V8 fine on LPG and poor on petrol

[blansk]

Hi
Defender 90 Anniversary 3.9 V8 petrol with gas conversion
starts perfectly on petrol, revs up as should when cold but when move no power, coughs, inlet manifold pops (sounds lean) but when switch to gas by revving to 1600 rpm then works perfectly all day long on gas
if try to switch to petrol when warmed up same problem, no power, more throttle travel required, definitely sounds lean.
Plugs must be OK. Not sure where to start (full tank of petrol there!). Do I start on fuel supply or could it be the MAF (its the older type of non-sequential gas) I am guessing the MAF isnt used on the gas circuit and the MAF seems to get a few hits on the forum

Its been a gradual problem. I used up the last tank of petrol 1 1/2 months ago no problems, been getting worse for last 6 weeks.


thanks

[SimonHobson]

Defender 90 Anniversary 3.9 V8 petrol with gas conversion
starts perfectly on petrol, revs up as should when cold but when move no power, coughs, inlet manifold pops (sounds lean) but when switch to gas by revving to 1600 rpm then works perfectly all day long on gas
if try to switch to petrol when warmed up same problem, no power, more throttle travel required, definitely sounds lean.
Plugs must be OK. Not sure where to start (full tank of petrol there!). Do I start on fuel supply or could it be the MAF (its the older type of non-sequential gas) I am guessing the MAF isnt used on the gas circuit and the MAF seems to get a few hits on the forum

Its been a gradual problem. I used up the last tank of petrol 1 1/2 months ago no problems, been getting worse for last 6 weeks.
Sounds a bit like mine at the moment :( Does your's use the same MAF sensor as the 3.9 Disco/Range Rover of the early 90's ? My thoughts are that the MAF isn't working properly, so when you use power, the ECU isn't adding the petrol required as it isn't getting a true reading of the airflow, and so the mixture goes lean and you get no power. Luckily I have a spare engine so can 'borrow' the MAF from that (and have a year or two to look for a replacement before I need it), but I'll be trying to clean it first.

A mate tells me that they get a lot of td5's in the workshop with bad MAFs, and a lot of them can be improved by cleaning - basically, just pour carb cleaner (or they use brake cleaner) through the sensing port. The sensor is a fine wire which is heated - and the rate of heat loss is related to air mass flow rate. If the wire gets sooted up, it affects the heat loss and gives an artificially low reading.

I'd guess that for starting, the ECU add's loads of petrol and this masks the problem at tickover/fast-idle power. Once you add power, and especially as the engine warms up and the ECU "pushes the choke in", then the problem gets worse.

[stevieh]

Guys until recently I had an MG ZT with the BMW diesel engine and the Bosch MAF's were notoriously poor. The best way to find out if the MAF was on the way out was to disconnect it and run it for a while. There is a guy called RoverRon who makes MAF compensators for BMW, Peugeot, Ford and Jaguar engines, look him up his website is a mine of knowledge for diesels and I assume the MAF does the same job in a petrol engine.

[SimonHobson]

... and I assume the MAF does the same job in a petrol engine.
Actually no, there are some very big differences. A diesel would run with no MAF at all, all the ECU has to do is squirt some fuel in - more fuel=more power, and low power=lean running. On a petrol engine, a MAF (or something that can act in some way as a proxy for mass air flow) is essential as the ECU must supply the right amount of fuel for the air going in - the mixture is critical to good engine running.

Apart from that, yes they do the same job - measure air flow and provide the measurement to the ECU.

[blansk]

Guys
many thanks for your advice. At this point I should mutter something complicated and exit quietly.
However I suppose others might find this useful.......
The story goes like this.
Being new to the car and lpg and surprised at the rate it gobbled 100 litres of gas I decided to see how far it went on a whole tank (the gauges are apparently not very accurate and I was only getting 50 L back on), so I ran it until it ran out, which meant it ran lean and backfired once. That blew the air filter box lid off. I clamped it back on. Come to think it has been running v badly since around then.
Independent LR specialist garage looked at it on saturday....couldnt understand diagnostics which I think he said showed MAF out of limits and lambdas high....he wiggled air hose from MAF to engine and it nearly stalled. Long circumferential split....from backfire? Taped up its much better.

I dont think its quite right and lacks power at around 50mph; could be old petrol. Will run the tank and put in some new stuff.

One other thing, being a pessimist I am still suspicious of the MAF, so this evening I disconnected it with engine running - hesitated, then picked up. Drives fine with it disconnected. Presumably it helps with economy a bit but couldnt otherwise detect any difference. Looks like the ECU has a very good limp home mode.

