AMF Limp Mode - Fixed

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AMF Limp Mode - Fixed

Post by Crofty » Tue Aug 06, 2019 10:17 pm

We have a Polo 6N2, 1.4 TDi (AMF Engine) which has long felt lacking in power, and occasionally suffers limp mode caused by negative boost deviation, IE it's not generating the boost it should be. I have replaced all vacuum hoses (more on this in a moment), cleaned the egr and tested its function with a vacuum pump, and ensured the waste gate is not stuck open, tested it's function with a vacuum pump (more on this too) and researched other peoples woes (no cure could be found) and crucially, how the system works.

Contrary to most peoples advice, the waste gate is operated by PRESSURE not vacuum, on the bulkhead are three valves, the left one and middle one control the EGR valve and the anti shudder valve (not sure which way around but it's academic) and on the right, the Boost Control Valve (A square valve, 028 906 283 F) I mentioned testing the actuator with a vacuum pump, and thought I'd found the culprit, as nothing happened (I figured maybe it was seized or stick open) but looking closer I couldn't see how a vacuum could operate it, bearing in mind the linkage, it needed to push not pull and the actuator was clearly not vacuum operated. Following the hoses, of which there are 3 on the Boost Control Valve, I realised none were connected to a vacuum source. Of those three hoses, one goes to the waste gate, another comes from the turbo compressor, and the other goes into the turbo's air inlet pipe.

My thought was now, is the boost control valve leaking boost pressure straight through to the waste gate, I removed it and sure enough, I could blow straight through, I double checked it with the solenoid plugged in and the engine running, and it still leaked pressure.

This explains the lack of power, the waste gate is always slightly open, losing turbine speed, and causing under boost.

Incidentally, with the valve fitted and still connected electrically (unplugging it caused limp mode) but the actuator hose disconnected, normal performance was resumed, we did get limp mode, but this time it was an over boost, which is to be expected without a functioning waste gate.

The misinformation, regarding the operation of boost control, and the fact that internet users who have problems never return to explain the fix has been a pain, so I am hoping this may halp people experiencing problems of their own.

I'll nip to the scrappy tomorrow and pick one up and I'll let you know
Last edited by Crofty on Wed Aug 07, 2019 9:06 pm, edited 2 times in total.

Posts: 6
Joined: Sat Jan 09, 2010 8:57 pm

Re: AMF Limp Mode - Fixed?

Post by Crofty » Wed Aug 07, 2019 8:54 pm


I got a valve from the local scrappy, and sure enough, limp mode has ceased and the 2000 rpm 'grunt' is back.

Boost pressure is introduced into the top pipe if the valve and is vented straight thru the valve to the inlet pipe to the turbo, unless overboost is encountered and the solenoid is activated, diverting pressure to the waste gate actuator. Testing my old one with a compressor caused the valve to eject a thick oily gloop, that must have been blown in by the turbo, and testing the solenoid electrically showed it to be temperamental, cleaning with solvent made no difference, it also had a rattle inside, and the replacement didnt.

I feel pleased to have only spent 40 quid all in, thats a solenoid, all vacuum/pressure hoses (6mm) and a replacement air inlet pipe due to a failed breather pipe connection. The amount of people who have had replaced turbo's for no improvement is alarming.

I simply hope someone finds this useful in the future.

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Re: AMF Limp Mode - Fixed

Post by alexperkins » Fri Aug 09, 2019 5:51 pm

Thanks for posting this useful chunk

These AMF engines are a rather complicated vacuum mess and a lot of people dont understand how they work!

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