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3.9efi V8 90 cold idle problem

Chouet

Chouet

In Fourth Gear
Hi all

This is my first post, having just purchased an '87 90 fitted with a disco 3.9 V8 and auto box. It is also multipoint LPG.

It won't idle when started from cold, but is ok when up to temperature. It starts first turn, but needs a bit of throttle to keep it going. I've looked for vacuum leaks etc and have found some things I'm not sure about.

The fuel pressure regulator has a pipe attached but the other end isn't attached to anything. I can only see two places a pipe could attach with nothing there - one on the RH rocker and the other on the distributor behind the vacuum advance pipe.

Any thoughts?

Thanks in advance

Nick

p.s. very much a novice - haven't tinkered for 20 years!
 
Is this starting and idling cold on LPG or petrol?

It might depend on the LPG installation as to what has been done to the engine. Some LPG installations bypass the Idle control valve. (Rear of the plenum with the 4 pin connector plug) This valve is what allows the engine when running normally (I.e on petrol) to maintain it's idle speed and also when starting from cold to admit more air thus causing the engine to inject more fuel.

The pipe from the fuel pressure regulator goes onto a connection when the Idle control valve is on the rear of the plenum.
 
Check the pipe Devon-rover suggested. May need to give the stepper motor (idle control valve) a clean or possibly a replacement.
 
There is an open port on the idle control valve (image below), though it looks though it's been exposed for a while. I've hooked up the pipe from the fuel pressure regulator to it - does it look correct in the second image?

I haven't tried starting it yet - it's too late to risk the wrath of the neighbours (it's loud!).

v8 90 Idle Control Valve.jpg v8 90 fuel pressure pipe.jpg
 
Reconnecting the pipe doesn't solve the cold idle on petrol and make's it run extremely rough on gas.

I wonder if it's been disconnected because of the gas or to get around a fault with the idle control valve? As you can see on my previous image, the port is corroded and looks like it hasn't been connected for a long time.
 
It could well be. As i posted before sometimes the whole idle circuit is bypassed to allow the LPG system to work. On my old 3.9 disco the stepper motor of the idle valve was disconnected fully closed and then the throttle cable was wound up to compensate. It gave a similar idle on gas and petrol when hot but was like you it did mean having to 'keep it running with the throttle' when cold.
But then i was running the single point cooker ring. A multipoint system should be more than capable of keeping it even when starting.
It's hard to think without seeing exactly what system you have.
 
The idle loop doesn't need disconnecting when fitting e "cooker ring" mixer to these as the take off for the idle valve is in the inlet to the throttle body - after where you attach the mixer. Also, as the OP has a multipoint system, gas is only added near the inlet valves so it doesn't matter whether the air went through the main throttle or the idle valve.

Now, to the poor running ...
There is something badly wrong with the injection setup as it should run like a dog with that pipe off - wrong fuel pressure and an air leak. It'll be worse on overrun with throttle closed when manifold vacuum is highest. So it really needs the petrol side sorting out, and only then can you look at the gas.
It might just be a matter of using it a while now the pipe is reconnected - if it's a system with lambda sensors then the fuel trims will have adjusted themselves way out and they'll take a while to reset. If not, then get the workshop manual for the vehicle the engine came from and work methodically through the test procedures.

Now, the gas system will be slaved from the petrol injection. Basically it disconnects the petrol injectors and looks at the signal, applies some timing adjustment, and then fires the gas injectors. If the petrol system is messed up, then the gas will be as well and it's no use trying to adjust the gas until you've sorted the petrol.
 
Thanks for the replies. I've taken the idle control valve off and it was pretty clean. however, there's no washer on it and the brass nose on the valve is marked which leads me to suspect it's been off before. Should it be possible to move the valve? It doesn't budge at all - appears to be totally seized.
 
The valve won't move by hand - it's driven by a stepper motor connected to the ECU.
Connecting it up and turning the ignition on/off should make it move, but don't do that before searching as IIRC it's one way to get it apart and have bits pinging out :eek:
 
I had tried turning the ignition on to see if the valve moved, but it didn't. I wonder if, given that it's an engine transplanted from a Disco, the valve wiring hasn't been connected to the ECU.
 
fortunately the wiring for the 3.9 hotwire is pretty simple, there is a big fat plug goes onto the ECU and a whole series of smaller plugs that connect to the various bits on the engine, all you need is the colour chart to go with them to determine what goes where, though I found that if I laid the harness on top of the engine it was pretty obvious which plug went onto which socket.
Luckily I managed to pick up a harness for a 3.9 without the connections for the cats.
 
I wonder if, given that it's an engine transplanted from a Disco, the valve wiring hasn't been connected to the ECU.
Click to expand...
That's actually hard to do unless you hack the loom about. Usual method is to just transplant the whole loom, ECU etc - and that only connects to the main loom with a few connections. In effect, the injection system is almost self contained and needs little more than a power supply and ignition switch signal.

On my 110, the connections are : large brown wire off the battery terminal on the starter, white wire (ignition circuit) off the fuel pump connection on main loom, fuel pump output to fuel pump connection on rear loom. Haven't got around to adding the road speed transducer (RST)yet, nor wiring up a MIL (Malfunction Indicator Light), yet - the RST would be a good idea as without it the ECU does get quite upset and turns the idle speed up to around 1100rpm.
 
And the earth connection which I read somewhere should be fastened to the block. Though I bolted mine to the chassis and it didn't seem to cause any problems

SimonHobson said:
That's actually hard to do unless you hack the loom about. Usual method is to just transplant the whole loom, ECU etc - and that only connects to the main loom with a few connections. In effect, the injection system is almost self contained and needs little more than a power supply and ignition switch signal.

On my 110, the connections are : large brown wire off the battery terminal on the starter, white wire (ignition circuit) off the fuel pump connection on main loom, fuel pump output to fuel pump connection on rear loom. Haven't got around to adding the road speed transducer (RST)yet, nor wiring up a MIL (Malfunction Indicator Light), yet - the RST would be a good idea as without it the ECU does get quite upset and turns the idle speed up to around 1100rpm.
Click to expand...
 
I may have made a bit of progress. On taking the idle control valve off again, I noticed that the lugs on the wiring socket have been cut off and don't match the cut-outs on the harness. I think it's the wrong part on the car. The closest I can find is the Lucas 70409A:
Idle_Air_70409A.jpeg

However, the plug on my harness has a cut-out on 3 pins, not 2. (as this image, but both left-hand ones need a lug). Could this be the right part?
 
Two new idle valves and two V8 specialists later, the v8 runs and idles properly on petrol when warm (idles a little low when in drive on lpg), but still won't run from cold (or a restart) without holding it on the throttle.

What controls the mixture and idle speed when the engine is cold? Is there a sensor I should be checking?
 
Idle speed is controlled by the ECU, which sends pulses to the stepper motor to wind it in or out.
What's supposed to happen is when you start, the ECU opens it up, the revs are high(ish) and the ECU then winds in the valve to get a fast idle - reducing it as the engine warms up I believe. There is an extensive section on diagnostics in the WSM which is worth reading.
 
Hi.

Grab a beer or two and read this to gain in idea of what does what on the 14CUX system http://www.britishv8.org/articles/rover-14cux-efi.htm

As described above at the last switch off the engine winds the ICV right back and then on start up the above procedure raises the idle and brings it back down. There is a base idle setting but the Rover V8 specialist should of done this for you as part of your initial complaint.
 
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