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3.9 EFi V8 backfire on LPG help


duncmc

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Hi all.

I am looking for suggestions for my problem;

My recent aquistion has a 3.9 EFI running on LPG with what I think is a single point OMLV system. I am getting backfires which are bad enough to split the hose from the pleunum chamber to the AFM and try to blow the trumpet bit off the air filter box. The engine has done 154k miles so will no doubt be worn. Initially the AFM-pleunum chamber hose was not fitted properly so I refitted it and it did run much better, but still gave me a backfire under moderate to wide throttle opening. Then today I had the big bang.

I intend to fit new NSK plugs, new genuine distributor cap and rotor arm, a Bosch coil. Then check the timing.

If that does no cure it, can anyone give me any other suggestions to try/check, or is it more likely to be just a worn engine and camshaft. There is a bit of tappet noise but not much. I am also going to check the compression in the cyclinders whilst at it.

Finally if it is down to the engine wear will it be easier to rebuild this engine or try to find a good second hand lower mileage unit? Bearing in mind that i have never done an engine rebuild or swap, although I am sure that I could manage.

Oh, and I did realise the likely problems of high mileage before I bought it, apart from the usual rust and running gear issues it is good.

Thanks for any advice.

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When mine did that it had burned out a valve.

Hopefully it wil not be that as it had the heads of, a decoke, new valve springs and the valves re ground in at 136385 miles in Nov 2008. (17200 miles ago). It also then had new plug leads and plugs at the same time. Interestingly that was done because of a misfire then.

Of course it still could be that though.

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I'd check the timing first :) I assume you know about LPG advance curves?

Thanks, I think that is a good call. I will do that, and have a look at the plugs/leads/dizzy cap etc.

I know that LPG needs advancing more than petrol if thats what you mean.

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Thanks, I think that is a good call. I will do that, and have a look at the plugs/leads/dizzy cap etc.

I know that LPG needs advancing more than petrol if thats what you mean.

As ThreeSheds said - timing. You can be a good 6 degrees more advanced when running on gas.

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Nomally LPG backfires are either stray HT or lean mixture, or both, so as above for the HT leads and cap/rotor etc, but it seems like you are half way there with that already.

And as above, check for air leaks, a can of WD40 or easy start is good for this, spray it all over the bits that you think could possibly leak (and other places too!) and if you hear the engine revs rise as it's getting extra fuel, you have a leak in the vicinity, then go hunting closer.

The other thing that no-one has mentioned is the actual setting up of the vapouriser and power valve -it could be that these are just out of adjustment and need resetting whilst attached to an exhaust gas analyser. The other thing that can happen is the filter in the LPG line can become blocked and therefore restrict the gas supply, causing a lean mixture, causing a backfire.

The last reason I can suggest, and I only mention it becuase you have had a lot of backfires, is the diaphragm in the vapouriser can split with continued backfires, this is what is used to regulate the LPG flow, which means if split it will throw your mixture all over the place. Inspection and replacement is easy, you just unbolt the back cover and have a look :)

HTH,

Pete.

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Thanks for the replies everyone.

Timing checked quickly and but there are no marks on the pulley, just the slot. It is advanced a fair bit though, and is advancing more with increasing revs.

I need to look at the manual later and check again properly once I have found out how to check the amount of degrees.

Edit; doing a bit of maths, it looks to be TOO far advanced - again I havent actually measure it yet though.

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Last night then, I properly checked the timing and it was at about 10 or 12 degress.

So then I decided to change the distributor cap and rotor arm to new ones that came with the sale (they were Britpart). It then wouldn't start. So checked and checked the order etc. Still no joy. Then I swapped back to the old, and it fired up. Taking it for a test drive though it was very rough and stalled as I pulled back onto the drive.

I have just stripped everything off again, and redone the leads/cap 3 more times. But it will not start. I haven't physically checked for a spark yet, but the induction timing light is firing using the pickup.

What a pain, really could do with it tonight, and off course it is parked in front of the garage where the Defender is!

I may have found the cause of the back fire though, the LPG hose to the injector from the vaporiser gets squashed when the bonnet is shut!

Just need to get it running now though. :angry:

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An afternoon not getting anywhere. Put the original Lucas arm and cap back on. Nothing, so checked a spark at a plug, positive. And getting fuel. Still nothing. Rechecked the timing statically, distributor out, checked and put back in. Found a loose wire on the coil. Fixed that but still nothing.

What gets me is with a spark, fuel, air, and the timing correct, with the same set up as before it should run. But it will not.

Currently charging the battery in case that could be it.

Ready to take the hammer to it!

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What gets me is with a spark, fuel, air, and the timing correct, with the same set up as before it should run. But it will not.

With the above it HAS to run.... ;)

A couple of questions -

Are you starting it on LPG or petrol?

Is it firing at all or is it totally dead?

Have you proven there is fuel there - ie: If starting on LPG, has it got gas coming through? (smell exhaust), If starting on petrol, is that getting through? (wet plugs)

A completely random thought: Blocked air inlet? (a couple of weeks ago a chap had a problem with a rag sucked up his air inlet)

If there are no timing marks you should create your own - ie find TDC after compression in No 1 cylinder * and then mark it yourself with tippex, and then re-time it again.

