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

92rrrandall · Post by **92rrrandall** » Thu Jun 24, 2010 5:06 am

Lucas

If just one of the spark plugs is black then you have a leaking injector.

If one of the grounding straps has been disconnected then there will be kaos under your hood. There should be a grounding strap going from the block to chassis. There should be a grounding strap going from(the back) one of the cylinder heads to chassis. There should be a grounding strap going from the starter bolt to chassis. Also check both ends of both battery cables.

You should use a vacuum gauge to see if there are any vacuum leaks or other basis engine problems.

Use a timing light to determine if your ignition is advancing or working correctly when you rev the engine. The neg terminal of the ignition coil is the most important input to the ECU. Any miss of the ignition will have a corresponding miss of the fueling.

Randall

Lugy · Post by **Lugy** » Thu Jun 24, 2010 10:04 am

Hi Randall, all the plugs are black.

I'll check all the earth straps, though I do still feel it's something temperature related with it running fine for the first few minutes, would I be right in thinking that it'd be unlikely to be the fuel temp sensor? From what I've read it's main purpose is with restarting a hot engine?

ramon alban · Post by **ramon alban** » Thu Jun 24, 2010 3:18 pm

Lugy wrote: 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.

Lucas, Just because the CTS looks OK on the kitchen table does not mean that is what the ECU will see.

You must take into account the connections between the CTS and the ECU and the CTS earth connection usually at the rear of the block - as mentioned in my earlier post and as described in the test process.

A broken connection or bad earth thereabouts, makes a good CTS look faulty to the ECU resulting in the ECU thinking the engine is cold, when it is hot and it over-fuels accordingly.

Result, black plugs.

Of course there are other causes as already mentioned, but replacement components are useless if the connections are duff.

92rrrandall · Post by **92rrrandall** » Thu Jun 24, 2010 3:31 pm

Lucas

I am assuming you have the 14CUX system, not the GEMS system??

The fuel temp sensor is used for hot start-up. It is also used to turn on the electric fans for 10 minutes if the temp gets too high.

Temp sensor esistance gets lower as the temp rises. So if the coolant sensor is not working due to the wire or sensor then the ECU thinks you are at the north pole(full enrichment).

Disconnect the FI harness from the computer and probe the connectors 25 and 7 with an ohm meter. It should give a resistance in ohms. If there is no ohms (or continuity) then the wiring or sensor is bad. Cheap and easy to replace anyway.

Randall

Lugy · Post by **Lugy** » Fri Jun 25, 2010 11:34 am

Well the earths seem good and changing the CTS has made no difference.

It drives fine for the first 1/4 mile whether hot or cold then it goes back into limp. If you turn it off it'll be fine again when you start it again.

I've also just limped off to a local garage I deal with who plugged their 2 test books in but neither would communicate, one was a Snap On one and t'other was a Sykes Pickvant.

Next on the list could be Lambdas the mechanic suggested.

Oh and it is the 14CUX!

Ian Anderson · Post by **Ian Anderson** » Fri Jun 25, 2010 12:57 pm

Pressure reliev valve not working
So fuel rail pressure increases causing overfuelling

If you have a tame mechanic ask him to put a pressure gauge in there to see what the pressure is doing
Ian

Lugy · Post by **Lugy** » Fri Jun 25, 2010 3:36 pm

By pressure relief valve, is that what I'd call the fuel pressure reg at the back of the fuel rail?

Would this be giving me my symptoms, IE starts and drives fine for a small while then goes into limp mode maybe as the pressure builds up too much?

Any info on your '40? Looks and sounds quite interesting

Ian Anderson · Post by **Ian Anderson** » Fri Jun 25, 2010 4:03 pm

Yes the fuel pressure regulator thingy at the back of the fuel rail.
On EFI fuel continuously passes through the fuel rail - through the pressure regulator and back to the tank

Also make sure the vacuum pipe is attached to it so that it adjusts the pressure depending on throttle opening / manifold pressure - need higher pressure running than at idle etc

The area between the fuel pump and regulator is at high pressure the rest is at low pressure

Or how about something stupid like a piece of paper / crud etc jamming in the air intake area - gets sucked in after few mins running and when engine turned off debris falls away.

Gt40 - yes loads of fun
Build log is here
http://www.gt40s.com/forum/gt40-build-l ... ld-up.html

Cheers
Ian

Lugy · Post by **Lugy** » Fri Jun 25, 2010 5:14 pm

Cool, I'll sniff around there tomorrow. I doubt it's something like paper as I've had a good bit of the inlet off as part of my "leak" check. I wouldn't be surprised if it did turn out to be something silly though.

Cheers for the link, I've started to read through. Maybe with my new job I'll have the pennies to build something like that!

ramon alban · Post by **ramon alban** » Sat Jun 26, 2010 6:43 am

Lucas, When you measured the CTS resistance from the ECU multiplug what reading did you detect. I should be typically something between 200 ohms and 6000 ohms, depending upon coolant temperature?

Lugy · Post by **Lugy** » Sat Jun 26, 2010 11:13 am

That ball park sounds familiar from when we took it out the hot engine then into a tub of cold water.

I've not done the new one yet.

Another thing I've found though, the throttle POT, which was tested before and seemed fine now has mysteriously stopped working, there's 5v going in(?!?) but no change when the throttle is opened. I've taken it out and there is no resistance change on it

Lugy · Post by **Lugy** » Sat Jun 26, 2010 4:02 pm

Well she lives again!

I managed to track down a replacement pot (no mean feat on a Saturday round this way!) and although it had an earlier connector I managed to improvise

.

The old one must have been breaking down inside but still working occasionally. I'll get another one during the week so it doesn't happen with this second hand one.

Ian Anderson · Post by **Ian Anderson** » Sat Jun 26, 2010 7:15 pm

Good news

Never let "Lucas - the prince of darkness" beat you - he'll try regularily though!

Ian

Lugy · Post by **Lugy** » Sat Jun 26, 2010 11:37 pm

Well it now runs absolutely sweet as a nut, it's used a fair bit fuel tonight though we've done a lot of off-road work, around 100 miles to a quarter and a wee bit of the tank.

At first it wouldn't idle very well, it had to be "coaxed" until it caught up otherwise would stall, not it must be used to doing it properly and it's learnt to do it by itself

Anyway, I'm happy enough, until it breaks again, just need to service it then see if any goodies can be bolted on

welshphill · Post by **welshphill** » Wed Mar 23, 2011 5:48 pm

can you help me, i have a VERY simular problem on my 95, 3.9 v8 discovery...

what do you mean when you use the term throttle POT???

is it the throttle position sensor???

many thanks