Hesitation when accelerating during warm up

[Thickpea]

I have owned my 1998 2.3 since 2000 and she has been a reliable workhorse, but recently an unusual problem has developed when warming up.

The engine starts fine, idles evenly at usual higher revs, around 1200, then if left idling will warm up and slow to standard idle after warm up, still running evenly.

However, if driven straight away, from cold, initially acceleration seems fine, but after a short while hesitation sets in.  This feels like fuel starvation, as if the choke on an old car had been pushed in too soon.

The hesitation only seems to be under load.  If the clutch is dipped the engine revs freely and evenly.  After a mile or so the problem suddenly stops and the engine runs sweetly as normal, with the usual brisk, even acceleration.  Once warm the problem is not evident at all.

I recently had an electrical misfire, which was cured by a new coil, plugs and leads. (as an aside, for information, brilliant price for the coil at Kimber Car Parts, Ã,£31.66 inc VAT and delivery - http://www.kimbercar...uk/Default.aspx)

Regarding the current problem - The MAF seems clean and clear, checked the engine temperature sensor; resistance reads as per Haynes manual values, no obvious leaks in induction evident.

I have scanned the previous posts, but can see nothing similar, any ideas or suggestions would be most appreciated.

Thank you in anticipation

[Mirez]

For the sake of a few pounds I would replace the coolant sensor as a first port of call - They have a habit of failing with intermittent readings which aren't always obvious with a simple resistance check.

After that I'd be looking at when it was last serviced? Plugs & filters? The engines using more fuel when cold then at any other time, so a failing spark plug/incorrect spark gap or choked filter can cause this symptom.

[Thickpea]

Thanks for the swift reply Mirez.

The servicing is up to date.  I will change the coolant sensor, as you suggest and see how it goes.

Fingers crossed...........

[Thickpea]

Update -

Temperature sensor replaced, no difference.

Further investigation on symptoms -
Starting from cold is fine and running is smooth and even.
Raising engine revs to 2k, using accelerator to speed warm up, results in even running for around 30seconds to a minute, then a sudden drop in revs and lumpy running for around 30seconds, (without accelerator movement), the revs then suddenly recover and the engine smooths out again.  Driving away after this is as normal, with no further sign of problems.

This leads me to think that there is a particular part of the warm up cycle, or component associated with it, that is the culprit.

Does anyone have any ideas, or perhaps can offer a flow diagram of the warm up sequence, with the key sensors employed at each stage.

Thank you in anticipation.

[Chrispb]

If you have had the car since 1998 do you know if relay 30 has been changed? older units suffer with bad solder joints which results in intermittent non starting and poor running, this relay gets quite warm even in normal running conditions, other than that coils or faulty engine wiring