No power at idle !

So my 1986 GTI cabby sits on the drive at idle absolutely fine. But then when I apply revs there’s just emptiness and then it cuts out. Is this fuel starvation? But then why does it idle ok? It has been sat for a few months but after a battery boost it started fine, just no power when the loud peddle is pressed, obviously the same when I apple the throttle cable/linkage manually under bonnet.
Is this an air issue? Fuel pump maybe?
Help please folks….? 

Do yourself a quick favor. Disconnect the Exhaust at the manifold or after the down pipe.  

A collapsed internal silencer can cause so much back pressure that on reving the car will stumble and die.

I had a 76 toyota that the wife claimed would only go about 40MPH…. I get in it and beat the hades out of it and I had no issue with it going 90…. (frigging women drivers and their diagnosis)… Then it was 30,,,,, Okay I get in and sure as hades 30 mph and stumbling.  I get it in to the garage, and well Everything looked super.  So I rebuilt the Carb, tuned it up, and it was purring like a kitten…. I gave it some gas and Fuel started to spray out of the Carb straight up.  

My Father and Brother said WTH, along with me, we checked everything timing, fuel pressure and looked at the total exhaust.. It all looked spanking brandy new….

So on a bet I took off the exhaust at the manifold and low and behold the car ran great, no more coughing, or gas out of the carb…..  

I cut the silencer off at the rear, and once I saw it, I know we had it fixed…….  The top of the muffler (silencer as you call it) had caved in at the top, the bottom looked brand new… So we decided to carve up to see the insides.  There was a piece of metal plugging the intake of the muffler, at idle it was open, but we knew it wasn't enough of an opening to allow full exhaust flow as it was about 1/4 of an inch gap… "The Old Potato in the Exhaust syndrome"….  

Ahhh ok....this makes sense. Thanks for that. I shall investigate. When it stops blooming raining !

So no joy after removing the manifold and inspecting the entire exhaust system. Still won't start until after 10 mins of cranking then it will idle but cuts out as soon as I touch the accelerator.
Lift pump works fine at the rear under the seat and the main fuel pump under the drivers side is making a whining sound but functions ok.
Checked injectors and fuel delivery all fine.
I can't think of anything else but can't help thinking it's something silly that's causing it.
Any ideas folks?

Needs proper diagnosing by measuring mixture.

I've just had idle issues and the car would not rev after the car was parked up for a few months, I fitted new spark plugs, HT leads, rotor arm and dissy cap before getting the car out of the garage, turns out the new out of the box dissy cap was faulty, the car would not rev and would randomly cut out.

I would be checking over the ignition system next if you think the fuel supply is ok.

I'm running out of options now. And the poor thing will go back in my garage in a week or so until late spring so I really want to nail this.
Replaced rotor arm, dizzy cap, plugs and leads but no joy.
I will add a a few litres of new fuel this afternoon to see if it might pull through and mix with the old stuff.
Clutching at straws now me thinks…..
 

fatboyjim said

Lift pump works fine at the rear under the seat and the main fuel pump under the drivers side is making a whining sound but functions ok.

If the main pump is whining, then you are having a fuel delivery issue. The primary pump is cavitating.

Main pumps only whine when they are restricted from getting the proper flow of fuel or having to push fuel that is restricted up the line.

Have you checked the fuel pressures?  86 GTI with the CIS injection has 2 pumps, the lift pump puts out 45psi and about 33 gallons per hour, whereas the Main on a 86 is @ 75 psi… with out a test then you are guessing that the fuel system is correct.  

On my digi's once I hear the Whine, I usually replace teh in-tank pump…..

The 86 in-tank pumps fuel to the sump (external) then the the fuel pump (main) to accumulator, then to filter and cis diz…..

Normally a good running fuel system the pumps don't whine.

If it is a Carb'd ride ignore this…. but since you mention lift pump….

Also years ago I had a fuel cap that was bad, causing a over pressurization in the tank…so you might try removing the cap and see what happens quick and easy.   

Yes mine is a K-jet 1.8 GTI. I've only recently replaced the in tank pump under the seat so I don't think it's that.
I might try and remove the old fuel that's in it. It must be quite old as I can't remember the last time I put fuel in ! 2 years or more maybe. Any tips on how to remove a quarter of a tank of old fuel anyone???

Put a hose on the right hose out of the fuel tank sender, then jumper the pump to 12V and ground…It isn't the violet wire that is the fuel gauge sense wire.

buy a cheap syphon pump be safer  if you
are unsure mate

Latest is…..emptied old fuel and put new clean fuel back in. Also replaced the in tank lifter pump, again, which seems to do the trick and let's it tick over nicely. But, as soon as I press the accelerator it still dies. I'm going to have to wait until next year now to get into a garage and get a full diagnosis, which is annoying. Any last minute help folks????

Nothing to add to the advice previously given. I don't think you can measure the mixture though, it would need a garage or tools you don't have….

Cheers Paul. I'll be taking it into my local VW garage in the new year

Have a look at the flap in the metering head, remove the rubber boot on the top of the metering head and check you can pull the flap up freely, there's a little metal pin inside which can seize or get gummed up over time.

Well I never ……! I did check the metering head a few weeks ago and it all seemed to function fine. All I did today was dismantle the unit and clean it and re-assemble and its all working perfect !!!!!!!!!!!!!!!
Only took about 6 weeks but got there in the end. Thanks Mark and thanks to everyone else for the superb advise.    

Fingers crossed it's fixed and trouble free now