How do I diagnose engine failure? Alfa 155 8v

[Balidey]

I have aqcuired an Alfa Romeo 155 that needs a new engine. I will be fitting a complete replacement, but I would like to diagnose the problem that caused the failure, if possible.

Previous owner describes a bang, severe loss of power, quite a lot of oil smoke and then the engine died. Lifting the bonnet shows the problem straight away, conrod sticking through a hole in the front of the block, about 4 inches wide. I think it was piston number 2.

Engine is a 1.8 litre Alfa 4 cylinder in line transverse, twin spark. Its the earlier chain driven twin cam 8 valve head. I am told this engine is very tough and 'bullet-proof', much stronger than the later 16v. Although this example seems to not be as bullet proof as others. Mileage is just over 105,000 which is not too high for one of these engines.

So, I would like to diagnose the cause of the failure when I strip it apart, but I am worried that a severe failure like this may not leave many clues to what actually happened prior to the failure. The previous owner is not any help in this detective work, so its purely what I can see during dis-assembly.

So, what do I need to look for, is there a step by step procedure I can follow? Or is it a lost cause and I should maybe just get the new engine in and forget about this one?

Car is a wide body 155 in purple. I am informed thats the rarest colour for the wide-bodies. Apart from the engine, the car just needs a little cosmetic tidying and then I can at last claim to be an Alfa owner. I've always had a soft spot for the 155 and this one definitly needed saving, I hate seeing cars scrapped for something as trivial as a conrod breaking free.

Regards,
Steve

[Guy Croft]

Assuming the crank didn't break in the first instance - potential causes:

1.Timing chain failure - link/tensioner or key/dowel drive to cams/crank. Of course the drive could well be consequentially damaged, ie: as a result of a failure elsewhere and may not be primary cause. Just note what you find there.
2. Head - remove. There will no doubt be massive damage in the combustion chamber with bent and fractured valves and you won't learn much there per se. But - check for cam seizure, spring fracture ie: valvetrain failure that might have led to valve to piston contact beyond what you'd expect from say, over-rev. If a valve seat came loose you'll never be able to ID as the fundamental cause because of all the damage in the chamber. Check the head gasket fire ring because detonation could have caused it. Detonation causes rupture to the fire ring, if that's the case it may be torn open and may look in places like it's been crimped with pliers.
3. As for the rod, failure can occur due to massive over-rev, in which case the fracture will be from fatigue and could have started at the small end where the beam joins the small end bush region, or in a bolt thread. It can also, of course occur due to bearing failure due to neglect in oil changes or low level. One expects the rod bearings to be beaten-up, but the pointer is often the condition of the main bearings. If they are in a pretty rough state you know that's the cause. Of course a dirty build or things like caps wrongly fitted can cause the same.

The trouble with a rod failure is you get so much consequential damage you can't make sense of anything. If you don't find anything untoward in the valve train (apart from obvious damage from things smashing around in the cylinder) you may never really know for sure what went wrong, but you can sure if that's the case that the failure started at the crank end of the rod. Worth noting that rod bolts - for example - are often blamed for failures that actually started in the bearing region. An oil analysis would be interesting, I guess oil manufacturers like Castrol and Millers can do that.

There won't be much worth saving unfortunately...

GC

[tmvolumex]

A broken rod can occur due to an inadequate amount of oil / pressure being supplied to the big end of the rod. An overheated big end on the connecting rod indicates an oil supply / pressure problem. Overheating of the big end is easy to spot as typically the big end of the rod will have turned blue and yellow in color. When the rod seizes on the crank, it is usually broken in half when it attempts to rotate with the crank and then goes through the side of the block. I am sure some unlucky folks have photos of this mishap!

[a-repair]

Steve Hi, you are right about the strength of these engines, these are very good engines. The problem is driving with to little oil in the sump. These engines can burn oil and when the previous owner(s) did not check the oil level often the bearings worn due to oil starvation. The shape of the oilpan is different from the old Alfa Nord engines (with the big sump) and must be run on max on the dipstick especially at hard cornering and braking. With normal use in a streetcar these engines can run easialy 300.000 km's or more. So if you put in an used engine, renew the main and conrod bearings and check for bearingplay. Also check the camshafts and followers for wear.
Greetings, Arie Vreugdenhil 's-Gravenzande Holland