I doubted that my answer was high octane gas when the mechanic told me that but who am I to dispute what a mechanic says? I go to this shop because it is within bicycling or even walking distance from my home. I don’t have the luxury of being able to easily have rides to & from a garage too far to walk to. I cleaned the outside of my idle air control valve. The plunger seemed to be tight so I compared it with a new one at a parts store but the new one was just as tight. Plus, no trouble code for that or anything else, so I figured that if I relaced the IAC valve, I would just be throwing even more money blindly at this problem. I don’t remember checking the IAC passage but there’s not much distance at all between the hole the IAC sits in and the air tunnel. Could the IAC valve be faulty though, even though it seems as good as a new one? Yet I would think there would be a trouble code, that’s the problem, no trouble codes at this point.
IAC valve failure won’t set a code.
That’s because it’s an engine control device.
Not an emission control device.
Tester
I had my engine anslyzed for $100 where no trouble code was seen but somehow he found thatcthere was random misfire in all but one cylinder. Wouldn’t he somehow also have found evidence of any IAC valve problem?
I ordered a new IAC valve online since that surging is getting worse now when the engine is at closed throttle. And at times it idles very noticably rough. I know that an IAC valve operates with a stepper motor so I can imagine that there may be intermittent faulty opetation. Can I have hope that a new IAC valve will make a difference for me? I spent $100 for diagnosis & all I was told was that I had random misfire and that I needed to run fuel system cleaner with highway driving. I tried Seafoam then Lucasoil. That did nothing for me, only made NAPA & Autozone a little richer. A buddy suggests STP Deep Clean.But I feel that something still needs to be replaced.
Check the book for installation. There was a calibration procedure for mine but exactly what slips my mind.
Just how much diagnosis did you get for your $100 . . . ?
Did the shop say “We know this is your problem” . . . ?
Or did they say “You need to start with this” . . . ?
On your particular vehicle, it’s easy to perform a fuel injector balance test with a pressure gauge and a pro-level scanner. You’ll very quickly know which particular injector is the culprit
Please refresh my memory . . . do you know the fuel pressure regulator is functioning correctly, or has it recently been replaced?
I don’t want you to run out and buy one, but they’ve been known to cause “random misfires” . . . I rather dislike that P0300 code, because it’s usually a very specific cylinder . . . or maybe even 2 or 3 . . . that are misfiring
They said “you need to start with this” which meant fuel system cleaner. A few years back, I saw my fuel pressure regulator had leaked out, gasoline all over the side of it. I replaced it with a new one but never could find out where it had leaked from. No evidence whatsoever. No trouble codes dor anything now.
It’s purpose is to adjust the fuel rail pressure for intake manifold vacuum. When there’s a high intake manifold vacuum less fuel pressure is required, as the vacuum tends to suck the fuel out of the injectors. There’s a rubber diaphragm inside those gadgets that separate fuel from the vacuum line. If the diaphragm breaks, first it won’t work correctly, and second, fuel will leak into the vacuum line. You wouldn’t normally see the leak unless you removed the vacuum line.
I had first sucked on my vacuum nipple on the faulty regulator & it held vacuum. Then, since I had replaced the regulator & now was just trying to see what had been leaking, I looked carefully all over the regulator & even ground the seam off & looked inside & could find no source of leakage. I never did catch it actually in the act of leaking either, the fuel dried up & never leaked out again.I also found no looseness where the vacuum tubing fit onto the nipple at the top.
When I test my Corolla’s fuel pressure regulator, first I remove vacuum line and look inside the line to see if there’s any gasoline in it. There shouldn’t be. Next I’ll connect a hand-held vacuum pump to the regulator’s vacuum input to make sure it holds vacuum to 20 mmHg. You can’t reach that level of vacuum otherwise, and it might not leak until that vacuum is reached. Suggest to secure yourself a hand-held vacuum pump. This is similar to the one I use. Mine doesn’t have all the attachments. If you can find one without the attachments for a better price, that’s your better version. Homebrew your own attachments if you need them. I use mine to check the car’s complete vacuum system as part of a bi-annual tune-up, usually find at least one vacuum problem. Easily pays for itself. It can be used to transfer small quantities of difficult to access fluids too. I use it for that , but infrequently. Some say it can be used to assist brake bleeding, but never had any luck with that idea. Gravity or pushing on the brake pedal works best for brake bleeding imo.
