Car runs better with MAP vac line disconnected, whaaattt? - G4+ Extreme

[Hilly]

Hey legends.

I've got a Holden VL Turbo that is suddenly running terribly. It idles OK-ish, but if you give the accelerator a quick stab the car splutters and stalls. If you get the revs up over a few thousand, the car revs OK, but again, if you give the throttle a quick stab, it bogs and dies for a second before slowly returning to a responsive state.

Given it's a 35 year old car, I've started the long process of eliminating possible causes. Have replaced plugs, checked for vac/boost leaks, cleaned many electrical connections, points, rotor button, etc (Crank Angle Sensor was next). However, while trying to remove/block vac lines in troubleshooting, I noticed the car immediately ran better when I disconnected the vac line to the Link. Revving the car was super responsive (like, almost perfect). I'm doubtful the MAP sensor is the issue, and suspect this is just masking a broader issue. But I'm not experienced enough to know what this might mean. I'm hoping this means something to someone. Like, someone might immediately recognise that this eliminates electrical issues since it runs almost perfectly with the MAP line disconnected. This might clearly indicate an issue with fuel, or a definite vac/boost leak. I really don't know, but I'm hoping someone might be able to understand the issue and explain what things might cause a car to run very poorly with MAP sensor connected, and greatly improve without to help me narrow my focus. Obviously I did not drive the car, and I realise the car will actually drive terribly without the MAP sensor connected, it just improves the throttle response so damn much that I can't get past it being a solid indicator to the right person.

Any help would be sooooo unbelievably appreciated.

Thanks in advance.

[Confused]

A copy of your tune, plus log files will help massively in trying to diagnose the issue.

[Adamw]

Agree with Confused, a log and map may help.

But to answer your question, MAP is a multiplier in the fuel equation, if you double the MAP you double the fuel.  A typical road engine will idle with a MAP around say 30-40Kpa, when you disconnect the vac line then the MAP will be measure atmospheric pressure rather than manifold pressure which will be around 100Kpa.  So effectively by disconnecting the vac line the ecu will try to inject 2-3 times the fuel by opening the injector for 2-3 times longer than normal at idle. 

This may suggest you have a fuel supply problem - low pressure, blocked filters, split pressure line in tank etc.  Or a very large vacuum leak.    

[Hilly]

Hey guys. Thanks heaps for the speedy responses. Unfortunately the map is password protected. Tomorrow, I'm going to try contact the workshop who created it and see if they will remote access my laptop to remove password. Then I will provide map and log.

I managed to actually drive it for a short period today. I noticed that if I adjust the throttle body open a little and start it keeping the revs above 1k, it seems to run OK. Started back firing and dropping power when approaching full boost the first time, so I babied it home, but it otherwise ran well. If you let the revs drop below ~1k though, it starts spluttering and dying and almost always stalls. Incidentally, I noticed the fuel pressure was 60psi at idle which seems high (1600cc injectors, E85).

Anyway, I hope to post more info tomorrow thanks. You guys are absolute wizards 

[Hilly]

Turns out the fuel pressure regulator was faulty. Reading 60psi one minute, then 25 the next. Thanks for your help guys 

[Confused]

Glad you got to the bottom of it!