I still think 4 litres of V8 should go a bit faster on neat petrol.

Oh. the other thing. Before going to the garage on saturday I thought I would just check the price of a main dealer MAF. He said it was quite expensive. At £700 odd (I missed the 40 or 50) +VAT he was right. The TD5 he said was about £65..........He did say the petrol one was the same as the RR 3.9 L V8.

Thanks for your advice guys,

[adam1977]

Hi
Defender 90 Anniversary 3.9 V8 petrol with gas conversion
starts perfectly on petrol, revs up as should when cold but when move no power, coughs, inlet manifold pops (sounds lean) but when switch to gas by revving to 1600 rpm then works perfectly all day long on gas
if try to switch to petrol when warmed up same problem, no power, more throttle travel required, definitely sounds lean.
Plugs must be OK. Not sure where to start (full tank of petrol there!). Do I start on fuel supply or could it be the MAF (its the older type of non-sequential gas) I am guessing the MAF isnt used on the gas circuit and the MAF seems to get a few hits on the forum

Its been a gradual problem. I used up the last tank of petrol 1 1/2 months ago no problems, been getting worse for last 6 weeks

thanks


The thing is though, if it's running well on LPG, then it should be ok on petrol, so it sounds like a fuel problem :rolleyes:

I had similar problems up until recently, I tried everything, all new ignition bits, resealing the plenum, filters, new MAF (£60 off ebay-new) in the end it was the fuel injectors, since fitting them the car runs great on petrol, same as when the LPG was working :o

[V8_Disco]

I was always told to run the car on petrol a bit if you want the ablity to run on this fuel as well, its helpfull in parts of the country and ireland where they dont know what LPG is

[SimonHobson]

I was always told to run the car on petrol a bit if you want the ablity to run on this fuel as well, its helpfull in parts of the country and ireland where they dont know what LPG is
Yes, it's a good idea.

With (petrol) injection systems, it's more or less automatic as they are normally setup to start on petrol and then switch to gas. Thus you get a "system check" of your petrol system every start - and of course get to exercise the injectors.

On carb systems, you need to do it manually. Particularly with the SU carbs normal on Rover V8 engines, the needles and jets wear (faster when running on gas as there isn't petrol in the jet to provide lubrication) and so the mixture goes richer. Also, gaskets and seals can dry out - it's 'interesting' to switch to petrol and find that it's leaking out the bottom seal fo the float chamber :rolleyes: I also found that one of my float valves started sticking open, leading to very rich running and pumping expensive petrol out of the overflow :eek:

[V8_Disco]

My Stage 1 is on Strombergs, If it has been running on Petrol and has any heat in the engine it refuses to run on LPG ... its one the reasons that i am tempted to go EFI with the truck (main one to be fair) I run about on petrol at least once a week/fortnight depending on usage of the truck

A

[SimonHobson]

My Stage 1 is on Strombergs, If it has been running on Petrol and has any heat in the engine it refuses to run on LPG
That's odd - you are allowing the carbs to empty before switching on the gas aren't you ?

[V8_Disco]

Yes, even to the point it stalls then try and turn over, left it for a couple of hours once and it still refused to run on LPG, next morning started on the button:rolleyes:

A

[SimonHobson]

Yes, even to the point it stalls then try and turn over, left it for a couple of hours once and it still refused to run on LPG, next morning started on the button:rolleyes:
That is quite odd. My only guess is that with the heat, the pressure is building up in the pipework and a solenoid valve can't open.

I've had that when the fill valve has failed to shut and I've completely filled a tank - temperature goes up, pressure goes up, and the solenoid valve can't lift. Rather annoying when you have to burn petrol while you have a full tank of gas :eek: Again, OK the next morning when it's all cooled down.

I have a suggested experiment. Close the manual valve on the tank or disconnect the solenoid, and run on gas until the engine stops. This will empty the pipework, filter etc at the front end. Open the valve/reconnect the solenoid but do NOT switch on the gas system. Then when the engine is hot, see what happens when you flick the switch. If my guess is right, it'll hesitate for a second or two as the pipework fills up, and then run on gas - if I'm wrong it won't run at all :confused:

[V8_Disco]

Thanks Simon, I might get some time this weekend to try that, though I am mid repainting the tanks from the disco and plan to refit them...

the multi valve on this tank looks to be neerer 90 deg than 60, any tips on measuring this angle (it will help when I have to bolt up the disco tanks as well) What I do no is I never seem to be able to get a full tank of fuel out it only lets about 35-40L burn before the car needs to swap over

Ahh but its all worth it when you drive a V8 land rover!:biggrin:

Thanks

Aidan

[SimonHobson]

the multi valve on this tank looks to be neerer 90 deg than 60, any tips on measuring this angle (it will help when I have to bolt up the disco tanks as well) What I do no is I never seem to be able to get a full tank of fuel out it only lets about 35-40L burn before the car needs to swap over
I tend to guess it ! Alternatively, they do make little levels for doing it, or simpler yet is to use a small square (raid the kids school maths kit ?) with 30˚/60˚ angles and just line it up against a horizontal or vertical.

