740 Turbo-turbo problems

Short answer: no. There are too many issues to deal with (the question has come up a lot before).

What indications of turbo trouble do you have? If it smokes badly at idle, and especially if it clears up after a minute cruising, my guess is the oil separator and/or hoses for the crankcase ventilation are plugging up.

Mike

Possible to convert the car; impractical to convert the engine.

As a practical matter, no. Sell the car to someone who doesn't mind fussing with the turbo and buy one that came without it.

And why would you want to? The turbo cars are much more desirable.

Good approach. Mine has been whining for a couple of years, and it's still okay. (I've been whining for more than 50 years....)

Mike

Ok, I got the car today and found the small hose leading down to the wastegate valve snapped off. Great! Reconnect and off we go, right? Nope. No change. It sounds like it is dumping all pressure as the turbo gauge does not respond as it should and turbo sounds like it is free wheeling. New wastegate actuator? Also, observed rod from wastegate as the wife revved and saw no movement. What should occur??

The hose has popped off the back of the boost guage, good thing the engine didn't blow up when it overboosted from the wastegate hose coming off, sounds like you were lucky.

You should get no movement of the wastegate actuator until you're approaching maximum boost. So revving the engine in neutral will tell you nothing. Look under the car up at that rod, and see if it's connected. Also, report the exact symptoms, whether the car seems sluggish or has lots of power. Let us know everything that is out of the ordinary and maybe we can help you better.

If it's an automatic you can observe the wastegate actuator by setting the parking brake fully, standing firmly on the brake, place it in drive and give it gas until the boost needle climbs. Doing this regularly is not adviseable but it won't hurt anything for a quick test.

Of course there's little reason to test this in the first place, just get out on the road, stomp it, and watch how high the boost guage goes (after you put the hose back on it)

Ok if that hose is still on there then you have a hole somewhere else, check the rubber hose couplings right at the intercooler, take them off and check with your fingers to see if a flap will open. Also check the rubber elbow at the throttle body, though any of that should cause lots of black smoke when you accelerate. It's also possible that the hose to the guage has popped off the tee at the overboost switch down on the pedal bracket or that the hose itself has split. If you hear air hissing then the turbo is producing boost but it's going somewhere. If the turbo was rubbing you'd know it, they make an awful metallic shriek like cutting metal pipe with a chainsaw.

It's worth pointing out that if there is a hole in any of the turbo hoses on the pressurized side, the engine power will exhibit a "fold-back" behavior - the power will drop suddenly when the turbo starts spinning and will stay very low (just more than idle) until you let off on the throttle enough to spin the turbo down, then everything will be fairly normal until the turbo spins again.

There is a valve mounted to the turbo that shunts the turbo output back to the input, allowing the turbo to freewheel. I understand it is to prevent overpressure in the ducts if you lift throttle suddenly when the turbo is spun up. Maybe it is staying open?

Mike

Also, low boost accompanied by hissing can be caused by an almost completely clogged exhaust, usually the catalytic convertor. However, if you have the black smoke problem as outlined above, then clogged exhaust is not the problem. However, continued driving with the above problem will overheat the cat, melting the substrate, giving you 2 problems.

If this is a Garrett turbo, they have a dump valve mounted on the front of the engine block right hand side. It has about a 1'' hose going to it. This valve can become loose and you will dump turbo pressure and not feed it to the engine intake. its worth taking a look at.

Harold

I'm not sure how, it's pretty simple mechanically, it's just a flap on a rod that covers a hole. The actuator pushes against it's internal spring and pulls the flap away from the hole.

One other thing to check, if this car has a compressor bypass valve is to see if it's somehow stuck open, the older Garrett turbochargers had a separate CBV mounted to the left of the timing belt cover, the Mitsubishi turbos have it on the turbo compressor housing itself.

That's a good point, I'd forgotten that valve. The bolts holding the flanges on the valve that the hoses are attached to loosen, allowing all the boost to leak out. One of the connections isn't obvious unless you look right at it.