Poor running (limp mode?!?) - 3.9 Rover V8

Lugy · Post by **Lugy** » Sun Jun 20, 2010 8:36 pm

Hi, first post from me after Google led me to this site.

I've just bought a Discovery 3.9 V8, it's a '96 3.9 EFi with autobox. I picked it up on Thursday and completed a 100 mile trip with no problems. The next day it developed an occasional slight misfire around 2250rpm but it would quickly clear itself and continue fine. Moving on to yesterday I took the car to help a mate break a car for parts, it was perfect on the way up (25 miles approx) then soon after we had finished we headed off. Doing some manoeuvring in his yard it began to stall, it'd start first time but as soon as load was put on it it would die. I got it going again and headed back onto the road.
After about half a mile it started to cut out at 1750rpm which I'm reading is the limp mode cut out, though I'm told that these engines do not have a limp mode. It drove as normal up til that point but would die back to idle and "bounce" to 1750 again.
I eventually got home (after crawling at 15mph up some hills!) and parked up. Some folks suggested a new fuel filter and luckily enough Halfrauds had one for me. This made no difference.

What we have now is it starts almost perfect though has stalled or dropped down to 0rpm and mysteriously started again. It is very lumpy when held at a fast idle and would seem to be running very rich, the oil has a slight petrol whiff and the plugs are black. It'll rev right up to 3-4k in neutral though will normally stall as the revs decrease.

So far apart from the fuel filter I've tested the CTS, cleaned the ICV, drawn the pump out the tank and cleaned the gauze, tested the fuel(?!?) temp sensor - well I assume that's what it is! Everything seems fine.

Any more suggestions? I read on another thread here there is a manifold temp sensor, will I have it and where would it be? Apologies about the long winded post, I must have covered everything!

Cheers

Lucas

Ian Anderson · Post by **Ian Anderson** » Sun Jun 20, 2010 9:29 pm

Sounda like Limp home

Disconnect the battery for 30 secs

reconnect it and it clears the Ecu - resetting it

If it then heads back to limp mode one of the sensors is failing

Ian

Lugy · Post by **Lugy** » Sun Jun 20, 2010 9:47 pm

Cheers, we've done this a few times today, it hasn't helped unfortunately. Any ideas as to what sensors to check?

Robrover · Post by **Robrover** » Mon Jun 21, 2010 7:47 am

Check the throttle sensor (TPS). My 95 Disco had something similar to this - only problem I've ever had with the hotwire efi system.

You may also want to check the MAF and the fuel pressure regulator and its vac hose.

Lugy · Post by **Lugy** » Mon Jun 21, 2010 9:51 am

The TPS would appear fine, when the throttle is closed it's 0 ohms then increases to around 4k ohms when open, I've read that could be about right. I tried disconnecting the MAF meter, it ran a little worse and when revved up it was much worse!

It's still using ridiculous amounts of fuel. Something else I've noticed is that the carbon canister has a hole in it (god only knows how!) and one of the take offs marked "AIR" has nothing attached.

Something else I've noticed is that lack of any ignition/warning lights. Is this right? Oh and I've read that there could be an on board diagnostic display which flashes a little LED, will mine have this?

DaveEFI · Post by **DaveEFI** » Mon Jun 21, 2010 10:06 am

Lugy wrote:The TPS would appear fine, when the throttle is closed it's 0 ohms then increases to around 4k ohms when open, I've read that could be about right. I tried disconnecting the MAF meter, it ran a little worse and when revved up it was much worse!

It's still using ridiculous amounts of fuel. Something else I've noticed is that the carbon canister has a hole in it (god only knows how!) and one of the take offs marked "AIR" has nothing attached.

Something else I've noticed is that lack of any ignition/warning lights. Is this right? Oh and I've read that there could be an on board diagnostic display which flashes a little LED, will mine have this?

The two sensors which have the most effect on the fuelling are the MAF and coolant temp.

Lugy · Post by **Lugy** » Mon Jun 21, 2010 10:15 am

The coolant temp seems to change well, I couldn't give an accurate reading on it because I don't have a thermometer!

The MAF I'm unsure of, it's giving a reading of 300 ohms when I take it off and is is affecting the running off the car. That's about as much as I can tell! I might just try and find another one, can't do any harm!

kiwicar · Post by **kiwicar** » Mon Jun 21, 2010 10:55 am

Two things make me worry here
"Something else I've noticed is that lack of any ignition/warning lights"
And
"I've just bought a Discovery 3.9 V8, it's a '96 3.9 EFi"
Where did you buy it? Private or trade?
I would find a copy of the official manual for the vehicle (not the Haynes) and start checking part numbers for items like the MAF and ecu. then start checking the loom for obvious mending and repair.
Best regards
Mike

Lugy · Post by **Lugy** » Mon Jun 21, 2010 11:20 am

It was trade seller, as for the engine lights I was led to believe only the diesels used them, though I was also told that there was no limp mode on the engines which it would appear there is!

DaveEFI · Post by **DaveEFI** » Mon Jun 21, 2010 11:53 am

Lugy wrote:It was trade seller, as for the engine lights I was led to believe only the diesels used them, though I was also told that there was no limp mode on the engines which it would appear there is!

What warning lights are you referring to? Have you got the driver's handbook - that should say which ones apply to the different engines.

Quagmire · Post by **Quagmire** » Mon Jun 21, 2010 11:59 am

Do you not even get an oil pressure light or battery indicator when key in and at position 1?

Not sure on what you get on a Disco (I have a 90), but you must have something like that before you start the engine.

Otherwise it sounds like the dash lights are not working, or have been disconnnected

Try asking about warning lights on LR4x4.com- very helpful bunch over there...

Lugy · Post by **Lugy** » Mon Jun 21, 2010 12:03 pm

Unfortunately the hand book is missing, there is a battery light and oil light, both go out as normal!

I'll head over there after work tonight

ramon alban · Post by **ramon alban** » Tue Jun 22, 2010 11:13 am

Lugy wrote:It's still using ridiculous amounts of fuel.

Hello Lucas, that is a clear indication of running rich and you need to check out the components that might cause that.

Coolant temp sensor, fuel pressure regulator and noisy throttle pot.

I am not familiar with your later Efi system but test processes for those three components are pretty much universal for the RV8 and you will find them here:

http://www.vintagemodelairplane.com/pag ... nts01.html

Note that when testing components one should also take into account the local wiring back to the ECU and/or earth.

A good component on the end of a duff wire is equally duff.

The ECU can also fail and the best method of testing is by substitution with a known good unit.

Lugy · Post by **Lugy** » Wed Jun 23, 2010 11:42 am

Well, I've just fitted a new MAF sensor, a quick run showed it's still in limp mode though the idle is better. I've disconnected the battery in the hope it'll reset any faults then I'll try again.

Lugy · Post by **Lugy** » Wed Jun 23, 2010 9:39 pm

Update for tonight, the idle was much better but we've still got limp mode, though for the first wee while it drives fine, then it stops being fine! Therefore it's something temperature related IMO. I've a CTS coming tomorrow which I hope will work, though I do have my doubts TBH, the resistance did change when we (unscientifically) tested it in cold water then hot water. The next step is a lambda probe I guess!