3.9V8 roughness

"new toy" is a bit iffy about cold starting: fires easy enough but drops back to below idle speed and stalls when stone cold.

once it's run for a bit to get warm (not up to running temp) it idles OK, although return-to-idle still drops below idle revs for a few seconds and then comes back up.

It's also slightly rough at certain rev/load conditions, most obvious is pulling off with not much boot. Not as smooth as I'd expect. better when it's up to temp, and it seems to pull OK if you give it some welly.

It is of course a 2-cat one, being a '95/96 model - so I'm not fully au fait with how the system works. I presume that initially, the system uses some default values until the lambda sensors are up to temp and working?

I'm thinking it might be mixture.

Any thoughts? I'll be looking at the plugs and likely replace them unless they look very good, and I'll alos fire it up in the dark and look for arcing in the leads, although they look quite good 8mm silicone ones, they're not original. ditto air cleaner element.

It's been standing quite a while, apparently - they got new motors a bit back and kept the discos as spares for several months - so it was MOTed in march and has done about 7 miles since. This may or may not be relevant - and it may settle down with a bit of use.

Other than that, it may have a slight appetite for coolant (need to monitor it), but then my 3.9 does that too, and so does the TD5 - I suspect a minor leak in each case, not enough to show as actual drips, which is just evaporating. Apparently, it had a gearbox replaced at about 75K miles, with a genuine dealer replacement no less. which is nice.

Reply to
Austin Shackles
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"Austin Shackles" wrote in message news: snipped-for-privacy@4ax.com...

Take the plugs out first, check for super steam cleaned ones. Odd it's using coolant, the 3.9's did have liner problems i believe...

Reply to
Nige

Check the leads......make sure they are ok and that they are not crossed incorrectly.

Reply to
Neil

Check also, all the magement leads and connectors to all the engine and related sensors. Airflow unit and lambda sensors especialy. People I know in the industry who service exchanged ECU's say that 99% do not have any "fault" themselves, but the internal logs are filled with details of sensor errors. Most will keep the engine running (when warm) with a "best guess" if a sensor seems to be playing up, but in turn the driver often notices odd problems such as you describe.

In turn, they say, that most problems are of the "Lucas Prince of Darkness" type, butchered or incorrectly re-assembled connectors, letting the weather in, and electrons out as a result. Most often caused by "gorilla tactics" of mechanics doing other jobs.

Of course, if coolant is getting out/in where it shouldnt, oooer..

I once had of all things (my first company car, passed down so I was not it's first victim) an early 2l Cavalier i with the same symptoms you describe. No coolant or oil leaks. But it kept burning out the airflow sensor, & they kept replacing it (each month under warranty!) No one wanted to accept that there was another problem somewhere.

My immediate past co-car, was perfect until it's first major service. After that, each time you changed gear, the stereo "Crunched" through all the speakers (worse at night with the lights on!) The main dealer service types refused to accept anything was wrong, I found (eventualy when I got p'd off enough to look myself) a loose bolt on a ground strap. Strap re-secured (very dificult to get at, I can see why it got left!) and problem cured. (And a "spikey" letter written to the service manager!) Never did figure out why that bolt was loosened either, it wasnt needed for the service.

Look for silly things like that, before digging too deep. It can be a long and involved job though. Keep records of what you find (if anything) and alter, so you can try and figgure out what affects it, better or worse.

Regards.

DJB.

Reply to
Dave Baxter

Austin,

Sounds like a bucket case. Stick it back on ebay.

Lee

Reply to
Lee_D

Mine did this after a rebuild - traced it to plug wires too close to the main alternator - battery wire, so it was probably spikes in the 12V feed to the ECU.

AC.

Reply to
Andrew Cleland

On or around Wed, 02 Dec 2009 10:35:54 +0000, Austin Shackles enlightened us thusly:

Nicked the leads off the other one, and that made it better.

changed the plugs yesterday, no signs of steam-cleaning (phew!).

leads made a noticeable difference; so might lash out on some decent new ones, will see how it goes. Might be worth getting the codes read, if I can find someone with a reader - could give insight.

plugs ahow rich if fired up for a minute or so and stopped - but the other day, when it'd been running for a while, they were more normal (though perhaps still a tad dark)

airflow connection looks OK. Lambdas would be a good check point. I might take it to the garage and get it on the EGA, see if that shows anything. don't need poisoned cats...

Reply to
Austin Shackles

coils breaking down?

Reply to
Nige

"Austin Shackles" wrote in message news: snipped-for-privacy@4ax.com...

Same issue with my '96 3.9 V8i, fires from cold but if I engage reverse and then go to Neutral the revs drop right off and it stalls. Once stalled it will not re-start for a good 5 mins, just turns over and won't fire. Almost as though it is flooded with petrol (If I am quick and get it into Drive before the revs drop, and press on the gas it is ok and again once warm runs absolutely fine.)

Could someone explain whey the leads being too close to things etc would cause these symptoms but only when it is first started from cold?

Reply to
Andy

In message , Austin Shackles writes

Worth checking their end-to-end resistance with a multimeter if you've got one to hand - showed up some 'gone-high' ones on my Minor 1000 back in the summer. Ran a lot better with new ones.

Reply to
Andrew Marshall

On or around Sat, 5 Dec 2009 12:21:18 -0000, "Andy" enlightened us thusly:

I'm still thinking sensors. Can the ECU be read for fault codes in-situ on these?

