3.9 Bad Misfiring

[tomg978]

Hi, this is my first post on here.

I have fitted a 3.9 V8 EFI from a discovery 1 to my defender, with an auto box. The engine had 70k miles when I installed it, had been sat for the best part of 5 years. An oil and filter change (20W50) and the engine ran really smooth, couple of lifters were a little noisy but not worrying. Now have covered a couple thousand miles , and last weekend travelled to Anglesey. By the time I had arrived, a small misfire had developed, not very noticeable, more noticeable at low revs. After the weekends competition, driving home, the misfire had turned into 2 cylinders, and engine is now down on power. Got it out today and its running very rough, permanent misfire, not intermitent. I have swapped all the plugs, leads, dizzy cap, and coil (these were all new when i swapped the engine) which made no difference. I am assuming that the camshaft has finally worn past its usefulness?

If so, is there anything I should do to help the oil feed to the camshaft before installing a new camshaft? I have average oil pressure (around 20-35psi when driving) , would it be worth renewing the oil pump gears at the same time?

FYI, I did a compression test 2 weeks ago, and got 160psi +/- 10psi on each cylinder.

Thanks

[ChrisJC]

I would take off the rocker covers and check that the camshaft has worn out (by checking all the valves move up and down the right amount). It might not be the camshaft at all.

Chris.

[tomg978]

Is it ok to run the engine with no rockers? No oil going to come flying out anywhere?
Thanks

[ChrisJC]

It is OK, the oil only dribbles out. But I was thinking to turn the engine by hand and use a Dial Test Indicator to measure the movement of each valve.....

Chris.

[tomg978]

Ok, is there a measurement I should be looking for? or just seeing which rockers move more than the others? I have been that it is usually number 7 and 8 lobes/rockers that suffer from lack of oil?

[ChrisJC]

I would measure them all and look for a few that are very different (perhaps not opening at all!)

I think the cam lift is specified somewhere though so you could see what it's supposed to be. Probably not worth it though.

Chris.

[Ian Anderson]

£20 infra red thermometer from Maplins
Check which cylinders are down on temperature on start up then inspect those ones. May be gummed up injectors or iffy contact in electrics feeding them

Ian

[tomg978]

£20 infra red thermometer from Maplins
Check which cylinders are down on temperature on start up then inspect those ones. May be gummed up injectors or iffy contact in electrics feeding them

Ian

I can get an infrared thermometer from work. Will give that a go. I have replaced the wires to the amplifier on the side of the coil. When I was changing the dizzy cap, i accidentally just moved the wires, and the engine wouldnt start. took me forever to realise, but the wires were just badly corroded at the connectors.

I have also been advised that the misfire could be just due to a slack timing chain? Is this a common issue?

Tom

[DaveEFI]

Can't see chain stretch causing a misfire.
Cam wear doesn't happen quickly - unless you had a lubrication problem. The first signs are the engine being down on power at high revs. It would take thousands of miles for it to wear down to the point where it misfired.
I've seen then worn virtually round and you still get a smooth idle and no misfire.
I'd be inclined to try a new ignition amp. They do fail - although usually completely rather cause a miss. But anything's possible.

[Spongo]

Coolant leak can cause plugs to stop working is it losing any?

[tomg978]

There is no coolant being lost, there is a lot of condensation on a cold start, but I am assuming this is normal for petrol engines?

If I remove HT lead number 8, with the engine running, the misfire doesnt change. however it sounds like it is missing more than 1 cylinder.

I am at the point of taking it out to replace it for a bigger lump (4.2 or 4.6). looking at the Land Rover Workshop Manual, the oil pressure for this engine is low, and I cannot source a replacement timing cover/oil pump for the engine, since it is a suffix B with the oil pump driven from the crankshaft.

Tom

[DaveEFI]

You do need to measure the oil pressure accurately first - the dash gauge is often wrong. And even then, there seems to be a lot of tolerance with sound engines.

A fairly good indication of bearing wear and really low pressure would be the pressure warning light coming on at any time on a very hot engine.

[tomg978]

You do need to measure the oil pressure accurately first - the dash gauge is often wrong. And even then, there seems to be a lot of tolerance with sound engines.

A fairly good indication of bearing wear and really low pressure would be the pressure warning light coming on at any time on a very hot engine.

I have hooked up a proper testing gauge for the oil pressure. Actual figures are

Cold @ 2500rpm = 35psi
Hot @ Idle 750rpm = 10psi
Hot @ 2500rpm = 25psi

These figures are not really a cause for concern, but it is lower than what the workshop manual states. I am concerned that since the oil pump for this engine is no longer available new, I could be spending money on a dead engine.

As someone said earlier down my post, the cam wear cant just happen like that. This engine has always been pathetic at high RPM, and I have always suspected that the camshaft would be on its way. However in the space of around 600 miles, it has gone from running perfect, to misfiring, not all of a sudden, it has happened gradually. It must be a mechanical fault , an electrical fault would be permanent or intermittent.

I have kind of made my mind up to get a new engine - there are still some saleable parts from this engine including the EFI system, and the block would make a nice coffee table

[DaveEFI]

A 3.9 in factory condition wouldn't be pathetic at high revs.

If it really has only done 70,000 miles the rest of the engine should be near perfect, condition wise. And fitting a new cam is an easy job with the engine in the vehicle.

[unstable load]

I am concerned that since the oil pump for this engine is no longer available new,

You can get the pump upgrade kit that solves the pressure issue, alternatively, the front cover from a GM 3,9 litre V6 is a bolt on part and you get the uprated pump with it.