1996 Buick Century With Rebuilt Finished?

I have a 1996 Buick Century 6 cylinder wagon into which I put a (second) rebuilt engine in August 2004. This second was a replacement under warranty because the first rebuilt had “all six spark plugs bridged with oil ash”.

15,000 miles into this second rebuilt, the engine was misfiring badly under load. The engine diagnostic codes were Random Misfire. The Fuel Pump tested OK. New spark-plug wires, new Ignition Module Assembly and 3 new Ignition Coils were put in. The mechanic said I may still need a new Catalytic Converter. The car ran good after this.

22,000 miles into this second rebuilt, the Service Engine Light came on. The code was P0305, Misfire Cylinder 5. The mechanic put in another new Ignition Coil for Cylinder 5. The engine ran good again.

30,000 miles into this second rebult, the engine misfired under load again on the highway. The Service Engine Light Came on again and the code was P0305, Misfire Cylinder 5. I had a different mechanic look at it and he replaced spark plugs and wires. The mechanic said that the replaced plug in cylinder 5 was badly fouled.

A week later it has misfired badly again under load and the code again was P0305.

More detail on the misfiring: When I was on the highway, the acceleration seems sluggish. Going up a mildly steep uphill (Route 93N in Boston around Montvale Ave), the car engine shook and made a fairly loud noise then recovered. A little way further down 93, the Service Engine Light came on.

This is the same thing that happened a week earlier, before the spark plugs and wires were replaced. Again, going up a hill on the highway, the engined shook, hesitated then recovered only to have the service engine light come on the next morning.

Can I run this car the way it is? The mechanic seems to think so, he is not the one who put the rebuilt in. Unfortunately the engine is now just out of warranty.

Could it be the timing chain/belt?

I don’t like to give up on cars and try to maintain them so they last.

You cannot continue to operate this vehicle with the Engine light on and running badly. It just creates more headaches.
These things work like a line of dominos. If the spark plug is being fouled it’s going to kill things all along the line, starting with the plug wire and coil.
This mechanic should not be telling you to motor on.

Since this seems to be a chronic problem the first thing I would do with it would be to run a compression test. There is absolutely no sense in continuing to throw ignition and fuel system parts at an engine that may have a mechanical fault.
The term “rebuild” is used pretty loosely also and it can mean a couple of dozen different things. There’s a proper rebuild and everything else after that.

Compression test next and go from there; and this should have been done already just to make sure.
Since some mechanics have a skewed idea of what constitutes “good compression” you might post any results back here for further discussion. Ideally, you want all cylinders on an allegedly low miles rebuilt engine to be in the 175 PSI area on all cylinders.

Thanks for the advice, I’ll do just that.

OK Here are the pressures:

Cylinder/PSI 1/160 2/150 3/160 4/150 5/155 6/162

Now I’ve driven the car another 100 miles or so. None of it on the highway. Around the town speeds of 25-40 mph, slow accelerations. The engine seems fine and no Service Engine Light.

But I fear if I take it on the highway and have to accelerate to 65 mph, that the engine is going to hesitate severely again, and the Service Engine Code of P305 is going to happen again (cylinder #5 misfire = P305). I need to accelerate fairly rapidly on the highway and that extra vibration, I think, accentuates whatever is wrong with the timing chain. On the other hand, could the extra fast acceleration be too hard on the fuel pump?

One other thing, the oil on the dipstick looks good.


If the engine has been rebuilt then it should have a new timing chain in it.
I believe if the car were mine I would take it out on the road and see what happens.
If it starts to run bad again, pull into an AutoZone, etc. and have them pull the codes to see if that 305 is back.
A 155 PSI is good enough to run with no problem so if that 305 is back again, and considering the plugs/wires/coils have been replaced, then I would start to consider a possible faulty fuel injector. Maybe a pintle (the needle) is sticking or it has an intermittent fault in the coil winding or the wire connector. Another possibility could be an intermittent fault in the ECM, or engine computer. I would not worry too much about the ECM at this point though.

If a faulty injector is suspected you could try swapping No. 5 with another one, drive it, and see if the problem reoccurs with a fault code showing a misfire on the cylinder the potentially bad injector is installed in.

The fast acceleration is not doing anything to the pump. A fuel pump or filter problem has crossed my mind but if this were the cause of cutting out, etc. then it should be affecting all cylinders, not just one.

Sluggish running can also be caused by a clogged catalytic converter but should not set a 305. It could set a code for a random misfire though.
Converters are easily checked with a vacuum gauge. Long term chronic misfires can contribute to clogged converters.
Hope some of that helps anyway.

So change the ignition coil if so equipped. See what happens then.