Petrol Turbo diagnostics

[Xantiaman]

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...

You will not get any boost if there is no load. This means you need to drive it, putting it on stands is cheating and wont work!

I disagree about boost being available from stand still. Its not. Load has to be generated to create enough exhaust gas volume. This varies in every gear. And varies even more once the ECU knows what load and rpm/road speed your doing. Load is monitored by the MAP sensor, surprisingly enough. The wheels can be spinning at 70mph but the ECU is not stupid because there wont be any load exerted on the engine to make boost.

I'm sure i've already said do a 4th gear test on this thread. *(have read back and yes this is the case)*

This is typically a 1-1, direct drive gear, that generates sufficient load throughout the rev range and the boost figures you obtain can be used as a bench mark.

Drive along at 30mph in 4th, then floor it. You want the engine to be doing about 1500rpm, and take it as far round the rev range as legally possible. Make a mental note of the boost achieved at 1500, 2000, 2500, 3500rpm and so on. You will see how quickly it builds and then settles, and see the point it drops off. Try doing it in 1st gear


Come back and let me know you have actually driven the car on the road with a boost gauge and i will stop lol'ing. I've given sound and useful advice and i would love to see you get to the bottom of this problem.

[Col]

well I've floored it a few times and managed to coax a bit more life out of it...

it still feels pretty sluggish, but to be fair it could do with a damn good service.

[mjb]

The boost gauge is only saying what the MAP sensor is reading, and I've had the PP hooked up and recording data while on the move - and it's vacuum all the way

Also, Increasing revs at full throttle from idle to 5000+rpm while in the air IS load. The injectors are working at the same duty cycle as while doing >100mph with your foot to the floor. Listen to any car with an atmospheric dump valve rev it - you'll hear the psssst on over-run. Not as much as when on the move, because the turbo's not spooled up to full speed, but it's still there, even with large laggy turbos

[Xantiaman]

I'm going now.

[mjb]

OK so I drilled a hole in my dad's car for the boost gauge which I blu-tac'd to the dashboard, got it off the axle stands, put it on the road and it ran at up to 1psi vacuum, exactly the same as in the garage.

[mjb]

Hang on a sec, there's an obvious thing I've missed - the dump valve!!!

Guess that'll be easy to test

Hmm 1424X4 £47.55 not as bad as I thought it'd be to replace if I need to!

[Xantiaman]

OK so I drilled a hole in my dad's car for the boost gauge which I blu-tac'd to the dashboard, got it off the axle stands, put it on the road and it ran at up to 1psi vacuum, exactly the same as in the garage.

What speed did you get up to then, or did you just leave it on the drive and revs the bollocks off it?

[steve_earwig]

Not that I know much about forced induckshun motors but I guess the point is revving it up in the air tells you almost nothing, why else do they make rolling roads? *runs and hides

[mjb]

What speed did you get up to then, or did you just leave it on the drive and revs the bollocks off it?

Yeah, I went through all that trouble to not bother driving it
35-40mph. As fast as the car can go without a downhill slope or a few miles of flat

[mjb]

Well it doesn't look like the dump valve - it's bloody hard to push open. Nevertheless I took it for a drive with the vacuum hose disconnected (still got the boost valve bypassed) and no difference - about 1psi vacuum at high revs

I guess this leaves two possibilities:

1. A *gaping* hole in the intercooler - big enough to not make a noise
2. Niz's theory that a seized turbo causes the flexi hose to collapse is wrong, and it does in fact have a seized turbo

I think tomorrow I'll get the car back up in the air and see if I can get to the compressor blades to see if it's seized. There's no obvious jets of air coming from the intercooler though that can be detected with cigarette smoke

[mjb]

A blocked cat would prevent the turbo from spinning wouldn't it... as well as the near-stalls I'm seeing when pressing the accelerator when idle

Anyone know how you test a cat? Take it off and blow into it?

[Col]

yes a blocked or melted cat would certainly bog the engine down somewhat.

I wondering if this is the problem with mine as there is a fair amount of rattling coming from either my cat or centre box

[mjb]

Well it seems an easy test is to pull the oxygen sensor out... guess what I'll be doing tomorrow evening

[jasper5]

I had a problem recently with a blocked cat on a Mazda MX5.

The symptoms were ....
1) Rattling from the cat (I had the car booked in for this rattle/new cat anyway).
2) The car wouldn't do more than 20mph.
3) When the engine was revved, there was a loud spluttering and spitting from the air filter box (the exhaust gas was blowing back).
4) When I held my hand over the rear exhaust pipe, there was very little effort to blow my hand off the pipe...there should be a lot of pressure when you stick your hand over the exhaust pipe and try to block it, using a thick rag.

To double check there was a blockage I unbolted the flange from the front of the cat, the engine revved as it should, confirming a blockage.
When I removed the exhaust to fit the new cat (the back box and pipe has to be removed to change the cat on that Mazda) I found lumps of cat internals blocking the back box and silencer, so, it was not the cat that was blocked, but the rear silencer in this case.

Removing the O2 sensor would give you the same effect as undoing the flange at the front of the cat as it would allow exhaust gas to flow out faster.

[mjb]

Aye, I think the symptoms are reasonably consistent, although having the turbo there does open up a pile of extra symptoms and the 50p shop exhaust masks any exhaust tone iffiness. I'll check the pressure on my coupe's exhaust before I leave work this evening so I know what to compare to. Had the back box off the tubby the other day (removing the towbar+mount which the exhaust was hitting when the mounts warmed up) so I know that's not got loose crud inside

If I need to replace the cat at least I think I can just drop it out without too much hassle, at least on the centre section end... How heavy are these things compared to the other exhaust parts? I don't want to end up in another situation like the subframe where I grossly underestimated its weight and ended up hurting as a result