high fuel consumption - dyno tune results

[mpwdhc]

hi guys,

some months ago i purchased a P38 on a bit of a whim, and in that time, have grown rather fond of Rupie. we've been on a number of adventures, bought a cheap little boat, and as a result, high fuel consumption has become a bit of an issue.

the computer says 17.5/100, which is fine for petrol. ive never measured actual consumption with petrol, but on lpg it started at about 23L/100, and is now up to 27L/100. it has vapour injection LPG.

i've given it a service and changed the plugs, and today, took it along to a dyno tuner. as a mechanic in a rather rustic workshop, today was utterly fascinating. based on today, the engine is essentially sound and there are no obvious issues with spark, vacuum leaks, etc.

some observations -

there seems to be interference with engine ground, and the wave matches the shape of that of the alternator. it is unclear as to whether this is a red herring or not, but i'm going to try shielding some wiring before next visit. or is this normal?

the mixture is rich, especially at idle. he was quite surprised by just how rich. overal on petroll it pumped out 90kw at about 4000. most others that had been on the dyno weren't too far off, but the best result was 100.

the LPG map was the default and though it runs better on petrol, my only real issue with the way it runs was consumption. the numbers required on the map to get it running reasonably are way, way lower than should be reflected in what comes out of the exhaust. he ended up going back to the default, and tweaked it a little - and wasn't very happy about it.

it is an april 96 car, and we were both surprised to see no sign of o2 sensor. i have seen mention of australian cars not being fitted with them - how, then, is the mixture adjusted? not something simple like on an old SD1 - screw on MAF?

any thoughts would be much appreciated!

thank you
davide

[bee utey]

Gday davide,

There are plenty of 4WDs and commercials sold in Australia before about 2004 without oxygen sensors, so the P38 is in no way unusual. The 1997-on P38 had sensors BTW. When I fit vapour injection to vehicles without OEM oxygen sensors I often fit one which can be seen on screen for LPG tuning. There is no automatic mixture adjustment possible.

As for the idle running rich, I suggest you remove the vacuum line from the LPG converter to the engine while it is running on gas or just stopped. There may be LPG leaking internally from the converter and this will appear from the vacuum line. Impco BRC "genius" converters are prone to this leakage and kits for them are under $100 trade.

[mpwdhc]

hey Utey,

thanks for the reply... interesting.

will check vacuum line tomorrow. the install was in 2009, so i suppose the converter is getting on.

the car has zavoli LPG gear. the theory is that as it determines what to do based on petrol, the petrol side needs to be right - but its rich overall and particularly at idle.

so on one of these, if no o2 sensor, where does one adjust the petrol idle?

as mentioned earlier, its unclear if this interference to the engine ground is of relevance - too much information perhaps; a red herring?

where's your workshop?

[mpwdhc]

couldn't resist, went outside to have a quick look - no apparent leakage from the converter, but if i remove (from coverter end) and plug the line, the idle drops a little

[bee utey]

couldn't resist, went outside to have a quick look - no apparent leakage from the converter, but if i remove (from converter end) and plug the line, the idle drops a little
The idle speed is probably dropping because of the rise in LPG pressure caused by removing the vac signal from the converter and making it run even richer.

I'll send you a private message with my contact details.

[benji]

The idle speed on petrol is done by first matching the voltage on the tps, then setting up the idle so that at 15 steps on the idle steper motor is around 750rpm. This needs to be done via diagnostics.

I'm pretty sure you can alter the fuel correction by diagnostics too - I've never done it though.

Sent from my SM-G900I using AULRO mobile app

[Graeme]

there seems to be interference with engine ground, and the wave matches the shape of that of the alternator. it is unclear as to whether this is a red herring or not, but i'm going to try shielding some wiring before next visit. or is this normal?In case its of any use, my 85 RRC had a very small voltage difference (around 0.5V) between the alternator case and the block on light electrical load which was traced to the alternator's belt tension stay's engine mount bolt not sufficiently tight. I was looking for the cause of fluctuating voltages particularly noticeable at night when stopped with blinkers on which was resolved by properly tightening the bolt.