checking engine after broken cam chain (non e30)

Post Mon Aug 08, 2005 12:18 pm

please please leave this thread up! i know it's not e30 but as bmws do seem to lose cam chains quite often i'm hoping someone here can help me!

my sisters 96 micra has broken its cam chain. only done 30k miles! happened while idling so either she lucky and needs a new chain, or unlucky and needs new engine. problem is, how do i tell? just put a new chain on time it up and hope it works?

the thing is by the time i've taken the head off i might as well have just put another engine in, there's plenty around. so don't want to take the heads off to find no damage cos then i'll have to buy hg kit, head bolts and all the rest

BLOODY JAP CRAP!!!!

Post Mon Aug 08, 2005 12:28 pm

You really can't risk it mate, even at idle it's doing 1000rpm so the pistons are going 1750 m/s (assuming 75mm stroke in a 325). Im my flat mates diesel citreon which was idling it actually broke the camshaft clean in 2. And my bros pug 205 gti most of the valves were bent even though it was very difficult to tell. So I'd just replace the engine, then maybe you can salvage some cash from the old lump if you are lucky.

Post Mon Aug 08, 2005 12:30 pm

Compression test?

A

Post Mon Aug 08, 2005 12:33 pm

Yeah but to do that properly you need to replace the cam chain!

Post Mon Aug 08, 2005 12:40 pm

i know what u mean widge, but a lot of jap 16v engines are non-interference design. don't totally know how it works but if the cam drive breaks the engine usually survives intact.

my 1988 mr2 has this design, it's not infallible but they usually survive a broken belt.
i haven't moved the cams and in the position they're in the bottom end turns over smoothly by hand with no clunks or nasty noises.

guess i need to find out if this engine is a non interference design or not, and if so put a new chain on and cross fingers. not too difficult hopefully as it's only the top chain that's broken (the main chain drives an idler below the cam pulleys and a small chain drives both cams from this idler)

Post Mon Aug 08, 2005 1:22 pm

There are a number of options.

Engine may be ok so don't just get a new one. I had the cambelt snap on an MOT test on a diesel escort. The engine survived ok - just needed new belt and rollers.

Option 1. replace cam chain etc, adjust and do a compression test. This costs some money.

Option 2. - IF POSSIBLE - remove camshaft so that all valves are closed. Then do compression test.

The results of both will tell you if engine is ok or if valves / pistons are damaged.

Karl

Post Mon Aug 08, 2005 1:26 pm

sounds like a good idea cheers karl

the cams are a doddle to remove on most jap 16v engines, the head bolts are usually under the cams, so the cam carriers just unbolt from the top and the cams lift off.

usually have to remove the belt/chain also, but i do not have that problem

Post Mon Aug 08, 2005 1:53 pm

Oh yeah good point mate forgot about the Japs making non interference engines, definetly find out, the fact you can turn it over is a good sign. Good luck

Post Mon Aug 08, 2005 2:04 pm

karlp wrote:There are a number of options.

Engine may be ok so don't just get a new one. I had the cambelt snap on an MOT test on a diesel escort. The engine survived ok - just needed new belt and rollers.

Option 1. replace cam chain etc, adjust and do a compression test. This costs some money.

Option 2. - IF POSSIBLE - remove camshaft so that all valves are closed. Then do compression test.

The results of both will tell you if engine is ok or if valves / pistons are damaged.

Karl

Reminds me whan a mate brought his Escort round for some work on it. Part of the job was a new cam belt, but i ran out of time. So i asked him to bring it round the next day for me to finish off..... you know what's coming!

the bloody cam belt snapped when he went to start it in the morning...

Bloody lucky thing was i slipped a new belt on and all was OK!

With a bit of luck the Micra will be OK. Then she can sell it and buy a proper car xxx

Post Mon Aug 08, 2005 2:12 pm

You should be able to find out on the Web weather (sp?) it is an interference engine or not. What age Micra is it?

Post Mon Aug 08, 2005 3:49 pm

1996. piece of junk! i've never heard of another micra that had a serious engine problem like that (not that i know a lot about micras i'm pleased to add!)

yep will check web about whether it's likely to be ok or scrap. i'm wondering if the tensioner caused it, need to have a look at that. can't see why the chain would break esp as it's such a short piece.

thanks for the help all.

Post Mon Aug 08, 2005 5:21 pm

comp test is a good method.,....

what id do is remove head and inspect it.... if its ok...assemble it... if its broke then get a new lump... of course ull have to get a HG kit if u do this

Post Mon Aug 08, 2005 5:25 pm

Karan wrote:comp test is a good method.,....

Surely you can't do a good comp test without the valves working?

Post Mon Aug 08, 2005 5:27 pm

Widge wrote:
Karan wrote:comp test is a good method.,....

Surely you can't do a good comp test without the valves working?

well u close them by remocing the cam/rockers as karl suggested earlier

Post Mon Aug 08, 2005 5:30 pm

Widge wrote:You really can't risk it mate, even at idle it's doing 1000rpm so the pistons are going 1750 m/s (assuming 75mm stroke in a 325)

don't mean to pick u up on small things mate (i'm bored ok!) but the pistons only do that speed halfway up their movement. when they get to the top of the stroke they do 0m/s for an instant. so how fast the impact is depends on how close to the top of the stroke the piston is when it hits the valve (which depends on how high the compression is and how much valve lift there is)

doesn't matter, i agree that at idle they can still break valves if the belt dies!

Post Mon Aug 08, 2005 5:31 pm

Hmmm......oh yeah - soz Karan I really must concentrate more when reading!

Post Mon Aug 08, 2005 5:37 pm

i'm not sure about the comp test thing

suely if the inlets aren't opening it won't suck a proper charge of air in to compress?

although i guess if each cylinder reads the same then one is unlikely to be damaged...

Post Mon Aug 08, 2005 5:39 pm

Yep your right, but what I was trying to illustrate is that even at idle there is a huge amount of energy in the system. My flat mates citroen stopped instantly when the belt broke with a loud clunk - all that energy at 900rpm idle was suddenly dissapated in to one valve, which as it was a shim head without rockers etc... (bloody french morons...) brake the cam shaft clean in half

Wish I had taken a photo now it was well impressive. So weather you are idling or doing 4000rpm makes little difference if it is going to destroy it's self - if idle was 10rpm it would!