Misfire under load - getting worse - help !!

My 1994 Disco 3.9 (dual fuel) has a misfire, particularly under load. Change down a gear and is seems much better. The misfire occurs on both LPG and petrol, although it is much more noticeable on LPG. The tick-over is slightly lumpy.

It feels "across the range" as opposed to a single cylinder.

To date I have done the following:
Compression test - all about 160.
New plugs.
New HT leads.
New distributor cap.
New coil.
New coolant temperature sensor.
Checked voltage at the coil.
Borrowed a Mass Airflow Meter - no change.

This is beginning to break my brain - any suggestions are most welcome.

Alan

Professor Alan said:

It feels "across the range" as opposed to a single cylinder.

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what does that mean?
all cylinders or at all speeds or what?

Put a new,(GENUINE) rotor arm in it.

Hello

what does that mean?
all cylinders or at all speeds or what?

I mean that it is more than one cylinder misfiring (as far as I can tell). It isn't a regular misfire - it can have run OK for a couple of seconds and then misfire several times in a couple of seconds.

Sorry if that is hard to follow but an intermittent misfir is a hard thing to describe.

Alan

eightinavee said:

Put a new,(GENUINE) rotor arm in it.

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Thanks for the suggestion - is this a common failing? The rotor arm certainly looks OK with no signs of burning or tracking.

Alan

i think i'd be more tempted to go along the fuel route, as rotor arms either work or they don't!. Intermittent faults would means something like a losse connection which is pretty hard to get with a rotor arm, unless yer dizzy is worn in which case a new rotor wont do ferk all. so if it happens with both fuels that should narrow it down to the parts that are common to both. injectors or such like. another thought
if yer engine has hydralic tappets then there is a possibility that some **** has got in there and affected the valve openning. i've seen this before where you can be driving along and all of a sudden you get a misfire. after a few moments its goes away. this is usually accompanied by tappet rattle.
one other thing what plugs have you put in and did it have the problem before hand?

I had a problem similar to this on one of my vehicles, and it was a crack on the plastic on top of the coil. Minor short circuit under load with the spark finding the easiest way to earth. A mate of mine had a similar problem and it was his new plugs were gapped too wide. That's just two of the hundreds of possiblilties that it might be. As they say in the classics... an ignition system is something that is capable of three times as many faults as there are components.

Hi Everyone

Progress (of a sort) at last.

The problem seems to be overfueling - took out the plugs today and they were black (sooty). After cleaning them the Disco was pretty good for about 5-6 miles then started misfiring again.

I have checked the oxygen sensor (front one) and it seems to be OK. As previously said, it has a new coolant temperature sensor.

Does anyone know if there is a way to check the throttle potentiometer?

Other suggestions are most welcome.

Alan

PS - does this model have an ECU and diagnostics socket? If so, where is it? The car is a 1994 model.
Also, should the 'check engine' light come on when the ignition is first turned on (as a part of the test cycle)? This isn't happening and I wondered if it should be.
Thanks.

Put A New Rotor Arm On It.

eightinavee said:

Put A New Rotor Arm On It.

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OK - done that - no difference - worth a try though.

Other suggestions welcome.

Alan

Ok,now you can do some proper diagnostic work on it - you say you checked the oxygen sensor.How did you check it? If your car is a Uk model then it should have an oxygen sensor screwed into each downpipe.These are heated sensors,and will not work properly under all driving conditions unless they are being heated.From memory the sensors are 3 wire Zirconia ones,with a red,black and white wire.Check the earthing on the white wire,battery voltage whilst running on the red wire and at a constant engine speed a cycling voltage of 0v then upto 0.8v and back down again,once a second.
Sorry about the long/poor sentences - its Friday eve with a large glass of wine !!
The critical thing is the output from the black wire,if it is staying at 0v then the ecu will think the engine is running lean and widen the fuel injector pulse width to try to richen it up.If it does not see a 0.8v,(rich) signal fairly quickly it will keep on trying to enrich the mixture - to the extent that it will foul the plugs and cause misfires.
So if the sensors dont switch this is why the sooty plugs etc,next you have to find out if they are capable of switching - do this by clamping the fuel return pipe off for a couple of secs while it is running.If they then dont show 0.8v and are being heated then you have failed sensors.If they do switch then you need to look around the engine for failed vacuum pipes/inlet air leaks etc.
Another thought is that it is possible to connect the coolant temp sensor onto the fuel rail sensor and vice versa.This makes the Ecu think the coolant temp is far lower than it actually is and will cause over rich mixture and misfire issues.
Yes you can connect your engine ecu to an external computer to read fault codes/watch "live" data etc,in practice the 14 cux system you have is such a simple/reliable system I rarely bother using Testbook on it unless I want to check / alter the stepper motor setting when dealing with idling issues.An oscilliscope is much more useful on a 14cux system as most poor running is to do with failing ignition components - which is why I said to change the rotor,they are not an either work/not work component as suggested,I changed one just this morning on an old 3.5 that would fire but not pick up - ran fine with a new one.Good luck with your car - if you need oxy sensors dont buy them from LR ! Get them from a motor factor or off the net - much cheaper !

