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Discussion Starter #1
Hi all,

OK truck is a 95 4.0 SE on multipoint lpg. In the last couple of days it has developed a really nasty backfire when you plant your foot. It takes off ok and will potter round at 30 and under light throttle, but anything over about half throttle there is all sorts of pops and banks from the cat/centre box area of the exhaust. No codes other than amfr at max correction and I know there is an issue on bank 2 as it's overfuelling and no.6 plug has been oil fowled, but this problem has been with the truck since I purchased it. Idle is rough when cold and it stalls repeatedly when in reverse.

Problem is there on both gas and petrol - though probably worse on gas.

I don't have the facilities to perform a pressure test but I do havea nano-evo which shows both lambdas switching properly.

Having searched through the forum I'm thinking ignition or low compression causing miss/no fire and fuel burning when it hits the cat instead - Anyone got any better ideas?

Dave.
 

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LEGACY VENDOR
1995-2002 Range Rover P38A
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Is the MAF reading within spec? Should be about 25kg/hr at idle.

If its reading the wrong airflow value, then it will be infecting wrong about of fuel, and will cause issues when the fuel trims max out.

I had issues like that on my 2001 when I first bought it and a new MAF sensor got it running better again. I bought aftermarket, and it reads a bit high, so got a Bosch one to put in to see if that sorts it out.

If you're getting oil in cylinder 6 then that could be causing issues on that cylinder - especially if plug is getting fouled..

Someone else with more experience in engines than me will probably have a load of other things to check!

Marty

Sent from my S4
 

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Discussion Starter #3
Hi Marty,

Thanks for the quick reply - I gave the MAF a good clean out last week so that could be connected, it's a second hand genuine having tried new pattern and it being rubbish. I'll check out readings later, getting 13.5mpg around town though so it can't be that far out.

There is defo something up with bank 2 - I just don't have the time to pull the head at the moment so I'm hoping this isn't connected.

Dave.
 

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Discussion Starter #4
Update with some data -

From cold start:

Air flow: 25.19
Intake temp: 3
Air flow sesnsor: 0.17

Bank1
LTFT -123.75
STFT 6.25

Bank2
LTFT -123.75
STFT 11.25

Almost stalled as it fired up and had a slighlty eratict idle between 500/700 then settled to an almost smooth 700. Air flow dropped to 18 over 5 mins at idle, disconnecting intake air temp give a reading of -61, reconnected back to 3. Disconnecting maf stalled engine.

I have no idea what the fuel trim figures mean but intake air temp looks suspicious given todays ambient is up in the teens. Reading was 4 degrees with engine off and 3 with engine running.

Dave.
 

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On the Bosch systems, the Air Intake Temp is actually done in the MAF aswell.

The long term fuel trims look as if they are maxed out - which is possibly causing the rough running - I find mine gets a bit lumpy with trims all the way maxed...

I also think, that (depending on the system you have) the LPG ECU 'fights' the petrol ECU in adaptations, which over a period of time will drive the fuel trims to their max. I have not exact data relating to it, so it's just a theory..

But on my sequential system, it has a feed off one of the Lambda sensors, so it will adapt itself whilst running to keep it running as efficiently as possible. Whilst this is fine, as it trims itself - the petrol ECU also still runs with the feedback from the lambda sensors, and it also 'adapts' it's fuelling (which is then mimiced by the LPG ECU for the petrol injector pulses, and passed onto the LPG injectors, the fuel burns, the system reads the sensors and adapts again...

I personally think that the fact both systems adapt/self trim whilst running - causes the petrol ECU (which stores it's adaptations - I think my LPG ECU just does it on the fly from it's default map which it's been tuned to - but starts from that default map every time) to - over time - drive the fuel trims all the way in one direction or another.

This is also hampered if one of the sensors which the ECU adapts anyway (like the MAF) goes out of range, because then it's compensating for that aswell, and when the fuel trims get to their max, and it can't adjust anymore - it starts running like a sack of bolts.

