Got a problem I am struggling with.

When I hold the throttle open at approx 3000rpm the revs cut/drop to around 2000, then rise to 3000 and drops again, and so on.
If I open the throttle more, the same happens BUT at a slower recovery to the 3000rpm, bit hard to describe I know sorry, it’s like a momentary pause before the rev back to 3000 then repeats .

Engine setup is 1998 Fiesta 1.4 zetec – standard ecu, standard induction (basically all as per the fiesta donor)
No fault codes shown
Got petrol in.
Checked for vacuum leaks – found nothing
Replaced Lamda sensor
New Coil pack at beginning of year
The car starts and drives fine up to the point where revs die off, where it kangaroos like its out of fuel.
Internet research points to practically every sensor on the engine. Throttle sensor or MAF sensor keep being mentioned. But wouldn’t these throw up an error code?

Does anyone have any ideas before I resort to calling out a mobile diagnostics guy?
Once I have sorted this I can apply for IVA, so I am keen to sort it!

Thanks
Richard

Posably a silly question, but do you have a vent to your fuel tank, and if so is it working. Easy test take of the fuel cap and then see what happens regarding the engine running. Cheers Ray

TPS or Vac sensor disconnected or U/S ?

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Originally posted by Chippy
Posably a silly question, but do you have a vent to your fuel tank, and if so is it working. Easy test take of the fuel cap and then see what happens regarding the engine running. Cheers Ray

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I was looking at my Focus manual the other day and noticed that it will cut back to two cylinders if it detects over temperature. Maybe your fan needs to come on earlier ?.

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Originally posted by Macbeast
I was looking at my Focus manual the other day and noticed that it will cut back to two cylinders if it detects over temperature. Maybe your fan needs to come on earlier ?.

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The only plug in diagnostic info that will of any help is looking at the live data from the sensors via the OBD port.
It is almost 100% certain it is one of the sensors or associated wiring.
Don't change sensors at random check the live data
My money would still be on the TPS or VAC sensors.

quote:

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Originally posted by britishtrident
The only plug in diagnostic info that will of any help is looking at the live data from the sensors via the OBD port.
It is almost 100% certain it is one of the sensors or associated wiring.
Don't change sensors at random check the live data
My money would still be on the TPS or VAC sensors.

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as BritishTrident says watching the live data will be a great help and will save you spending time & money on sensors etc

Both the Tps and the temp sensor share the common sensor earth (Brown wire). If your temp shows -40 then it is not seing the temp sensor at all. Its the default temp for open circuit.

So look at the tps and the wiring back to the ecu.

quote:

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Originally posted by big_wasa
Both the Tps and the temp sensor share the common sensor earth (Brown wire). If your temp shows -40 then it is not seing the temp sensor at all. Its the default temp for open circuit.

So look at the tps and the wiring back to the ecu.

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Since my last posting, i have found a split vacuum tube on th inlet manifold which i have replaced.
I have cleaned the MAF which had a little dirt on it.
I haved swaped back to orginal coil pack from donor (just in case)

These have made no change other than it idles a lot smoother now (probably the vacuum tube).
Still cuts out when revs reach 3000 and revs back up on its own.

So I have tried to capture some OBD data to see if anyone can spot something.
The first three are engine idling to warm up.
The second two are me trying to do a screen grab during the reving problem, when the revs cut some sensor data seems to disapear, also the system reverts to "open loop" then if i let it idle a few second it goes back to closed loop.








Reving problem






If anybody has any ideas they would be most welcome as I dont know what to do next. Getting a bit fed up with this now

Cheers
Richard

[Edited on 11/9/12 by monkeyarms]

I have now got the following OBD error which is reproduceable when the cutting/reving happens -
"P1351 IDM Input Circuit Malfunction/ Ignition Coil Control Circuit High Voltage"




As the coil pack is provided with +12v from the switched 12v from ignition (going to C1 P6 in the battery box) this is part of the wiring I have made, so I will test for bad connections in my wiring.

Alternator provides a good 14v at fast idle so that seems ok.

I have recorded the laptop screen with sound to better see the fault.

http://youtu.be/df07QknCrAg

[Edited on 15/9/12 by monkeyarms]

Have you got your ETB tacho connected? What does that read compared to your OBD output? I'm wondering if it is reading twice as much as it should 7000rpm being the max (IIRC), and your OBD reader says 3500 before it cuts out.

Try disconnecting the feed to it and then trying again - resetting the P1351 code first.

Does it actually cut out or just reduce power?

Rich, the ETB tacho shows same revs as OBD. I have tried cutting power to the ECU to clear it, also reseting the P1351 code.
Cut out or reduce power? hard to say really I have only driven the car on the street and when the fault happend its very much like running out of petrol. Turn the engine of and on again and its ok, to the 3000rpm point of the problem.

Im half wondering if a fuel pump problem could behave like this?

quote:

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Originally posted by RichN
Have you got your ETB tacho connected? What does that read compared to your OBD output? I'm wondering if it is reading twice as much as it should 7000rpm being the max (IIRC), and your OBD reader says 3500 before it cuts out.

Try disconnecting the feed to it and then trying again - resetting the P1351 code first.

Does it actually cut out or just reduce power?

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Could be the inability to supply enough fuel at higher revs.

Have you tried changing the coil pack back to your new one?

I got an auto electrician out in the end, and he found that the problem
was one of the connectors in the fuel pump relay base was spread open
(by me testing in the past jabbing a test probe in) As the revs and vibrations
rise the fuel pump relay was being shaken and interupting the pump supply!

I don’t think I would have spotted that. All sorted now thankfully.

Glad you have fixed it, and such a small fix at that!