4.6lt V8 Fuel issue

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daveincov

New Member
Posts
29
Hi everyone I have a 1998 4.6lt V8 running on LPG which has started to develope a servere thirst when running on petrol. Usually £35.00 worth of fuel last 350-400 during the mildst of winter however lately I'm only getting 160-200 miles.
My initial thought was maybe the petrol injectors werent closing fully when switching over to lpg and thus using both fuels. I also noticed that the average mpg calculation was changing (about 1-2 mpg depending on how i was driving) when running on LPG, as I thought the sensor for the fuel usage was on the petrol injectors, this shouldnt keep changing.
So I hooked up my nanocom last night to see if I could see anything on the engine ecu, but as complete novice not exatly sure what I was looking for, however I did get a fault code P1187 - Oxygen sensor heater circuit inferred
open circuit, upstream sensors which possbily implies I have a lambda sensor issue and needs replacing but doesnt state which.
Looking at RAVE, changing Lambda sensors doesnt seem too difficult but before I go head and spend a few £££, would like your advice.
thank you in advance
David
 
Sensor for fuel usage is not on the injectors as such, but is calculated in the ECU from the injector opening period, so should still read as I understand it when the LPG is in use, but will be inaccurate as the petrol ECU does not know the the injectors are switched off and the car is running on LPG.
All that could be wrong, but that's how I understand it.
 
daveincov said:
Hi everyone I have a 1998 4.6lt V8 running on LPG which has started to develope a servere thirst when running on petrol. Usually £35.00 worth of fuel last 350-400 during the mildst of winter however lately I'm only getting 160-200 miles.
My initial thought was maybe the petrol injectors werent closing fully when switching over to lpg and thus using both fuels. I also noticed that the average mpg calculation was changing (about 1-2 mpg depending on how i was driving) when running on LPG, as I thought the sensor for the fuel usage was on the petrol injectors, this shouldnt keep changing.
So I hooked up my nanocom last night to see if I could see anything on the engine ecu, but as complete novice not exatly sure what I was looking for, however I did get a fault code P1187 - Oxygen sensor heater circuit inferred
open circuit, upstream sensors which possbily implies I have a lambda sensor issue and needs replacing but doesnt state which.
Looking at RAVE, changing Lambda sensors doesnt seem too difficult but before I go head and spend a few £££, would like your advice.
thank you in advance
David
Click to expand...
Your calculations must be wrong that's about 70 mpg?
 
dove said:
Your calculations must be wrong that's about 70 mpg?
Click to expand...

I think they mean that's how much petrol is being used before the switch over to gas, although I could be wrong.

Could be your temp sensor is faulty and not switching over to gas as it should or switch over temp is set too high in the gas ecu.

Regarding the MPG read out, my understanding is that whatever it reads take off the approx 15% extra you will be using on gas (depending on how your gas system is working, could be more not likely to be less) and that should be your mpg on gas, very approximate of course. For example if it reads 20mpg then you should be getting approx 17mpg on gas.
 
Sorry to confuse you, I meant to say petrol is only used to warm the engine up prior to switching over to LPG. This will vary depending on petrol prices and outside temperatures but lately there has been a dramatic increase in petrol consumption which is causing concern.
Thank you Datatek I too was unsure about the black art of how the computer calculates MPG and normally dont bother monitoring it because its useless for LPG, and have to use my brain instead.
I'm guessing if its a faulty Oxygen sensor then its probably running rich hence the increase in consumption, unless of course there is a leak in the fuel line I cant see. The lpg seems to be ok 260 miles on 75-80 litres(always try running your lpg to when it automatically switches back to petrol, surprised me how much further I could go).
 
additional info, tonight I cleared the P1187 fault from the engine ECU using nanocom then monitored the fuel inputs.

Bank 2 showed
Pre-Cat 02 Sensor 5.05
Fuel Trim Long Term -38.75
Fuel Trim Short Term 38.75

Bank 1 showed the same except the Fuel Trim short term was very inconsistent showing from -5 to +20 then after I had pulled up on the drive it eventually climbed to 38.75.
The fault code P1187 also came back, so should I assume the 02 sensor on Bank 2 is faulty? I really don't want to have to replace the pair if I have one working.
Cheers
David
 
Last edited:
On a previous p38 I owned which had lpg I had bad running in the winter. It was so cold that the lpg evaperator wasn't getting sufficiently warm to do its job properly. The cure was to lag the water pipes to it. Might be helpful? If you have lpg software it could be worth noting the temp.
 
Took the plunge and replaced both Lambda sensors, cleared fault code, and reset the adaptive valves. All appears to be working correctly, getting readings from both sensors, and fuel consumption is improved.
 
I would have reset the adaptive values first....

The ECU reads the Lambda information and adjusts fuelling to suit...over time it adjusts the injection timing to kept the Lambda readings within parameters..

If running on LPG, the ECU is measuring the burn data from the Lambda sensors based on the LPG, when it does run on petrol it is using fuel trims set for LPG which is different to Petrol.....

LPG is less efficent than petrol, so more LPG needs to be injected to get equivilant power out of the engine by about 10-15%, so when running on Petrol, the ECU starts injecting more fuel giving reduced MPG as it is still basing the trims on LPG....

This is one of the drawbacks of running multipoint LPG systems on modern engines...the ECU gets used to the LPG burn data from the Lambda, and when it is asked to run on petrol it starts using more fuel or running a bit rough...
 
Its unlikely to be an oxygen sensor issue,far more likely that the enigne is running so far away on its long term trims on LPG,that the poor ecu cannot trim it back far enough to get it correct on petrol.
Get the LPG checked and serviced,then have the long term trims reset.(Known as an adaptive reset) Then you are in a position to get the LPG ecu trimmed to match what the petrol trims do all across the load/rpm range.This is the most common LPG fault I see and is the most ignored,often because people cant be bothered or dont understand how to do it.
 
eightinavee said:
Its unlikely to be an oxygen sensor issue,far more likely that the enigne is running so far away on its long term trims on LPG,that the poor ecu cannot trim it back far enough to get it correct on petrol.
Get the LPG checked and serviced,then have the long term trims reset.(Known as an adaptive reset) Then you are in a position to get the LPG ecu trimmed to match what the petrol trims do all across the load/rpm range.This is the most common LPG fault I see and is the most ignored,often because people cant be bothered or dont understand how to do it.
Click to expand...
Dat is wot i says...:D:D:D:D

(only you put it better then me!)
 
Hey guys thank you for your feedback, I understand what you are saying but it was the sudden drop in economy, plus the fault code and the lack of feed back data from the sensor on bank two which let to my conclusion that must be hardware fault. However why the hell I didn't try to reset adaptive values in the first place, I don't know... we live and learn. On a side note 12 months since last lpg service so time to book it in, but that is a date stamp on the lpg tank of 05/12 which is about six years since the conversion and heard rumour's the tanks do have an expiry date can anyone confirm this? If so I'll think I'll pay a little extra and have a donut tank in the spare wheel well to give me more space in the boot, as 95lr cylinder one takes up half the space.
Thanks again for all you're help always appreciated.
 
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