If your MAF if reading 18kg/hr at idle (even after 5 mins) then that is too low, so what the petrol ECU will do is put less fuel in, because it thinks that there is less air than there is - which will then burn lean, and cause rough running.

The Lambda sensors will detect that it is running lean - and the petrol ECU will try and compensate. But if the fuel trims are at the Max already, then it can't compensate anymore - and it will just keep running badly (or stall)

The Bosch MAF sensors don't seem to respond to cleaning as well as the GEMS ones do, so I think it would be worth buying a brand new Bosch MAF, and then get the adaptive values reset (should reset the fuel trims and mean that the ECU will learn the 'new' values quicker). Hopefully this will cure the issues.

What further complicates things a bit is that the IAT reading is also used in the ECU's calculations on how much fuel needs to be injected - so this value being off probably isn't helping things either!

What happened when you used a new pattern MAF? what make was it?

I use a Bearmach one (about 1/2 the price of Bosch - couldn't afford a Bosch one at the time). and it runs fine with it - the idle does get a little bit off at times - and I think I have the opposite issue to you, where my MAF reads high - so the ECU injects more fuel than it needs, and I think my fuel trims max out the opposite direction to yours.

However, it's gotten me through about 18 months - and I now have a new Bosch one sitting at home to fit when I get back from work tomorrow... I'm interested to see what the MAF values are when it's fitted!! (Island 4x4 is the cheapest place I've found a genuine Bosch MAF - £100 including VAT.. and because it's over £50 ex VAT, you get free next day shipping aswell!! No affiliation to them, other than a happy repeat customer! They have a disclaimer on their website saying 'not for use with LPG' due to backfire problems - but with a multi-point sequential system, you shouldn't get a backfire in the air box, as the gas is injected in small quantities directly into the manifold, rather than the main air intake via a gas ring, like the single point systems).

Hope this helps...

Marty
 

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Discussion Starter #6
It's a genuine (second hand) Lucas maf. I did buy a new pattern (no makers mark) which was fine but there was a stutter when starting from stationary which was cured with the Lucas part.

Dave.
 

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These ECUs are very sensitive to MAF voltage changes: A few 1/10ths of a volt output variation can affect the fuelling considerably: Some folks seem to do OK by cleaning them, some don't... Still worth checking the actual output voltage levels though, eg:

http://www.rangerovers.net/forum/7-range-rover-mark-ii-p38/41999-info-mystery-bosch-maf.html

For some reason this MAF output sensitivity is worse on some LPG rigs, too....particularly if there are other problems present.

The bad/wrong IAT figure/output could cause this problem (depending); Investigate further for sure....

and if the Bank 2 Hegos really are OK as per your nano-evo tests then it tends to point at a HT issue (?)
 

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Sorry - for some reason I thought you had a later model, but re-read the whole thing now and see you have GEMS... So ignore all the cr*p I wrote about Bosch ones!

The theory is still the same as dar as the fuel trims go though, and the idle reading does still sounds a bit too low...

The IAT sensor could be dirty (or faulty) - think its mounted in the air box on a GEMS (been awhile since I looked at one of them!) So maybe a clean of the sensor and connections may help with that part.

The GEMS MAF sensors seem a lot less prone to failure than that BOSCH ones, and from experiences of other posts that I've read, seem to respond to cleaning a lot better than the Bosch ones too.

Sorry if I put you wrong in the earlier post!

Marty

Edit** Probably what put me wrong was the mentioning about the O2 sensors changing voltage a lot... From memory (once again been awhile since i looked at one) the GEMS sensors should output a fairly linear voltage and not jump around too much..

Rave has a good overview of how it all works in the 'sequential multi port injection' section if I remember rightly..

Probably best to look it up in RAVE as my memory is shot from long work days and delayed flight back home!

Sent from my S4
 

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... and then I was led astray, too, Marty !.... oh well... should have picked it up by the Lucas MAF comment for sure (and yes the IAT is on the airbox on the GEMS).

