Dang it, I hate it when I do that. Accidentally flipped a near full response off and had to re-do it a second time.

Excuse my ignorance, as I am not sure what you mean when you refer to 'UG'? And maybe I should? I think I see what you are saying though?

Disregarding the sea level testing atmospheric conditions variable, the PCM interprets the MAP signal as a slightly different value of VOLTAGE for you to see, by a scanner with a live data stream reader?

2.03V - 1.96V = 0.07 VOLTS
What does this equate to in PSI?

1.76V - 1.6V = 0.14 VOLTS
What does this equate to for PSI?

Or, a normal key on with engine off value? Is 4.49 normal? Probably, as it sounds right?

I am not sure if these would be significant enough differences or not between what the PCM interprets vs what the VOLTAGE actually is at the MAP, to cause noticeable symptoms? Or, what the normal expected value or values would be? It does not seem too far off just off-handedly thinking about it though?

Maybe I should look at that graph you linked..

Yes, the BUS does that. There is a fuse in my 1997 ZJ and it says it is for the 'cluster'... Translation = Your Jeeps computers. Particularly, the BCM. Although, also the PCM to a more secondary degree. The ZJ Grand Cherokee has the BCM, which is the next in line of command, standing behind the PCM. I had an extremely strange electrical problem, and I traced it down through using the BCM pins and its terminal connections with a jumper.. and a multimeter on the battery and the cluster gauges fuse terminals.

 

Is there anything soo simple it could be overlooked?

 

By UG I meant a reading from my Ultragauge. It is reporting MAP PSI pressure from the PCM via the OBD2 VS my reading the MAP signal voltage on a VOM.
The MAP voltage VS pressure is from a chart at the link I listed.

: As a new wrinkle, I went to set up a gage page on the UG to show the long and short term fuel trims and the UG only had a choice of bank 1 fuel trims and no bank 2 fuel trims and no bank 2 O2 sensor voltages either. I set a paqe with bank 1 short and long term fuel trims and the UG is reporting 0 for both.

I am starting a new thread to ask what the numbers mean on the PCM and if I might have gotten one set up for the 4.0 straight 6 engine.

MAP PSI on Ultragauge at "idle"


Map Voltage on VOM at "idle"

 

If I understand a UG (which I don't), this means your bank #2 oxygen sensor(s) are off-line? The bank #1 oxygen sensor(s) voltage is giving you a value, or just the corresponding long and short term fuel trims?

 

What would normal fuel trims be on a Jeep with a replacement PCM when it has only just been started and moved around the yard?

Would a PCM be zeroed-out before sent off to you when you bought it?

(edited)
Short term fuel trim should move though. You can check it by running at idle, 1500 rpm, and 2500 rpm for thirty seconds each.. to see the STFT change values. I guess your idle is already at 1500 rpm though, so maybe adjust accordingly?

 

Think of the Ultragauge as a scanner displaying live data stream information. Check it out at http://www.ultra-gauge.com/ultragauge/ It has 7 different data "pages" that you can set to display up to 6 data items.

I did hold the 2500 RPM for about a minute and the short and long term bank 1 fuel trim values did not change from zero which makes me think the PCM is not reading or reporting them or not making any changes in them regardless of the sensor inputs. There was an already set up gage page on the UG to report oxygen sensor voltages, and although it was "factory set" for both banks, only bank 1 voltages are shown and both bank 1 O2 sensors were the same 1.000 and did not change with the engine running (I was scrolling through the pages at the time to get to page 7 that I had set for the values I wanted to track.)

From the large black spot on the grass under the tail pipe I have to assume a rich fuel condition that might account for the high idle, but there is also surging that is not accounted for in the zero fuel trims and unchanging O2 levels, and then there is still the dropping out of the tachometer needle on a random but often basis.

 

It sounds like there is maybe a problem in the oxygen sensor circuit? The results look like they show an issue that needs to be sorted.

As long as the UG is working correctly (I am sure it is).. then, it is either the O2 sensor side, or the PCM side has something wrong with it seeing the results, right?
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Maybe start at the O2 sensors themselves, and work along the length of their wires to the PCM looking for a short would be appropriate. There should be a 12 VOLT heater element supply, a ground, a conductor going into the O2 sensors from the PCM, and a signal voltage going out of the sensors to the PCM.

I know a short to ground of the oxygen sensors signal wire would be likely to cause a signal VOLTAGE = '0' VOLTS. I am not sure but, a short to VOLTAGE say with the heater element circuit to the O2 signal wire.. something like this might cause an oxygen sensor signal VOLTAGE to be stuck at '1' VOLT, but I am not sure?

I know a short in the O2 sensors circuitry can cause issues sometimes with the CkPS to act like it is acting funny, and also somehow it can cause issues through the same to affect the fuel gauge, and sometimes even the VOLTAGE gauge, poor abnormal running conditions, no-start conditions, etc...

What is the actual VOLTAGE at the sensors compared to the UG results?

 

Update:
The 2nd replacement PCM was due today but has not come yet. I did however receive my order of new Champion RC12YC spark plugs so I installed them. I had cleaned the Autolite 3924 Plugs that were in it just before installing the eBay replacement PCM, and after running at "idle" on that PCM for a dozen or more tries at solving the 2000 RPM idle, the plugs came out all black and sooty from both banks. They were definitely running very rich where previously before the original PCM failed they were a nice normal tan/gray color when I pulled them for cleaning. Fingers crossed that another PCM will be programmed for a V8 and be able to control the fueling correctly.





BTW, the Champions came gaped to .030" and needed to be re-gaped to .035". After changing plugs I removed the bad PCM for return and won't foul the new Champions with it.

 

Those plugs look fouled to me.

Is the side labeled as RIGHT; are they the plugs from out of bank #1 (the side with 1.00 VOLTS oxygen sensor readings per the UG..)?

 

I didn't remember which side was bank 1 so I labeled them by side and front to back.
All the plugs were equally sooty from both sides, IE from both banks.
The PCM was not changing the fuel trims and either not reading or not using the O2 sensor info.
I am sure it should have set codes running that rich if it was working.
It is on its way back to the seller now, and my "new" one has been delayed until the 12th.
Maybe the USPS is letting packages sit for the estimated time the virus can live on cardboard before it moves them on.
The original promised date was yesterday; I hope this one is actually for a V8 and in working condition.

 

To wrap this thread up, the problem was the PCM and not any of the sensors. After all the investigations and testings, the original PCM died (probably due to one or more capacitor failures), the first PCM I bought on eBay was defective or not programmed for my Jeep even though it had the same part number, and the 2nd PCM from eBay with the same part number and the same T0526 number is working just fine with a 1000 RPM initial cold idle and clean exhaust, and an eventual 750 RPM warm idle. The fuel gage is still intermittent, but the check gages light stays off, the tachometer needle no longer dips, and the engine no longer surges or runs rich.

One thing I did learn when inspecting the O2 sensor wiring is that my Jeep has only two O2 sensors and they are on the single exhaust pipe on the right side and down stream of where the two bank's down pipes come together. I don't know if this is normal or someone's previous change, but the working PCM has only bank 1 data for O2 sensor and fuel trim readings which apparently affects fueling on both banks equally. In any case it is back to its previous normal now with the advantage of a nice clean throttle body, fresh Champion spark plugs, and the vacuum connection to the cruise control restored.

I still don't know what the vacuum device under the battery is nor where the capped off T in the cruise control vacuum line that the manual says is for the climate control is supposed to connect, but for now I am just happy to have a starting/driving Grand Cherokee Limited again..