Background
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I've had my Mk3 Golf for around 4 years the total milage is 115,000 miles, around 12 months ago it started to not fire correctly under load at low RPM. The condition got gradually worse over the 12 months until when running down the motorway at 80 MPH you could feel the misfire through the floor pan.

At speed it feels a bit like knocking and at low RPM the engine just would not pickup correctly especially from a standing pullaway, it would splutter into life.

During this 12 month time I continued to run the vehicle it passed all exhaust emissions tests without any problems (the government emission tests do not put the engine under load they are performed in neutral gear).

I've also had the cylinders compression tested and cyln No: 1 and 3 were down, No1 by as much as 66% and No3 about 20% lower than the rest. All other cyln's were within VR6 manual tollerance. We also put oil down into the chamber and repreated the tests to work out if the problem was with piston rings or valve closure, the indication at the time was with valve closure. Since the readings did not change.

I've changed an renewed the spark plugs and leads this didn't really make any difference. Although the engine runs a lot better for around 2 weeks after the plug has renewed on the No1 cyln. I suspect because the plug isn't cocked up.

Other things noted during this time was the apparent low MPG (British Imperial Gallons that is) the electronic reading was giving me, I am used to expecting 30 to 32 MPG however 26 was about the top amount I got with 22 being the norm. Yet during the 12 months I didn't feel like I was putting any more fuel into it to get about.

In July 2003 I took the vehicle off the road to fix the problem and improve the cars overall handling.

After initially getting the engine out and stripped down, the state of the No1 cyln chamber showed a lot more coking than all the others and the colour was not the same. I can't remember what the colour difference was, maybe my colleague can if this would help.

Other than this there was no obvious mechanical problem with the engine. In fact there was little or no play or noticable wear in any part of the engine.

This lead me to believe the problem is with fuel supply into the cylinders.

I've had the injectors taken in to be cleaned and checked they have all come back as being OK.

Having now cleaned and painted all the engine parts to remove rust, corrosion. The head's been cleaned up and lapped in.

I am left with one final decision, that is to have the block re-bored and a new set of over-sized pistions. Before I'd taken the engine apart I've expected this to be necessary but after looking at the bore wear (which is non-existant) and the overall state of the rest of the engine its has not seemed necessary so far.



Piston / Bore Question
======================

However during last weekend we checked out the No1 piston against the bore and found the following strangeness.

We first checked the old compression rings and oil seal rings for wear. It is not possible to get even our smallest feeler gague between the ring and the piston when checking for play in the piston gap it sit in.

We then checked the wear between the ring and bore by pushing each ring down the bore with the upside down piston so its under the wear lip of the bore and then mesauring the gap between the two ends of the ring. We worked this out to be 0.025mm, and the limit from the manual is 0.15mm. Our reading from the old rings is inline with what new piston rings would be.


The VR6 tech manual claims the following:

Standard Piston size 80.985mm, this is also stamped into the top of the piston itself, just to confirm we are working with the right sized pistons.

The pistions themselves still have the original machine marks (like cross hatching) visiable all around below the rings. There is minimal wear on one surface (the outside of the V surface, prob due to gravity) but not enough to rub out the machine marks. These machine marks are around the base part of the piston not the head part where the rings are.

The bore has no feelable (with fingers) lip over the top part of the chamber.

There manual also states the maximum deviation in piston diameter to be measured over 3 points of the piston is 0.04mm.

With a Michrometer we measured the diameter of the piston and to our suprise we found the piston to be slightly oval in shape. The deviation between the wide and narrow sides is just 0.38 mm, but this way outside of the 0.04mm tollerance the manual states.

Also in measuring the piston size itself my measurements indicate (taking into account the accuracy I believe I can obtain with my Michrometer) I get a reading of between 80.00 and 80.02 mm for overall piston diameter. This is a long way off the 80.985mm it should be.

It is also possible to easily put large feeler gauges (between the piston and bore), upto 15/1000 or 20/1000 of an inch. The piston also (without rings fitted) and mostly degreased (once lined up at the 6 degree incline) slide in and fall through towards the crank area easily.

The VR6 manual says the piston to bore gap should be no more than 0.025mm.


Questions
=========

So I'm seeking advice with someone on the following:

1) The slightly oval / mis-shapen pistons, is this normal ? Are new pistons perfectly round ? How much do pistons wear over 7/8 years 115,000 miles. Its had a partly synthetic oil in it in the time I've had it.

2) Given the above information confirmation that a re-bore and new pistons is the way to go.



Other Notes
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While its been down I've taken the time to renew all linkages, balljoint and complete set of poly bushes for the suspenssion and steering rack, taken off both front and rear subframes and sand blasted, painted and re-assembled. New front wheel bearings, brake discs have been skimmed, new CV joints, rack boots, track rod ends.

Also on the engine side of things around 2 years ago my water pump failed, this is the plastic impellor pump driven off the belt. When taking the engine down my water pump now has one plastic leaf on it to push the water around (think there is 6 to 8 in total normally). This shall be renewed but I suspect there must have been fragments of water pump in the coolant system smashing up the leaves. This might have lead to over heating ? Even through the water gague temp was always its nominal value.

I've not yet to check out the electrics to each injector, this shall be done before the engine goes back in. Since the misfire could be with ECU <-> Injector signalling.

The only last thing is I am thinking of putting a "struct brace" over the front suspenssion mounts. My thinking if that a 7/8 year old cars metal will have fatigued and the additional support will improve handling and reduce the stress on the cars body ?

Other than that I've spent very little on the car in the 4 years I've owned it, set of new tyres, set of shocks, set of brake pads, new battery, water pump, water thermostat, wheel bearing and thats it.


Thanks for taking the time to read my ramblings,

Darryl