Can someone maybe tell me what happened?

I have a '99 GMC Jimmy, 4.3 L engine, 150,000 miles. I was driving to work with cruise set on 70 mph, i noticed the engine revving like it was trying to maintain speed, slowing down to 65. I kicked the cruise off and the engine died, and once i got it pulled over steam/smoke started billowing out from around the radiator cap and from the front of the engine......once i had the hood up it looked as though the radiator cap was almost blown off...there was steam but it looked like smoke too, with a coolant smell, but also a burning smell. The temp gauge never showed hot, but a couple of days before i noticed the heater would only blow hot when i was giving it gas, sitting still no heat. I only noticed coolant coming out around the rad. cap, nothing underneath or anything, no white smoke from the tailpipe. The engine would turn over but not start, would almost catch but not quite, sounded like it had lost compression or something. After this happened i added coolant, checked the oil and it had gone down to "add" on the dipstick, but there didn't seem to be coolant mixed with the oil, but the coolant that had spilled out had a greasy feel to it.....could it be a blown head gasket? Can anyone give me an idea what may have happened?

Saturnthegiant, sounds to me like maybe the engine had an air pocket or got low on coolant and overheated possibly causing engine damage. Will it start now? Did you check the coolant level when the heater only blew warm on acceleration?

Yes, i did check the coolant level first thing when i noticed this.....coolant level was fine. I admit, i did not notice what the temp gauge was reading, but as soon as i got stopped i flipped the key back on and it read around 160.

Saturn when you turn it over now does it start? If not you said it sounded like it lost compression, so it turns over different than normal, Did you notice any fluctuations in the temp. before this?

Still doesn't start.....no i never noticed any temp fluctuations. It always ran cool...i never had any problems with the engine before this.

Well, do you have fuel pressure?

Do you have spark?

Check for codes?

Injector pulse? Will it start if you spray carb cleaner into the intake?

Think you need a new rad cap, but that has nothing to do with a cold start.....

Maybe you do have two different problems but it is strange they occured at the same time, by all means spray some carb cleaner or fuel into the intake and see if it starts, if not check for spark. When you crank it does it turn over normally, slowly or very fast? If it almost blew the radiator cap off something is very wrong.

A friend of mine says he thinks it sounds like the timing belt may have broken....thus the water pump quit, causing the engine to overheat. Any ideas? I can't look at until the weekend, i've had a bad cold keeping me on the couch, and going out of town to work today.

Saturn the engine may have jumped time due to a bad timing chain, the chain does no run the water pump on your particular vehicle though. The timing chain also could have skipped a tooth or two causing the timing to be retarded which would cause an overheated condition, just a thought. Let us know, best of luck.