Full Throttle Miss

[Superman]

I would like your best guesses on what is wrong with my car.

On Friday morning, the car spit out the #6 spark plug. This may be unrelated to the current problem, but it has spit out the #6 plug twice now. Perhaps I forgot to tighten it. Twice. I got the plug back in.

Then, on that same morning it developed a substantial miss on full throttle. Anywhere on the tach, it will handle 1/3 throttle okay and runs smooth, but when I give it full throttle, it acts like it’s running on three cylinders. It almost acts like the rev limiter is cutting in. I’ve switched out the CD box, cleaned up the cap and rotor and replaced the little ice-cube relay under the seat. I am starting to think it is not an ignition problem. There is no smoke, black or otherwise, when this happens. Again, I suspect a fuel delivery problem but I wonder what you guess it might be. I don’t have a fuel pressure gauge.

[Superman]

Are you all chickens? Let's hear your best guesses.

[stlrj]

If you're parking it outside I'll bet you got some moisture inside your distributor cap.

[Scott R]

Control pressure dropping under load?

EDIT: Sorry missed the "no gauges"

[Zeke]

I think you better figure out the spark plug thing before you go any further. I've never had a plug come out of any car and I've left a couple loose for a bit until I discovered what I had done. This wasn't a long time, either. If a plug is left loose in an aluminum head, I think it's possible that the gases escaping through the threads could irreparably damage them. Normally, a plug won't spin around in the threads to the point of coming out. Your plug is coming straight out, IMHO. That's a bad thing.

[Jeff Alton]

I agree with Milt. Was there any aluminum on the spark plug threads when you went to put it back in? Any signs of detonation on the plug? Was the electrode hammerd flat?

Cheers

[brett25]

how does a spark plug get "spit out"? i never heard of that before. are you using inserts or something?

[vash]

a buddy's car once spit out the same exact #6plug. strange.

plug wires? or maybe something like timing advance?

[Superman]

Nope. No inserts. No aluminum on the threads. No hammered electrode. No sign of detonation. Just a spark plug that unscrewed. In reinserting the plug, nothing unusual was detected. I still had to turn the plug twenty times to get it seated. All the threads are still there.

Oh, and no moisture in the distributor cap.

A drop in control pressure is the best suggestion I hear here.

[vash]

how would you check the control pressure at WOT? with or without gauges, it will be a mothereffer. my money is on ignition.

[Scott R]

Originally posted by vash
how would you check the control pressure at WOT? with or without gauges, it will be a mothereffer. my money is on ignition.

Decklid secured, but open a tad. Pressure gauge racing taped to the back glass is how I have seen it done.

[vash]

cool!

[RoninLB]

Beru wires suck. Maybe try a cheap set of auto store wires. If you need almost new Beru wires let me know. I make deals you can't refuse.

Blown spark plugs happen on al heads. Just ask any boat wrench. Those threads are just to delicate. I use Loctite Graphite-50 for plug anti-seize instead of copper. It's made for spark plugs, the copper isn't. Just read the labels. I don't care that the factory many wrenches do not recommend it.

For a wild guess.. the plug thread transfers combustion heat to the head. If you are not dumping the heat the plug should be running hot. If the plug is super hot it may be pre-ignition? Not sure if that's happening? The plug grounding may also be disrupting?

[Superman]

I appreciate the input.

I also suspected ignition, but am moving away from that suspicion. For example, when the #6 plug exited its home in the #6 head, I was on the freeway. It made a noise like one cylinder had failed. I was driving a 5-cylinder Porsche, and it felt that way. I could still accelerate, etc, but it sounded awful and was lacking some 40 hp or so.

The current problem is different. The car runs great at steady throttle. Runs great at 1/3 throttle. At full throttle, it feels like the rev limiter is kicking in. Not like it's running on five cylinders, but rather like it's running on two.

Again, I think it is significant that there is NO smoke when this happens. As a contrast, when my ignition timing is just a little too aggressive, the car runs great almost all the time, except occasionally, in high RPM situations, at full throttle, the engine will make a terrible rattling noise very briefly, and smoke comes out the exhaust.

Again, I suspect fuel pressure issues, or an RPM limiter malfunction. My hope is on the latter. (my car's RPM limiter shuts off the fuel pump as a way of limiting engine speed)

I am still very open to your thoughts.

[Jeff Alton]

Hmmmm, what about the coil?

Cheers

[Superman]

The coil is the ONLY ignition component that is the slightest bit suspect. But the reason I'm not as suspicious of it is

1) They are ordinarily as reliable as a hammer and

2) No smoke. If ignition cutoff were causing this, then the unburned fuel would burn in the exhaust system, causing smoke.

At least, those are my thoughts.

[john walker's workshop]

clogged fuel filter?

[JP911]

Can you disconnect the rev limiter to test your theory? I think it's in the dash somewhere behind the clock.

[morrisjm1]

I had a similar problem. It turned out to be an ignition wire. It only missed under near full throttle conditions. I spent a lot of time and money trying to run the problem down. I installed the wires as the last thing to try before going to a real mechanic.

I think when the cylinder pressure increased with increased MAP, the spark found a path through a bad terminal on the wire.

[ruf-porsche]

Originally posted by john walker's workshop
clogged fuel filter?

Clogged screen in the fuel tank

Do a search for SOUK Fuel Tank