:21 AM

well roadrunner I do get fairly poor mileage, but I thought it was a jeep thing, I get 14-20ish and its the 4 banger, I think the map is worth taking a look at, it has been stumbling too

95heepyj 08-14-2012 09:11 AM

well I replaced the map sensor, it seems to run smoother but it did not fix the check engine light, still comes on randomly! any other ideas, I am going to check the vacuum lines again

0III0forlife 08-14-2012 09:21 AM

vacuum lines or possibly an exhaust leak near the manifold. IAC sensor is another route. If its stumbling on a start up. IAC is Idle Air Control, its 1 or the 2 sensors on the throttle body. The other is the TPS, Throttle Positioning Sensor.

If the IAC is going bad, it will cut off the air to the throttle body, causing it to run rich, which will throw a code from the oxygen sensor because of the rich mixture.

check the plugs as well, if you have them all burning the same color then the injectors are working the same, if 1 is blacker than the rest, it might be a stuck nozzle for that injector.

I'm just throwing out ideas. Its kinda hard to trouble shoot a motor by just reading what you type about it, without actually being there.

95heepyj 08-14-2012 09:46 PM

yeah, its not acting like an iac sense it idles perfect, i can pull the plugs to check them, it drove like new today, last night after putting in the map sensor the CEL popped on going to the gas station and popped on once more on the way home from the gas station. today it didnt pop on at all

jokerchief462 08-15-2012 12:13 AM

In my 89 2.5 it had an emissions timer set to go off at like 81500 miles. Then a constant check emgine light. PO let it burn out. Well I had to get a smog so I needed the light to work. Found the emissions timer and did a reset I found here iin the forum. No more light passed smog. So look next to your gas pedal on the fire wall for a little box with a couple of wires. If it is there unplug, unbolt and remove the box. Turn it over there is paper sticker on the back. If you pierce it with a small screwdriver you will see aall the timers. Rest happens when you lift the wire from the top of the spool and set back at the botoom. Pretty crazy. So if you got one reset it light goes out. Had me going crazy.

I see you are working other issues there was an excellent thread I followed to fix my 2.5 aalso. Actually followed it to the end. Ended up with new exhaust, cat, O2, map, rebuilt throttle body and fuel pressure regulator. That was it the fuel pressure regulator PO had screwed with it and put it back together wrong there is a fuel return hole in the diaphram he had it blocked. End of problems passed smog and still no more check engine light except at start up when all the lights show.

95heepyj 08-17-2012 07:33 AM

It threw a mat sensor code yesterday so I will try that, I dont think it has an emissions timer so thats probably not the cause, yesterday it did a couple random backfires on take off but other than that the light didnt come back on yet (since the morning, and the light doesnt come on when it backfires)

roadrunner58 08-18-2012 06:05 AM

Is the engine back firing through the intake manifold or the tail pipe? Check your fuel pressure if it is back firing through the intake. Have you ever replaced your fuel filter?

95heepyj 08-19-2012 07:22 AM

sounds like it the exhaust to me, yeah I replaced the fuel filter, I will throw a MAT at it and see what it does.

95heepyj 08-20-2012 01:14 PM

I installed the MAT sensor this morning, cleared the codes and so far it appears fixed also after installing the MAP last week I got 50 more miles to the tank of gas

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