yer man said his was an intermitant misfire which is not gonna come from a rotor arm. granted you can have a misfire from a rotor, and i would class that as not working. so like i said your rotor arm either works or it doesn't. you just don't get intermitant rotors faults. but do pardon us for trying to help this poor chap sort his problem.

slob said:

yer man said his was an intermitant misfire which is not gonna come from a rotor arm. granted you can have a misfire from a rotor, and i would class that as not working. so like i said your rotor arm either works or it doesn't. you just don't get intermitant rotors faults. but do pardon us for trying to help this poor chap sort his problem.

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Come and fix some of the crap garages throw at me then if you are that confident - I need a holiday ! (why do I keep 2 spare rotors in my Disco - its a Tdi)
The rotor causes an intermittent misfire because under light load a poor spark will do - under heavy load a big fat spark is needed,he said it misses under load.

The TPS shoul read 0.5-0.9 when closed and increase steadily as the throttle is opened untill it reaches open value. I don't reckon its this though, I reckon you'vr got and airleak on the inlet.

Check the flexible hose from the airflow meter to the plenum. Check the joint between the plenum and the velocity stack housing. check the joint between the velocity stack housing and the inlet manifold. Check the joint between the inlet manifold and the heads.

The easiest way to listen for an air leak is to cut a short length of hosepipe, about 24" -30" long, put one end up to your ear and search around the joints with the other end.

If its the plenum-velocity-inlet manifold joints then just take it apart, clen the surfaces and reassemble with a thin smear of dow-corning. If its inlet manifold-heads than you'll need to buy a gasket.

ahhhh .... there was a second page!!

I've read page 2 now .... eight in a v ..... your a God. Can I come and be your apprentice?

i still have a sense that its fuel related which could be a sucky gasket. and i still don't believe a rotor arm would cause the fault as described, especially since its got a new one. any rotor arm i've seen that causes problems its usually pretty obvious has bits missin or burnt or even worn through.
except for them ones with the spring for a rev limiter. unless you know what to look for.( once fixed one of haynes' racing 911's cause their mechanic didn't think of the spring)

I said all that for good reason,Rover v8's run perfectly for long periods - which leads to people to "if it aint broke dont fix it"
Trouble is ignition components on them are stretched even when in good nick and aftermarket bits compound the problems.They are very fussy about their sparks - anytime a running issue on one is presented the quickest way to sort it is to ensure the sparks are happening properly - without an oscilliscope and timing light all you can do is fit new bits- they are probably overdue for a change any way.
Oxygen sensor activity is the second area to go to - it is the only way you can really tell what is going on in real time,it has made diagnosis so much easier its not true.
I find it a very satisfying way to make a living,cos there is always something to learn - no two faults are the same.

Right, as eightinavee said - time for some proper diagnostic work.

I am working with two multimeters - an accurate digital one and a less accurate analogue one.

I have checked both oxygen sensors. The readings are similar. When running on petrol the readings are 'all over the place' but when switched to LPG they both show a fairly regular (once a secondish) voltage pulse. It is too quick for the digital meter to read accurately but the analogue one shows about 0.7 to 0.8 volts.

It took a while to test because some previous owner had a 'tie-wrap fetish' and the wiring harness was obviously his favourite practicing ground.

Eightinavee, I hope you didn't hit the wine too hard last night - I need your clear head.

Everyone, please keep the suggestions coming.

Alan

Arrrgh - you didnt mention the LPG before,does it misfire on LPG ? What system is fitted ? Normally we ignore the LPG system till it is running 100%on petrol,as the oxy sensors are switching on LPG it means they are being heated and are capable of switching - this means we can believe their outputs and watch what happens while running on petrol.Can you quantify what you mean by all over the place,it isnt the best trying to monitor their outputs with a multimeter - can you get hold of a scope?
It might be an idea to write up a flow chart to remind you what you have checked. Keep going !!