Does this sound like a burnt valve?

Post by **kcodyjr** » 21 Sep 2016, 20:00

No. You'd have terrible or no boost with a bad intercooler hose.

My instincts are pointing to the electrical - a misfire at low RPM could indicate insufficient spark, either due to faulty high-voltage side components that you're already going to replace, or faulty coil, or weak alternator, or weak ground strap, or possibly faulty battery, but if the battery was that bad you'd surely know it already.

I'd swap in the parts you've got, and inspect/clean the ground strap and coil connections.

If you still have symptoms, take a voltmeter to the system at idle. Measure across the battery posts and from coil primary positive to chassis. If those two measurements are inconsistent, you have a wiring fault. If they are consistent and low, you have an alternator problem. If they are consistent and correct (search the forums for the correct number), you have a coil problem.

Post by **erikv11** » 21 Sep 2016, 22:13

Yes. A small split in an intercoooler hose, or a loose clamp, the system can hold boost ok but stumble badly only at idle due to unmetered air getting in. And a big split or tear will act like what kcodyjr describes. But he has a point, so maybe this points to the vac lines and away from the IC path. The fact that there are no fuel trim codes does tend to argue against the vacuum leak, however. Stumble at idle = vac leak, mantra, mantra

I'm also not second-guessing the idea to check electrical, just to keep your options open. Another thing is, I would expect a spark or ground problem to be exacerbated at high RPMs (under load and when the engine bay is hot), not to get better.

Earlier this week someone here had symptoms like this that turned out to be a combination of a small split in an IC hose and a bad fuel injection relay (or FI relay connector - I don't remember which). There have been a bunch of those FI problems lately, getting more common as these beasts age. As long as you're being thorough, check the connections on the FI relay. I know you listened to them click but you didn't listen while it stumbled, did you? Just give the connector a look, see if it is in good shape or falling apart.

Post by **kcodyjr** » 22 Sep 2016, 08:35

Most ignition issues would be worse at high RPM as erikv11 says, but not all of 'em.

I have seen a bad alternator or a bad battery produce low voltage at idle, but strong voltage at higher RPM. In that case, revving it up would smooth it out.

A loose or corroded wire *could* have symptoms like a bad alternator, depending which wire. Or it could get worse with RPM.

A bad coil could also go either way, depending on the nature of the failure. It's a fight between input voltage to the coil, and how much time it gets to build up a magnetic field before it's collapsed to generate the spark. Higher RPM = less time between sparks, but higher RPM = more power available.

I would generally expect a high-side problem to get worse with increasing RPM.

Jazzer · Post by **Jazzer** » 22 Sep 2016, 12:55

Quick update: put in new Bosch cap and rotor, new bougicord wires. Problem remains...what's peculiar is the misfire is in cylinder 4 only. Waiting for the plugs to arrive, but the existing plugs still look pretty good, especially for being 15+ years old.

Post by **kcodyjr** » 22 Sep 2016, 12:57

It isn't losing coolant, is it?

Post by **erikv11** » 22 Sep 2016, 14:35

The plot thickens. Specific to #4 does not sound like a vacuum or hose issue unless it is a torn intake manifold gasket right at #4 (no reason to suspect that).

And does the #4 plug look any cleaner or dirtier than the others? Along those lines you may want to be thinking to do a compression test soon, after all.

Here's something relevant and free to check: you could swap the injector leads on 4 and 5, to rule out faulty injector wiring. Just swap the connectors, you should have enough wire to do it on 4 and 5 (or 3 and 4)? They all fire at the same time. Consider also a faulty injector in #4 but that's a little more work to check.

Jazzer · Post by **Jazzer** » 22 Sep 2016, 15:11

It is not losing coolant, nor is it overheating. I'll try swapping the wires on the injectors, that seems simple enough. The plug looked the same as others; I'll try a compression test test tomorrow, hopefully.

Post by **rspi** » 22 Sep 2016, 15:15

Jazzer wrote:Quick update: put in new Bosch cap and rotor, new bougicord wires. Problem remains...what's peculiar is the misfire is in cylinder 4 only. Waiting for the plugs to arrive, but the existing plugs still look pretty good, especially for being 15+ years old.

OMG! Please swap #4 and #1 plugs and see if the problem follows the plug.

Post by **Clemens** » 22 Sep 2016, 16:39

Yeah, sometimes it hurts to let good friends go. I had to swap out my bourgicord wires just months ahead of their 20ieth b-day...

Honestly: like Robert says, get rid of the old plugs.

Post by **erikv11** » 22 Sep 2016, 20:55

lol ... read the thread, plugs ordered several posts back and are on the way, hang in there folks.

Does this sound like a burnt valve?

Re: Does this sound like a burnt valve?