Yet another 3.9 v8 with rough idle - advice please!

Hi there,

I have a 1996 Disco 1 3.9 V8 automatic with LPG conversion. I purchased it in October 2012 and its my first Land Rover. I am an amateur DIY mechanic, with some ability. I depend on the Haynes Book of Lies as the forum refers to it. I have managed to fix a bunch of issues with the car so far. I have to say that the Disco was not well maintained when I got it. That's OK - I knew that I would have a challenge to fix this car and I wanted a project to learn new skills.

There is one issue that seems quite common out there in the Landy v8 world and it is really frustrating me.

I have a rough idle issue (that can stall the engine) after the engine is warmed up to normal operating temperature (after about 15 mins of running).

I have read through tons of posts, but I'm not sure what to check next.

Here's what I have done so far to try and address this issue:

1. Put in new spark plugs
2. Installed new HT leads
3. Installed new Distributor cap with new rotar arm (a mechanic did this for me)
4. Installed new coil and amplifier module (as Haynes refers to it)
5. cleaned the breather tubes that go from flame trap/filter thing on rocker arm cover to plenum
6. cleaned flame trap/filter - which was quite manky (have ordered a new one - I got it much cleaner, but it is still quite dirty) Note that it seemed air could pass through the filter easily after I cleaned it.
7. Cleaned out the plenum end of the breather hose to rocker arm case / flame trap filter thing.
8. The speedometer was intermittently working so I replaced the speed sensor / attenuator today. I understand that this may have something to do with the fuel injection system.
9. When I bought the Disco, the battery was pretty well goosed so I replaced that back in October. The alternator seems fine - I have not tested it though - but I don't think there's a problem - it starts every time and the idle is unaffected when the lights, rear window defrost, etc. is all switched on.
10. the vacuum hoses look to be in good shape, they are fitted well and I don't believe there are any leaks

Lately I have been running the Disco mostly on Petrol only. However, if I recall correctly, I think the rough idle issue / stalling after 15 mins of driving happens on LPG, too. I will need to check this again.

Today I did items 6, 7 and 8 above. I have bought a new fuel filter but have not yet fit it.

When I started the car today after working on it, it ran very sweetly. She just purred and I kept it running for 20 minutes and then shifted into gear (both D and R) and guess what? The idle went right down to about 500 RPMs and the engine suddenly started running quite rough at idle. It also stalled at one point as the idle went way low. As a side note, when the rough idle starts in I also notice that acceleration is not as smooth as it was previously.

It seems like there is some sensor that told the ECU to change the fueling / air mix once a certain operating temperature has been reached.

Are there specific sensors that could be misinforming the ECU? If so - which ones? And where are they on the engine (if you would refer me to Haynes sections I'm good with that!) Would the airflow sensor be sensitive to temperature of the engine?

I don't think there is an issue with the fuel injectors, because the motor runs fine from cold for about 15 to 20 minutes. If the injectors were blocked, I would expect that the Disco would alway run poorly.

I don't think this is a base idle issue - but maybe it is. I have been running on petrol rather than LPG - it could be tuned for LPG - do you think?

I reckon that the timing may need checking again - I would like the collective opinion on that. I don't have the tools or skill to do this - so I will have to take it to a mechanic.

Any suggestions would be very much welcomed.

Thank you for your time.

I have received a few suggestions of what to investigate - I reached out privately to a veteran V8 Land Rover owner and I thought I would post the suggestions so that others may benefit or comment on them:

>> 1. the throttle potentiometer. It needs to be correctly set and check that it scales properly and smoothly.

I'm dependent on the Haynes manual - it explains how to remove / refit the throttle potentiometer but mentions nothing about how to set it and test it scales correctly. Any suggestions here from the community?

>> 2. Ignition timing might be worth having a look at but the Rover V8 is very tolerant and will run even with timing way off. How does it rev? If it picks up quickly and smoothly the primary issue is unlikely to be timing.

My Disco picks up smoothly. Even after the idle problem kicks in after 15 to 20 minutes of driving from a cold start, it still accelerates smoothly but there is a slight hint of roughness during acceleration (only after the idle problem kicks in).

>> 3. If the base idle is not correct there might be idle issues but theoretically, the idle stabilisation valve should take care of that but it might be worthwhile to remove and clean the stepper motor,

Haynes Book of Lies issue again: no mention in it about a "stepper motor". What is it? Where is it? Can someone translate stepper motor into Haynes manual speak please?

>> 4. remove and clean the plenum and check and set the throttle disc

I'm good with removing and cleaning the plenum - but the throttle disc?? Is that the same as the "throttle linkage assembly" as Haynes calls it?

