3.5 EFI engine help needed

Gunny · Apr 23, 2009

Hi chaps 3.5 EFI 1988 When I start the engine from cold it runs up fine, ticks over as it should, throttles up and down ok, but as soon as the engine has warmed up it starts to hunt then eventually cuts out, I can start it up again but it will hunt, I can rev it but as soon as the revs drop it cuts out again. Leave it off for a couple of minutes and it will start and run fine again until it reaches that certain temperature then cuts out. I have changed the temperature sensor
Thermo sensor
Coil was new 8 months ago
New dizzy cap and rotor arm
I have checked all wiring cleaned up all relays, I have been told maybe its the air flow meter, but surly if it was that it would play up from cold not just at temperature, I have also been told maybe its the ECU, I have just about come to the end of the road and have nothing concrete to point at as the problem, Please can someone point me in the right direction.

swampdonkey · Apr 23, 2009

if it was a carb then id be looking a choke staying on from what you described....and it may be a long shot(but ive heard of it before now)but if the inlet manifold(s)are leaving a slight gap when the engine warms then again you could be getting probs only once warm from air weakening the mixture

Gunny · Apr 23, 2009

I see wear going, and it sort of makes sense because if air were getting in it would make the engine lumpy, but why only when warm

njh2 · Apr 23, 2009

Hi.
I don't want to give you too many things to think about but.........when my 3.5 efi did this it was the ecu overfueling the engine so when it was cold it was fine but as soon as it got up to temp it would hunt/cough and sometimes cut out but as i was running it on lpg it helped to isolate the problem. I gave this advice the other day on another thread for someone who was having the same prob and suggested he found a good non franchised dealer who would have a known good unit to check it with.
Cheers, Nick.

swampdonkey · Apr 23, 2009

as the engine warms and expands at a different rate from the manifold tyhen its not unheard of if the manifold isnt quite tightened down correctly for it to allow air in past the gasket therefore upsetting the fueling

Gunny · Apr 23, 2009

ECU I read somewhere I think if your getting under 16 mpg then the ECU is faulty, I think Im getting about 15 mpg, but the car has always run ok until now.
Swampdonkey
Thanks for that imfo Ill check that in the morning, I did replace head gaskets about 18 months ago, Ill go through air pipes as well.

ramon.alban · Apr 24, 2009

Hello Gunny, You have probably developed a rogue air leak into the plenum, and this compromises the mixture below a sustainable ratio as the engine gets hot and the temp sensor stops any cold running enrichment.

Your '88 system will be similar to the SD1 in many respects and you can find a comprehensive air leak detection program on my website here:

Rover SD1 Efi Plenum Chamber

there are other possibilities, and from the component index you can read about all the efi components and how to troubleshoot them.

iconic · Apr 24, 2009

Just to clarify, are you saying runs OK when driving along but when idling it cuts out?

..also, not sure about the 3.5 but the later 3.9 have 2 temp sensors, one for gauge an one for ECU?
I checked Paddocks website and couldn't see a seperate ECU sensor for the 3.5, although this doesn't mean there isn't one!

3.5 EFI engine help needed

Gunny

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swampdonkey

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njh2

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