Low compression, but...

2007 Solara 6 cyl 3.3 engine with 130k mi, worth about $2500.

What's happening:
Code P0302 misfire all cylinders, clear them, returns misfire #2 cylinder.
#2 cylinder compression 100 with range 146-187
Cold start runs ok - after a few minutes, rpms drop & it starts coughing running rough, but then it might shift to full power and run just fine with excellent acceleration for a few more minutes, then repeat. Stepping on gas doesn't do much as rpms drop.

While driving, RPMs just drop, power drops, then catches and either regains full power or stalls. Also stalls or comes close near idle. I was told that this looks more like fuel line issue (O2 sensor, injectors, plug wires, fuel filter/pump, timing belt) than compression, but need to know if it's worth troubleshooting fuel line if the engine is beyond repair.

A year ago, I replaced two of four O2 sensors, cleaned throttle body and replaced air filter. 8 months ago replaced valve cover gasket, all 6 coils & plugs (Denso & NGK) & PCV.

This week, the above trouble started. Replaced the #2 ignition coil and plug The removed plug tip was white (no gunk). After replacing #2 coil/plug, car ran ok for 2 days, then issues came back. Removing the plug showed a little oil on the threads. Wiped the threads, but the issue came back immediately along with engine light. Doesn't seem to be burning oil or leaking coolant.

My mechanic said the white plug tip indicates a coolant leak into engine and the compression issue can "only" be fixed by taking apart the engine to find and fix any damage or by replacing the engine. This old car is not worth that investment. Before I junk this car that I love:

What diagnostic tests might isolate the problem(s)?

Is the low compression fatal for a car worth $2500? If not, how might it be fixed relatively inexpensively? As a last ditch effort, is it worth trying engine restore (or similar) or, Seafoam in the cylinder?

Lots of expensive and vague help from mechanics so far and I'm low on funds so any help will be very much appreciated. Thanks so much.

Ideally, you would perform a compression test on all the cylinders, not just the one with the misfire, (maybe you did? It's kind of unclear if the range you posted is perceived good compression or the abridged results of a physical compression test), then if a cylinder shows significantly less compression than the others, perform a cylinder leakndown test to determine where the compression is going. Then you can decide if the expense of the repair is something you want to commit to.

Thanks. On the 6 cyl Solara, the 3 in the rear require removing tons of stuff that makes it costly to check those. The 3 front are easy and I'll make sure they look at those 3 along with the leakdown test. The compression numbers were just cranking one and checking the acceptable range from min-max. My reading was much lower than the stated minimum. Hope that clarifies.

Thanks for your reply. I'm wondering if you have any tips to see if I can get a discussion started here since I do need to find out if what's happening is fatal for this car so I can make plans. I've already spent quite a lot on this and gotten lots of differing opinions from mechanics who are happy to charge me, but are a little short of a real strategy. Thanks.

So let's say it has low compression in one cylinder. It either needs head work or an engine.
You're not getting out of it for less than a thousand dollars, and that's VERY conservative.
Low compression is always inside the engine. There's not going to be one part you can bolt on and restore compression. If you can't crack it open and fix it, then replacing the engine is the way to go.
If you want to stop beating around the bush, start getting quotes on a used engine installed, then decide if it is in the budget to fix it or if it's going to the junk yard.

www.car-part.com/cgi-bin/search.cgi?user...er=&iKey=&userPage=8

$600-800 for the engine, depending on miles, build date, your location etc...
Probably another $600-$800 to install, plust oil, antifreeze, plugs, filters, miscellaneous shop and disposal fees gets you to $2000 in a hurry.
Think of the condition of the car, is it rotted? Does it leak water? Tires? Suspension?
If it's a trashcan on wheels, it still will be with another engine. No offense, but it is frustrating to pay for an engine, get in your car after it's been down a week and make it around the corner just to hear the old familiar junk struts and loose stab links.
If it's a decent car, $2000 is probably less than you will spend another car, plus, the devil you know is better than the one you don't.