3.9 V8 Cutting out at 3000rpm

Recently fitted a new idle air control valve which has sorted out my fast idle.
Quick road test and she now dies as soon as she hits 3k rpm. A couple of seconds off the pedal and she's ok again.
Sitting still in park I can freely rev it to 4 or 5k with no issues. Is this a mass air flow sensor issue?
I had to disconnect battery to get the IACV to teach the ECU the new settings and since then I've got this new issue. Any ideas?

Sounds like you're running out of sufficient fuel flow? Does it bog down or 'switch off'?

No bogging down, everything is normal to around 3000 then throttle goes dead. Engine doesn't stop, it continues to run. After about 3 seconds when the revs have dropped off a bit, push the pedal and the power comes back in.

Honestly sounds like fuel pump or filter issue (air filter too) more than sensor related, you seem to get sufficient fuel/air when not under load but loaded at 3k rpm it struggles. Fuel pump issues can be quite misleading because you'll never really know untill it fully fails.

Fugawi said:

No bogging down, everything is normal to around 3000 then throttle goes dead. Engine doesn't stop, it continues to run. After about 3 seconds when the revs have dropped off a bit, push the pedal and the power comes back in.

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If its engine speed related issue, theres no engine speed sensor as such but the EFI ECU receives a pulse from the ignition coil, so the issue maybe as simple as iffy coil top connections or the coil its self, only checking and a coil test will we find out.
I assume you have check the connection to the valve and is functioning in and out also a new seal was used.

Given that it revs OK with no load and bogs down with load, I’d suspect the fuel pump, fuel pressure regulator, or fuel filter.

By the way, did you re-set the adaptive values after fitting the new IACV? If not, that might be your problem.

Thanks for the suggestions. Coil is fairly recent and fittings looks new and clean but I will have a look as not double checked there. The valve had a new gasket and was working as it should when tested. With regards to re setting the new adaptive values for the IACV, not sure how to do that but disconnected battery for 30 mins to try and clear ECU.

Never suffered fuel issues with other vehicles so it's new to me if it is this. I will check fuel filter. Air filter recently cleaned and re oiled. I didn't think it could be a fuel issue but I'll bow to far greater knowledge on here. Would there not be at least some spluttering as the fuel started to run out rather than an instant cut off with no symptoms or warning.
Because it seems to be at a certain engine speed (dictated by the throttle pedal) I keep harking back to the TPS mis reading the throttle position. I think I read the old RR Classics had a cut out at a certain point but can't remember the circumstances.

AFAIK replacing the idle control valve does not require any reset of the ecu on a 14CUX hotwire injection system.
Have you got cats?
Maybe a bit left field but how good is the battery and alternator and are the main earth points clean?
Discool is correct there is no speed sensor, no crank position sensor either. The ignition and injection systems are separate so you need to decide if this is an ignition problem or an injection problem.

If it is possibly ignition, check coil and coil wiring - the wires often corrode inside the insulation.
Next check ignition amplifier and wiring.
Check pickup resistance and reluctor air gap, check rotor cap and rotor arm are in good shape. Pattern ignition parts are known to cause issues.

If it is an injection issue check/test air flow meter. Check throttle potentiometer (base voltage, resistance and scaling).
The 3000rpm bit may not be relevant but could depend on whether or not this is a catalyst engine. Is there any difference between the engine being hot or cold?

Hope that gives you some food for thought.

Ok, thanks for that, a few bits to check there. Engine temp has no bearing on it, cold or hot it's the same.
Just took it out. Another observation.....if you're in D at say 2300rpm and move gear selector to 3, the revs increase as it selects a lower gear. This has the same cutting out effect, even when your foot is off the throttle as the revs are pushed to around 3k rpm.

Fugawi said:

Ok, thanks for that, a few bits to check there. Engine temp has no bearing on it, cold or hot it's the same.
Just took it out. Another observation.....if you're in D at say 2300rpm and move gear selector to 3, the revs increase as it selects a lower gear. This has the same cutting out effect, even when your foot is off the throttle as the revs are pushed to around 3k rpm.

Click to expand...

Could try the same thing in low range and determine if it's when it's under load or just throttle position? Not really sure if it'd work that way but it's just the process of elimination at the moment, electrical gremlins are a bastard...

That's a good idea, hadn't thought of that. I'll give that a try.
As others have said there is no road speed sensor that quashes another theory of something I'd seen on the net about the old Range Rover
"For safety reasons, Range Rovers have a "kill-joy" function built into the ECU program. If the Road Speed Transducer provides too fast a signal, the vehicle will stop accelerating."

It is true there is a limiter but the main function of the road speed transducer is to tell the ecu if the car is moving and then engage (or stop) idle stabilisation. The speed limiting function is indeed present but it is doubtful this is the issue here. If you suspect it is a road speed transducer problem then try swapping the transducer. The road speed transducer just sends pulses to the ecu; the ecu counts the pulses and equates the count value to a speed. Apparently on a Range Rover Classic, the trigger point for idle control to switch on/off is about 3mph. However, 14CUX ecu's used in TVR's didn't have this set up and so a conversion circuit was used.

I've never experienced what might happen if the road speed transducer triggers the upper speed limit but I have had a transducer fail and the effects were I had no speedo, the EFI warning light came on but apart from that there were no discernible running issues at motorway speed.

I know this is an old thread and you might have this sorted by now.

But, the vac advance pipe to the dizzy can cause this if disconnected, as can a blocked or incorrectly fitted non-return valve that's in the vac pipe. If the non-return valve is fitted the wrong way around, then the issues you see have been reported before.

Jon

Thanks for the reply Jon. All inputs appreciated. The issue is all sorted now but after several dead ends I took it to RPi Engineering.
The underlying fault was a bad TPS and the idle speed was likely varied due to several incorrect EFI settings. The ignition timing was rather retarded from where it should be which was also making her run a bit hot after a short journey.
Running an absolute treat now. With the running properly sorted she's about to start being stripped down for a full makeover and totally different final appearance over the next few months.

Glad you got it sorted!