I'm not a Haynes manual zealot - really! - it is just that it is the only reference I have to work with.

I would very much appreciate feedback and suggestions from the community.

The short answer with any old v8 EFi is to remove the air filter and pipework and place aside, and then everything else on the top of the inlet manifold (not the manifold itself just the trumpet base) needs removing, stripping and cleaning.

If you remove all this you will find the stepper, you will access the throttle, you will be able to make sure that the little T piece in the flame trap breather is clear and that all the ports in the trumpet base are clear etc etc.

There is a coolant temp sensor just behind the dizzy at the front of the inlet manifold, fitted vertically and this tells the EFi what temp the engine is so it can work out enrichment.

Rough idle on a RV8 usually means lean but it could also be poor air metering thus impossible idle control.

I see you attend a university therefore you can read?

READ THIS TWICE AND IN FULL

Rover 14CUX Hot Wire Mass Flow EFI: Service and Troubleshooting

This tells you everything you need to know about 14cux and it will seriously help you understand your car, which is a simple beast at the end of the day.

I am not an LPG fan so I shall not taint this post by discussing it at length, but basically get it perfect on petrol first and then look at getting it running on LPG.

There is an igntion timing offset required between petrol and gas but that is for you to decide on.

Around 6-10 degrees BTDC at idle is nice for a rover but I have run up to 12 or so, be jaws I wanted more at WOT.

Hope that helps anyway, definitely give you something to get into.

Read that web page, it's awesome!

Thanks so much Noisy! That was a great explanation and this has increased my confidence. I have a clear idea of what to do.

It's colder than a well diggers a*** right now up here in Scotland, so I think it is time to get a cuppa and start reading the web page you refer to. And yes, I will read it at least 2 times!

Cheers!

Hi Gregle, did you sort the issue (I have the same problem!).

cyd said:

Hi Gregle, did you sort the issue (I have the same problem!).

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Lol.

cyd said:

(I have the same problem!).

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me too mine wouldn't pull the skin off custard at the mo

Hi there,
A quick update in the continuing saga - I read the material that Noisy suggested - quite helpful. Also had a suggestion to go to www.ecumate.com and download the user manual and read the troubleshooting section. This was a really good read. Does anyone have one of the ECU mate devices? I am seriously tempted to buy one.

Anyway, I cleaned the plenum and the stepper motor - which was very gummed up. I went through and cleaned out all the breather tubes that lead up to the plenum.

Someone privately asked me what kind of HT leads I put on - I bought the cheapest ones I could find and it was suggested that I should not use cheap and nasty HT leads but go for LR original kit or high quality aftermarket items - I did the latter.

All that work paid off as after starting the engine it ran smoother - that's for sure. I had to leave and could not do a test drive. The next day I went out to start the engine and NOTHING. The starter turns over but I have no spark - checked HT lead from coil to distributor while turning over the starter and there's no spark whatsoever. I got out my voltmeter and battery is OK, turned on ignition and tested at coil / LT leads and no voltage at all.... Fuses seem fine. I guess it must be a wiring issue. I am looking through forum posts now and starting at wiring diagrams.

Any help here would be appreciated!

So long story short - I think the low idle issue is now sorted and the main things that helped that were new fire trap / rocker cover filter, cleaning out breather tubes to plenum, cleaning up the plenum and stepper motor and putting on a decent set of HT leads.

Now trying to figure out why I am not getting power to the coil. Again - any advice is welcomed.

Gregle

try running a feed from ign live to coil +. will just be a dodgy connector probably.

3.5 efi said:

try running a feed from ign live to coil +. will just be a dodgy connector probably.

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Thanks for that - any suggestions where best to get an ignition live feed from ? There are two +'s on the coil - which one should I tap onto? (15 or 1? or the nut in between?)

My engine has an LPG conversion - but I want to sort out all the issues and so I'm running it on petrol only just now. I'm new to Land Rovers and I don't understand the wiring - although I am pouring over wiring diagrams just now trying to learn. It seems to me that whoever did the LPG conversion butchered the wiring. I was following the wiring around the engine compartment and I found a bunch of 'hacks' and splices.

If you would not mind giving me a hint on where best to connect in to get a ignition live feed and which +'ve LT on the coil to attach to, I'd appreciate that.

Cheers,

Gregle

It sounds like you're either losing you power feed to the coil or the ignition amplifier isn't switching.

Have a careful look at the wires on to the coil and check for breaks or corrosion near the spade terminals. The spade terminals on the wires also corrode and break have a look at them.

I had a similar sort of problem recently and couldn't find anything wrong, it was driving me mad so opened up the loom because there is a splice on the power feed for coil and ignition amplifier - had a look couldn't see anything but cleaned the joint anyway and it's been fine since so might be worth a look.