Hopefully something in this lot will help

Roger

* Finding TDC after compression on No 1:

  • Take out No 1 plug - the one at the front of the near-side (uk) bank of cylinders.
  • Turn the engine using a ratchet/socket on the bottom pulley bolt while covering the plug hole with your thumb. You will feel the compression.
  • Ease off near the top and feel for TDC through the plug hole using something soft. (I pull off a piece of vacuum pipe and use that)
  • Paint your TDC mark.

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With the above it HAS to run.... ;)

A couple of questions -

Are you starting it on LPG or petrol?

Is it firing at all or is it totally dead?

Have you proven there is fuel there - ie: If starting on LPG, has it got gas coming through? (smell exhaust), If starting on petrol, is that getting through? (wet plugs)

A completely random thought: Blocked air inlet? (a couple of weeks ago a chap had a problem with a rag sucked up his air inlet)

If there are no timing marks you should create your own - ie find TDC after compression in No 1 cylinder * and then mark it yourself with tippex, and then re-time it again.

Hopefully something in this lot will help

Roger

* Finding TDC after compression on No 1:

  • Take out No 1 plug - the one at the front of the near-side (uk) bank of cylinders.
  • Turn the engine using a ratchet/socket on the bottom pulley bolt while covering the plug hole with your thumb. You will feel the compression.
  • Ease off near the top and feel for TDC through the plug hole using something soft. (I pull off a piece of vacuum pipe and use that)
  • Paint your TDC mark.

Thanks Roger.

It DOES have timing marks, I just couldn't see them at first. Once cleaned up they are ok. I have statically set the distibutor to 10 degress BTDC after taking it out. I think the rotor arm had might moved round when I initially tried changing it as it was very dificult to get off.

I am starting on petrol. The change over switch will not switch to LPG whilst not running.

It is turning, sparking, but NOT starting to fire.

I also thought about a blocked air inlet. It looks ok, I have had the air filter off, AFM off, and the hose off to the pleunum vhamber and looked in. All clear.

I have not checked for wet plugs, put you can smell petrol in the exhaust, and when I removed a plug to check for a spark.

Next I am going to follow the Lucas Constant Energy System-V8i fault diagnosis path in the LR workshop manual and see what that finds, together with new plugs, leads, rotor arm, cap and coil.

Fingers Crossed. My gut feel is that I have just disturbed a poor electrical part of the system in messing before.

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Duncmc, when you changed the dizzy cap, you didn't take the leads off all at once did you?

Whilst I's sure you've checked the lead positions more than once, are you 100% sure that when No1 pot is at TDC, the arm is pointing to No1 HT lead?

I wouldn't be the first time, and won't be the last, that someone in the past has had the dizzy off, not put it back correctly, and countered for this by moving the leads round a post or two.

I know this, because I spent a whole weekend, yes a whole weekend once trying to start a Range Rover that I had changed the leads on, fitted them by the book.....

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I have now fitted the new distributor cap, rotor arm, NGK plugs, HT leads, Bosch coil, new condensor and cleaned up the earths under the coil. Also I have resoldered some LT terminal connections.

I checked the position of the rotor arm relative to No1 cylinder at TDC with a bit of wire, and noted that I had to rotate 180 degrees to get to point at no 1. So onto the diagnostic list, 12+V at the battery etc, all that was good. But is stil would not start, all it could be was 180 degrees out!

So I removed the distributor and turned the engine over 180 degrees, and refitted it.

Of course then it started! :) But it is running rough. When I checked the timing there are no marks, so I will have to make some.

Now the problem is it keeps dying and will not run smoothly. :( The HT keeps dissapearing when it dies, as the timing light stops. And I suspect the battery is on its way out together with probably a failing amp pack on the distributor.

So next is a new battery, new amp pack and I need to fit a new vacuum advance unit as that isn't working.

Hopefully then it will be ok. Fingers crossed.

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For LPG the ignition system needs to be perfect.

Also, on single point systems you should gap the spark plugs to 0.7mm.

Don't mess about with ignition timing until you have done this.

webber

Thanks for the suggestion, it has the magnetic pickup type of distributor rather than the older points though. I still have to check the gap though. I need non ferrous feeler gauges for this apparently.

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Thanks for the suggestion, it has the magnetic pickup type of distributor rather than the older points though. I still have to check the gap though. I need non ferrous feeler gauges for this apparently.

The gap should apparently be 0.20mm to 0.35mm. I have found 2 bits of plastic the correct size to check with later.

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Already done that. New NGK plugs, and amazingly all came out of the box with the correct gap? Never had that before.

Did you set the gaps to the standard (by the workshop manual) or reduce it for LPG (0.7mm or thereabouts)? The standard ignition isn't strong enough with LPG and the standard spark gap as LPG is harder to ignite and the ignition is pretty marginal anyway. If you fit modern distributorless ignition (much bigger project obviously, but the best single modification I made to my Range Rover) then you can run larger gaps.

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Did you set the gaps to the standard (by the workshop manual) or reduce it for LPG (0.7mm or thereabouts)? The standard ignition isn't strong enough with LPG and the standard spark gap as LPG is harder to ignite and the ignition is pretty marginal anyway. If you fit modern distributorless ignition (much bigger project obviously, but the best single modification I made to my Range Rover) then you can run larger gaps.

Hi Geoff, I set the plugs to the standard setting. I did not reduce them for LPG. The current trouble I am getting on petrol too.

I have ordered an A+R amp from RPI to get the bigger spark. Hopefully will come early next week.

Dunc

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