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3.5 efi V8 - cold start not working?


simon90
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Does anybody know about the ‘cold start’ on a 1986 3.5 efi V8 (from a Range Rover)? Apparently it’s the reason why the engine is a little irate when first started, but sorts it self out when warm. Has something to do with the automatic choke?

It’s a Landrover 90, but with a Range Rover engine…….so what section do I use!!!!! :blink::blink::blink::blink:

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stuck in the rear of the plenum (ie near the firewall) is a black blob, this is the IAC (stepper motor). It opens and closes a valve to let air into the engine and thus increase tickover (when using a/c etc) or as a choke.

Take the plug out and it will probably be all gunged up- give it a good clean (i did mine in petrol, but i'm not sure if you're meant to- it certainly got it clean) and check to see if it works. when the car is cold, start the engine and it should open and then close as the car warms up (although with a big hole in the plenum it might run a bit oddly).

It does lots of other odd things iirc but you would have to ask a proper grown up about the advanced stuff.

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IIRC the stepper motor is only for the hot wire system, and if the injection system is from a 1986 this shuld be a flapper type.

Quickest way to tell is to look at the plenumn chamber. If it is flapper, there is a 9th injector sticking out the side of it on the drivers ( RHD) side.

Rather than re hash all the excellent advice already given, have a look at the tech archive as there are some very good PDF files for each system

HTH

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I agree with roverdrive - full diagnosis is the only way to sort efi problems. If you decide it is one problem you will end up going in circles..

To answer your question the cold start device (for a flapper) is a small aly cylinder at the front left of the engine as you look at it. When you start the engine the valve inside is open and lets more air though. This valve closes fairly quickly with the ecu controlling mixture.

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For a cold start (cold engine) on the flapper extra fuel is injected into the plenum chamber by the injector that sticks out of the side of the plenum (top centre on the drivers side. This is controlled by the thermotime sensor which is the front of the 2 sensors on the top of the inlet manifold. When the switch goes faulty the usual result is overfuelling which may cause starting/running problems. (The rearmost of the 2 sensors is the temp sensor & when that plays up it also usually causes overfuelling & is the cause of many mysterious running troubles for which the ECU is usually wrongly blamed) Generally the cold start injector serves little purpose in temperate climes & many just unplug the electrical leads from the injector. I did just that when I bought mine & it makes no apparent difference to the starting.

The extra air valve opens to allow extra air to maintain satisfactory cold running until the engine reaches proper operating temp. The air is taken after passing through the air flow meter so it is registered by the ECU & returned to the plenum on the engine side of the throttle butterfly. The valve closes as the engine reaches normal operating temp.

What exactly do you mean by 'a little irate'?

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Hear goes! When first started it sounds like its going to stall (battery warning light flickers), then it picks up. The revs seem to rise and lower as if its trying to settle down but something is throwing it off. When you press the accelerator pedal, it almost stalls as the revs instantly drop. After a few mins, it seems to settle down to normal idling.

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Cold start on a 3.5 flapper is done via the 9th injector, and that only fires during cranking. It is controlled by the thermotime switch (brown topped sensor) situated next to the coolant temperature sensor………….. but that is not your problem……..

It sounds like the coolant temperature sensor may be faulty ( the blue topped sensor), or the EAV has seized closed.

Always, on a cold engine the eav (being a bimetal valve) will allow extra air until it is closed by internal heat from the coil and then engine heat to keep it closed………. The extra air gives you the fast tickover needed for a cold start.

Also check all the hoses around the plenum for splits …………. Loss of manifold vacuum will give the same symptoms.

:)

Ian

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The 9th injector doesn't seem to make much difference, connected or not :rolleyes: it's a factory bodge anyway.

Dodgy running could be a few different things, your best bet is to go through the PDF file in the tech archive methodically and check everything - otherwise you end up going round in circles.

Since it's traditional, I'll say now that it is usually quicker to fit MegaSquirt than diagnose Lucarse EFi :P

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