If you don't want to go straight in and opening up the loom, try wiggling the sheathing near the alternator and see if it them fires up - if it does I suspect you'll have possibly a dry joint (at the splice) or a broken wire.

Also one other thing to check, make sure the connectors on the ignition amplifier are properly connected and are nice and tight.

Hope that helps, good luck.

Excellent - thanks guys. I have my work list now that I'll start on here shortly.

I will take some photos of the wiring because it has been seriously tampered with. I will trace the wiring around but I think it had to do with the cowboys that fit the LPG. From the looks of it, an ignition live wire has been spliced from the set of loom wires that come out near the steering wheel column in the engine compartment. The splice then goes to feed the coil and something else, I assume LPG kit. Honestly, I am super irritated by that conversion and I am millimeters away from tearing it out.

I would like to get back to original wiring at some point. Anyway - I am off to get some breakfast and head out to start working. I want to get my machine working again.

Gregle

There is no greater pleasure for me than ripping out bodged up **** and getting back to a nice tidy original loom.

I genuinely feel your pain!

UPDATE:

I stayed away from the loom because I was hot on the trail looking at the wiring to the coil.

Tracing the wiring to +'ve on the coil -- I found an in line fuse with a 7.5 A fuse that was blown. I replaced the fuse and improved the dodgy splices and yes! Power was reaching the +'ve on the coil.

I checked the connections to the coil 'tabs' to make sure they were clean and also made sure that the connections to the ignition amplifier were fully connected. Then I did further testing.

Here are the first set of voltmeter results:

1. Battery ( - to +) : 12.18 V
Yes, I know, a bit low and it needs a charge...

2. Battery - to Coil + : 10.62 V
Not good, but better than 0 - which is what it was.

3. Battery - to Coil - : ~ 2.0 Volts
WTF? This is not right. My understanding is that the volts should be at or maximum 1 volt below battery voltage. Something is wrong here - coil itself or ignition amplifier?

4. Ignition Amplifier / module to earth : 0.02 V
Seems OK.

Because of test 3 above - I did a further test as was suggested by the RAVE. I removed the LT leads from the coil on the coil negative side and re-ran the tests. This is what came out:

1. Battery ( - to +) : 12.35 V
Interesting, a better reading.

2. Battery - to Coil + : 12.3 V
Now the reading is in spec.

3. Battery - to Coil - : 12.3 Volts
Another measurement in spec. Seems that the coil is fine. Conclusion: I suspect that the ignition amplifier is goosed.

Would folks agree with this conclusion?

Finally, I tested the LT leads going to the negative side of the coil - the one lead that comes from the ignition amplifier was very low - like 2 Volts - and would not settle down - it was moving up and down. The other lead was pretty much at 0 V.

One question: with the line fuse blowing - there must be a short along the way - could a bad ignition amp short out the circuit?

If folks think a new ignition amplifier is required - I have looked through posts and I have read that Britpart and Lucas parts are not great - and Intermotor is a good choice. I cannot find Intermotor for Land Rovers on Rimmer or Island 4x4 sites. There are Intermotor ignition amps for Jaguars at Island 4x4 but no photos so I cannot tell if they are similar to Land Rover ignition amplifiers or not. Any suggestions?

Thanks for any comments and guidance.

Gregle.

Intermotor is available from motor factors, so speak to your local parts people.

Also Intermotor have their catalogue on their website, it's quite an interesting browse!

SMP EU - Intermotor

Thanks Mr Noisy! I have the Intermotor part number (15410) from the parts catalogue at SMP EU and will order this up.

So I take it that you agree with my conclusion - that I need a new ignition amplifier?

--Gregle

Ignition amplifier is a common failing but I would also suggest you check the pick up in the dizzy as it is this that connects to the amplifier. You need to check resistance across the pins. If you have been inside the dizzy and had it apart you should check the air gap. Do a search, I know I have posted on checking air gap before. Anyway, it sounds like you are making progress. Hopefully you have found that once you get your head around it, the ignition system is actually quite simple.

gregle said:

Thanks Mr Noisy! I have the Intermotor part number (15410) from the parts catalogue at SMP EU and will order this up.

So I take it that you agree with my conclusion - that I need a new ignition amplifier?

--Gregle

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It is a sensible possibility but Master Tech Kev is guaranteed to guide you in the right direction

Mr Noisy said:

It is a sensible possibility but Master Tech Kev is guaranteed to guide you in the right direction

Click to expand...

Thanks for that Noisy although I'm not sure I deserve "Master Tech" status, maybe "Good Amateur" is more like it.

I aim to please