Help diagnose head problem...

VaSteve · 03-13-2006, 05:30 PM

I took my 944 n/a out on the track this weekend. In the last session, I must have done a number on it or missed a shift because now it it making a "bad noise"

. When I pitted, the 911 experts (you see where this is going) told me that it was a bent valve.

I have had others (non 944 guys) tell me that maybe I broke a spring or damaged a lifter. Car has 174K miles - first time on the track.

The car still runs and drives (I did another lap, thinking the damage was something else

) but it is quite noisy. I'm not competent enough in my diagnosis to know what I'm dealing with.

A friend is going to lend me a compression tool, but I can't get it for a couple of days. Once I have it, what am I looking for? I have also been told to take off the valve cover and just look at what's what. What to look for?

Give me some data points, so I can think about it for the next several days.

AFJuvat · 03-13-2006, 05:47 PM

Well, what did you do to it? Did you miss a shift or seriously over-rev the motor?

If you missed a shift, you probably whacked a valve\broke a spring - get the revs up high enough and the valves will "float" - the engine is rotating too fast for the valves to close completely - particularly if the springs are weak.

You are looking for significantly lower compression on at least one cylinder. If you have a bent valve, the car should be running noticably worse - almost like you are missing on a cylinder.

You can also idle the motor and pull the plug wires off one by one - if you pull a wire and the engine does not change pitch, you found your cylinder.

AFJ

VaSteve · 03-13-2006, 06:03 PM

Quote:

Originally posted by AFJuvat
Well, what did you do to it? Did you miss a shift or seriously over-rev the motor?

I was working on my heel and toe coming from 4th gear into a tight turn from which exits well in 2nd to come out of with any torque. I know on at least one lap I revved it up really high and wasn't smooth into 2nd...but I can't recall if the problem occured directly afterwards on that lap. I started noticing it between 3 and 4, again can't recall if it was the same lap. There was no significant money shift that stands out in my mind...

I was still able to make it around for another lap, through the paddock, to the tow truck and off the tow truck. I don't recall a significant loss of power, but I did dial it back...kept it in a higher gear through each turn.

I didn't mess with it today; it seemed to idle OK..just noisy. Is it possible that it's not a bent valve, just one of the other parts gone bad?

AFJuvat · 03-13-2006, 06:05 PM

You may have broken a valve spring..

Compression and leakdown aren't all that different than it would be for your SC.

AFJ

VaSteve · 03-13-2006, 06:28 PM

Maybe there's light in my tunnel.

Thanks.

Razorback1980 · Razorback1980's Garage

Yep, I did the same thing to mine several years back. I overreved the engine and bent a valve like AF suggested. That's when I figured out that I just don't have what it takes to drive on road courses....so I went back to driving straight. But mine wouldn't idle properly if at all. If yours is idleing, then I would guess it's not a badly bent valve like mine was, but it could still be bent a little and leaking compression. The compression test will tell the story on that. Not sure what the tension was like on your timing belts but if they would have jumped, then you would have lost a lot of power. Did you lose power? I don't think I would remove the valve cover until I looked at the results of the compression test. What I would do first (because I have no patience) is go to my local auto parts store and see if they loan tools. Our parts stores here (both O'Reilly and Auto Zone) will loan for free or rent tools and then you could do the compression test sooner. If your compression is even across all cylinders, then I would start looking other places. A good stethescope might also help you pinpoint the noise. One thing that also comes to mind is the water pump. It makes a terrible noise if the bearings are out....and you don't want to run it with a bad water pump or you will have bent valves. Is the noise a constant noise or an occasional knock? Does it increase with RPM's? Bent valves or broken valve springs should be more of a knock and increase with RPM's. Water pump will be more of constant noise and will increase in loudness with RPMs. Are any of your gauges indicating something is wrong...like low oil pressure?

VaSteve · 03-14-2006, 08:51 AM

It's a knock that increases with RPM. I don't recal what the guages were telling me at the time. I can't imagine what thay would be saying.

pokey · 03-14-2006, 09:31 AM

When I first saw this thread, I was waiting for someone to reply "Don't use your teeth," or "Try to suppress your gag reflex." Oh, well. I guess I had to be the one to go there...

nynor · 03-14-2006, 09:32 AM

destroyed rod bearing on cylinder #2 at the crank. oil starved while cornering at high speed. i bet if you use a stethoscope you will find that the noise is coming from the bottom of the engine. pull the plug wire on #2 and i bet the noise goes away.

VaSteve · 03-14-2006, 10:30 AM

Why would pulling the plug wire make the noise go away? All the parts would keep moving...

Razorback1980 · Razorback1980's Garage

I was pretty much looking for what nynor just explained....what is your oil pressure?

nynor · 03-14-2006, 11:18 AM

the noise will go away or lessen because there will no longer be an explosion in the cylinder slamming the piston and rod onto the bare crank journal.

nynor · 03-14-2006, 11:25 AM

if i am correct, not saying that i am, and you keep driving this car, the rod is going to end up disintigrating as well, destroying the rest of the engine with it. pull the oil pan plug and look for shavings, etc.

Razorback1980 · Razorback1980's Garage

The noise probably won't totally go away but it will decrease by a significant amount. And you will PROBABLY have less oil pressure than what you are used to. What nynor suggested is the easiest and best way to look into it. Check for shavings in the oil. If there are none, then we'll go back to the valve theory and do the compression test.

VaSteve · 03-14-2006, 12:16 PM

I'd rather initially do a compression test than dump a buncha oil...

I don't recall the engine losing pressure while I was driving it. I will pull the plug wire to night and report the results.

nynor · 03-14-2006, 12:22 PM

if the oil is good, put it back in!

Razorback1980 · Razorback1980's Garage

Yep

nynor · 03-14-2006, 12:33 PM

it seems that dumping the oil will take a hell of a lot less effort than checking compression.

VaSteve · 03-14-2006, 12:36 PM

Quote:

Originally posted by nynor
it seems that dumping the oil will take a hell of a lot less effort than checking compression.

You're getting to point when you see just how lazy I am. Compression is checked from above the car

I've spent too much time under that car the past 3 months.

nynor · 03-14-2006, 12:48 PM

your compression numbers may be bad if the bearing has gone, as debris may (read probably) scored the piston walls, etc. regardless, let us know what happens.