Broken connecting rod

[pager14]

I recently got a 96 Bonneville SE with 85k on it from my brother. It had some sub-frame issues. I fixed that with new metal and got it to pass state inspection. I drove it for a thousand miles and didn't have any issue with it. My son used it the other day and called me to say he was broke down. When I got there and checked the motor out it wouldn't crank. The starter engaged in the flywheel. I had it towed home and tried to turn the crank from the balancer. It wouldn't turn normal. I pulled the inspection cover for the flywheel and spun it backwards. I heard something drop in the oil pan (knowing the sound to be a rod cap dropping). I pulled the oil pan and found a bolt that had backed out with threads intact, the cap itself, a piece of the rod (#1) and a broken bolt. My brother owned this car since it had 20k on it. It was his wife's car. They changed the oil religiously every 3 k. It set for a year and a half. I don't see that to be the cause. What would have caused this?

[xxdabroxx]

I recently got a 96 Bonneville SE with 85k on it from my brother. It had some sub-frame issues. I fixed that with new metal and got it to pass state inspection. I drove it for a thousand miles and didn't have any issue with it. My son used it the other day and called me to say he was broke down. When I got there and checked the motor out it wouldn't crank. The starter engaged in the flywheel. I had it towed home and tried to turn the crank from the balancer. It wouldn't turn normal. I pulled the inspection cover for the flywheel and spun it backwards. I heard something drop in the oil pan (knowing the sound to be a rod cap dropping). I pulled the oil pan and found a bolt that had backed out with threads intact, the cap itself, a piece of the rod (#1) and a broken bolt. My brother owned this car since it had 20k on it. It was his wife's car. They changed the oil religiously every 3 k. It set for a year and a half. I don't see that to be the cause. What would have caused this?

I'll give you a hint. How old is said son?

[pager14]

He's 18. I was 18 once too and can't see a little hot-rodding breaking a connecting rod. I kind of have a lead foot myself. The car didn't burn or leak oil.

[RJolly87]

Has the car been using coolant? Were the UIM issues ever addressed?

[crash93ssei]

The 3800 is an extremely durable engine, stock or even lightly modded if it still has the factory rev limiter I can't see any amount of "normal" hot rodding doing any damage like that. I would lean towards a failed UIM or LIM gaskets caused massive internal coolant leak that hydrolocked the engine and broke the rod, that has happened many times. If there is no coolant in any of they cylinders then I would just lean towards bad luck and *shoot* happens.

[Jrs3800]

I agree Ryan... The stock rev limiter on a 96 is 5450 meaning roughly a 5650 Rpm shift at WOT.. No enough to kill a 3800..

It sounds more like to me that it inhaled coolant and bent/snapped the rod as coolant/water does not compress.. With a 96 the mileage was sure right for an upper intake failure.. Not going to lay any blame on the son with this one..

If you replace the engine, Look for an 04-09 3800 Series II or Series III.. You could use an 04-05 Bonneville, Buick Lesabre series II L36( Vin K ) or even a 06-08 Pontiac Gran Prix/Buick Lacrosse, Lucerne series III L26( Vin 2 )

[pager14]

I thought the same thing at first. There wasn't any coolant in any of the cylinders. There was hardly any oil on the block, intake or in the vicinity of the rear main. I pulled the motor and found a connecting rod bolt in the pan. The threads weren't damaged. The other one was broke. Probably from all the stress on that side of the connecting rod. That was the only thing I found. I'm not saying it can't happen, if the bolts hadn't been tightened properly one would think it would have come apart long before this. I got a motor out of a 04 grand prix with 91k on it. Looks like I'll have a little parts swapping to do (motor mounts, alternator/tensioner assembly and possibly the exhaust manifolds). It'll probably be my weekend project. I want to get it done as soon as possible. This cars drive like a dream. I appreciate the replies guys. I guess it was just time for this motor to go.

[Jrs3800]

Yeah, it's hard to say what happened.. It's one of those things, sometimes they last forever other times they dont.. It was an engine that was produced by the millions, so there is always a ticking time bomb somewhere..

My 95's original motor was never very balanced, and in the higher revs you could feel it.. With the current motor it as smooth at 2500 as it is at 6000.. But the 96 motor is swinging near 20 years and a hard life.. several years ago I noticed a light " thud thud thud " when changing the oil.. That faithful sound of a bearing a bit out of clearance until the oil gets there.. I think I noticed that 5 years ago.. it's gotten a tad worse in that time, but still sports great oil pressure.. I do know it's on borrowed time at this point..

Now, with the 04 grand prix engine... I can't remember if that oil pan is stamped or cast aluminum.. Then you have the oil filter adapter, the bonneville one points down at an angle where the gp one should sit between the tripod housing and the balancer.. So you will have some things to swap over.. I highly suggest new aluminum gm intake gaskets and a new upper plenum, and if you need sealer I highly recommend permatex gray..

If you have any questions just ask

[pager14]

Well after a lot of work this weekend, I finally got the car running again. I had to do a little more swapping than I originally thought. The fuel injector plugs were different so I had to use the ones off of the blown motor. I broke the elbow on the belt tensioner (not the plastic one either) so I had to buy a new tensioner. I went ahead and replaced the front pump seal on the tranny while the motor was out. For some reason I had a hell of a time putting the torque back in. I was at the point that I was reaching for a hammer. I had to get a new a/c compressor. The original one leaked behind the pulley. The one I got with the grand prix motor looked pretty clean and good, but after trying to hook up the lines I found out that the Bonneville compressor has a larger port on the suction side. The grand prix ones were the same size. It was 1:30 Sunday morning when I fired it up. After letting the fuel pump build up pressure, the engine cranked over about 5 times and fired up. The oil pressure was at 40 psi at idle. The TPS sensor connector came off and the engine jumped to 2000 rpm. I reconnected it and reset the check engine light. I drove it to work this morning. It seems to run great. Oh, and the a/c is nice and cold.