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Out of ideas

robcom

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Hi everyone, I,ll get straight to the problem at hand and thanks in advance to the people who get to the end of this post.

I just got my car dyno tuned (VH with VN injection unregistered), when I pulled it off the trailer it was running great up and down my street with not a hint of a problem. Then something went wrong, here are the sequence of events.

Between putting it in the garage and starting it the next time all I've done is wrapped some cables with heat sheilding tape and double clamped my fuel lines, inlet and return being careful not to clamp the fuel supply.

When I went to start it the next time the battery seemed dull but it started and I let it idle for a while and then gave it a rev and it ran like a bag of **** like it was flooding or missing real bad.

So I did some preliminary checks and found the alternator was shagged so I replace that and still no good.

Then I started checking leads and coils but the spark seems good and the leads have good resistance values and there is a good spark.

Then I checked all the sensors for resistance values and voltage values as by the manual. and all check out fine. Yet to do the o2 sensor.

Fault code 12 showing in diagnostic mode so all OK.

When I rev it, if I give it a quick stab to full throttle the thing bogs down and won't rev over 2-3 thousand and black smoke coming out of the exhaust. But if I gently bring the revs up it will got to full revs but coughing and spluttering all the way.

The plugs are black as the ace of spades so I am definitely getting fuel, just too much of it or not enough spark.

Here is the tricky part, when I disconnect the MAP sensor wiring the thing comes back to life, revs freely and no bogging down.

Remember I haven't checked the o2 sensor.

Does anyone have a suggestion as to what this is?
Has anyone seen this before?

A bit confusing when the MAP sensor wiring is disconnected and it goes good seeming that this sensor is a major part of the system.

Thanks
Rob.
 

vt1538

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When the map sensor is disconnected the ecm will go to a look up table referencing throttle position to meter the fuel.
Sounds like you may have caused a wiring issue when you wrapped the wires.
When you run it with the map sensor disconnected then did you get a trouble code?
 

robcom

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Haven't checked that, I've been too busy trying to find a hard fault that may be the problem. I will check that this arvo when I get home from work. I may have to unwrap those wires and see what has gone on there. The funny thing is that the wires I wrapped were only feed and ignition wires to the starter motor which also incorporated the o2 sensor wire.

Thanks for the input and I will let you know what comes of it.

Rob.
 

robcom

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No, I have checked and double checked hoses, sensors etc. Would you know if the o2 sensor would cause it to run so rich. Or is it just a sensor to let you know that you have a rich or lean condition?
 

vt1538

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The O2 sensor has no input until the engine gets over 40 deg & the exhaust is hot.
When it starts generating a signal it is used to trim the fuel to the set air fuel ratio. So basically no. While a faulty sensor can cause the engine to use excessive fuel it doesnt appear to be your problem.
Have you checked your fuel pressures ?
 

Chuff

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I agree with vt1358. Sounds like the MAP hose has been disconnected from the manifold so the MAP sensor is receiving no vacuum indicating to the ECU that you've got it floored, therfore the ECU pumps heaps of fuel. Removing the MAP wiring plug fixes the problem as the ECU no longer relies on the MAP value.

You said you have checked the hoses. Have you actually disconnected the hose from the MAP sensor at the MAP sensor, not the manifold, and checked for a valid vacuum signal?
 

robcom

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No I haven't, and I haven't the facilities to do so. Are you thinking that the fuel regulator has **** itself and is letting in more fuel than it should?

If so that wouldn't explain the car behaving normal when the MAP sensor wires are disconnected .

It's like everything I've tried so far, think it might be spark or fuel but then it run well without the map sensor. It sort of blows every theory out the window. This is the frustrating part it feels like it should be the MAP sensor but then when I back probe the MAP sensor it has 4.8 volts with no vacuum and when I suck on the pipe the voltage goes down like it should so that tells me that the MAP sensor is good. When it is backprobed during idle it wavers between 2.7-3.2 ish because of the bad idling.

Here's another theory to run by you, the wires that run to the distributor if they have been disturbed could that mean that the timing is not being advanced and therefore perform like it is.

Hang on disregard that last theory because when I disconnect the MAP sensor it all run OK.

See how frustrating it is.

Appreciate the feedback
Rob
 

robcom

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Yes I have disconnected the hose and there is a valid vacuum signal and this is indicated by the voltage received when the MAP sensor is backprobed when idling in the earlier post you may have missed, 2.7-3.2 ish volts. The one thing I haven't done is backprobed the MAP sensor signal at the ECU but it should have the same voltage as at the sensor itself but stranger things have happened.

The way it looks is the MAP sensor supplies approx 5.0v with no vacuum and approx 2.5ish at idle so I believe the signal from it to be correct.

Thanks
Rob.
 

vt1538

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First thing that I would do now is remove the shielding from the wires & see what happens.
Running a current next to a signal wire can sometimes cause interference. This mainly happens with the coil or pip signal wires.
I am not sure on the V8 set up for wiring. Is there a start signal wire from the starter to the ecm? If so has this been put back on the right terminal?
 
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