What is the effect of diagnostics mode on the pcm

[Cameron9595]

New to this computer controlled car stuff and looks like I need to learn quicker then I thought because my car is not running.
I have a vr 5litre commodore auto sedan has shift kit (not sure what it is) but changes real hard.
The car will run in diagnostics mode up to around 2500rpm and then starts to miss, out of diagnostics mode car will only idle really rough with the accelerator cracked. While it is idling with the accelerator cracked rpm jumps up and down about 400-500 rpm (surging hunting) and will not increase rpm no matter how much I put my foot down. Hooked it up to tuner pro all sensors seem to be putting out signals even the oxygen sensor has cross counts. What I have checked is timing is 10degrees in diagnostics mode, have checked that the harmonic balancer hasn't spun, changed ignition module, new plugs leads and fuel filter (fuel filter needed changing).
I believe whatever the problem is bypassed in diagnostics mode eg- computer doesn't adjust timing in mode.
So is there anything else that you can think off that the computer bypasses in diagnostics mode, also has no fault codes showing. I did record it running with tuner pro 1 hot and 1 cold if anyone would like to look at it (confuses the shit out of me)

[Dylan]

Post the logs maybe someone can see something post the adx as well.
What tune is it running? Any engine mods?

[Gareth]

Logs may help see the issue, but the problem sounds more basic than ecu related.

What is the fuel pressure?
If the filter was blocked the surely the injectors will be poor also?
What condition is the spark in?

PS - in diag. Mode the fans come on, the MIL is on and timing is fixed at the base setting. Im sure i've left something out...

[Cameron9595]

Will post it ASAP not on the computer at moment.
I also understand that fuel pressure might be a issue but I drove it home from where I broke down in diagnostics mode gentle throttle (max 2500rpm) but if fuel pressure was an issue it should be able to do the same Rpm out of diagnostic mode. Whatever is the issue is bypassed in diagnostics mode.

[Cameron9595]

these are the files I recorded. one is when the car is cols. than I warmed it up in diagnostic mode and recorded again.

[Dylan]

The issues described are a sign of running lean. Also to much timing can also show these signs especially on a leaner mixture. In diagnostic mode the timing stays at 10° untill 3000rpm from memory.
I'll have a look at those logs tonight.

[Holden202T]

yeah that's right, makes me think there is some sort of tune issue or a timing issue if it runs in diag mode...

what I'd try and do, put it in diag mode, get the timing light just to confirm your seeing 10 degrees ... then remove the diag jumper and see what the timing does.....

looking at your hot log its around 22 degrees advance when your holding it with throttle cracked and should maybe get to almost 30 degrees at proper idle .... it also looks like its maybe not got enough butterfly opening at idle, as soon as you let the throttle off it jumps up IAC steps a fair bit just before stalling....

it does look like the idle is maybe a bit rich, but its hard to know because it never really gets to where it should be idling.

try screwing the butterfly stop in a few turns, to get it to idle without the accelerator, then slowly back it off and see how low you can get the idle with it running nicely.... you should be able to get it to idle at 750ish with about 10-20 iac steps at idle, I have a feeling how it is now its going to be a lot more than that.

but confirm the timing and iac step will be a good start.

[Cameron9595]

The idle is at 10 deg in diagnostic mode. Engine idles fine and smooth and runs fine to around the 2500 rpm mark in diagnostic mode. I have only had the car for about 4 months so not sure of condition of all parts and sensors and would assume to be reasonable as no fault codes are being logged. To check pcm logged codes we induced one by cranking over with cas disconnected, fault logged so we cleared it. The car was running well when it cut out, thought I had run out of fuel. That was not the issue and only way I could drive car home was in diagnostic mode.

The reason I ask what other things does diagnostic mode override is because eg: diagnostic mode overrides timing advance. So I am thinking a sensor or setting or something that pcm does not pay attention to or use in diagnostic mode.
So if car runs sooth In diagnostic 750 to 2500 rpm I would assume:
fuel pressure is enough to that rpm, (out of diagnostic mode O2 shows cross counts)
idle is stable, so tb adjustment is ok I would believe.
Spark is good, and any advance by pcm would let the spark jump with even less pressure in cylinder at idle,
So I believe the issue or main issue to be something that diagnostic mode over rides, may be even in the pcm itself or chip.

Is it possible through tunerpro to enter into diagnostic mode?

All help is appreciated.

[Gareth]

fuel pressure is enough to that rpm, (out of diagnostic mode O2 shows cross counts)

the fault you are describing doesn't add up, how can you be sure that the engine is getting to correct volume of fuel via cross counts? surely when it hits the 2500rpm wall this would not be the case...

A wide band would tell the truth.

It is possible that the PCM and/or calibration file has gone bad, but would be the first to fail in this fashion I've ever heard of

[Cameron9595]

Hi and thanks for reply biggvl, as I said I am new to ecu computer control. I would assume as the oxy ge sensor does not actually control the car but only reports back to the pcm what is happening that every time it shows a cross count the mix has gone past stich, rich, lean, etc... If was a constent lean or rich the sensor would show a constant voltage above or below stich. And the condition would still be present in diagnostic mode but engine idles and runs smooth to 2500rpm in diagnostic mode.