Going Lean

[RenoF250]

My engine has been running fine and then a few days ago had a fuel problem and died. We let it cool and nursed it most of the way home but the time it would drive was getting shorter so we ended up having it towed. Anyhow, it appeared that problem was mostly a dirty pre-pump filter. I changed that and the pump sounds happy and fuel pressure is where it is supposed to be but now it keeps going lean. It used to run with the air/fuel bouncing around optimum but now it keeps shifting down to lean. I put the gauge back on and it has 30 psi at idle but it is slowly going from lean to rich and then it will go down and stay lean for awhile. Any ideas? I am not running it like this.

BTW when it had the fuel problem, it went lean and died within a few seconds it was not lean long at all. I did try disconnecting the battery for awhile to clear the PCM but that does not seem to have done anything.

[sly-jeeper]

30 psi seems low to me I would suspect the pluged screen ate up the pump

[RenoF250]

30 psi seems low to me I would suspect the pluged screen ate up the pump

Book says 30psi with vac on regulator up to 39 psi with no vac which is what I am seeing.

I check the voltage on the MAP sensor and it was 1.15-1.3V and book says it should be 1.5-2.1V so I ordered a new one. Worst case I have a spare.

[SilverXJ]

30 psi seems low to me I would suspect the pluged screen ate up the pump

I've seen 31 psi spec'd, but a range wasn't given. I don't think 1 psi is going to give him the problems he is having.

Have you had the pressure gauge attached when the problem is happening?

[sly-jeeper]

Guess I should have read his sig. I was thinking obd ll fuel pressure

[RenoF250]

30 psi seems low to me I would suspect the pluged screen ate up the pump

I've seen 31 psi spec'd, but a range wasn't given. I don't think 1 psi is going to give him the problems he is having.

Have you had the pressure gauge attached when the problem is happening?

Yes, and it is solid. Only moves if I rev it an vac drops.

MAP was definitely bad resistance number were way off from the new one and with the new one it no longer burbles and pops under deceleration just goes full lean. I believe that is correct as the book says on decel PCM turns off injectors. TPS also appeared bad from the meter so I replaced it as well. Unfortunately mixture problem remains. It starts at optimum and then drops to super lean and then starts going back and forth between lean and rich.

Here is a video:
http://youtu.be/yYx8RC8OXM0

It used to sit steady green and idle. I know you cannot read the gauge at the end but the number blobs you can see are 20 and 40, the needle is pointing right around 30. Also, you are correct the book does say 31 psi.

Could the IAC be open too far and the PCM is leaning to lower the idle? It is idling around 700 rpm. I pulled the vac line to the regulator off to give it a leak and make it idle higher and it went rich and then slowly went lean and started doing the same thing.

[RenoF250]

Thought about it a bit more and really doubt my IAC theory because it does it at speed as well. Hitting the throttle used to make it go full rich but now it will bounce down to lean and sounds different.

[RenoF250]

I just tried something else. Start, rev to 2k and hold. Mixture stays in the green then falls to lean. What would make it want to do that? It is still warm so it should be running closed loop. Seems like the loop response is too slow and it is oscillating.

Also, checked codes - 12, 33, 55. Nothing exciting.

[FlyinRyan]

What are your fuel trims like?

[SilverXJ]

That would be a good indicator, but I don't know if he has the tool to read that on a Renix. It would be a good investment though.

[RenoF250]

That would be a good indicator, but I don't know if he has the tool to read that on a Renix. It would be a good investment though.

It is a 1994 so OBDI, is it possible to read the trims?

It did work just fine until it had the fuel issue and now it does this. It is also odd to me that it holds at optimum just fine for awhile, then drops and starts oscillating. I am starting to think it is the PCM.

[SilverXJ]

Oh, your profile says 1985 CJ.

Its possible to read Renix fuel trims, so it should be possible to read OBD I trims. IIRC Mau'dib used a MT2500 to read the fuel trims on his Renix.

Going back through this thread, what AFR gauge are you using? It looks to be one of those ones that use a narrow band sensor. If so, this " back and forth between lean and rich. " May be normal.

[RenoF250]

Oh, your profile says 1985 CJ.

Its possible to read Renix fuel trims, so it should be possible to read OBD I trims. IIRC Mau'dib used a MT2500 to read the fuel trims on his Renix.

Going back through this thread, what AFR gauge are you using? It looks to be one of those ones that use a narrow band sensor. If so, this " back and forth between lean and rich. " May be normal.

Sorry, engine came from a 1994 Cherokee.

It is an Equuis that uses the same sensor as the PCM. It did bounce through the middle before when it was working right but it would basically light all of the green optimum LEDs and one rich and one lean. Now it is going full lean to full rich and the engine sound is changing. It will even go lean under acceleration and I think I may have heard it ping (exhaust is loud so it is hard to tell). I measured with the meter and it is going from 0.2-0.8V on the O2. I do not think that is normal.

I tend to think it is related to the original fuel problem but I do not know how since the pressure is fine now. There should not be any air left in the fuel line either and I think that would be more sporadic, this is fairly predictable.

[SilverXJ]

I don't have much faith in those narrow band gauges for other than a light show. There is no real quantifiable data.

Is there a possibility you could have disturbed something during pump removal that could be allowing air in?

I think there is something more to your original issue besides simple a clogged filter. I haven't seen many filters clog and cause an immediate problem like what you are describing. One thought is that perhaps something came off the pre pump filter or fuel pump and possibly hit the after pump filter. What about some kind of vacuum leak in the emissions system created as well?

What happens when you drive it and get on it a bit?

[RenoF250]

I don't have much faith in those narrow band gauges for other than a light show. There is no real quantifiable data.

Is there a possibility you could have disturbed something during pump removal that could be allowing air in?

I think there is something more to your original issue besides simple a clogged filter. I haven't seen many filters clog and cause an immediate problem like what you are describing. One thought is that perhaps something came off the pre pump filter or fuel pump and possibly hit the after pump filter. What about some kind of vacuum leak in the emissions system created as well?

What happens when you drive it and get on it a bit?

I would think the narrow band should be in the ballpark and consistent with what it was before. It is showing what the PCM is seeing. It holds perfectly for the first 15 sec or so so I do not understand why it goes off in the weeds. It did not do that before.

It is possible there is air but after I changed the filter I jumpered the relay and ran the pump for 10 min or so. You could clearly hear air at first and then it went quiet.

It is a pretty small filter so it does not take much to clog, also the tailpipe ends at the axle and it seems the exhaust was heating everything up back there. I could barely touch the fuel line so I think it was a heat soak/filter problem.

I have checked all of the emissions stuff over and over and do not see anything. I just went through and individually pulled and capped all of the vacuum lines and it did it the same thing with each.

Also unplugged coolant temp and got a CEL and was hard to start but then did the same. IAT gave CEL and did the same.

I would like to find something that effects it.

It does the same when I drive it and get on it, that is what really concerns me. I do not want to run it lean under power.

If you watch the video you will see the oil pressure seems to jump around at the same rate but not in sync with the O2. Tach is fairly stable, does not seem to be moving enough to justify the oil pressure movement. I cannot even come up with a theory for the oil pressure change.

It seems something has pushed the PCM out of its normal control loop and it is trying to control O2 with the slower O2 loop and oscillating. I suspect it is intended for the MAP and TPS to be the high speed control loop and the O2 is just for fine tuning and it has moved out of that for some reason.

Will changing ignition timing change the O2?