86 GTX Misfire Problem

[GTXGTXGTX]

Gave My faithful 86 GTX a bit of a service the other day, oil and filter change, drained and flushed cooling system and re-filled, new plugs (Bosch). Did not do air and fuel filters as they were done just a few months ago.
Drove car to Brighton and back 120 mile round trip and all perfect purring like a contented cat (it's only done 97k miles)
Then today 2 things happened;
Water pump bearing started rumbling (99% sure water pump as have had this on other scirocco's) assume this is just a coincidence as I drained an refilled the system the other day.

Also the main problem is misfire or lumpy running when accelerating, a definite mis-fire or partial fuel starvation maybe? and exhaust appears to be smoking slightly, seems ok at low revs but definitely rough when trying to accelerate? car has never done this before.

Any advice or suggestions appreciated.

Thanks

[unknownmale]

Could be fuelling or ignition. Easiest to check first is for spark. I would try

1. Holding the revs at say 2500rpm ish until it starts misfiring and then pulling each lead off one at a time. Assuming it's constantly misfiring and it's a spark issue, the lead you pull where there is no change will be the offending cylinder.
2. With the engine running at the RPM which causes the misfire, pull each lead off and ether hold to the engine block to look for a spark, or connect a test light to battery ground and the other end to the lead (Bit safer). If you don't get a spark on a lead you'll know it's likely an issue with the lead or distributor cap.

If this only occurred after changing the plugs my first thought would be to check all of your leads as it's the most likely place to cause a misfire.

[connorj19]

Exhaust smoking slightly as if it's burning oil ? Did you drop any of the plugs? Might be worth checking gaps I've done this when I first serviced a petrol motor. Probably be something else but it happens to me once, car would run rough and would run on 3 pots as the gap was to small for a healthy spark

[mark1gls]

Did you chance the HT leads, dissy cap and rotor arm? If not how are they?
HT leads don't last forever..

[GTXGTXGTX]

Thanks for the advice everyone. Rotor arm is newish. Going to check plugs and replace HT leads. Will post an update.

Quick question, are spark plug testers a worthwhile investment? Thanks

[unknownmale]

Thanks for the advice everyone. Rotor arm is newish. Going to check plugs and replace HT leads. Will post an update.

Quick question, are spark plug testers a worthwhile investment? Thanks

Not unless you do it for a living. you can just plug the spark plug into the lead and hold it a few centimetres from the engine block to test. If its too light to see the spark you could also hook up a test light in series.

[mark1gls]

Agree with unknownmale.

[connorj19]

Take it to VW, I'm sure all the computer technicians will sort it out haha. I took the rocco to my local VW dealer once as I wanted a test drive of a Passat cc. All the sales team come out to look at a 1991 gt2. I was amazed how they all rememberd selling mk2 golf gti's but never an iconic car as a scirocco. Bit off topic I know...

[GTXGTXGTX]

A quick update on the problem.

I changed the plugs and made no difference. I tried "unknownmale's" suggestion of increasing the revs to the point when it starts to misfire then checking each lead. The strange thing is the problem does not occur whilst the car is not being driven. Whilst stationary the engine revs perfectly from idle to 6000 with no hint of a problem. It's only when driving the car and you accelerate past 2500 rpm the engine is rough and feels to be misfiring, or petrol starved? or something. If you increase the revs very slowly it's a lot better. It's when you put your foot down above 2500 rpm instead of the usually immediate response the engine is very rough. Strange I know.

Any suggestions or thoughts appreciated. Thanks

[mark1gls]

Check and clean the little filter on the bottom of the lift pump inside the fuel tank, does the main pump sound noisier when you have the problem?
The lift pump helps bring the fuel out of the tank to the main fuel pump underneath the car.

[Village Idiots]

Try replacing the plug leads.

[unknownmale]

Does this happen on a full tank of fuel?

[GTXGTXGTX]

Thanks for the responses.
A brief update, I have changed plugs, HT leads (Bosch) rotor arm and distributer cap. The problem persists, rough engine and misfire when accelerating between 2500 and 3000 revs. No problem if accelerate gradually, nice and smooth but if I put accelerate hard the engine is very rough. If you back off the pedal slightly it's fine, it's when you want a quick response the problem occurs. I am now thinking insufficient fuel getting through when required. I have ordered a new fuel filter (Mann) and will start with that, I will then move to the (lift pump as suggested (and tank, maybe the filler neck is rusted.

Can someone please tell me, is there just the one large fuel filter? or is there a smaller transparent in line one somewhere?

To answer Mark question above I can not hear the fuel pump.

The petrol tank is 3/4

Any advice appreciated. Thanks

[the edmundator]

Can someone please tell me, is there just the one large fuel filter? or is there a smaller transparent in line one somewhere?

Only the single metal one on a fuel-injection system.

Make sure your fuel filler-neck isn't rusty and check inside the tank for any rust fragments. These can easily block the gauze on the end of the fuel line inside the tank, especially when there is increased demand for fuel.

[GTXGTXGTX]

Thanks Edmundator. I will check.