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My polo has developed reduced power - it runs as though it isn't getting enough of something - very noticeable uphill - second gear only to make the last hill home and that is a real struggle. It doesn't want to go over around 3500 revs but will when not in gear.

It also is making a hissing noise which increases with the revs - it is a definite hiss not wind. The hiss seems to be coming from the side/s of the car - its really odd - when you put your head in the engine bay you can't hear it.

Lastly it takes ages to start in the morning, it struggles to run and then is fine - and will be for the rest of the day.

Could there be one unifying fault?

I have:

Changed plugs, Changed HT leads, Changed air filter, Cleaned throttle body.

Thanks

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    Check the vaccuum booster pipe from the inlet manifold to the brake servo as these have a habit of splitting on these cars. – Steve Matthews Jan 31 '17 at 15:06
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Your changed components have nothing to do with the problem. Plugs and HT leads don't fix a hissing sound. Air filter and throttle body are closer, but still no cigar.

This seems to be a clear case of a vacuum leak. But where it is, that's the problem. Steve Matthews said in the comment (should probably posted an answer instead) that the brake booster pipe could be the location of the leak. Better to check that. But really, the leak could be anywhere.

There are basically two ways to locate a vacuum leak:

  • Inspection. Inspect all rubber parts for cracks.
  • Starter fluid. Spray it at a possible leak location, and see if the engine revs up. If it does, you just found your leak! Just be careful with the fluid; don't spray too much or else it could ignite.
  • The only exit points I can identify from the inlet manifold are: a)to the brake servo b)to the charcoal filter (although this is from the throttle body) c)at the injectors I have had a good look and tested with propane and can find no leak. Also I still have my old polo (spare!) and swapped over the 'brake servo to inlet mailfold pipe' - no difference. I can't hear the hiss at idle - only at the higher rev range - after watching yotube videos of how to test I feel the noise is more of a sssss which comes from the sides of the car rather than a whistley hiss from the engine bay. – Kleptonite Feb 2 '17 at 10:04
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Consider a clogged catalytic converter. Is the hiss coming from underneath the car?

Have a friend rev the engine while parked. Feel the exhaust coming from the tailpipe to be sure exhaust is coming out freely. There should be no "pops" of exhaust coming out, only a steady stream. If the exhaust gas is restricted, the cat. converter is bad.

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Solution: As time has gone by a slight amount of power has returned, and the exhaust has become louder. Today the joint between the mid and rear silencer selection literally blew apart at the join. After removing the rear silencer section and upending it lots of small pieces of exhaust inner bits fell out.

The fault appears to be a partially blocked exhaust due to the disintegration of inner workings. Power has now returned. Thankfully. Thanks for all the suggestions.

Exhaust

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