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My son has a 1999 Grand Prix SE with the 3800 series engine. A little while ago he started to see the low coolant light coming on. We bought a gallon of Dex Cool premix and he went thru all that in about a 2 week period. One night at a friends house he went to start the car and it would not turn over. Once I got it towed home I discovered the engine must have hydraulically locked because the end broke off the starter. We pulled the plugs and found that #6 cylinder was full of coolant.

After doing alot of reading we have narrowed it down to the intake plenum. This seems to be common for this engine series and car. We bought a maintenance book and tore down the top of the engine. After pulling the plenum off we saw that the top of the intake manifold was covered with coolant. From all this it appears that the plenum is the culprit but we cannot see any cracks!

Any suggestions on how to check the plenum for cracks with it out? Are we barking up the wrong tree??

Any help or suggestions would be appreciated.

MJ  

wow... i bet you will try to find coolant leaks from now on... before they go this far...

there is a huge problem...

most coolants will destroy the engine bearings... main, rod and cam...  so as long as it is torn down.. you might ask other techs if they have had problems with the orange coolant eating bearings..

i also worry that the cranking of the motor may have damaged the cylinder wall, piston,. rings, bend the rod.. break the rod . break or bend the crank...

since those motors use multi layer steel head gaskets.. you also want to order a tube of hylomar for them from harbor freight or goodman...  

I don't have it torn down quite that far.... We only went far enough to take the plenum off the top of the intake manifold. If we have to go further than this,it may be beyond our capability. The coolant only seems to be on top of the manifold and inside of 1 or more cylinders. The big question is where did it come from? Is the plenum cracked and should we be able to see it?? Is it possible that the gasket under the plenum was the culprit?

Any other guidance or suggestions would be appreciated.

MJ  

This is a common problem on the 3.8. You're wasting your time looking for cracks. You have to replace the entire plenum. You can buy aftermarket plenums. Go to www.rockauto.com and look up your vehicle. Dorman Products makes a decent one that has been redesigned to eliminate the original problem.

However, the bigger issue is whether you have already damaged the bearings. To check that, you may have to pull the oil pan and remove a rod cap to see the condition. Or, you can buy the new plenum, drain the coolant and oil, and take your chances.  
I'm a retired ASE Master Tech and keep up with current technology. I have over 400 auto fixes on my blog at ricksfreeautorepairadvice.com

You can close this one out.

We took the plenum, turned it upside down and filled the coolant area with water. After holding our breath for 30 seconds or so we started seeing the water drip down inside the throttle body so this is definitely the culprit.

Dumped the oil (it didn't look bad) and put in some fresh stuff. Took all the plugs out, put in a used starter, hooked it up to a battery and blew all the cylinders out. Bought a plenum and gasket kit and took a couple nights putting it all back together. It only popped on the first start attempt so we pulled the front 3 plugs again and heated them with a torch. Popped them back in and she lit off on the first try! It took a while for the exhaust system to dry out but it seems to be running fine. My son has been driving it for almost 2 weeks without issue.

Total cost of the repair was less than $250

Thanks for the help!

MJ