Petrol Turbo diagnostics

[Chaz]

Was going to post about my wife's Turbo too. When we bought it, it generally went well but the exhaust was falling off. Had that replaced and since then the car does 0 - 60 in 2 years. No boost at all from what I can tell.

Ill disconnect the battery to reset the ECU as a start - get a engine light on at times but dont have any tools to check it with. The car needs a service, no doubt but where else can I start to check to see if there is an issue with the turbo, piping or other? Any help appreciated.

[STALLED]

I know this may sound a tad dumb, but has the cat collapsed?

[Xantiaman]

Fairly easy one this. If the actuator has lost most of its pre-load (and there isnt much as standard) then the wastegate could be open slightly, enough for boost to build very very slowly. Get a boost gauge on, and disconnect the hoses to the boost control solenoid, bypassing it but keeping the multi plug connected. This way, it will only run 'actuator' boost.

Intercoolers can sometimes split the end tanks slightly, causing low boost. It would make a noise if doing this.

[mjb]

Fairly easy one this. If the actuator has lost most of its pre-load (and there isnt much as standard) then the wastegate could be open slightly, enough for boost to build very very slowly. Get a boost gauge on, and disconnect the hoses to the boost control solenoid, bypassing it but keeping the multi plug connected. This way, it will only run 'actuator' boost.

Actuator boost?

I'm under the impression that it's (supposed to be) on boost unless the solenoid chucks compressed air down to the wastegate actuator, and if there's never enough pressure in the manifold, it should never open the wastegate... hence my theory that the wastegate's jammed open

Intercoolers can sometimes split the end tanks slightly, causing low boost. It would make a noise if doing this.

I'm reasonably sure it's not the IC or any leak in the charge section. There's no noise of escaping air, and pressure drops slightly at revs whereas I'd expect it to raise slightly with a leak.

[Xantiaman]

The wastegate cannot easily get jammed open in my experience. This would suggest that the actuator has seized. More often than not, its because the spring inside goes weak.

The actuator has a spring and diaphragm inside, which has a pre-set tension against the wastegate lever, it is always shut until boost is produced.

When you drive it under load, and as long as the wastegate is firmly shut, it will start to make boost. 'Actuator boost' is the base setting, bypassing the boost control valve will rule the valve out of the equation and allow you to pin point if the wastegate is loose.

If you only see 2-3 psi above 3000rpm, the actuator spring has gone weak, allowing the wastegate to be slightly open until there is enough load and exhaust gas to close the valve. Its a very common problem on a T25 turbo with a low base boost setting, because even from new, the actuator spring has low tension.

Its a simple enough task to shorten the actuator arm and restore this tension. You should see about 4.5psi boost around 2000rpm in 4th.

The BCV will not allow boost past until the ECU senses the engine producing boost. Once the desired boost has been reached it will open and shut at various rates. It fools the actuator into thinking there is less boost than there really is, 'bleeding' the excess air back into the boost tract (the 3rd pipe that goes to the top turbo inlet pipe)

I've seen the BCV fault code come up before, i'm not sure why it does it on a perfectly good valve, but if a physical fault such as a loose wastegate was the case, the ECU might thing it was the BCV at fault.

As said, you must get a boost gauge on to diagnose this fault, or your just pissing in the dark.

[mjb]

Yeah so I blew £25 on a boost gauge and it told me pretty much the same as the MAP sensor did - under 1psi vacuum on revs, even with the valve bypassed.

Going to have a play tomorrow and see what the wastegate's like. I just mopped the garage floor and it ain't dry, so I don't fancy doing any jacking up when it's slippery...

[Col]

any chance you could take a few pictures of you investigations mjb

I know my Xsara like the back of my hand... but the 406 is a total mystery to me

[mjb]

Will do. I always try to take pictures to help others (and my memory )

[Col]

what rpm does the turbo start to kick in on the SRI ?

[mjb]

You don't feel the turbo 'kick in' - it's pretty seamless, unlike most turbocharged cars. When you floor the accelerator it should pull with force from about 1500rpm to about 4200rpm.

Right I've had a look today and the wastegate rod is hard to move by hand but does move. It's spring loaded and snaps shut. I've given the actuator end a good helping of WD40 to clean it up a bit too. It's still producing a mild vacuum (1psi) on revs though

I'm at a complete loss with this

(excuse the mspaint scaling )

[Col]

You don't feel the turbo 'kick in' - it's pretty seamless, unlike most turbocharged cars. When you floor the accelerator it should pull with force from about 1500rpm to about 4200rpm.

right, that's what I thought.

apparently my Dad spoke to the mechanic who did some work on the car in the very recent past...

he said the turbo was working fine and pulling hard. Surely it's not as simple as me being too gentle on the accelerator

[mjb]

The car doesn't feel as quick as you'd think it would - the engine's designed to be 'lazy' and the soft suspension and deep 406 seats absorb a hell of a lot of the feeling of rapid acceleration.

Of course if the turbo's not working, you won't be able to keep up with a Micra

[Col]

well I think I'll just go out for a quick spin and give it a bit more gas

I swear I remember it being a bit more punchy than it is now... maybe old age has dulled my senses

[Xantiaman]

Yeah so I blew £25 on a boost gauge and it told me pretty much the same as the MAP sensor did - under 1psi vacuum on revs, even with the valve bypassed.

Going to have a play tomorrow and see what the wastegate's like. I just mopped the garage floor and it ain't dry, so I don't fancy doing any jacking up when it's slippery...

Sorry if you wasted £25.... Your saying your getting no boost at all, what gear did you test in?

The actuator is very hard to move by hand. The best way to test it is using compressed air direct to the actuator, not too much of course, 10psi is enough.

If the actuator moves freely, the wastegate is firmly shut and your still getting no boost, then you have a split pipe somewhere.



Performance wise you should not feel a kick, its a small turbo constantly modulated by the ECU to produce a flat torque curve, it feels a bit boring like this, which is why i went bigger turbo and separate electronic boost control with adjustable gain.

[mjb]

Your saying your getting no boost at all, what gear did you test in?

Neutral - it's on axle stands in my garage. The ECU has no idea what gear it's in unless the wheels are turning, and then it takes a few seconds to work it out from the speed sensor in the diff. Given how you can be on boost from a standstill in a working car, I don't think being in gear has any effect.

The actuator is very hard to move by hand. The best way to test it is using compressed air direct to the actuator, not too much of course, 10psi is enough.

If the actuator moves freely, the wastegate is firmly shut and your still getting no boost, then you have a split pipe somewhere.

I don't have a compressor, and I could probably buy a new tubby for the price of one... although I have seriously been considering one...