steam under engine plenum cover

[davidson5557]

RE: 1999 Pontiac Montana

Two days ago when I was driving to work the cabin heater did not work very well. (low heat output, fans ok) Then it seemed to worked well just before I got to work. (10-15 minute drive). On the way home I noticed steam coming from under the top part of the hood. I was not in a position to stop and check what was going on. The temperature gauge indicated normal operating temp so I drove about a mile home. When I got home I opened the hood and there was no steam to be found. (nor any indication that there was any steam). The overflow was normal. I thought the radiator cap was not very hot. I took a chance and took it off ( no problem ). The fluid level was full and somewhat hot, but not blistering hot as I expected after a 20 minute drive.

The next day I drove about 10 miles and again steam was seen. I stopped and the steam was coming from the back side of the plenum cover. I left the engine running and could see the steam continuously but not from the source due to obstruction from the plenum cover. After about 2 minutes the flow of steam stopped, to my surprise, even though the engine was idling.

I have not seen the steam again although I have a very watchful eye. This morning I let the engine idle in my driveway for about 20 minutes but no steam appeared. I have also driven it of short 10-15 minute trips. Again, no steam. I have been trying to read in my Haynes manual and Pontiac service manuals a possible explanation. The heater hoses that connect to the heater fittings look fine; there is no evidence of leakage. I don't know if the poor heater performance on my way to work is connected to the steam appearing, but the heater has worked fine yesterday and today. I added about a cup of water to top off the radiator.

At this point I am stumped. Any ideas.


BTW; I had to upper an lower intake manifold gaskets replaced in the summer. I think the mechanic did a very good job. I went on road trips totalling 8000 miles without incident. I did not need to add a drop of radiator fluid.

My mechanic suggested a stuck thermostat or partially plugged radiator but offered no explanation for the appearance of steam.

[rockwood84]

the heater hoses are in a bundle and one of them has a 90 degree quick connect on it and the one that goes from the metal by-pass tube that runs accross the front of the motor sits where it can rud on this 90 degree q/connect. mine did the same thing the steam will vent out under the hood then stop. the dealer wants $209.00 for the quick connects and hoses but take the outer crimp off of the factory crimp and there will be a nipple you can put the heater hose back on with useing a 80 cent # 10 hose clamp.