Interference engine problems need help

[9za4ck4]

Ok so I have a 1988 944 porsche non turbo sohc. So heres the problem timing belt jumped some teeth, I replaced it and put it back in time and now it wont start at all. I did a compression test and I got 0 compression all the way across. My question is what problems am i looking at? I realize valves are probly going to be bent but I'm looking for all possible problems. While I'm in there I want to know what else should I look for? Thanks ahead of time

[stevieturbo]

9za4ck4 said:

Ok so I have a 1988 944 porsche non turbo sohc. So heres the problem timing belt jumped some teeth, I replaced it and put it back in time and now it wont start at all. I did a compression test and I got 0 compression all the way across. My question is what problems am i looking at? I realize valves are probly going to be bent but I'm looking for all possible problems. While I'm in there I want to know what else should I look for? Thanks ahead of time

most damage should be obvious when it's stripped.

Head damage, valve guides, valve seats, piston damage....there could be quite a bit really.
or you could be lucky and just be valves.

[Steve_D]

I would double check the timing of your new belt install before stripping down. Posssible to have wrongley timed it giving you the no compression.

Steve

[Pumaracing]

Rather depends on whether it happened at 7000 rpm caning it down the bypass or at cranking speed doesn't it? Why are we guessing?

["That croc was going to eat me alive."
"Well, I wouldn't hold that against 'im. Same thought's crossed my mind once or twice."]

Edited by Pumaracing on Tuesday 25th January 13:37

[bertelli_1]

They have a tendancy to bend valves in those circumstances.

"my girlfriend said she wanted a tutu. I said surely you mean a four by four".

[D_G]

Pumaracing said:

Rather depends on whether it happened at 7000 rpm caning it down the bypass or at cranking speed doesn't it? Why are we guessing?

["That croc was going to eat me alive."
"Well, I wouldn't hold that against 'im. Same thought's crossed my mind once or twice."]

Edited by Pumaracing on Tuesday 25th January 13:37

Makes no difference. As long as you now have the timing right my guess would be bent valves, get the head off and have a look.

[stevieturbo]

D_G said:

Pumaracing said:

Rather depends on whether it happened at 7000 rpm caning it down the bypass or at cranking speed doesn't it? Why are we guessing?

["That croc was going to eat me alive."
"Well, I wouldn't hold that against 'im. Same thought's crossed my mind once or twice."]

Edited by Pumaracing on Tuesday 25th January 13:37

Makes no difference. As long as you now have the timing right my guess would be bent valves, get the head off and have a look.

Makes no difference ?

Do you think the same damage is possible at idle with the car sitting stationary, as 7000rpm driving flat out ??

Ive seen catastrophic damage due to valves not doing what they are supposed to at high rpm. It would be physically impossible for that same damage to occur at idle.

[oakdale]

If the belt breaks at low rpm, the cam will stop rotating almost immediately, usually with the valves on one cylinder on the rock and the valves on cylinders 90deg away partly open but sometimes not enough to cause damage, at high rpm, the cam keeps turning causing inevitable damage.

If there is no compression at all, it's not likely to be the timing, as with a single cam wheel driving the valve train, it's not possible to position the timing to give no compression at all.

Unfortunately, in my experience a belt that has jumped a few teeth does the most damage, as the engine keeps running while the valves are hitting the pistons, sometimes causing a valve head to break off and embed in a piston.

Edited by oakdale on Tuesday 25th January 23:37

[D_G]

What I'm saying is either way valves will meet pistons and damage will occur on an inteference engine. The level of damage is only evident once the head comes off, I've had cams / guides snapped at low RPM, but also very little damage at high RPM belt failure. The engine speed isn't a given as to what damage there might be, engine design more determines that.
Also if the belt fails the car will still drive the crank as long as the car is in gear and the clutch not depressed, doesn't take long to damage the valves / pistons.

[oakdale]

D_G said:

What I'm saying is either way valves will meet pistons and damage will occur on an inteference engine. The level of damage is only evident once the head comes off, I've had cams / guides snapped at low RPM, but also very little damage at high RPM belt failure. The engine speed isn't a given as to what damage there might be, engine design more determines that.
Also if the belt fails the car will still drive the crank as long as the car is in gear and the clutch not depressed, doesn't take long to damage the valves / pistons.

If the cam has stopped in a position where the pistons can't hit them, it doesn't matter how long the crankshaft keeps turning for.

[9za4ck4]

Well I've checked the timing several several times just to make sure its in time. Because i kept thinking it was out of time still when I wasn't getting any compression. But it is in time and very dead on and still I'm getting no compression. Oh and I wasn't banging the rev limiter when it jumped teeth, I was casually driving and I do believe it was right a 3000rpms. Anyways I dont think that really matters though, But I was just going to replace the head with a new complete head. Does this seem like a smart idea or is that going to be a waste of money. I think what I'm trying to ask is whats the possibility of this engine being a complete loss?

