P38 Diesel occasional cold idle 'missfire' (with Nanacom FIP info)

PCPics · Feb 19, 2016

Hi all - my first post - though have fixed a myriad of problems on my P38 diesel auto (1998, 190k) from the collective knowledge in this forum - particularly Wammers and Datatech - over the past three years.

Here is the problem - occasional cold idle 'missfire' with associated white smoke. The problem does not occur immediately after a cold start, but appears after a short drive (a mile or two) and pause at junction... RPM steady at 750, then a single occasional miss (roughly every five to eight seconds)... Once all is warm, no problems.

The usual culprits seen to: leak off hoses, MAF hose, new filter, new glow-plugs, no bubbles in the hose, new tank pump.

This morning, looking at the fuel pump info on a Nanaocom Evo (the little touchscreen one) the immediate unusual reading (before starting cold engine) was the fuel temp and water temp show -15 and -13C respectively when the overnight temp has been at or close to 0. Air temp and hevac temp both indicated about the same (3C) and my accurate photographic thermometer the same... Can both fuel and water temps be that wrong?

On start up initial revs are just over 1000 and slowly drop to 750, engine running smoothly, water temp comes up quickly, fuel temp far more slowly. 'Missfire' occurs when water temp is partially warm (say in the 35-45C) zone and the fuel temp still in the -8 to -5C area. Eventually when all is warmed up, water is in the 82-85C zone and fuel at about 27C....

Other fuel pump info which may be 'out' is fuel quantity at about 9-10 (on warm 750 idle with hevac on). Timing looks good at 52.

So - I would like to ask any of you with Nanocoms if you also see unusually low temperatures on the fuel and water temps before the morning cold start - and should I be looking at replacing the relevant sensors?

The only reference to the v.low fuel temp I can find was a guy who saw about a 15C lower reading on the fuel temp (with Nanacom) compared to another diagnostic system (can't recall details) which would make the fuel temp read actually about right?

Is there a specified correct setting for the fuel quantity setting. Seems from forums that about 5.5 is what to aim for?

Would any of these be causing my problem?

Should I just live with it?

N2O · Feb 19, 2016

Occasional missfire is probably duff injector.

Grrrrrr · Feb 19, 2016

PCPics said:
Hi all - my first post - though have fixed a myriad of problems on my P38 diesel auto (1998, 190k) from the collective knowledge in this forum - particularly Wammers and Datatech - over the past three years.

Here is the problem - occasional cold idle 'missfire' with associated white smoke. The problem does not occur immediately after a cold start, but appears after a short drive (a mile or two) and pause at junction... RPM steady at 750, then a single occasional miss (roughly every five to eight seconds)... Once all is warm, no problems.

The usual culprits seen to: leak off hoses, MAF hose, new filter, new glow-plugs, no bubbles in the hose, new tank pump.

This morning, looking at the fuel pump info on a Nanaocom Evo (the little touchscreen one) the immediate unusual reading (before starting cold engine) was the fuel temp and water temp show -15 and -13C respectively when the overnight temp has been at or close to 0. Air temp and hevac temp both indicated about the same (3C) and my accurate photographic thermometer the same... Can both fuel and water temps be that wrong?

On start up initial revs are just over 1000 and slowly drop to 750, engine running smoothly, water temp comes up quickly, fuel temp far more slowly. 'Missfire' occurs when water temp is partially warm (say in the 35-45C) zone and the fuel temp still in the -8 to -5C area. Eventually when all is warmed up, water is in the 82-85C zone and fuel at about 27C....

Other fuel pump info which may be 'out' is fuel quantity at about 9-10 (on warm 750 idle with hevac on). Timing looks good at 52.

So - I would like to ask any of you with Nanocoms if you also see unusually low temperatures on the fuel and water temps before the morning cold start - and should I be looking at replacing the relevant sensors?

The only reference to the v.low fuel temp I can find was a guy who saw about a 15C lower reading on the fuel temp (with Nanacom) compared to another diagnostic system (can't recall details) which would make the fuel temp read actually about right?

Is there a specified correct setting for the fuel quantity setting. Seems from forums that about 5.5 is what to aim for?

Would any of these be causing my problem?

Should I just live with it?

Can you take a trace with the Nanocom for a few minutes for each of the 3 fuel menus? Needs to be warm and just idling. Unfortunately mine is the older diesel and I know some of the newer ones have some weird default values (might be air temperature?) but most should be similar.

PCPics · Feb 19, 2016

Grrrrrr said:
Can you take a trace with the Nanocom for a few minutes for each of the 3 fuel menus? Needs to be warm and just idling. Unfortunately mine is the older diesel and I know some of the newer ones have some weird default values (might be air temperature?) but most should be similar.

I will try and do a trace over the weekend (have not tried that before and use a Mac so who knows what will happen!). Mine is the older non-egr engine. What specifically are you looking for on the trace?