Incidentally highmiler if the IAT is out the fuelling will be out, too, of course - and the trims (but that still does not explain the Bank 2 differences; STFTs are based mainly on the Hego outputs..); What DTCs are showing up ?
 

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I think that injection/loop info Marty is referring to (or at least a summary of it) is as below: Again, not sure how seriously/literally to take those readings c/o your nano-evo (#4) but with trims/temperatures as indicated the ECU would have reverted to open loop (and in particular those highly-negative LTFTs show the ECU is/was trying to lean out the fuelling....) ?
 

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Discussion Starter #11
Thanks guys - evo reports both banks running closed loop, but I know there is a problem on bank2 due to fowled plug and a suspicious exhaust/gasket leak sound. However I have had those problems for a few months and this problem has occured recently, so am hoping this is seperate to that.

I seem to remember reading that non testbook diagnostic machines struggle to interpret fuel trims properly? So LTFT readings may be a red herring.

The Gems lambdas follow a slow switch between approx 0v & 5v - both have done this from new last year and I think this is as should be.

IAT and MAF are first call though at £800+ for genuine Lucas I think I'll go salvage!
 

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..OK, and now assuming closed loop:

Most non-testbook diagnostics kit should surely give better results than that for trims (?); You should basically get a reasonably-accurate (digital) 'snapshot' of the STFTs -perhaps you are doing- but also a good idea of the LTFTs ... minus 124% is bizarre.. that's quite a big red herring !! [I prefer the ELM327/OBD methods - as then at least you can produce some decent graphs (over time) to interpret....] Assuming your STFTs are showing correctly I wonder what the real LTFTs are on your rig then ? - as I did not know this is possibly such a weak point with nano-evo/GEMS...

Yes, plug issue pot 6 and maybe an exhaust (air) leak could certainly account for the Bank 2 discrepancies .... so perhaps the leak became much worse (suddenly) - or not ! But still not sure what those (also meaningless) IAT nano-evo readings of yours are though !.... what exactly does it measure/show accurately then ?!

Worth checking IAT temp/resistance characteristics first - although if it is (way) out-of-spec it defaults to 50C on GEMS; Again, are there any DTCs indicated ?

- The GEMS MAFs are pretty resilient in fact - could be that (do you still have the old one ?) but the IATs are (fairly) cheap if that helps !

As you say at 13.5mpg it can't be much wrong.... maybe ECT (or other temp. sensor ? ) - but your stalling etc makes me think IACV/throttle (what is your nano-evo showing, step-wise ?) However the backfiring/overfuelling problem you describe usually means something more fundamental....

EDIT: Just noticed your other posts, and even worse LTFTs; What were those figures showing up on (if pre-nano/evo) ?

http://forums.lr4x4.com/index.php?showtopic=78012

...and do you recall any DTCs from way back then, too !?

Incidenatally the reason I keep asking about DTCs is that if the MAF or CKP is iffy you don't get any codes....
 

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Discussion Starter #14
You'll have to pardon my ignorance but I don't know what DTC stands for?

All posted data is from the same evo - LTFTs have always been in the region given. Truck drives ok but as it's the only one I've driven I have nothing to compare it to.

I'll pull the IAT sensor and see how it is behaving when I get a chance. What would be the most useful data set to look at in terms of diagnosis?

Thanks for all your help so far -to date I've dealt with all the usual p38 gremlins, I'm only just beginning to get to grips with the motor!
 

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Discussion Starter #16
OK Thanks for the spoonfeeding - hadn't linked DTC with fault code, duh. Anyway no codes just 'AMFR at maximum positive correction', in fact the only code I've ever seen is a lambda fault.

Still on a steep learning curve ref efi petrol motors having come from oil burners but every problem brings new learning opportunities!

As mentioned above I'll compare my IAT sensor behaviour to rave specs as a first move, source another MAF then delve deeper.