Ahh but its all worth it when you drive a V8 land rover!:biggrin:
Oh yes :D All those "tractor" drivers should try it before they knock it :cool:

[tyson23]

simply unplug maf and see what it is like then should run ok once unplugged if maf is faulty.

[V8_Disco]

Thanks Simon

A

[p5bowner]

jumping on the bandwagon,..87 range rover 3.5 efi. flapper 2am maf..
bit of history .had it for 12 years on lpg,backfired once a year if that ,then it just pushed a pipe off as a weak link,petrol /gas fine.
3 years ago,i blew the engine,replaced myself..everything still okay...bare in mind this is using the same old champion plugs/leads for last 3 years.
THEN THIS NEW YEARS EVE,
head gasket went,heads sKimmed,valves cleaned,put back together,and it runs like an absolute pig on petrol(wanted it running right on petrol before the gas).so checked for air leaks,new magnagor leads,new dizzycap rotor,new ngk plugs,still running like a pig.
took advice reading on here,drained the fuel,fresh fuel,injector cleaner,took for run but was just dying on me,unless i floor it,and i mean floor it,just to keep it running.Runs better cold but i guess thats due to the cold injector flooding it with fuel,spray carb cleaner down the filter it runs well but i guess same as cold injector....
Enough was enough two weeks down the line,i attempted it on lpg.....RUNS LIKE A DREAM.
I,ve wiggled injector wires,maf wires(did unplug but dies even at high revs,as it wont tick over on petrol anyway)
SO,(SORRY THIS IS SO LONG),what am i left with and how the hell do i check???
has my injectors packed up???
has my wiring on injectors packed up???
has the maf packed up???
How do i check the above,injectors at £80 each/maf at £loads,how do i pin point????
thanks all in advance

[SimonHobson]

jumping on the bandwagon,..87 range rover 3.5 efi. flapper 2am maf..
...
SO,(SORRY THIS IS SO LONG),what am i left with and how the hell do i check???
has my injectors packed up???
has my wiring on injectors packed up???
has the maf packed up???
How do i check the above,injectors at £80 each/maf at £loads,how do i pin point????
Best advice would be to get the manual for your setup, and follow the diagnostics/checks given in there - they are quite comprehensive. You can find them on the LHP2 CD, or there might be other places on the big bad internet if you search.

Your problem doesn't sound like fuel supply (such as clogged filter or clogged suction strainer on pump) - that tends to cause little (or none at all) power with large throttle openings but fine at low power.

[p5bowner]

feel a bit dim here,i have the disc but cannot find where diagnostics are ??

[SimonHobson]

feel a bit dim here,i have the disc but cannot find where diagnostics are ??
There isn't a program for them, there's a set of instructions on how to test individual components. Not sure what things to click on as I don't use the crappy menu system they probably wasted loads of money on, I just keep the PDFs on my hard disk. You want Book 2, Section 6 Fuel - I'd guess it will be under "Workshop Manual", "Fuel System" or something similar.

BTW - knowing your way around the manuals is possibly one of the most useful skills these days :D

[TEMPL4R]

The injectors work constantly in pulses and are wired as 2 sets of 4 IIRC. I doubt they are the problem, one maybe, but not the lot. As Simon says, it doesn't sound like it's a fuel circuit feed problem, more wrong input to the ECU.

The Flap sticks and the tracks go faulty where the variable resistor for the position sensor is. Are the temperature sensors OK. No air leaks after the Flow meter?
If you pull the cold start injector plug off, does the engine stop?

I'd start by taking off the sensor plugs and check for wiring problems and air leaks. Something is not right since the rebuild.

If it runs OK on gas, the ignition system is OK. Simon is the lad to talk to about gas, so I won't go down that road as I know bog all about it.

Chris

[p5bowner]

thanks for the time and advice.i know theres no air leaks,ive been there about 10 years ago,caused me all kinds of grief.ive replaced all pipework,and rubber taped where they adjoin,efore cliping.i also tested with a can of easy start,spraying everywhere,but no sign of leaks.
i have a decent test metre the wife got me xmas,so now ive a reason to use...its nice to have something different to test/check.now feel theres hope in pin pointing the problem...