Sudden Miss/ Valve Noise

[Shadoobie]

Hey guys, I've got a real puzzler for you. I was driving from work today at about 15 mph when my spider suddenly got a rough idle and I began to hear what sounded distinctly​ like valve clatter at more than half throttle. Temp is fine, oil pressure fine, just like the flip of a switch things went bad. As soon as I got home I noticed some excessive blow-by. I also pulled the plugs to find the rough idle/miss and the engine ran slightly worse when plugs 1 and 2 were pulled, but not nearly as badly as 3 and 4. Im waiting for the engine to cool down to check the valves. Any ideas?

[RRoller123]

Blown head gasket? Timing belt slipped? Start with a compression test, I would think.

[lglade]

The magnetic pick-up on my distributor had a bad wire and it gave me some similar symptoms, including what sounded like valve clatter. I thought my car had something really bad going on, but replacing the pick-up cured it.

[Shadoobie]

After doing some digging, I discovered that the cams are out of time with the crank by approximately one tooth and the timing belt has a bevel on it where it's been rubbing against something. So I believe I have found my culprit.

[RRoller123]

If you have the timing belt cover installed, it is common for the spacer at the bottom mounting stud to be missing, which will pull the cover too close to the engine front and allow the belt to rub against it. Mine was missing, added an ~1/2" phenolic spacer to the mounting bolt and it solved the rubbing problem.

[Shadoobie]

Well it looks like I was wrong. I rechecked the timing marks at 0° and they appear correct as per the photo.

After this, I ran a compression test. The test came up as follows from cylinders 1 to 4: 65, 60, 165, 170. I checked the coolant to see if it was the head gasket, but I've got no oily feel or look and a whole cup I drained out was totally clean. I guess I'm checking the valves tolerances next?

[sptcoupe]

The head gasket has gone away between your two bad cylinders. Hopefully you haven't scored the block or head surfaces between the cylinders.

[Shadoobie]

The head gasket has gone away between your two bad cylinders. Hopefully you haven't scored the block or head surfaces between the cylinders.

Do you know any way I can confirm that before I pull the head off? Because I've got no coolant in the oil or vice versa, no smoke from the exhaust, and it's a less than 5k old Goetze head gasket from Autoricambi. It seems a logical conclusion otherwise, but I would really like to be 100% sure before I dig too deep.

[klweimer]

I would start by checking the valve clearances on the first two cylinders. I can't imagine both cylinders being screwed up at the same time, but at least you could rule that out before pulling the head.
Kirk

[Shadoobie]

A leak down test confirmed it. My practically new head gasket from AR is blown. Oh well. Does anyone have recommendations on a replacement kit for a 1979 motor?

[sptcoupe]

I don't know the build history/specs of your engine, but there are a number of causes for a new (relatively) head gasket to go bad. One of course is a faulty gasket, but most of the gaskets sold by our vendors are quite good, including those from AR.

Incorrect torque sequence in tightening the head bolts, incorrect torque values, not torqueing the head bolts in a series of steps, failure to re-torque the head bolts after a few hundred miles of driving, re-using old head bolts, imperfections on the head and block mating surfaces (corrosion/pitting, slightly warped, old material gasket material not cleaned off, etc) are the most common.

I would talk to AR about it and see what they say, and review the above to see if any one of these common causes could have led to the failure. So be sure you review the rebuild to eliminate any of the above before wasting the time it will take to pull the head to install another head gasket. Just swapping to another brand gasket may not solve your problem.

Good luck.

[So Cal Mark]

uh, no on a faulty head gasket. Damaged during handling/installation is a possibility but not faulty. Was the head checked for flatness before it was assembled? The other cause to blow between cylinders is detonation, and that isn't always audible

[sptcoupe]

I agree with Mark. The odds of a faulty gasket are really slim.

[RRoller123]

I am going with the theory that the visible rubbing on timing belt has caused it to jump and that the valve timing is off enough to disrupt compression. The fact that it is only on two cylinders is not good, indicates valve damage, yet your signature says you have a 2L non-interference engine. Odd.

[sptcoupe]

If the timing was off, the likely cause is detonation caused the head gasket to fail. The fact that you lost two adjacent cylinders simultaneously is a pretty good confirmation of the gasket going away between those two cylinders.

Your biggest issue may be that the flame/heat front running between the cylinders after the failure has scored either the head or block mating surfaces, or both. I don't think you have a valve problem, but a visual check will tell you that.

Be sure you are absolutely sure there is no scoring on the head or block. It doesn't take much time for that to happen after the gasket fails. If there is any doubt, get the damaged surface milled (if still possible), or you will have the same issue. Most of the time, it is the softer aluminum head that suffers the damage.

And you may have gotten lucky and all you have to do is clean the surfaces and put it together with a new gasket.

Keep us posted.