Lack Of Boost

[Chubby]

Hi all,

Started the car this morning, waited for it to warm up, took it down the road, 'LOADS OF BOOST' [img]images/graemlins/grin.gif[/img], got to the traffic lights, pulled away, no boost. [img]images/graemlins/confused.gif[/img]

Where has it gone. Could it be the waist gate stuck???

Help!!!!!

Regards,

R. [img]images/graemlins/29428-red4.gif[/img]

[johneroberts]

You need to pressure test it, more than likely the hose has split before the dump valves, or one of the dump valves is faulty.
Cheers
jr

[Riz_RS4]

Yes sounds like a DV (Diverter Valve) has gone.
Take it off and do a pressure test using your finger and something to push the valve back, if it holds this proves there is no pressure leak.


Riz [img]images/graemlins/tung2.gif[/img]

[Chubby]

have changed dump valves - no change.

Sounds like waste gate to me....

[Sql_RS4]

i sometimes have something similar in my car.

but in my case it is a temporary lack of boost. Sometimes it comes back after a couple of tries but usually i have to restart the engine - sometimes more than once to get the boost back.

What should i check?

[Riz_RS4]

Ok chaps now try the MAF (Mass Airflow Sensor) if this is faulty is incorrectly reads the wrong air-flow figure and could cause pressure loss on the turbo.

This sensor wont give a error code under the VAG computer either, i would recommend you get a dealer to look at it as they are not cheap £300 approx plus fitting.

Riz [img]images/graemlins/thumbs.gif[/img]

[Chubby]

Thanks Guys,

Car fully warrantied so it will be in to John Fox Audi Nottingham tommorrow, although I don't have a lot of faith with Dealers, most will know who to change spark plugs on a A3 1.6, but problems with a RS4 is quite different.

Most people would drive the RS4 now and say it was quick, they have no idea of what a 480bhp goes like...

regards,

R. [img]images/graemlins/29428-red4.gif[/img]

[Chubby]

What a morning - forgot what date is was - contacted John Fox who told me to contact Audi Assistance, which I did and in 75 mins a Dedicated Technician was on my door step admiring the car.

Nice chap, explained this was the first RS4 he had worked on, this was confirmed by him not being able to locate the plug in port (in rear ashtray).

His wonderful computer said something about too high boost, which he cleared without making any notes, and then tried to do a full check on the system but the computer could not identify the car / engine configuration.

The boost has now returned, although not as good as it was, so we have taken it into the Audi dealer to see if there computer can give it a full diagnostic check.

More to follow when I hear from the dealer.

[Chubby]

In times of trouble it nice to talk to someone who know's what they are talking about - Kim Collins QST has said that it all points to the boost control solenoid.

Thanks Kim.

regards,

R. [img]images/graemlins/29428-red4.gif[/img]

[ER785]

Boost plays a very important part in a turbo engine making power. As such, VAG owners should always know the condition of their turbo engine - when it is making boost, and how much boost is being made. It is a known fact that after driving one's car for a while, one may become desensitized to its power and so eventual loss in boost and power may be overlooked and assumed to be a person "just getting used to the power". However, it is better that we rely on quantitative numbers to check up on our cars as our mind's perceptions are subject to distortion due to many factors.
The ideal solution in monitoring boost pressure is a boost gauge that taps off a vacuum line off the intake manifold. But if one only intends to monitor his/her car's health and boost levels only periodically, then the VAG-com 1551/1552 clone is sufficient. Reading off certain measuring blocks off your VAG car's ECU (which in turn gets its info from the cars sensors), the VAG-com will be able to tell you lots about the performance of your engine.

Using the VAG-com, do a three block read on blocks 115, 118 and 120.
Block 115 will show you a reading in the 3rd and 4th column, requested boost and actual boost in millibars (1000 mbar = 1 bar). You will have to subtract about 1000mbar from both columns since atmospheric pressure is around that level. What is left is the boost request (boost that the ECU wants to "see" in the intake manifold) and actual boost (actual boost pressure at the intake manifold). The ECU controls the turbo wastegate and will modulate it to get actual boost as close to requested boost (following the rev/load map of the chip) as possible. Under sustained full throttle, actual boost should not deviate more than 100millibar from requested boost. If it does, then something is wrong. It is normal to have larger differences in these two numbers under shifting and under throttle application/de-application as it takes some time for the wastegate to react to commands. However, under sustained full throttle, assuming the road gradient doesn't change much, the two numbers should be very close to each other. The closer, the better.


