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My gut is telling me the same thing about the fuel system, Jon. I don't even want to speculate about the possibility of a harness problem -- Ugh. As stated, the ECM swap yielded the same problem (no change). I think this weekend I'll pull the timing cover and get the task of checking cam timing over with; just to rule it out.

If I can get good pressure gauges on my leak down tester I can bring that if you haven't done that or don't have one to use. I know you've done a compression check but leak-down is a better idea of the health of the seal.

It's just so odd how it acts - There is no rhyme or reason to it. It comes and goes. It never really has any power but the sputtering can come in across the whole RPM range, regardless of load. You can even hear the miss/sputter at idle. I suppose it could be a physical engine issue but based on how it acts, I'd expect it to be more of a constant issue.

I'm picking up another engine this weekend just 'cause I found a deal on a screamer (Bilmar-esque)... and having an extra Quad4 or two never hurt anything.

Don't let Shawn read that... LOL And yes I do agree with how sporadic it is it's very hard to lay blame on a mechanical setup or part.

I can bring my tester if you can't get yours working Jon.

Was any of the threads in the head helicoiled? Or "repaired" ?
Might have a crack spreading open when it sees pressure and some heat. Try a carbon test in the coolant? Does the temp gauge seem to move around more with rpm change more than it should?

If I've not got this thing figured out before y'all get here later this month, the Quasar won't be attending Bfest and we'll have to look into taking the Garnet and maybe the GT. I'm hoping to get it figured out soon so I get the A/C charged. We'll see though...

As for the head, yes - A single point was helicoiled -- But that was on the cam tower so it shouldn't really effect combustion gases getting in the coolant. The head is also studded. But - I do have a block tester - I will have to get some fresh testing fluid but I can do that. I'd be extremely surprised to find any gases in the coolant.

I feel like a cracked block would not only flip the check engine light but the engine would run so poorly you wouldn't be able to even have it idle. Since the fuel system tested good, I think your down to checking the grounds and especially double checking the timing.

Well, today I checked a couple things. First, I replaced the large vacuum line between the intake manifold and the air/oil separator - The one that used to have the built-in heater inside it. Then I swapped out my dipstick tube and dipstick - For some reason I never noticed it before - The car came with an 89-91 style dipstick and I swapped it for the correct 92+ style. Nothing major. Took it out for a drive and it seemed very slightly better; Still not right and was still stumbling slightly but it was better.

Got it home and traced all of the vacuum lines. Checked the vapor canister, purse solenoid/valve and the vacuum reservoir. EVerything looked fine except I did notice that I had my purge valve lines backwards. Corrected that and took it for a drive and it was a disaster. The engine was generator so much vacuum that you could hear a whistle coming from the oil cap from the suction. With the oil cap/dipstick open the engine would run but would idle high - With it closed the engine would bog down and create insane suction/vacuum.

Any idea what the hell just happened? I swapped the lines on the purge valve back and it made no difference - I did notice as well that the vacuum line coming off the TB (to the purge valve) had absolutely no suction on it. Very odd. I'm lost and at approaching the limit of my patience.

Do Quads have that tiny little PVC hole between the runners(on the head) that gets clogged like the Eco's do? If you look at an intake gasket on the Eco's(and the 2.4) there is a little passage/channel on the bottom that goes to that small pin hole from the runners. Damn thing is only like 1/8 inch wide, but runs the length of the gasket.

Charcoal canister itself leaking? Stuck purge valve?
Cap the oil dipstick at the tube and then cap it at the intake manifold temperarily for a quick drive. I would suspect one of the tubes to intake it leaking or cracked at the intake manifold where it goes into the aluminum.

What about the air/oil separator itself? Could that be clogged? We see that a LOT on the Volvo 2.3s -that sucker builds up with carbon, eventually clogs shut, crank-case gasses build pressure, and the engine seals blow out. I don't recall that the Quads had a PCV valve, but I would go through EVERYTHING on that breather. Double check everywhere that intake can suck air from the engine. That stupid little hose that connects that little box to the intake always rots out too. Spray brake or carb cleaner at the engine and see where it effects idle, if you can get it idling smoothly enough to get a good test out of it. Moneypit you might be onto something with that little port -I know the 2.4 Twin Cams have it (as you said), but I can't recall it being on the Quads for sure... And I think it is smaller than 1/8 inch too, so very easy to clog...

Never had the oil/air system clog. Always a first though. The most common issue with the Q4 2.3 O/A-S is dry rotting of the soft lines, mainly the ones you cannot easy see. Most exposure to heat and moister are those difficult to see items, than easily accessible items of the system.

As time has revealed decomposing rubber parts, I've replaced them with silicone or 'metric' sized heater hoses. I wasn't wanting to find NOS or reusing old scavenged parts.