N2O · Feb 19, 2016

Those older ones are much better, no egr, airflow meter and much more responsive engine with lower fuel consumption.
Air temperature is weird value on both versions.

PCPics · Feb 19, 2016

N2O said:
Those older ones are much better, no egr, airflow meter and much more responsive engine with lower fuel consumption.
Air temperature is weird value on both versions.

Thanks for your comments N20 - however the air temp readings all look to be good - it's the fuel and water temps that seem way too low on initial start up. Your earlier comment re: injector - would that not cause a miss all the time?

...and as for the responsiveness - I've always found the initial pull-off from standstill a little weak! Could that be improved with fuel pump quantity setting being adjusted?

N2O · Feb 19, 2016

Yes, it can be better if set correctly, it should run even if you set it around 4mg/stroke(if pump top is in good shape).
Currently i'm running with 2,7mg/stroke on one of mine, but it's a bit overkill, as revs are dropping slowly below 1200 rpm. It should be ok with 4-5mg/stroke hot engine with no electrical load.
What's your trip computer fuel consumption value? It will read low mpg if pump fuel quantinity is out of adjustment.

PCPics · Feb 19, 2016

N2O said:
Yes, it can be better if set correctly, it should run even if you set it around 4mg/stroke(if pump top is in good shape).
Currently i'm running with 2,7mg/stroke on one of mine, but it's a bit overkill, as revs are dropping slowly below 1200 rpm. It should be ok with 4-5mg/stroke hot engine with no electrical load.
What's your trip computer fuel consumption value? It will read low mpg if pump fuel quantinity is out of adjustment.

Mpg reading is only a little lower than brim to brim calculation like 22 indicated and 24-25 actual, but an improvement in initial pull off would be welcome. Is there a specific fuel qty setting? All comments I've seen say 'about this' or 'this should work'! Most point to the range 4-5.5 though and the VWs that use a VE pump say 3-6... and bringing us back on topic... is my current setting of 10 likely to be the cause of the 'miss' rather than the low fuel and water temps?

wammers · Feb 19, 2016

PCPics said:
Mpg reading is only a little lower than brim to brim calculation like 22 indicated and 24-25 actual, but an improvement in initial pull off would be welcome. Is there a specific fuel qty setting? All comments I've seen say 'about this' or 'this should work'! Most point to the range 4-5.5 though and the VWs that use a VE pump say 3-6... and bringing us back on topic... is my current setting of 10 likely to be the cause of the 'miss' rather than the low fuel and water temps?

To alter fuel quantities other than idle the pump would have to be taken to a specialist and put on a calibration machine. Miss will be down to an injector.

PCPics · Feb 19, 2016

wammers said:
To alter fuel quantities other than idle the pump would have to be taken to a specialist and put on a calibration machine. Miss will be down to an injector.

Thanks Wammers - re: miss, I have a spare head with all of the injectors in it - will fiddle!

Re: fuel quantity (at idle) - would changing it from 10mg/stroke down to say 5mg/str on pump top make any difference to the initial pull off? It's an auto and has always felt pretty lifeless as it pulls off the line - though quite acceptable once the revs come up. Inaccurate mpg readout doesn't bother me (seems they all do it to a degree) - so not thinking it needs a pro recalibration.

Finally - do I need to be concerned about the v.low fuel and water temp readings on cold start?

N2O · Feb 20, 2016

Altering fuel quantinity wil do big difference with off line torque, but you do in at your own risk - you may end in runaway engine if you tap pump top too hard..
Proper remap gives best results, and does not eat gearbox like plug in chips/boxes do.

PCPics · Feb 20, 2016

N2O said:
Altering fuel quantinity wil do big difference with off line torque, but you do in at your own risk - you may end in runaway engine if you tap pump top too hard..
Proper remap gives best results, and does not eat gearbox like plug in chips/boxes do.

Warnings noted N2O! Will look at it next week (waiting for warmer weather!)

wammers · Feb 20, 2016

N2O said:
Altering fuel quantinity wil do big difference with off line torque, but you do in at your own risk - you may end in runaway engine if you tap pump top too hard..
Proper remap gives best results, and does not eat gearbox like plug in chips/boxes do.

Only thing you can do to alter fuel quantity at idle (Throttle pot below 9%) is change the idle setting to raise or lower the idle speed. Pump injects what the engine needs to maintain 750 RPM subject to load. Less fuel lower RPM, more fuel higher RPM. Only other way of altering fuel quantity with idle switch disengaged (Above 9% on throttle pot) is to chip the ECU. Moving the pump top manually on the car is not the way to adjust fuel. The pump top however can certainly be in the wrong place if someone has played with it. The only way to really set this up properly is to have the pump calibrated off the car on a very expensive calibration machine.

