96 dual fuel V8i intermittent misfire

[calakas]

I've had my 96 Disco for nearly 5 years now and I've not been able to find a clear answer to the intermittent problem of ignition misfire which causes a reduction in power and blowback (backfire through the intake).
The following account covers my experience in investigating this fault over the years I've owned the Disco. Please feel free to ask me to provide further detail on any areas I might have missed or just glossed over. I’ve also included the occasional digression to other issues that might be related.

The misfiring is intermittent and when it happens it's level varies, it could be light where I don't feel anything but I can hear an occasional bump in the engine, or it can vary up to a high level where it almost feels like the ignition key is being momentarily turned off.

When it is happening it is at it's worst when moving off from standstill. Whilst running on petrol (I'm an Aussie) the misfiring has to be extreme before a blowback occurs, but on LPG it happens very easily. I normally use petrol for cold starting, otherwise I'm running on LPG.

Soon after I bought the car, I found that when turning right at a corner, it was more likely to blowback on LPG or lose power on petrol than when traveling straight or turning left. I found the cause to be from a very poorly installed LPG system. The mixer had been badly spliced into the rubber hose between the MAS and the intake plenum. When I turned left, the weight of the mixer caused a split in the rubber to open up and drastically alter the air/fuel mixture.

After I replaced the hose and clamps, all was good for a couple of months until an unexpected series of blowbacks. The first one was a real heart-stopper. I now think the previously poor LPG installation was working as a pressure relief for the blowbacks. Now that the hoses are all good and tight, the blowbacks almost disintegrate the air filter housing and you can actually see the bonnet (hood) lift almost two inches. If I'm driving beside someone when the blowback happens, they almost run off the road. I now need to use cable ties to hold the air filter housing closed, all the clips are broken and I’m not replacing it until the cure has been found.

I've taken the MAS apart to check for damage and it is still OK. On a side issue about the MAS, the car was also having idle speed issues where it would sometimes idle at 2000 RPM on LPG or petrol. I found the multi-pin connector on the MAS was unreliable, so I soldered the 4 required wires onto their respective pins, problem fixed. However, this was after replacing the idle control valve on the rear of the intake plenum and the throttle potentiometer on the front of the plenum. I think the engine ECU was using the false information from the MAS to adjust the idle mixture which affected both fuels. Having dual fuel can be useful in fault finding.

Back on the misfiring subject, I've checked or re-joined all the vacuum tubes I could find. I ran extra cabling to the earth/negative of all of the major components in the engine bay and in the cabin. I've cleaned every multi-pin connector I can locate. I've pulled all ECU like devices apart (except ABS) and cleaned the edge connectors. I’ve replaced the plugs, distributor cap, rotor button, coil. I later found three ignition leads were intermittent. I hadn’t replaced them because they looked OK. I replaced the faulty leads and voila! The engine has never run so smoothly. Starts instantly when cold, considerably more power, idles and accelerates smoothly.

I have found that success with the Disco never lasts very long. A few months later, BANG. Blowback had returned with a vengeance. Nothing had obviously changed in any of my previous work. Something I hadn’t mentioned earlier is that after a blowback, the engine almost always runs perfectly for a short while, or a few days, like it’s cleared out a buildup of some kind inside the engine. Plugs are always dry and brown, unless I’ve been using petrol more than usual, then they’ve got some soot on them. The petrol mixture is a little rich. I haven’t sorted that out because I use it for about 30 seconds a day. A tank full will last a few months. Oil is always clean. Driveway’s not though, British car, lots of drips.
With the return of the blowbacks, I have noticed that they’re much more likely to happen in damp conditions. I then found burn marks on some ignition leads, ah ha! This explains a lot. Some HT is intermittently being grounded or getting into the next lead causing the next plug to fire when the intake valve is open, hence strong blowback.
This time I replaced the leads with Bosch ones that have thick insulation and used many after-market spacers to keep the leads away from each other and metal.

18 months later, still a perfectly running engine, until last Wednesday when it was time to come home from work. What a nightmare drive through peak hour traffic. If the police were nearby, they’d have thought I was brandishing a gun. Blowback after blowback for a few kilometers, then I switched to petrol and limped home in a foul mood. Once above a speed of 40kph or so, the car appears to accelerate smoothly, but become rough again when cruising. The upcoming Australia Day long weekend had just been ruined.

The problem appears to be electrical. The engine will run perfectly one moment and rough the next. Like a switch is being randomly operated.

I did find that one of the hoses on the air intake had worked loose. I replaced it with a new one and there has been some improvement. The car is at least drivable now. I just have to accelerate slowly, like in the early days.

90% of the time, when the engine is cold, it is missing.
When the engine is warm and has been misfiring, if it is turned off for 30 seconds or so, it is smooth again when restarted, for a while.

I bought a brand new timing light and brought home a high quality Fluke multimeter from work to try and track down the fault. I followed the steps for checking the ignition components in the service manual, all good. Timing is rock solid, 2 degrees BTDC for the Australian market. Vacuum advance doesn’t work and never has, diaphragm has always had a hole. Leads and spacing are all still good. However, I have read that I’ve been using the wrong plugs, I was lazy and just swapped old for new, assuming the previous owner had used the correct ones. The original Champions are no longer available, at least not from regular suppliers, so I thought I’d try the recommended NGK plugs. Being a local holiday weekend, I’ve only been able to locate 7 plugs so far. I think I’ll put them in to see if there’s a difference.

If I think of any points that I’ve missed, I’ll post again.

Thanks in advance for any suggestions. Thanks heaps if anyone out there actually knows the answer.

Craig

[pavel]

Hello there Calakas: can you find out what problem codes the vehicle has thrown? You mention she had the wrong plugs installed - what other incorrect components might there be in place? Have you tried to have the problem(s) diagnosed by the dealer or a good shop? I have no specific useful comments as I have no idea what's going on under your hood/bonnet....but, could your advance-less distributor be involved?
I like the way your LPG system seems to run leaner (brown plugs not black) than the stock petrol system as this might allow your exhaust valve stems to stay cleaner (this is a big problem with these engines in N. America)

[calakas]

Hi All,

Towards the end of my first post I mentioned that I had the wrong plugs in the car and I was just about to change them. Well, I'm embarrassed to say that I had forgotten that I had changed the plugs to the correct ones (NKG BPR5ES) 18 months ago. This was when it ran well for the first time.

I changed the plugs over anyway, even though the ones in there seemed to be in good condition and brown and dry. As I was removing the plugs, I noticed that the small barrel nut, the one that the lead clips onto, was loose. I always make a point of tightening these when I install plugs, so they've worked loose since they were installed. The leads wobbled very easily on the plugs.

Unfortunately, I didn't have time to systematically check for misfire after each new plug was installed, so I don't know for sure what the cause was. I can only assume that it was related to the loose barrel nuts on the plugs. Anyway, the engine has not missed a beat since the new plugs went in. Starts instantly, idles smoothly, and I'm actually pushed back into my seat when accelerating (haven't felt that for a while). I still have to get used to not pushing so hard on the pedal.

I will post again if there's any hint of the problem returning.

Great forum.

[pavel]

Great to hear she's running properly - this is a perfect example of the old law that you should check the simple things first! New plugs often solve problems. Cheers