Misbehaving Defender 4.2efi

Hi all, anyone done battle with a troublesome V8 efi?
Details: 4.2EFI hot wire 1994. In a defender 90 truck cab.
Problem: poor cold start. Fires up straight away then dies immediately but fires up and keeps going with throttle. Once it's been idling for a few mins it either suddenly cuts out or it sttumbles nearly stalls then idles rough and massively overfuels, acrid black fumes from exhaust. I switched off the fuel pump whilst it was doing this and it idled alot better and kept idling for about 10 mins before it finally stopped!!!!
Summary: seems short of fuel on start then when warm massive overfuelling. Keeps running for an unusually long time with fuel pump switched off! Any ideas? Changed water temp sensor all plugs are now sooted up as you would expect, I'm thinking fuel pressure maybe anyone had similar?? Need help the fumes from today alone must have knocked years off my life expectancy!:*)

You are on the right lines with fuel pressure and strange as it may seem it's not that you have to much pressure.
If the fuel pressure is to low the engine ICU try's to prevent a damaging lean mix by going into a fixed over fuel setting.
So check fuel filter/pump.
I have not had much to do with EFI's but this turned out to be the problem with a mates trialer I drove.

tottot said:

You are on the right lines with fuel pressure and strange as it may seem it's not that you have to much pressure.
If the fuel pressure is to low the engine ICU try's to prevent a damaging lean mix by going into a fixed over fuel setting.
So check fuel filter/pump.
I have not had much to do with EFI's but this turned out to be the problem with a mates trialer I drove.

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Thanks! I never thought of that, pump and filter are fairly recent, so I'm thinking regulator?!

Back up a bit!!!

Symptoms sound exactly like an inlet air leak somewhere. What happens is that it will fire up then the mixture goes lean so it dies, add a bit of throttle and the effect of the air leak is less pronounced so it will run with a bit of throttle if you see what I mean.

I would be inclined to pop off the plenum/ram housing clean it all up then re-assemble. Check all the vacuum lines including the vac advance to the distributor (if it has one). It might be useful to check the ECU for any fault codes. If you don't have a fault code reader you will have to do things the hard way with a multimeter. You will also be needing to re-set the ecu.

Assuming you have cats and o2 sensors (should have in a '94) an air leak will make the idle mixture too lean. The o2 sensors, once warmed up, will detect the lean mixture and tell the ecu to inject more fuel hence the overfuelling. You need to resolve this ASAP or you risk killing either the o2 sensors or the cats or indeed both.

Forgot to say, leaking exhaust might also cause o2 sensor to tell ecu it is too lean so dump in more fuel but that shouldn't cause stalling or idle problem when cold as the ecu doesn't look at o2 until warmed up a bit. Also, puzzled how your V8 can run for 10min with fuel pump switched off???? Mine will stop in about 10 seconds! Hey ho. Anyway, you might find more info if you jump in to the V8 section. Good luck!

Hi thanks for reply, it doesn't have cats but still has o2 sensors. I have followed your advice and booked it in for a diagnostic on Friday, I won't run it until then. I couldn't afford to financially (drinking fuel) or for my health, it almost choked me to death afew days ago under the carport! Come to think of it I haven't heard next doors dog bark for afew days either!!

Let us know what they say. My heart tells me it is a small inlet leak somewhere.

I would go with an air inlet leak also.
What condition is the hose aAFM to plenum housing, take it off have a good look at it.

Cheers I will have a better look tonight.

classic kev said:

Let us know what they say. My heart tells me it is a small inlet leak somewhere.

Click to expand...

Ok will let you know! Thanks

Well I got the washing up done and the kids in bed early, bought a bottle of wine and nibbles for the wife. That bought me a couple of uninterrupted hours with the landy! So this time I actually removed the inlet tube and had a good look it looks in perfect condition but when I flexed it I noticed a 2 inch slit running parallel to one of the wire ribs it was facing the bulkhead so was missed on my first 'inspection' I wrapped some black tape around it to see if it made any difference, after a splitter and a cough she roared and idled without overfuelling!! She just seems to be idling a bit quicker now though 1000rpm. I think a plug change is now in order and a good drive to see if the ecu will settle things down! I cannot thank you enough for putting me on the right lines. 423"]I would go with an air inlet leak also.
What condition is the hose aAFM to plenum housing, take it off have a good look at it.[/QUOTE]
Hi, well did the washing up got kids in bed ear

Good job then,it does not take much to upset an efi.Trialer threw a wobler the other day because manifold gasket blew.

I've taken the day off tomorrow to 'tinker' so fingers crossed she will be back to normal. Cheers

Just catching up on this one!

Glad to hear it has been sorted and without the expense of a garage visit. Did you remember to reset your ecu? Just unplug it for a couple of seconds then reconnect or disconnect the battery if you fancy.