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Hotwire EFI running problem


tiddler
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Sorry if this has been covered before. I did a search and couldn't find anything so here goes.

I have a late 3.9 serpentine v8 running hotwire.

After standing for 6 months whilst we did some work to the car including plenty of welding, it would not start/run/idle well. We've gone through the test procedures that HFH posted and replaced fuel pump, cts, plugs, dist. cap and rotor,lambda sensors and AFM. We have not however tested the ECU or been able to try a "known good" alternative. All earths have been made good and there is good voltage all around the system, now here's the problem:

If you start it as normal it dies or pops and bangs on opening the throttle.

If you remove the AFM plug, start it and rev it, then replace the AFM plug it idles and runs/picks up very well EXCEPT the idle CO reading will not go below 15%.

Any ideas welcome. I have a spare Megasquirt ECU which is starting to look very tempting! :angry:

Thanks,

Giles

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:o

Erm

When one did the welding one did disconnect the ECU from the RR didn't one ?

If not then high chance your ECU is fried sir

Nige :huh:

Oh dear,

That is the simple answer I was sort of expecting, but did not want to hear :(

No, it was not disconnected. yes it probably is toast.

Can the ecu be tested or repaired? Any recommendations? Mark Adams maybe?

Thanks again,

Giles

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Sorry to be stupid

but if the battery is disconnected would the ECU still be affected.

My LSE has been fine after loads of welding work with the battery removed having said that the Air Flow meter did fail very soon after.

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In short - yes Jules. There are all sorts of strange effects when passing high current through something as weird (in electrical terms) as a Land Rover chassis.

You're throwing a lot of electricities at the thing, all trying to find the shortest route from welding tip to earth clamp. It doesn't take many to go via the ECU to do damage. Think of it like when your engine earth strap gets old and all of a sudden the starter motor is earthing through the sleeve of the throttle cable instead :blink:

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Sorry to be stupid

but if the battery is disconnected would the ECU still be affected.

My LSE has been fine after loads of welding work with the battery removed having said that the Air Flow meter did fail very soon after.

I believe the battery positive was disconnected, but earth still attached. My welding friend is more used to working on diseasels and didn't think it would be an issue, until he came to start it!

Time to track down a nice working ecu and try it I think. :(

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If you start it as normal it dies or pops and bangs on opening the throttle.

If you remove the AFM plug, start it and rev it, then replace the AFM plug it idles and runs/picks up very well EXCEPT the idle CO reading will not go below 15%.

Giles,

This sounds very similar to problems that I had with mine after the car swallowed a load of water on a Hillrally stage. We eventually tracked it down to mud on the hotwire in the AFM.

My understanding is, when you replace the AFM plug after you have started it, the ECU has already swapped on to the limp home map - which overfuels to be safe - and will effectively continue to bypass the AFM. I would personally try swapping out the AFM for a known good one, before trying the ECU. You may have got away with the welding!

H

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Giles,

This sounds very similar to problems that I had with mine after the car swallowed a load of water on a Hillrally stage. We eventually tracked it down to mud on the hotwire in the AFM.

My understanding is, when you replace the AFM plug after you have started it, the ECU has already swapped on to the limp home map - which overfuels to be safe - and will effectively continue to bypass the AFM. I would personally try swapping out the AFM for a known good one, before trying the ECU. You may have got away with the welding!

H

Not sure of the exact model you have but fully disconnecting the ECU plug can reset some units

Worth a try

To see if the AFM is faulty a quick test is when it is running take the AFM Plug out , this will trigger max air and max fuel, and it should run hugely rough n rich, if there is NO change then suspect AFM rather than ECU

Nige

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Tiddler, the initial symptoms before you play with the AFM plug sound like a lack of fuel. Have you checked the fuel pressure with the engine static and running? It may pay to do so. As Henry suggests after you remove/replace the AFM plug the ECU has probably swapped to the limphome map and the longer injection times will pass more fuel through allowing the engine to run.

Ideally it would be good to eliminate the ECU and AFM by "known good" substitutions. Even though they are "Lucas" they are fairly well protected internally.

Do you know the history of the AFM you swapped ? Was it "known good" or just an available spare of unkown parentage?You can watch the operation of the AFM with a voltmeter, again, worth trying.

jw

Afterthought: Have you checked the operation and connectivity of the throttle position sensor?

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Easy fuel check is to splice a cheap inline filter on the return line (from the regulator), then you can see how much fuel is returning (or not). It should be a good flow at tickover/static revving. If it's not, suspect lazy fuel pump (which would cause underfuelling) or sticky regulator (which would cause overfuelling in my experience, they tend to stick shut).

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