Thanks for this useful info
I changed the Idle Air Control Valve. A youtube video said that to calibrate an IAC, start the car, slowly accelerate in park, let it idle for 5 seconds, shut the engine off for 10 seconds, restart, check idle. No improvement even after driving the car around hoping the IAC would “learn” different speeds.
Poor gas mileage would be caused by a mixture that is too rich, or insufficient spark advance, or a slipped timing belt or chain causing valve timing to be incorrect. Or a worn out engine that has low compression. Was a compression test done? Bad valve timing would alter the compression test.
Are the oxygen sensors good? What do the short and long fuel trims look like?
Thursday, I did a compression test. Looked ok. With pressure building up at what seemed to me the proper rate, 1, 2, 3, & 5 cylinders were 150 psi, 4 was 158, 6 was 155. I had replaced the O2 sensor by the converter a couple years ago due to a trouble code, and the sensor by the exhaust manifold last month or maybe June, due to a trouble code. I had the engine diagnosed for $100 and no mention of any fuel trim data. They only said that even though I didn’t get a code 0300, there was still the vague indication of random missing in 5 of the 6 cylinders. They told me to use fuel system cleaner, which I did twice, no improvement. The car has rough idle, and the engine idling in pulsations. Instead of a steady “RRRRRRRRRRRRRRRRRR” sound, most of the time at idle, the sound is more like a well defined “RURRURRURURURURUR”, and the car gets to rocking when this happens. Yet no constant trouble codes. Although last week I did get the code 0300, like the mechanic said it had, random misfire. I cleared the code and it hasn’t returned.
And the mechanics found no vacuum leaks. This condition is intermittent, yet running uneven like this most of the time. Valve timing would seem to be ruled out as I’m thinking if that were a factor, the problem would be constant. New plugs & wires this summer, boots greased with silicone. New evap cannister also then, and the purge and vent solenoid vales tested ok. Yet I don’t hear the “click click click” shortly after engine warmup that I know to be the evap purge system purging the evap cannister. Maybe these solenoid valves have wiring trouble. Could sticking engine cylinder valves be intermittent? I’ve read where oil additives could free sticking valves. I’m wondering if that could be due to oil fumes from the oil lubing the camshaft working on deposits on the valve stems and softening them up? I’m hoping a reader of this post has had a car behaving just like mine does as described here, and would share what he did to correct it…
Please refresh my memory, as this thread is pretty long . . .
Your car has 3 separate coil packs, correct?
Have the coils ever been replaced?
Are coil towers . . . where the wires connect to the coil . . . in good shape and can you feel a solid engagement when you connect the wires to the coils?
You’re doing a great job, by the way! 
One coil pack. New plugs and wires greased with silicone and seated securely. Perfect acceleration and flawless cruising. Here’s the thing that causes me to feel that this is not an ignition problem- the tips of each spark plug looks perfect. Dry, no soot. No radio static from plug wires shorting out. Doesn’t seem to be a transmission issue, acts up like this even when idling in park.
Your post title refers to removing fuel tank. Apparently you’ve either already done that, or decided it wasn’t necessary. Suggest to review the details of the current symptoms you are trying to diagnose/solve.
If you think engine runs ok at higher speeds, but isn’t idling properly, take a dollar bill and hang it behind tail pipe at warm idle. The bill should constantly flutter away from the tailpipe. Does the bill ever move briefly towards the tail pipe?
I wonder if the crank sensor is bad?
Tester
I wonder about the CKP sensor also, since my car sometimes shows some of those symptoms mentioned. But I’m wondering if a bad CKP sensor would show symptoms intermittently. Yet I suppose there could be a loose connection externally or internally with the sensor. I should remove it and check it as best as I can. I’m also wondering if I have a wiring issue with my purge solenoid or vent valve. I checked each of them for solenoid operation and leak by and they were ok. Yet I dont remember hearing the clicking noise I used to hear after the engine has started and the charcoal canister is being purged.
George San Jose, I removed the fuel tank and checked purge canister which was on top of the tank. It was ok. If a dollar bill shows flutter both out and in of the tailpipe, would that indicate a sticking exhaust valve issue?