No power and misfire/backfire

Hi

I have a year 2000 P38 RR Vogue.

I have had an ongoing severe lack of power and misfire/backfire (through the intake) problem for some time.

Details...

A cold morning start causes the engine - after a few seconds of normal running - to almost stall and run quite rough. The ECM seems to keep the engine running.

After about 10 seconds or so the engine runs quite smoothly.

Attempting to drive away is difficult until the engine reaches a close to normal operating temp.

There is almost no power. Pressing on the accelerator causes the engine to almost die. Very reminiscent of a lack of vacuum advance in the old distributor based engines.

I have to "putter" along the road at max 40kph for at least 400-500 meters before the RR even begins to improve.

I can eventually drive somewhat normally but without any "passing power" as the engine warms up.

At normal operating temp the RR runs along "ok" but any attempt to accelerate induces the "lack of vacuum advance" problem. If road has a slight incline, the RR tends to slow down and any attempt to maintain normal speed frequently induces the same lack of "vacuum advance" problem.

I noticed the other day that as I (attempt to) accelerate the RR loses its power, but as I release the pressure on the accelerator the revs just then start to increase. This is a little difficult to describe.

I read through some of the RAVE manual today and it seems that the coolant temp sensor could also be influencing the problem.

The coil packs and HT leads were changed approx 12 months ago so I anticipate that these are still ok. The is no visible arcing of the HT leads and no consistent misfire.

The local service centre was unable to diagnose the fault. My RR had been running fine since purchase 18 months ago, so there was until now no need to become overly familiar with its "innards". Last week I purchased the (P38) ECM module for my Rovacom Lite and performed a quick diagnosis. Apart from the ECM logging a few misfires, there was nothing untoward in my first but brief look at the RCL data.

As yet, I still do not know enough about the P38's needs and settings etc so I am here seeking some advice. This question has been posted elsewhere and he general consensus has pointed me to the MAF.

I hope to obtain a little more info if possible on what maf values I should get and/or how the maf values are likely to change. At the moment I have been advised that the maf reading should be approx 19kg at idle and approx 90-100kg at 3000rpm.

The O2 sensors appear to be ok. I intend to try a more "in depth" diagnosis tomorrow (Sunday the 6th Sept).

What other advice can you offer me, thank you in advance for your anticipated problem solving solution

As you have the ECM module, the settings can be set to "Default" in the "Other" menu.
As you probably know, the ecu is "Adaptive" in other words, self tunes to suit the engine condition etc.
The ecu learns the various weaknesses in the system and applies the relative offsets to maintain optimum performance and economy. Whilst there are basic settings to get the engine running, there is no easy way of just putting in figures and all will be well. If your engine is running rough,it is probably down to component weakness/failure rather than the ecu having "Lost the plot". Monitor all sensor feedbacks and if need be, reset the settings to original...bear in mind though that will take a good few miles to re-learn and adjust, it doesn't happen instantly. To repeat, reprogramming will not eliminate or override a faulty component

Hi

Irishrover said:

As you have the ECM module, the settings can be set to "Default" in the "Other" menu.

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That's a good idea, I didn't think of that. Thanks for the suggestion

As you probably know, the ecu is "Adaptive" in other words, self tunes to suit the engine condition etc.
The ecu learns the various weaknesses in the system and applies the relative offsets to maintain optimum performance and economy. Whilst there are basic settings to get the engine running, there is no easy way of just putting in figures and all will be well.

Click to expand...

Yes, I understand.

If your engine is running rough,it is probably down to component weakness/failure

Click to expand...

Yes, that's really what I expect, it's just a little difficult to find the troublesome components.

To repeat, reprogramming will not eliminate or override a faulty component

Click to expand...

That is also understandable, I will be pleased to find the problem component(s), though it might be expensive . I was quoted AU$1000 for a MAF sensor, so that's something I would prefer not to put in just for checking purposes (as the suppliers as expected will not give a refund). I will see if I can find a source of a maf sensor I can swap in and out to see if mine is faulty.

Thank again for the "set to default" tip.

Hi

An update.

I cleaned the maf sensor (with maf cleaner) that made a difference, although it did not really improve the running, just "different".

As IrishRover suggested, I reset the adaptive values. Whoa! THAT made a BIG difference in the way the engine ran! The engine would idle but only just.

After idling for a few minutes I could hear the engine was "adapting" I guess as it began to run better.

I took the RR for a short test drive, approx 5km. It was not too good, especially in the first 600m. By the time I came back there was a marked improvement. At this time I'd like to think it is at least substantially better, however I have not had the engine endure a cold start yet and I imagine that the adaptive process requires much more drive time to adapt itself (at least I hope it does get better).

hi pete have you unpluged the mas senser and taken it for a drive?

This fault does sound very much like the Maf is failing,with a hot 4.6 you need 22-26KG at idle,and around 90-100 at 3000rpm running free.
With your diagnostic gear connected up watch the oxygen sensors in live data whilst someone else drives.If its the Maf failing the Oxy sensors will stay low at 0v when you ask for power.This is because the injector pulse width for the low airflow reading is simply not wide enough to feed the engine properly so the reading is low and the engine stutters.
Have you tried disconnecting the Maf and letting it run in default ?
If you decide you need a new Maf dont be tempted by the cheap ones,go genuine from LR or Bosch.I have a contact in Perth who I seem to remember deals direct with Bosch so it may be worth asking them for a price.PM me if its any help.

Hi

j the range said:

hi pete have you unpluged the mas senser and taken it for a drive?

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The engine would not run without the maf. This may have had something to do with any adaptive setting in the ecu. I have not tried it without the maf since cleaning the maf and resetting the adaptive values.

Hi 8inav

eightinavee said:

This fault does sound very much like the Maf is failing,with a hot 4.6 you need 22-26KG at idle,and around 90-100 at 3000rpm running free.

Click to expand...

Thanks for the info.

With your diagnostic gear connected up watch the oxygen sensors in live data whilst someone else drives.If its the Maf failing the Oxy sensors will stay low at 0v when you ask for power.This is because the injector pulse width for the low airflow reading is simply not wide enough to feed the engine properly so the reading is low and the engine stutters.

Click to expand...

I'll see if I can try that later today.

Have you tried disconnecting the Maf and letting it run in default ?

Click to expand...

Yes it did not idle.

If you decide you need a new Maf dont be tempted by the cheap ones,go genuine from LR or Bosch.I have a contact in Perth who I seem to remember deals direct with Bosch so it may be worth asking them for a price.PM me if its any help.

Click to expand...

Thanks for the advice. I saw the eBay items but based on others' comments I considered them to be false economy. I obtained a price of AU$395 from Repco. I discovered though that this price is for a maf of a different model number. Repco's (Bosch part) number is 0280218010 whereas the maf in my RR is F00C 2G2 029

After driving about 25km, the RR is definitely driving smoother and with more power than before I began the clean and reset. However I do not believe it is as good as in used to be (at least not yet). Perhaps this will improve with some more driving?

FWIW, just after the reset and prior to any "fresh" adapting of values by the ecu, I noticed that as I depressed the accelerator to increase the revs, as it reached 2000rpm, the rpm would drop back to 1750. I continued slowly pressing down (further) on the accelerator and each time the rpm reached 2000, it would drop to 1750. Even though the accelerator was slowly and continually being moved downward (to increase the rpm) it would keep dropping back around 250rpm and did not rise above 2000rpm for some time. When I eventually managed to increase the rpm to 3000rpm it would still drop back 250-300 rpm.

After the brief driving session I don't think this "problem" remains.