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3.5 Efi Over Fueling


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1986 3.5 efi has been running 100% perfect until last week.

I had been out for a drive on the way home it started to cut out at low revs. To get home I increased the tickover on the throttle cable. after about two miles the tick over revs went up high so I put the cable back to it's original position and it ran perfect with good tick over. I did notice when it was playing up that it was smoking.

When I got home I started it a few times and the tick over was normal and no smoke. So I thought that was the end of it.

I got in it to go to a RTV trial on Sunday and as soon as I started it was obvious the problem had returned cutting out at due to lower than normal tickover and excess smoke. I drove the 20 miles to the trial site hoping it would fix its self again but no joy. When I got there it was still cutting out and excess smoke. Decided I could not drive an RTV like that so adjusted the throttle cable to give me normal tic over and set of home. Half way home the tick over revs went up. I got out reset the throttle cable and drove home about 10 miles and it was driving perfectly with no smoke and normal tick over. Got home switched it off and started it half a dozen times and all was normal. Got in it yesterday to try it and guess what the problem is back.

I'm convinced the problem is due to it over fueling. BTW I've had the ninth plenum injector disconnected and blanked of for years.

So the question is.... what will cause a 3.5 efi to overfuel after different running times?

I've just tried it with a borrowed ECU and Air Flow meter and it's still the same. These were known good units (thanks Nige T) so that rules them out. I also shorted out the temp sensor plug and that had no effect. The temp sensor also gave the correct readings, so for the moment I'll assume it's OK.

I tested the throttle potentiometer and had 4.35 volts between green and yellow (ECU voltage) which is correct. But when I measured between green and red I GOT 4.35 volts, the reading should be 0.3 - 0.36 volts. It would only adjust down to about 4 volts. Very strange was that as I opened the throttle (reved up) the voltage (green to red) went down so that on full throttle it was down to 1.5 volts. This is the reverse of what should happen the voltage should rise as the throttle is opened. All very confusing. I then tested the voltage at the Potentiometer plug from the ECU (potentiometer unplugged) As expected I still got 4.35 volts green to yellow but I got 2.7 volts green to red. I'm not sure if this is correct. I'm going to buy a new throttle potentiometer tomorrow but I would be very interested to know what voltage I should have on the green to red from the ECU does anyone know or could you check what your reads for me please. I can't find this info in any of the efi test sheets I have.

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Throttle pot should be 0.29 - 0.36 volts with throttle closed rising smoothly up to 4.6 - 5.0 volts as the throttle is opened up. Thats with the ignition on measured between pin 20 of the ecu plug and earth.

Sounds like throttle pot is most likely fooling the ecu into thinking the throttle is wide open hence over fuelling.

You can also check the overall resistance of the unit. With ignition off measure the reistance between ecu pins 3 and 25. Should be about 5Kohms.

To set the position of the new throttle pot with the ignition on, and the new pot connected measure the voltage between the red and green leads at the pot plug. Rotate the pot until you get a reading of 325mV + or - 35mV.



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Thanks for the replys guys.

I fitted a new potetiometer and it was still the same. fitted new coolant temp sensor and still the same.

I then went round all the other components in turn and found that by almost removing the over run fuel relay the problem was temporaraly cured. So I reckon it's something to do with that or the vaccum switch that controls it.

It's run OK since, I'm waiting for it to play up again. I will then replace the relay with a shorting wire if that cures it it will prove that the fault is in that circuit. As far as I'm concerend it is just another thing to go wrong so if it is that I will probably just leave it wired out.

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  • 7 years later...

Hi, I know this is an old thread but can anyone tell me what was the end result. i have the exact same problem and have run out things to change. the list of things change as follows, AFM, ECU, Fuel temp sensor, Injectors, TPS, fuel pressure 37lb through an FSA regulator, All earths cleaned and the sparks come from an edis 8 and megajolt system. The engine is an 1986 3.5EFI flapper. Any help or suggestions would be much appreciated.

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