740 240k miles looses power on highway

[efinet]

I have a 740. It has been driving great for the past 4 years when I got it.
Oil and water are ok. No nocking sound.
The car starts properly, however I can not drive faster than 30mph??
There is a hissing sound comming from the front??

What could this be? I would apreciate help as I need this car for work and can not drive on a highway with it!

Thanks

[gc]

At this age, the converter may be used up and clogged, limiting the engine's ability to breath.

Can you bypass the converter and see if the problem disappears?

[ber]

I had a similar issue with my '88 740 gle and I was told the mass air flow sensor might be faulty... When you shut off the car it should click twice right by the air filter. If not that sensor might be bad, stopping your engine from getting the proper air/fuel mix. Or maybe you just have an vacuum leak. Check your hoses...