auto box not changing up properly?

My 1999 4.6 autobiography has just had it's coil packs, leads and plugs changes as it was developing a nasty misfire and I was unable to get more than 20mph out of it when cold before it went in for the work. It felt so much better when I picked the car up and really had some good acceleration - but this was only around town. I did'nt get the chance to try it on a bypass or anything at speed. The garage also added some fuel injector cleaner. The car had around a quater of a tank at the time. I added some more fuel the following day and the car seemed to go back to the 'warm running' state it has done all along. The car routinely runs at 30mph at 21-2200rpm which seems a little high? But at 50mph is running 1900rpm Could it be that the auto gear box is not changing gear at the optimum time when it should?
At speed, the car is still a constant problem:

The engine will tick over well, rev freely upto 3k whilst driving and get a flat spot and then rev to 4k and get another one. Though engine will rev past this - which it seemed incapable of doing before the coilpacks etc were changed but still with no increase in speed. The flat spot was only at 4250 before the parts were changed but I think that now the engine is sparking and revving more freely (though going nowhere), it's just highlighting this mystery fault all the more frequently. That's my guess anyway. The kickdown, when applied makes the whole thing 100 times worse and I dare not try and overtake anyone as the car just revs up and does not increase in speed no matter what speed I'm doing at the time.
I can drive it very very gingerly and perhaps get it to 70 if there is no real load on the car, and it flies going down hill(!) but the kickdown is still a problem.
I have struggled to figure this one out and when the garage who changed the stuff above, they mentioned nothing of any more 'fault codes' other than what they found with the misfire on all 8 cylinders - which is why the coilpacks etc were changed.
It's getting so frustrating having a 4.6 that has the power of a mini a lot of the time!
Any thoughts anyone? Help would be most appreciated.

KJT said:

The car routinely runs at 30mph at 21-2200rpm which seems a little high? But at 50mph is running 1900rpm Could it be that the auto gear box is not changing gear at the optimum time when it should?
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Torque converter lock up occurs at around 50mph which accounts for the drop in revs and is normal

Sounds very much like it's under fuelling. According to Rave, a misbehaving throttle position sensor or more probably a misbehaving Mass Airflow Sensor would give the effects you're experiencing. Other than changing them, I don't know of sure way of testing them other than with the diagnostics hooked up whilst you're driving.

Thanks very much! So you don't think the garbox has anything to do with it then?
Where do I find the TP Sensor and MAS thingy?

You're welcome!
No, I don't think the gearbox has anything to do with the engine misbehaving. I'm not at home at the moment so I can't directly compare mine but it sounds like it may be lingering in 3rd at 30mph although the revs at 50mph are fine. That wouldn't have anything to do with having flat spots. I think you're going to have to bite the proverbial and take it in somewhere to see if there are any faults logged in the ECU. Otherwise, you're just going to be guessing and could end up spending more money still not curing it than the diagnostics would cost.
Your throttle position sensor is mounted on the throttle body on the engine. If you say kick down is OK, then this sensor is probably OK as this gives signals to the gearbox as well. It's main function is to tell the engine ECU that the pedal is being depressed and to alter the fueling accordingly for acceleration. If the ECU is not receiving this signal, poor acceleration will be the result.
The Mass Airflow Meter is the aluminium part mounted on the outlet of the aircleaner. This cannot be tested other than by sustitution.
Diagnostics is definitely your first step.

I have changed the throttle sensor and it has made the engine seem a little better.I also read that you can test the mas by disconnecting it when the engine is warm then drive and see if there is any change. I've done this and it stops the car from starting. I plug the sensor back in and away we go with the faults again! Is this telling me something?

You can sort of test the MAF by disconnecting yes - You have to disconnect it whilst the engine is running and then go for a ride. If it runs much better with it disconnected, you may have found your problem. To be honest, I'd forgotten about the disconnection test - sorry about that. And yes it will stop the engine from starting.

I did try that but as soon as I pulled the plug off the engine cut out. Would this suggest that the MAF is actually NOT faulty?

KJT said:

I did try that but as soon as I pulled the plug off the engine cut out. Would this suggest that the MAF is actually NOT faulty?

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The test implies that you can remove the MAF when the engine is running and go for a run. This then means that in practise the car should continue to run to allow you to do this, but if your vehicle is cutting out when you take the MAF out of circuit then this implies something else is wrong, as you end up with the same result as if you removed it when the vehicle wasn't running. i.e. a dead engine. If you see what I mean.

Have you checked to see if you are getting enough air in to the engine, how old is the air filter?, fuel filter?, what condition are the plugs in after a run, are they wet?

woogoo said:

The test implies that you can remove the MAF when the engine is running and go for a run. This then means that in practise the car should continue to run to allow you to do this, but if your vehicle is cutting out when you take the MAF out of circuit then this implies something else is wrong, as you end up with the same result as if you removed it when the vehicle wasn't running. i.e. a dead engine. If you see what I mean.

Have you checked to see if you are getting enough air in to the engine, how old is the air filter?, fuel filter?, what condition are the plugs in after a run, are they wet?

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the engine cuts out cos it need time to rejust, you must keep the engine running, to give it a chance..

Hi Guys,

Thanks for the advice. Leads n plugs changed 3 days ago but just put a trolley jack on my acc pedal and removed MAF sensor - car ran perfect! Just popping out to buy a nice replacement.