Column 3 on Block 118 will show you percentage duty cycle on the N75 valve. The N75 is the valve that plays a large part in regulating the wastegate. An unenergized solenoid (0% duty cycle) permits full charge pressure on actuator diaphragm. So the turbocharger naturally regulates itself. As boost builds, the wastegate opens. The ECU can operate the N75 to vent some of this pressure so the wastegate either does not open, or opens less. For example if reading is 62.8% duty cycle it means over a 100 millisecond period (1/10 of a second) the charge pressure in the actuator line was filled for 37.2 milliseconds and vented for 62.8 milliseconds. An orifice in the charge pressure connection restricts the flow rate of charge air into the actuator system. If you are seeing a large difference between the two reading off block 115 (request high, actual low), and you see that duty cycle on the N75 is high (90-100% range), then you are probably leaking boost from a pressure hose (turbo outlet to intake manifold) or the diverter valve (DV). You should then check all clamps and hoses along the whole pressured tract as well as swap DVs

Column 3 on Block 120 will show you calculated torque at the crankshaft in Nm. This is calculated by the ECU by taking readings from 5 different sensors and applying a formula to the 5 readings. I am not sure exactly which 5 readings are used in the calculation, but block 120 has been known to be accurate on all stock and chipped cars except the B5.5 Passats. There seems to be a compatibility issue between the European B5.5s and certain chips. This problem remains unsolved at this point, so the only way to actually check torque on these cars is by dyno-ing or by checking 0-100 timings and making sure they are consistent.
So after reading/logging these 3 measuring blocks, if boost, torque, and N75 duty cycle numbers are all strong, then the car is ok. If for any reason boost isn't as high as it should be, you should check all pressure and vacuum lines for loose clamps or cracks. If that doesn't solve the problem, then it will require you to start troubleshooting step by step by swapping certain car parts with a friend who has the same car in working condition.
Here are the following steps to take. Always go in this order as it goes from easiest to hardest. Hopefully the problem will be solved in the early easy stages: -


1) Swap Electronic Control Units (ECUs)
2) Swap Diverter Valves (DVs)
3) Swap N75s
4) Swap Mass Air Flow sensors (MAFs)
5) Check intake/filter box for blockage
6) Check exhaust for blockage or melted catalytic converter
7) Check turbo wastegate rod and wastegate diaphragm


Your boost problem will most definitely stem from one of these sources. If not, take note of all readings and try to create logs. Post on forums and describe in as much detail your problem. Eventually there will be someone who has had the same problem as yours, who'll come along and help you.

Good luck!

[quovadis]

actually ...when i had this problem one of the turbos had blown or was about to blow.
We noticed a huge lack of boost, and there was an abundance of oil in the pipes as well as after the turbo.

[Chubby]

Thanks for the info. although it isn't much good to me as I don't have a VAG-com. Have passed on your details to local Audi dealer who has informed me that they can find nothing wrong with the car on there computer, and now they want to drive it around until they can replicate the fault.

So now I've got some hairy arsed mechanic racing around the back lanes of Nottingham, in a modded RS4 trying to get the turbo boost to exceed the maximum. [img]images/graemlins/033102bad_1_prv.gif[/img]

How do I feel........... [img]images/graemlins/FIREdevil.gif[/img] [img]images/graemlins/rocketwhore.gif[/img]

Regards,

R. [img]images/graemlins/29428-red4.gif[/img]

[Nige_RS4]

Chubby, it sounds like you are in 'limp' mode, which doesn't necessarily mean over-boost (it can be any number of things which the ECU decides might harm your engine). In my opinion, your dealer needs to start using their VAG 1551/1552 to it's full potential and not just to reset the service interval! What's the chance of the problem re-occuring by driving round, you know yourself that it invariably won't re-appear!!

1. Get them to check the MAF readings at WOT.
2. Get them to pressure test the system - as per jr's advice.
3. The N75 valve is an inexpensive part (£35) - they should be prepared to replace this (but it has to be said that unless you have been fiddling with it, it is usually very reliable)
4. Get them to check your lambda sensors - they will have been subject to higher temp extremes (especially with the recent weather) if you are running 460bhp.

What cats are you running (100 or 200 cell) as these can sometimes cause CELs - but unlikely to cause limp mode - anyone have any experience of this ???

Finally, the audi tech that first diagnosed the problem at your house needs his ar$e kicking, as there would have been a fault code present and he deleted it without making a note of what it was!

Hope this helps, Nige.