J30 Engine autopsy help requested.

[rwhyde]

Hello,

I'm new to the list but have been lurking for about 3 weeks. I bought a '93 J30 with a bad engine. It has 120K on it and 59K on the current timing belt.I pulled the heads (what a bear) and found that the exhaust valves had been in violent contact with the pistons to the point that two had broken their heads off and were laying in the cylinder. Very little damage otherwise. What puzzles me is that the timing belt appears to be perfect, no shredded teeth. I've never seen one jump time without shredding teeth before. Is there another cause for such a failure??Thanks for the input.Wayne Hyde

[Q451990]

Any chance that the last belt was installed with the timing off? It's possible that they put it back together and it didn't run... or maybe it was only one or two degrees off and didn't scramble until someone got it to higher RPMs.

If the pistons and heads aren't damaged, I'd just replace the valves and seats on those two cylinders and re-assemble with a new belt, tensioners, etc.

Good luck!!

Heath

[elwesso]

For the price of a VG30, id consider getting a new one and rehabbing it...

THe VG can be had for like $500......

[rwhyde]

Q451990,

Thanks for the input. This engine ran for 59K miles on this belt, which was replaced by the dealer.It appears the hydralic tensioner might be original to the car (1993) do they fail suddenly? It appears firm when I examine it.I've replaced many failed timing belts but never one that jumped without destroying itself. Do lifters pump up if over revved enough to crash valves?? Anyone heard of this?Thanks,Wayne

[rwhyde]

elwesso,

Thanks, and good suggestion,I do have another '93 parts car with an engine. I am just intrigued with fixing this one since the bottom end appears OK but I'm probably going to drop the pan and take a good look while I have it on a stand. I'm reluctanct to just stick any used engine in a car without at least a new timing belt, water pump and hoses especially in one of these engines. They are not easy to work on at all, with the engine in the car, if you know what I mean. I sure don't want to do this again anytime soon.Wayne

[elwesso]

Hmm, this is intriguing... im going to have one of the Z guys take a look at this thread.....

[boileralum00]

It is not uncommon for the belt to slip without stripping the teeth off. I have hear dof it happening to a few Z guys on tt.net. I'm not sure what would be more depressing though - having it slip a few teeth like yours did, or having an idler pulley stud snap like mine did (bending 18 valves in the process...)

[rwhyde]

WOW! 18 valves.

Well, the one comforting thing is that you know the cause of your disaster and right now, I'm just not positive that I know mine. I REALLY don't want to fix it and not get to the root cause of the problem only to get to do the job again.

Thanks for the input.Wayne

[Nismo_Freak]

A complete failure of the tensioner would cause the belt to skip badly. I have seen them without sheered teeth on RB's, VG's, and CA's.

[boileralum00]

Well, since you are going to be in there anyway, I would replace the tensioner, both pulleys, and the studs to be safe, along with the timing belt. That way you know that all the timing components are new and aren't going to fail again. I would say that the tensioner was probably the root cause of it, since it has been 59k since the belt was changed, and chances are that the dealership didn't change the tensioner at that time in the first place.

[rwhyde]

OK,

What does a failed tensioner look like??This one is very stiff. Any tests?? Or just replace for safety's sake (I'm thinking yes)I have the factory manual but don't see a very good description except to look for oil leakage.

Thanks,Wayne

[rwhyde]

Right,

Looking at some white stamped markings on the tensioner, it appears to be original. The marking appear to be a date stamp. This is a '93 model so maybe it just let go in some way. I'm familiar with Porsche 911 failed tensioners and MB, but this one appears to look fine. Maybe it is the Bastard!?Thanks,Wayne

[boileralum00]

OK,

What does a failed tensioner look like??This one is very stiff. Any tests?? Or just replace for safety's sake (I'm thinking yes)I have the factory manual but don't see a very good description except to look for oil leakage.

Thanks,Wayne

I always recommend replacing them at 60k intervals along with the timing belt, water pump, and everything else that you have to get to at the same time. I'm not sure what a failed one looks like for sure, but at 59k on the belt, it could be a combination of inconsistent tension from the tensioner plus the slight stretching of the belt from the 59k miles of use, plus having the tension adjusted past the limit in the first place.

[rwhyde]

OK, a new tensioner sounds good.I'm feeling a bit more comfortable that it just had to have failed in some invisible fashion now, (except crashed valves).What a bummer. Most cars I see that persist in using hydralic tensioners have wised up and made them a one-way type device. Once they are extended they lock and won't retract without beign released from the backside. A kind of fail-safe design.Thanks,Wayne

[s13sr20chris]

shoot, thats a tough diag now that the heads are off. would be interesting to see it at tdc and note whether the cams line up or not. i also always reccomend a tensioner with the belt. no sense in having a snazzy motor without snazzy maintenance.

[evildky]

I've never had this issue but on my 85 300ZX I followed the recomendation to replace the tensioner and stud while I had it apart, with the heads off surely you've seen the tensioner to verify it's condition?