V8s are known for problems with shorting and cross-firing (firing the wrong plug) if the leads are dodgy - which will be more likely if they're damp - once the engine's running it gets warm and dries the leads out.

I still reckon the problem is a fuelling one though. This one is not quite as smooth as it should be on the road (although a lot better with the decent leads) - could still be leads, at that, although the set that's on it has never shown issues on the other motor. At certain revs/throttle it's a tad vibey, and I'm not sure it really pulls right in full-welly mode. Might risk a compression test...

My cruise control is now similar to yours, fixed the vacuum pipes and not working :-) I don't know if you get vaccuum unless the vehicle's in motion. Cruise doesn't work under a certain speed, I've an idea about 28 mph BICBW. Not that bothered, although like yours, it'd be nice to have it working just because.

Reply to
Austin Shackles

The pump is electric and should create vacuum once the master switch is engaged, I have metered the 3 pins and none go to 12v so I think mine is something to do with the inhibitor switch on the brake pedal or transmission select. (It lets me remove the key with engine off and in any gear on the selector whereas my 300 tdi only allows the ignition key to release if the box is in park so I suspect there is some link here).

If it turns out that the pump is shot then it's staying shot as they are now £300+ from LR!!

Reply to
Andy

Breaker?

Lee

Reply to
Lee_D

for a copy?

Reply to
Nige

Who let you off the play station! ;0)

Lee D

Reply to
Lee_D

When cold, the rich mixture needs a higher voltage to break down and make a spark. That in turn creates extra high energy interference that could just tip something over the edge into misbehaving. Other systems (the alternator for one) will be working hard too, trying to recharge the battery, and run everything else. That can (it can be measured) if the engine ground strap is not 100%, cause other things to be susceptable to the ignition interference.

Very lean mixtures too can create a need for higher voltatge spark, but by then, everything else has warmed up, so all the operating levels and timing could be different making a big difference in what it can mess with.

In general, keep HV ignition leads well away from any other lead/cable/wire if posible.

I once (many years ago) had an old V4 Transit (remember them?) where the alternator would often die for no reason. Eventualy, someone told me to move one of the plug wires away from it. It never needed another alternator replacing after that simple "fix".

Modern cars are a collection of network'd computers, powered by an engine. That engine in turn relies on all the compuers to be able to communicate reliably with each other.

Bring back moggie minors, bobweight timing advance mechanisms and SU carbs! Of course, a different era, and different faults and trouble. The difference is, they are a lot easier to understand for most of us!

Cheers All.

Dave B.

Reply to
Dave Baxter

On or around Sun, 6 Dec 2009 09:13:09 -0000, "Andy" enlightened us thusly:

On ours, the brake inhibit is a vacuum switch - there's a pipe to it; pushing the pedal lets air in and stops the vacuum being there. dunno if there's an electical switch too, mind.

Reply to
Austin Shackles

Jacob, the 3.9 engine's 14CUX injection system doesn't have a 9th injector, only the earlier 3.5 type with the 4CU "flapper" injection system.

Austin, I had similar issues on a customers 3.9 disco - tried replacing everything, including fuel pressure regulator and every sensor on the damn thing, right down to the ECU (and that's a right Royal pig to get at!) but all to no avail. I eventually sussed it's starting issues....... read on.

Something for you to try before you spend too much more time on it.... Next time you switch off for the night, open the bonnet and kill the engine by pulling the fuel pump relay (or fuse), NOT by switching it off at the ignition. Once it's stopped, turn off the ignition and refit the relay or fuse. Next morning, just turn the key straight to crank and see if it fires up any better/smoother. DO NOT turn the ignition on and wait for the pump to run or you will defeat the whole purpose of this test!

Did it start easier / better / smoother? If it did, then your injectors need replacing because they are leaking and allowing fuel to pass after you've turned the engine off, possibly to the extent that the pump priming prior to starting is enough even to give an exceedingly rich mixture.

Did it make no difference at all? Well, at least you haven't spent any money and you've eliminated any serious issue with the injectors.

Badger.

Reply to
Badger

On or around Sun, 13 Dec 2009 23:32:13 -0000, "Badger" enlightened us thusly:

I'll try that, might need some nifty work with the immobiliser that's on it. Although I could lock it and unlock it, that leaves the immobiliser off for about 30sec.

Started it today, first few times it fires and dies, then it fires and won't reliably idle or pick up off idle, then after a bit it will pick up and run OK.

It feels like lack of fuel, I could believe something like the pump being idle or the pressure regulation being iffy or something, making it slow to build rail pressure - so the first could of attmepts there's not enough fuel.

I'll be changing the fuel filter (since it's likely it hasn't been chanegd for a while) in a bit. I'd have though that a slow pump would be more problem all the time - last time I had one of those, it'd only run at idle - but the symptoms were a bit similar to this - dies as soon as you tried to increase the revs. However, on that one, the symptoms were persistent and didn't go away.

Reply to
Austin Shackles

Thanks Badger, I didn't realise the 3.9 only came with the hotwire, I was more worried it was a carb motor and I was totally out in left field rather than only being 'out' in the infield!

sdom and experience come with age, they say, but I do wish I could remember the darn question

Reply to
JacobH

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