PCPics · Feb 22, 2016

Grrrrrr said:
Can you take a trace with the Nanocom for a few minutes for each of the 3 fuel menus? Needs to be warm and just idling. Unfortunately mine is the older diesel and I know some of the newer ones have some weird default values (might be air temperature?) but most should be similar.

Grrrrr - here are the traces done, about two minutes each after a 20 minute drive, all nice and warm, vehichle in Park, no throttle application - with no misfire (that occurs only when partially warmed). 4 files: Fuel with and without AC, Swtiches and General (both with AC). Don't know if any of this reveals much as the idle felt really smooth! Fuel quantity definitely looks higher than the recommended 5-ish from N2O (see earlier posts) which I hope indicates the poor intial off-the-line grunt?

Missfire - awaiting injector socket to swap injector/s out of spare head.

Link to Nanocom traces on Dropbox:
https://www.dropbox.com/sh/2mkiq0635owlv99/AAB3I70lpGzsed3J91O7gKywa?dl=0

wammers · Feb 22, 2016

PCPics said:
Grrrrr - here are the traces done, about two minutes each after a 20 minute drive, all nice and warm, vehichle in Park, no throttle application - with no misfire (that occurs only when partially warmed). 4 files: Fuel with and without AC, Swtiches and General (both with AC). Don't know if any of this reveals much as the idle felt really smooth! Fuel quantity definitely looks higher than the recommended 5-ish from N2O (see earlier posts) which I hope indicates the poor intial off-the-line grunt?

Missfire - awaiting injector socket to swap injector/s out of spare head.

Link to Nanocom traces on Dropbox:
https://www.dropbox.com/sh/2mkiq0635owlv99/AAB3I70lpGzsed3J91O7gKywa?dl=0

Fuel quantity is whatever the engine needs to maintain idle RPM. Drop it to 5 even if you could do and the engine will stop.

PCPics · Feb 22, 2016

Wammers - thanks for your input and I hear what you are saying about the pump adjusting it'self to supply the correct fuel for the 750 idle speed. The reason I am curious about adjusting the Fuel Quantity at idle is partly led by N2O's observation that a lower reading on the Nanaocom (giving more fuel) will improve the initial off the line pull along with the info in this video (its a 4-cyl VW, but pump is a VE-EDC) where it's hooked up to diagnostics for the adjustment and is done in a very gentle and careful manner.

Starting at 6m40 in this video:

I'm not trying to make it into a Ferrari - but if an adjustment like this can help the pull-off, I think it would be worth doing? Why do you think dropping it to 5 would cause the engine to stop? N2O is saying he has one set at 2.9 and though he admits it's too much, it's running ok apart from the slow return to idle. Naturally I'm not looking to go that far!

wammers · Feb 22, 2016

PCPics said:
Wammers - thanks for your input and I hear what you are saying about the pump adjusting it'self to supply the correct fuel for the 750 idle speed. The reason I am curious about adjusting the Fuel Quantity at idle is partly led by N2O's observation that a lower reading on the Nanaocom (giving more fuel) will improve the initial off the line pull along with the info in this video (its a 4-cyl VW, but pump is a VE-EDC) where it's hooked up to diagnostics for the adjustment and is done in a very gentle and careful manner.

Starting at 6m40 in this video:

I'm not trying to make it into a Ferrari - but if an adjustment like this can help the pull-off, I think it would be worth doing? Why do you think dropping it to 5 would cause the engine to stop? N2O is saying he has one set at 2.9 and though he admits it's too much, it's running ok apart from the slow return to idle. Naturally I'm not looking to go that far!

What is the idle setting on the pump settings screen? Has the mid section ever been off?

PCPics · Feb 22, 2016

Pump idle setting is at 128 (as is standard I beleive)
Pump has not been opened since I've owned it and the tamper-proof bolt is still in place and un-damaged. I have got the correct tool and a gasket/seal set if it leaks and am aware of the alignment of the rod to collar when refitting (if I need to change seal)... and of course pre-marking everything before starting!

wammers · Feb 22, 2016

PCPics said:
Pump idle setting is at 128 (as is standard I beleive)
Pump has not been opened since I've owned it and the tamper-proof bolt is still in place and un-damaged. I have got the correct tool and a gasket/seal set if it leaks and am aware of the alignment of the rod to collar when refitting (if I need to change seal)... and of course pre-marking everything before starting!

Ok carry on have a play but don't moan if things go tits up. These pumps can only be set properly on a very expensive calibration machine.

PCPics · Feb 22, 2016

wammers said:
Ok carry on have a play but don't moan if things go tits up. These pumps can only be set properly on a very expensive calibration machine.

Your honesty is appreciated Wammers! I will be dealing with the 'miss' in the coming days (when the 27mm deep socket for the injectors arrives) and will then look again at the fuel quantitiy readings.

P38 Diesel occasional cold idle 'missfire' (with Nanacom FIP info)

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