Thanks,

Dave.
 

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Not intended as spoonfeeding per se, just thought you may have seen a bunch of Codes already with your symptoms !....

However on GEMS the MAF generally does not flag up that many Codes; 'AMFR' is an exception, as it will at least give P1178/P1179 if it exceeds certain limits. (Those limits being max. flow rates of +/- 5.5 kg/hour).

- Your problem would for example then have probably shown up as P1178; This definitely points back at your MAF; Check the inputs to it, too (notably reference voltages as per Rave). As you may know if the MAF limits are exceeded the ECU will usually then ignore these readings and use other means (eg. RPM) to 'guess' the Flow, and fuel accordingly. This can give 'less than ideal' running (even though it can still be Closed Loop)

- Note also that there have been a few instances of LPG installations that can detrimentally affect the MAF output (eg. depending on just what the LPG ECU is monitoring and how it is cabled in)
 

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Most Multipoint LPG systems (luckily) don't have a lot of affect on the MAF, as when they are installed, they have their own dedicated MAP sensor (Manifold Absolute Pressure) sensor fitted on the intake manifold, and the LPG ECU uses this to determine fuelling.

LPG gets a somewhat bad rep occasionally for causing all sorts of issues (not trying to argue with you RRfanman! as there are cases, definitely of shoddy LPG installs causing havoc of both single and multi point systems!), but a lot of this bad press comes back to the earlier 'single point' systems which have a gas ring in the air intake, and the LPG is then sucked into the engine with the air.

The downside to single point is that if there is a backfire, as there is gas in the main air intake, it can ignite and blow the intake and airbox off, and damage the MAF. The newer sequential systems have their own injectors and inject the LPG right into the inlet manifold. I haven't ever managed to get my multipoint system to backfire (luckily)!

The only other connection my LPG ECU has to the vehicle (other than the obvious splicing into the injector wires to mimic them to the Petrol ECU whilst running on LPG, and to get the injector timing for the LPG ECU to fire the LPG injectors off) is to one of the O2 sensors so it can do it's own adaptations on the LPG injector timing pulses.

Let us know what the results are with another MAF regarding the airflow rate and the IAT sensor.

Marty
 

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Most Multipoint LPG systems (luckily) don't have a lot of effect on the MAF, as when they are installed, they have their own dedicated MAP sensor (Manifold Absolute Pressure) sensor fitted on the intake manifold, and the LPG ECU uses this to determine fuelling.

LPG gets a somewhat bad rep occasionally for causing all sorts of issues (not trying to argue with you RRfanman! as there are cases, definitely of shoddy LPG installs causing havoc of both single and multi point systems!), but a lot of this bad press comes back to the earlier 'single point' systems which have a gas ring in the air intake, and the LPG is then sucked into the engine with the air.
Not too worried if you are arguing Marty as long as we find resolutions to such problems ! The quality of LPG installations differ quite a bit indeed; Some don't have a MAP though and just modify the fuel injector pulses (dictated by the MAF etc) for gas, instead (?)

Anyway, the OP's problem is both on petrol and gas (and worse on the latter); The question remains what common components are the most likely culprit/s ? eg. Might be the IAT, hence my DTC question.

Incidentally as a nano-evo chap yourself (albeit Motronic) have you noticed such inaccurate LTFTs as reported or is this just a nano-evo-GEMS issue ?
 

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For reference, Rovacom reads the LTFT on GEMS from +160 to -160. But this is just a bit count, and doesn't mean 160%.

As for the LPG-trouble, with a sequential system any running problems that are not directly linked to fuel (i.e. pressure or injectors) will occur on both fuels, because the same sensors are used and most LPG systems use the fuel maps, including trims, as a base. LPG being a more difficult fuel when it comes to mixture and ignition, typically is affected first/most.
O2-sensors are a likely suspect, but the values you recorded seem correct (switching from almost 0 up to 6V). A compression test could yield more information, but also bad news...
 
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