[powerstroke]

9za4ck4 said:

Well I've checked the timing several several times just to make sure its in time. Because i kept thinking it was out of time still when I wasn't getting any compression. But it is in time and very dead on and still I'm getting no compression. Oh and I wasn't banging the rev limiter when it jumped teeth, I was casually driving and I do believe it was right a 3000rpms. Anyways I dont think that really matters though, But I was just going to replace the head with a new complete head. Does this seem like a smart idea or is that going to be a waste of money. I think what I'm trying to ask is whats the possibility of this engine being a complete loss?

my guess is depends on the cost to repair the damage compared with a second hand engine?? .. pull the head just a few bent valves= happy days!!! ... valves embeded in pistons,bent rods, smashed guides, and chewed seats= FUBARed..... maybe

Edited by powerstroke on Wednesday 26th January 09:00

Edited by powerstroke on Wednesday 26th January 09:02

[powerstroke]

powerstroke said:

9za4ck4 said:

Well I've checked the timing several several times just to make sure its in time. Because i kept thinking it was out of time still when I wasn't getting any compression. But it is in time and very dead on and still I'm getting no compression. Oh and I wasn't banging the rev limiter when it jumped teeth, I was casually driving and I do believe it was right a 3000rpms. Anyways I dont think that really matters though, But I was just going to replace the head with a new complete head. Does this seem like a smart idea or is that going to be a waste of money. I think what I'm trying to ask is whats the possibility of this engine being a complete loss?

my guess is depends on the cost to repair the damage compared with a second hand engine?? .. pull the head just a few bent valves= happy days!!! ... valves embeded in pistons,bent rods, smashed guides, and chewed seats= FUBARed..... you did check the cam isnt broken??

Edited by powerstroke on Wednesday 26th January 09:00

Edited by powerstroke on Wednesday 26th January 09:02

[stevieturbo]

oakdale said:

If the cam has stopped in a position where the pistons can't hit them, it doesn't matter how long the crankshaft keeps turning for.

Except when a valve head comes off and pulverises the entire engine. So it matters quite a bit how long the crank continues to turn.

[GavinPearson]

9za4ck4 said:

I think what I'm trying to ask is whats the possibility of this engine being a complete loss?

It's probably a dead cert.

I'd start looking for a new engine.

[oakdale]

stevieturbo said:

oakdale said:

If the cam has stopped in a position where the pistons can't hit them, it doesn't matter how long the crankshaft keeps turning for.

Except when a valve head comes off and pulverises the entire engine. So it matters quite a bit how long the crank continues to turn.

You're being silly now, if there has been no contact between the pistons and the valves, a valve head will not have come off.

[stevieturbo]

oakdale said:

stevieturbo said:

oakdale said:

If the cam has stopped in a position where the pistons can't hit them, it doesn't matter how long the crankshaft keeps turning for.

Except when a valve head comes off and pulverises the entire engine. So it matters quite a bit how long the crank continues to turn.

You're being silly now, if there has been no contact between the pistons and the valves, a valve head will not have come off.

You do realise what an interference engine is ?

Yes, one that there will always be a position where there is contact when the belt fails.
Hence the name "interference"

[anonymous-user]

GavinPearson said:

It's probably a dead cert.

crickey, that's really messing with my head.............. ;-)


Even famous probabilists like Wang Zikun would struggle with that!

[oakdale]

stevieturbo said:

oakdale said:

stevieturbo said:

oakdale said:

If the cam has stopped in a position where the pistons can't hit them, it doesn't matter how long the crankshaft keeps turning for.

Except when a valve head comes off and pulverises the entire engine. So it matters quite a bit how long the crank continues to turn.

You're being silly now, if there has been no contact between the pistons and the valves, a valve head will not have come off.

You do realise what an interference engine is ?

Yes, one that there will always be a position where there is contact when the belt fails.
Hence the name "interference"

The term interference means that the valves and pistons will make contact when the valves are at full lift and the pistons are at TDC, it doesn't mean damage is inevitable because if the belt breaks at low rpm and the cam stops rotating almost immediately (with no valve fully open) damage may not always occur.

Edited by oakdale on Wednesday 26th January 20:58

[stevieturbo]

oakdale said:

stevieturbo said:

oakdale said:

stevieturbo said:

oakdale said:

If the cam has stopped in a position where the pistons can't hit them, it doesn't matter how long the crankshaft keeps turning for.

Except when a valve head comes off and pulverises the entire engine. So it matters quite a bit how long the crank continues to turn.

You're being silly now, if there has been no contact between the pistons and the valves, a valve head will not have come off.

You do realise what an interference engine is ?

Yes, one that there will always be a position where there is contact when the belt fails.
Hence the name "interference"

The term interference means that the valves and pistons will make contact when the valves are at full lift and the pistons are at TDC, it doesn't mean damage is inevitable because if the belt breaks at low rpm and the cam stops rotating almost immediately (with no valve fully open) damage may not always occur.

Edited by oakdale on Wednesday 26th January 20:58

Have you ever seen a camshaft or worked at an engine with more than 2 cylinders ? They are called interference for a reason !!!