Big Dave

As I'm working on repairing my engine, I keep wondering about the sequence of events that took place before the chain tensioner snapped free from the cylinder head. On the way home from SITM 2006, our caravan got caught in a massive traffic jam in Kentucky and sat still in traffic for over an hour in 90+ degree heat. While were there, I noticed 2 things...First, my temperature was rising. It never got into the red, but I suspect I may need to look at my cooling fan and see whether it needs work. Second, my oil pressure began dropping. Normally it never drops below 2, but this time it got between 1 and 2 while were sitting. When we started moving, the oil pressure took a while to climb back up to the 4-5 range. That struck me as odd, since it normally spikes to 5 when revs pick up. I never received a temperature or oil pressure warning.

Fast forward a couple hours to when we had been driving at normal 928 highway speeds. Around Dayton, Ohio, after a couple stops along the way, I got a belt tension warning. We quickly pulled off so I could check and adjust. I gave the belt a 1/4 turn and started it, and that's when the awful noise started (generated by the teeth on the cams hitting the tensioner that had broken free and turned sideways).

When I rebuilt after the timing belt failure in '04, I replaced my rod bearings. I freaked myself out today wondering whether I had put the end caps on incorrectly, but I confirmed tonight that all the numbers match up. However, I did notice marks on the crankcase identical to those pictured here (I didn't have my camera in the garage tonight..these pics are not from my engine).

Are these marks that looks eerily like grooves made by the crank normal?

Also, I've got a chicken-or-the-egg issue....what happened first....did the low oil pressure cause the tensioner issue...or did the cylinder head crack back in Kentucky, getting debris stuck in the oil passage and cause low pressure...or are they unrelated? Was the low oil pressure due to the high engine temp?

Sorry for the long post. I'm just freaking myself and don't want to wonder whether I'm not fixing everything that's wrong.

BC

I have seen about 5 blocks apart like this, and all of them have had those marks. When the mains are in, the crank has its meat on the thrust bearing surface which is on the lower level of the step that you see going towards the crank journal itself.

When someone has "thrust bearing wear" that bearing surface gets worn down, almost until it allows the crank to hit where your arrows are pointing - BUT by that time, most have found out the problem.

Does everyone concur?

Mrmerlin

those marks look like factory machine marks and what Brendan said

AO

Those are just the machining marks. I had that too when I did my OP gasket and rod bearings. Nothing to worry about. I'll see if I can pull up my pics.

AO

See in the 3rd pic down...
https://rennlist.com/forums/showpost...2&postcount=39

Rob Edwards

They were still machined the same way in 1990:

Bill Ball

Yes, factory, especially if they are on the side facing forward. TBF marks are worse and of course the TB would be worn out on the rear face.

Big Dave

OK...thanks for the reassurance!

Now to figure out the oil pressure question...

AO

Dave-
How's this... The boss began to fail/or had failed completely. Tthe oil feed line (IIR there is one to the tensioner) held it together for a bit, but began to fail thus causing your oil pressure to drop. When the oil pressure dropped to zero when you turned it off, the tensioner pad fell and allowed the entire tensioner to tilt and get chewed up.

So my theory is that the boss failed and the oil pressure drop was a result of this action, not the other way around.

SharkSkin

I was just reading up on the operation of the oil thermostat earlier and noticed that it operates much like the coolant thermostat. I just double-checked and the 85 seems to have the same setup, according to the "Service info by year" PDF.

What this means is, if for any reason the thermostat is not closing fully when hot, you will not get flow to the oil cooler. Take a look and see if it looks like it has been seating properly inside the block, if there's crud in there, etc.

One possible explanation for your description of the oil gauge behavior is that the oil wasn't going through the cooler, became hot enough to drop pressure while you were sitting in traffic, then took a long time to cool down because it was not being run through the cooler.

The thermostat opens at 87°C, should be easy enough to confirm operation of the thermostat in a running car with an IR thermometer, for those wanting to check this system out.

johnb

Hi
Funnily enough I had the sump off my '86 4.7 16V Euro manual yesterday to change the sump gasket.

I had heart failure to see scouring marks on the main ladder, having had a previous auto TBF. I quickly rushed to check the crank end float and found it to be 0.27mm, well within spec, car has done 110K miles. (Spec. = 0.16mm - 0.41mm taken from WSM)

If you look at the second picture the crank web is 'miles' away from the ladder and therefore I concluded that the marks must of been made at manufacture.

Being a manual, any possible 'damage' scouring would be on the other side of the ladder caused by excessive clutch actuator pressure. Or at least I hope thats the case!

Vilhuer

Pics in post #1 are from my GTS engine. Crank in it was and still is fine. Thrust bearing was sligtly worn on one side but counterweights were still far away from touching block. There were other problems in block but they were higher up in cylinder walls.

In case of TBF both upper an lower half of the block have wear marks that are continuous over the border. In case of these factory machining they tend to be only on one side or another and abruptly stop at where half ends and another starts. This is most likely because half were machined separately when these spots were done. Based on very limited sample base I think later engines have more machining marks in these areas from the factory. This could be due to casting moulds.

Mike Frye

If I'm missing something obvious, please let me know because I'm a newbie.

Does anyone think there could be a link between the TB warning light coming on and the rest of this mess?

Did you check the tension with a tension tool or did you adjust it until you got continuity on the circuit?

It just seems weird to me that this happened after the TB tension light came on and the belt tension was adjusted. Is it possible that the engine heated up in traffic and expanded so much that it stretched the belt, then when you were running it later on, the engine cooled down to normal operating temp and the loose belt started lapping and caused the TB light to trigger?

Is this even possible? If so then the low oil pressure (caused by lack of viscosity in this case?) and the tb light were caused by the excessive heat. If not, I'd be curious to know what caused both problems.

ErnestSw

NJShark, TBF = Timing Belt Failure AND Thrust Bearing Failure as in this thread. Common mistake.

cfc928gt

Change the oil thermostat, it's cheap and the symptoms you describe of low oil pressure buildup when hot point to that. I found my engine runs a bit cooler now that the oil thermostat is working because the oil is flowing to the radiator and I no longer have pressure issues when hot.