1.8T auto - drop in performance
 

Driving my 75 1.8T auto the other day I noticed the performance had dropped off slightly. I'd filled the tank with Sainsbury's premium and was pootling home when after a few miles I opened her up as the limit went to NSL and she felt just a bit flat, as if there was no boost.

The car is running spot-on otherwise. Slightly uneven idle as normal but no misfires, drives perfectly, will happily rev if I boot it, but has lost that 'kick'. The car's in good order overall, having recently had an oil change and IMG to resolve the coolant leak so I'm puzzled.

I've had a poke around the engine bay and there are no obvious damaged or loose hoses. The turbo is quiet, the exhaust slightly louder than normal, possibly as I'm applying more throttle to overcome the apparent lack of boost. Oddly the auto box is slightly smoother, perhaps because it's dealing with less torque.

I've hooked up my cheapy bluetooth OBD reader and no fault codes (historic or pending) are being brought up. I've done a battery reset but no change.

Any ideas for what to check?

RESOLVED!

A further poke around the engine bay (stone cold this time), reaching down beneath the exhaust manifold to the rear of the turbo I found the actuator rod flapping around. The retaining circlip had gone awol and the rod has disengaged from the wastegate arm, meaning the turbo has not been boosting as I suspected.

Reattaching the rod has returned performance back to normal, with the lovely torque fill returning, and MAP readings (courtesy of a borrowed ScanGauge reader) showing at around 22PSi at full throttle.

https://i.ibb.co/yf231GH/turbo.jpg

:eek: 22 PSI :shrug: I hope it's the accuracy of the Scanner at fault - around 12 PSI would be a Good maximum Boost Pressure ;)





Paul.

Nothing wrong with the scanner. MAP is 22PSi, not Boost. I suspect Boost @ 22PSi would result in an exploded engine.

MAP sensor takes into account atmospheric pressure which I believe is around 14-14.5psi.

Which would give you around 8psi which sounds a bit low to me? To be honest, I'm not sure what boost a 1.8T should be running at.

:cool: that will be something new i've learned


I think it makes sense - Positive Boost is measured in Relative Pressure

Absolute Pressure includes the Relative Pressure Vacuum (negative readings on the Boost Gauge)


My Boost Gauge (reading from the Inlet Manifold next to the MAP sensor) shows Idle around -10 PSI to Full Boost around +12 PSI.

this would give an Absolute Pressure Reading of 22 PSI. ?





Paul.

Sounds about right, if your getting 8psi your boost solenoid is working correctly.

I observed a MAP reading of 14.6 at full throttle (before I refitted the actuator arm and the car was essentially running as a Nasp 1.8) which is atmospheric pressure. Under partial throttle or at idle it was running as low as 4.5.

Now it's fixed, I've observed a MAP of 22.8 under full load, equating to about 8 PSI boost which I understand is about right for a stock 1.8T.

The difference between having boost and not is remarkable. While it doesn't make much difference pootling around town, at higher speeds the car was unnervingly slow. I really couldn't like with a nasp 1.8 auto.

I'm delighted that the solution was so easy and cost £0, and experiencing the car in the slower form has proved interesting.

:cool: I think i'm catching up now :D


0 on my Boost Gauge should read 14.5 using the MAP Sensor Values.

-10 on my Boost Gauge at Idle would be 4.5 via the MAP.

+12 on my Boost Gauge at Full Throttle would be 26.5 via the MAP.


lol.... I read some misleading info r.e Absolute vs Relative Pressure that had me confused.....easy to do I know :D


I think it's more accurate to say - the MAP Sensor Values do NOT account for Atmospheric Pressure - rather the Boost Gauge does as it is Zeroed at Atmospheric Pressure.


out of curiosity what MAP values do you see at Idle now ?




Paul.

I'm not sure. I've returned the ScanGauge to my mate who lent it to me, so now I'm left with my cheapy bluetooth OBD2 reader and Torque app, which I don't trust quite as much. I'll take some readings and let you know.