Engine miss after high rev 6200 rpm?

[pmead]

After a spirited drive tonight I have an engine miss. Immediately after hard acceleration in 1st gear (6200-6300) rpm the car began to miss under load. I don't hear it at idle. There is no metallic sound, no knock, no tick, but a definite miss and slight loss of power. 1983 sc 150K was running well before incident. Any similar issues out there? I am assuming a compression test and leak down should be where I start. I may try to run engine and check exhaust temps to isolate miss. Where would you guys start?

[speedo]

Sounds like you have an angry valve. Do a leakdown. Maybe something came loose or maybe there was an over rev...why did you run it to 6300 in 1st gear? You have no torque left on stock SC cams?

Speedo

[pmead]

I don't have a good reason for the high rpm, just stupidity! I hope it is something simple. I will have time to look at it Wednesday.

[Quicksilver]

Originally Posted by pmead

I don't have a good reason for the high rpm, just stupidity! I hope it is something simple. I will have time to look at it Wednesday.

Let's be more specific. Did you down shift and over rev the motor or did you just put it in first and keep accelerating until it went past redline?

The "down shift method" can be destructive. The "keep accelerating into the red method" isn't destructive in of itself.

The redline on the tach actually marks where you should have shifted because the engine is past its powerband and you would be faster in the next gear. The rev limiter doesn't kick in until around 6500. The Club Sport DME chip raised the rev limiter a couple hundred more RPM so there is some safety there.

[boyt911sc]

Originally Posted by Quicksilver

Let's be more specific. Did you down shift and over rev the motor or did you just put it in first and keep accelerating until it went past redline?

The "down shift method" can be destructive. The "keep accelerating into the red method" isn't destructive in of itself.

The redline on the tach actually marks where you should have shifted because the engine is past its powerband and you would be faster in the next gear. The rev limiter doesn't kick in until around 6500. The Club Sport DME chip raised the rev limiter a couple hundred more RPM so there is some safety there.

Quick,

The OP has a 3.0 liter SC CIS with ECU and does not have DME (Motronic).

Tony

[pmead]

I was accelerating hard, and shifted into 2nd near redline (6300) then when I continued to accelerate in 2nd I heard the miss. The miss continued under acceleration the rest of the way home. (2 miles) There was no metallic sound and it almost sounds like an exhaust leak. The rev was under acceleration, not mechanical as in a missed shift or down shift.

[DanielDudley]

Look at your plugs, and do a compression test. Lots of things can cause a misfire.

[pmead]

I just started car and checked exhaust temps at each exhaust flange and they are all close to the same. No obvious cylinder not firing. I blocked the exhaust with a rag momentarily to check for exhaust leak, no change in sound. Tomorrow I will start with plugs and then check compression or pull valve covers. Would a broken valve spring create this problem? I do not get any smoke. It sounds more like a small exhaust backfire and is in rhythm with the rpms. What are the symptoms of broken head studs?

[turbo owner]

Originally Posted by pmead

I just started car and checked exhaust temps at each exhaust flange and they are all close to the same. No obvious cylinder not firing. I blocked the exhaust with a rag momentarily to check for exhaust leak, no change in sound. Tomorrow I will start with plugs and then check compression or pull valve covers. Would a broken valve spring create this problem? I do not get any smoke. It sounds more like a small exhaust backfire and is in rhythm with the rpms. What are the symptoms of broken head studs?

Check your plugs, then the valves, maybe a broken spring? Could be head studs too, have they ever been replaced? Look for oil build up where the cylinders meet the case, if you have some it could be bad news, head studs.

[Quicksilver]

Originally Posted by boyt911sc

Quick,

The OP has a 3.0 liter SC CIS with ECU and does not have DME (Motronic).

Tony

Geeze, how did I miss that...

Either way it doesn't sound like he had an expensive episode. Looks like he has a somewhat normal tune adjustment/troubleshooting in front of him.

[Ronnie's.930]

Originally Posted by pmead

What are the symptoms of broken head studs?

In addition to what turbo owner mentioned about signs of oil & carbon goo around cylinder mating surfaces, there is typically an exhaust leak type sound than can almost sound like knocking or tapping - can usually be heard at idle and then goes up in tempo with the throttle.

[pmead]

Its not looking good. The spark plugs look great. All dry nice uniform tan color on all plugs. Cold compression test is another story. Cylinders 2,4,5,6 are all 175-180. #1 is at 120. #3 is at 100 and makes a totally different sound when cranking the engine with my tester hooked up. (There is no "air noise" when I crank) #3 is the only cylinder with the strange sound. Stuck valve? Broken valve? broken valve spring? Can any of these scenarios be repaired with the engine in the car? I think I may drain the oil and pull the passenger side valve covers next. A leak down test on #3 may tell me more.

[turbo owner]

Originally Posted by pmead

Its not looking good. The spark plugs look great. All dry nice uniform tan color on all plugs. Cold compression test is another story. Cylinders 2,4,5,6 are all 175-180. #1 is at 120. #3 is at 100 and makes a totally different sound when cranking the engine with my tester hooked up. (There is no "air noise" when I crank) #3 is the only cylinder with the strange sound. Stuck valve? Broken valve? broken valve spring? Can any of these scenarios be repaired with the engine in the car? I think I may drain the oil and pull the passenger side valve covers next. A leak down test on #3 may tell me more.

Don't let pulling the engine scare you, these cars were meant to have the engines removed to be maintained. Lot easier to work on out of the car.

[pmead]

Just pulled valve covers found 2 broken head studs. One broken stud found in valve cover, there is no sign of the other. I find it hard to believe that my pre-purchase inspection that included a valve adjustment and head stud check did not show this 8 months ago. The one stud may have broke while in my possession, but the other with no remains had to be there. No visible signs of valve train issues. I will do leak down test and pull passenger side valve covers later this week. The head studs do not explain the miss/backfire. To mad to look at it right now. Thanks everyone for your insight. Any advise you can give is appreciated.

[RedCoupe]

The high RPM miss could also be ignition wires. Whenever I have my wires off, I like to check the resistance terminal to terminal. Have found one or two bad wires on several cars.

[wayner]

Sorry to hear about head studs.

As for 6500rpm, I don't think that is particularly high.

[patz]

3.0s have a rev limiter. My euro SC had it in the rotor, it is spring loaded and fail over time resulting in a lower rev limit.

[turbo owner]

Bite the bullet and pull it, get it done right.

[pmead]

Motor coming out after holidays for rebuild. I am to busy to do anything at this time. I have a healey bugeye to reassemble first. (promised to buyer so it comes first) I have an MSD 6AL ignition on the sc with rev limit set at 6500. The PO had this conversion done and I like the reliability. I have used MSD boxes in many projects.