'92 Camry 5SFE missing/stumbling

This thread is a long time coming for me. My Camry developed an intermittent miss or stumble back in May after using some Marvel Mystery Oil in the gas. I do not know if they are related but that is the nearest change that I can see that could be the cause.

The miss started about 1/2 way into the tank, and mileage has dropped since from the normal 28-30 to ~23 mpg. At cold idle, the miss is less noticeable, but becomes more frequent as the engine warms up and idle rpm drops...usually one cylinder (#4?) but occasionally two. When accelerating, the missing does not usually occur under light throttle, but abruptly happens above 1/3 to 1/2 throttle.

After installing the pressure gauge (inline from the filter to the fuel injector rail), I was getting slightly high readings at idle (45-46 psi) when tested per the Haynes manual (same as test procedure in the factory manuals stickied). Fuel pump operation tested okay with the wire jumper from FP to +B terminals and key on. After engine start, pressure was also high after disconnecting the vacuum sensing hose from air intake. Hence, the change-out of the pressure regulator.

When it first started, I checked the plugs firing using a timing light pick-up on each cord. #4 did show intermittent missing, but since the plug replacement stumbling has not improved but firing is consistent.

So far, I have checked or replaced the following:
-Spark plugs (#4 looked running a little rich)
-#4 fuel injector (based on spark plug #4)
-Cleaned IAC while still on TB
-Removed & cleaned EGR
-Installed permanent in-line fuel pressure tester
-Checked/adjusted the TPS
-Replaced the fuel pressure regulator (was getting slightly high pressure readings).
-Fuel has been consumed and refilled with fresh gas w/o additives.
-Various vacuum lines that do not run down the firewall side of the engine.

Also recently replaced items not related to this:
-New Toyota ignition coil
-New spark plug wires (Bosch)
-Replaced distributor shaft seal & o-ring seal (separate write-up)
-Replaced fan temp control switch

What I have not done yet and intend to do (given time):
-Remove and clean the IAC (needed a new gasket ordered...probably in but haven't picked it up yet)
-Replace remaining vacuum lines, especially larger diameter ones from pressure regulator. Small diameter lines look good and pliable.
-Haven't done the carb spray on vacuum connections trick yet.

If I am missing something, let me know. Any and all help is appreciated.

This is just a wild guess. It's possible that the Marvel Mystery Oil partically clogged 1 or 2 of the fuel injuectors. It is pretty thick to be running through the gas tank, if it's the same stuff I remember from my youth. I would try running some Chevron/Techron fuel injector cleaner in the tank to see if that helps.

Mike

MMO is a little thinner than mineral oil as far as I can tell, and is supposed mix in with the gas. With all I went through to check the injectors, I'm suprised I forgot to write this in. I usually give the occasional treatment Seafoam, but heard/read good things abt MMO as a good maintenance additive. Been using it in the Envoy and ol' truck with no problems, so I figured why not?

I actually pulled all of the injectors out, rigged a 12V power supply to hold open the injector solenoid, squirted some cleaner in each, and used pressurized air (set to ~40psi, same as fuel pump pressure) to force the cleaner through the injector. Also installed new o-rings and grommets after the cleaning and reinstalled. I did not see any odd spray patterns or effects of wear.

I also forgot to note that the fuel filter had previously been changed as well, just due to age. Thanks for the response!

Compression check? Doesn't exactly fit your issue but who knows, maybe something will pop up. Maybe the inlet strainer on the fuel pump (if equiped). Um...Maybe electrical? TPS or cam/crank sensor? I've been away from Toyotas long enough to forget what tells the ECM to inject more fuel at tip-in. Beyond that I'm scratchin.

Did the new fuel pressure regulator effect the pressure? You said "firing is consistant" after the plug change. Does that mean the miss is gone and you just have the stumble now or you still have the miss but can't detect it with the timing light? No CE light right?

Ah yes, the compression check. Forgot that fiasco as well. I did try to run a compression check on #4 (since it was the suspect one at the time). I do not remember what number I got because the threaded adapter got stuck in the plug port...the hose crimp just let it spin instead of unscrewing the adapter. Had to remove the valve cover and spark plug tube to get long-nose pliers in deep enough to loosen it. The tube is not easy to remove.

Anyways, I haven't tried again for fear of getting it stuck again unless I can secure the hose to the adapter.

And I did forget to note that the CE light has never come on.

Before the plug change, I tested the pulse in each wire with the inductive timing light. Only #4 had intermittent misses. After the plug change I didn't see any misses at any plug.

In other news, I did spray all the reachable vacuum connections with carb spray with no changes detected.

As noted before, after changing the fuel pressure regulator the pressure went from too high to too low, now at 33 psi at idle. However, if I remove the vacuum line going from the VSV to the pressure regulator, it jumps to 44 psi. And when I replace it, pressure drops back down (I forget the number) and the engine stops stumbling for a bit but returns shortly after I start to drive. After that, driving performance is unchanged, with it chugging under more than light throttle. I think it is due to something becoming unstuck and then binding up again. Could it be the VSV?

Looking around the Internet for it, I don't know that it is called the VSV. It is a small, barrel-shaped part that has two vacuum lines going to it and is screwed into the firewall side of the intake plenum near the passenger side. I see no electrical connections to it. I remove the left line (closer to the driver) and the engine dies. I remove the right one (going to another electrical vacuum sensor which connects to the regulator...is that the VSV?) and it increases fuel pressure and seems to calm down after I reinstall the vacuum line. My only guess is it is a vacuum regulator for the VSV but there is a vauge reference in the FSM about it being the MAP sensor...am I not seeing the connector? Arrgh, the MAP sensor I've seen is bolted to the firewall in front of the driver. Very strange and confusing!

Well, I have finally found the problem. The #4 spark plug wire boot was arcing inside the spark plug tube...there was a nice whitish scorch mark on the plastic piece that connects to the spark plug. I only saw this after running the car enough for the miss to be consistent.

Old Bosch wire set style with arc through edge
40778

New Bosch wire set design with fully rubberized end
40779

Hope this will educate everyone on the issue...if you have the old style Bosch wireset, have them changed out with the new design. Mine had a lifetime warranty through AZ, so no charge.