Is this bad? mbc just installed...

ian2000t · #1 11-24-2011, 01:53 AM

Just installed my ball and spring mbc in the hose between turbo and waste gate. Its obviously worked because I can now get 15 psi instead of 10. But, the problem us above 15 the boost gauge goes crazy and bounces round to about 10 psi vacuum!

http://static.photobucket.com/player.../VIDEO1143.mp4

Any ideas??

ian2000t · #2 11-24-2011, 02:53 AM

The problem is definitely not the gauge either - I've just tried putting my finger lightly over the end of the hose to the gauge. Pushing plenty of air through, then all of a sudden nothing at all.

Could it be some kind of overboost limiter? Its a 1990 UK 760tic. There are boost lines running to 2 weird devices by the glow plug relay.

ian2000t · #3 11-26-2011, 11:25 AM

Had no one had this before when upping their boost to 15psi?

anders · #4 11-28-2011, 06:06 AM

Do you notice a power loss when the gauge drops? It sounds like you have a overboost valve somewhere. I don't know how the D24TIC's are but all the D24T I have seen have had a dump valve that dropped the boost if it reached over 12psi or so. If The D24TIC do have one it will be somewhere on the charged air piping.

ian2000t · #5 11-28-2011, 06:44 AM

Yeh, there is a power loss when it happens. I've disconnected the boost line to the gauge and you can feel suddenly boost stops coming through when it happens.

I think one of those devices in the picture must be something to do with it. I've read the dtic should have an overboost warning light on the dash, but nothing is lighting up on mine...

Think I should just disconnect them and try it again?

anders · #6 11-28-2011, 07:45 AM

I don't think it will be any of those devices you pictured. I'm pretty sure you will have a dump valve somewhere. Post some pictures of the intake, turbo inlet, and piping.

ian2000t · #7 11-28-2011, 08:10 AM

Pic of turbo and inlet manifold (sorry not close up of wastegate):

Then straight from turbo into Intercooler:

Then from intercooler to Inlet (this pic isn't my car but mines exactly the same).:

http://image.bayimg.com/gagfcaado.jpg

Then there are thin boost lines going from inlet manifold to:

- 2 devices pictured in 2nd post
- boost gauge in car
- fuel pump

anders · #8 11-28-2011, 11:53 AM

I didn't see the picture of the turbo, but if it has the wastegate attached to the compressor housing, try to tie the actuator rod in the closed position and see what happens, you should get 20PSI or more.

ian2000t · #9 11-30-2011, 05:30 AM

Ok, will try that - which way is "closed" - towards the wastegate or away from it?

v8volvo · #10 12-01-2011, 12:59 AM

Whoa! For a minute I was trying to figure out what all that weird stuff on the right side of the engine bay was... pretty weird to see the steering shaft and master cylinder sitting over there.

It does sound as though you have a safety override situation going on at your elevated boost level. A stock US-spec D24T is limited to about 10psi, I think the D24TIC is closer to 12 due to a different wastegate setting. The safety dump valve on a D24T opens at about 12psi, so dumping at about 15 would make sense on a D24T where 12 is the stock pressure.

However, since you say you are actually seeing vacuum indicated on your gauge, I wonder if your situation is not an air dump but in fact some kind of wastegate override or fuel interrupt? Are you sure that the gauge is really measuring vacuum, or could it just be the needle bouncing down into the vacuum range from rapid movement?

There are some diagnostic tests you could do. The one anders suggested would work but needs a little modification if I am remembering correctly. On a D24T, unless the TIC setup is different, the wastegate mechanism is all internal so there is no external linkage you can secure in one place for testing. However, you could achieve the same effect by removing the pressure signal hose that runs from the compressor housing to the wastegate servo. Just slip the hose off on the compressor side, and plug the opening, then with a warm engine GENTLY go for a test drive. As you get up near 15 psi, watch and see what happens. Either your issue will be gone or will still be present, which will mean, respectively, that the symptom is being caused by a wastegate control override at high boost, or by a fuel interrupt or an air dump in a location you haven't found yet.

Note though, I would be careful running with the wastegate disabled -- you could get a boost spike way higher than 20psi under certain load conditions. If you try that test, then be gentle with the accelerator so that you build boost gradually and don't end up with more than you want. Don't want to pop that nice new headgasket you recently installed. ;-)

If your issue is still present with the WG disabled, then you can re-enable the WG and do some more searching for where the safety dump valve could be. In the back of the intake manifold? In the EGR plumbing somewhere? If it is not to be found, then perhaps on the TIC motors this is accomplished by cutting fuel. To test for that, you could disconnect the 12V+ supply to the IP fuel cutoff solenoid and attach a jumper from battery positive, then go for a gentle drive and gently see if you can gently get the boost up above 15 lbs.