Does this sound like a burnt valve?

[Jazzer]

Stull hard to believe that nailing a car to merge into traffic burns a valve. If that did it it would have died soon anyhow.

I'm now wondering if that had anything to do with it or if it was just a coincidence...the good thing was I was going to replace all of these parts anyway, so it's nice to know when I do get it sussed out I'll be that much closer to stage zero. I did find a torn vacuum line that I replaced, and I was amazed at how much the boost improved. The gauge went from barely in the white to 2/3 full; now when I step on the gas it really responds. I'm gonna try some seafoam in the next day or so, along with an "Italian Tuneup". We'll see what happens...

[rspi]

At this point can I assume it's a valve problem then?

I would say that is a safe assumption. These cars (like many others) have a history of burnt valves.

The reason it runs so smooth about 1500 rpm's is due to the good 4 cylinders and the overlapping of the I5 engine.

I was recently told by a mechanic that the sensor on the subframe is what detects the misfire and which cylinder it is. The sensor is counting the explosions and their timing, communicating that to the ECU. When the knocks are missing, it tells the computer that one is missing and which one it is. When at high rpm's, it may be running to fast to sense that it's missing.

I would keep the injector unplugged and avoid the Italian tune up. When one valve burns (chips) there are usually a couple of other valves not far behind and running it hard will likely cause another to burn.

[erikv11]

... I was recently told by a mechanic that the sensor on the subframe is what detects the misfire and which cylinder it is. The sensor is counting the explosions and their timing, communicating that to the ECU. When the knocks are missing, it tells the computer that one is missing and which one it is. When at high rpm's, it may be running to fast to sense that it's missing. ...

Do you know what sensor he is talking about? That sounds strange, not sure I would swallow it hook line and sinker. But interested to hear more if it is correct, or even just part of the misfire detection system on these cars.

Misfires are usually picked up by when the deflections in the crank ankle (or even the cams) don't show the right surge to correlate with the firing order commands coming from the ECU. If the value from the crank sensor isn't what the ECU expects, then it reports that there is trouble (a misfire) with the cylinder that was firing at the time. I always assumed P80 cars would use that system, too?

[kcodyjr]

The compression result is telling; there's a hole in the cylinder that isn't closing. Probably valve. At this point, it could also be a hole in the piston, or completely shot rings, in descending order of likeliness.

The valve doesn't have to be burnt to act this way, it might just be sticking, but it could be burnt too.

Point is, it's all conjecture at this point. No ifs ands or buts, the head has to come off.

OK, I suppose you could start by sticking a fiber optic camera down the spark plug hole...

[abscate]

Interesting one to debate. Is misfire detected by crank sensor or knock sensor?
Inquiring minds wish to know...

[Jazzer]

Once again, thanks for all of the insight; I appreciate your help trying to figure this out. At this point, I know the head has to come off; there are some great videos and tutorials (thanks again, Robert) that I will pore over. When I have the head off, then what? I know to look for the valve problem (probably an exhaust valve), but I've never done anything like this before. Do I try to fix the valve myself, or take it to a shop? I've never used a machine shop before, nor do I know what to look for. Does a shop fix all the valves, or rebuild the head? How long would something like this take? It is my daily driver, so down time is a real problem. Would a remanufactured head be worth getting? Sorry for the barrage of questions, but I'm trying to manage my schedule in the next few weeks to accommodate the repair.

[kcodyjr]

Once the head is off, you need to identify the problem. If it's a valve, it's a head job. If it's rings or piston, the engine has to go on a stand and rebuild the bottom end, or just swap in a used engine.

Simply taking the head off, replacing one valve, and putting it back on, is a wasted opportunity to refurbish the head, but I don't see any inherent reason you couldn't do it. But let someone else chime in, there might be a reason. At bare minimum, you would need to replace the head gasket and head bolts during reassembly.

In any case it's a bit of a twitchy operation, so be sure to do that reading.

What you really want done is to have all the valve seats ground and the valve guides redone, and then replace any marginal valves as well. I have mixed feelings about machining the mating surface; if it's warped, it's warped with the block. In either case this means taking the head off, stripping it down, storing any remaining good valves in oil, and sending it off to a machine shop.

If time is a bigger factor than money, you could get a rebuilt head, but then there's potential mating issues.

[rspi]

Do you know what sensor he is talking about? That sounds strange, not sure I would swallow it hook line and sinker.

The one on the front of the subframe. I guess that could be tested easily. Unplug it and see if the misfire is still detected. A misfire can even be created by unplugging an injector and that sensor and see what happens.

[Jazzer]

As I plan the tear down, here's a question: I have new front cam seals that I was going to put on, planned before all of this went down. Since I'm now rebuilding the head, WHEN during the job should I do that, while the cams are still on, or after I've removed them? It doesn't look like they can be replaced without removing the cam gear.

[101]

I replaced my front cam seals as I reinstalled the cams on the head. I didn't remove the cam sprockets. I took great care that the seals remained positioned correctly on the camshafts and in the head, and that they stayed in place as I reinstalled the cam cover.