Just bought a 90 745 turbo, and while I have plenty of experience working on my 90 245 I have no experience with either 740s or turbos. I have a couple of questions, and will likely have more as I start digging in to my new ride.
First, I noticed there's an opening on the engine compartment side of the stock airbox. On my 240 I have a similar hole in the bottom of the airbox that goes to the pre-heat hose off the exhaust manifold. Is this car supposed to have something like that or does it just draw warm air in from the engine bay when it's open?
Second, the cruise-control mechanism inside the engine bay is broken, the rubber diaphragm or whatever it used to be is shredded. Is this going to cause any problems other than no cruise-control? I'm thinking of potential vacuum loss issues.

 

shouldn't even be open on a turbo, at least it isn't on my 1992 T. (but they did make some changes in 1992). With the turbo, there's no place to put a heat riser.

cap off the vacuum hose going to said dead cruise control modulator, then no chance of a problem.

 

There should be a preheat hose coming out of the box.
You can get that cruise control diaphragm cheap from a PickNpull type yard, any 740, 940, even 240 or 960 is a good donor--nice to have cruise control working.

 

240's have the cruise actuator at the gas pedal rather than under the hood.

I guess they deleted the preheat hose on 1992+ turbos, my 92 745T never had one, and there is no hole in the airbox where it might go.

 

745 '89 turbo I had used a pre-heat hose. I think my brother took it off. Maybe I did... but I don't think it is there now. At any rate, it isn't needed unless your smog inspection guy says anything.

cruse control actuator--though placed in different locations--look identical to me. (on my 240 it is as pierce says, by the gas pedal, but on the '89 745T it was under the hood). But the cruse control uses a vacuum pump separate from the engine vacuum, so it should not make a difference.

 

Thanks for the replies everyone! I remember getting hit for no preheat hose on my 240 when getting it smoged, but it was a little more noticeable than this one. I'm toying with the idea of fabbing an additional cold air intake using the extra hole in the shoebox, but I'll have to take a good look at it first. Good news about the CC vacuum line, I'm planning on heading to the wrecking yard this week, but I don't know what they'll have available.
Thanks again!

 

the stock airbox inlets is plenty big enough for way more air than your car will ever draw, at least until you get up to the stage IV turbo world of super-boost and massive engine mods and 300+ HP.

if you're subject to california smog, and your car came with a preheat hose, it will be on DMV's checklist for the smog guy to verify. I *HIGHLY* recommend wiring the flap closed on the inside of the airbox, they don't check that, that temp actuated flap has a tendancy to stick open, and can cook your MAF with hot air in hot weather.

 

I second that. Unless the flap thing has been replaced in the last 5 years, it has probably failed, and doesn't function properly anyway.

 

Just a quick update- the car passed smog with no problems, the numbers looked pretty good. Turns out the airbox has a 'knock-out' style plug in it on the engine side where the pre-heat hose probably ties in on the N/A cars. When you look inside the airbox it looks like there's an opening, but it's completely sealed off from the engine compartment. Haven't made it to the wrecking yard for the CC actuator yet, but my shopping list of bolts and clips and the like is growing

 

There's a module that controls vacuum to the cruise control. If it's not on there wouldn't be a vacuum leak.

My cruise went out too. I need to check that diagram.

*TIP* get a vacuum pump with a gauge. if the used one your interested in doesn't hold vacuum get another one.