levee-11

Ok ok, I know there's a million threads on this topic, but mine is unique, by the fact that it simply can not be solved. Here's the backstory:

Last weekend picked up a 1992 4Runner SR5 for my younger brother turning 16 from the original owner with all the paperwork, and I mean all the paperwork. It has 400k miles, but they are all freeway miles. The thing runs like a top, you'd think it's brand new. It's cosmetically flawless too. The holy grail. Now get this, drove it home after picking it up, drive was 140 miles, and we stopped at my work on the way home to show everyone. (I work at NAPA). When we got there, I quickly pulled the two codes it had, 14 (Ignition module sending bad or no signal to ECU), and code 24 (Intake air temp sensor sending bad or no signal to ECU). We then started it up to show how good it ran, and it idled flawlessly. Shut it off, and went inside, and came out about five minutes later to go home, and it would crank, start, then shut down. It did this a couple times before it would not start at all, and just crank. That's where our story begins...

Guy at work has owned a ton of Toyotas, and knows these engines inside and out. He immediately blamed the Fuel Pressure Regulator. He said if we jumped FP and B+ and it ran, that would confirm his suspicion. We did that and it ran until we pulled the jumper out. We believed his diagnosis, and I immediately had a regulator sent in for pickup on Monday (Today was Saturday). I had a friend tow it home, but we could not drive it onto the trailer (It would not start with the jumper anymore). Got it home and Sunday and Monday I did a ton of diagnosis ad reading. By this point I had an amazing understanding of this truck's circuitry and systems, and ruled out the pressure regulator as they rarely go out, and jumping FP and B+ has nothing to do with that, it merely bypasses the COR and VAF/AFM. I confirmed it was getting fuel, and I tested connections all over areas of suspicion and ruled that COR was good, fuel pump was good, VAF was probably good, and this had to be an electrically caused fuel issue. For the heck of it I pulled the return line on the regulator to see if I had fuel there, and it started right up! I had to shut it off because of the fuel, but this made me question if it was the regulator after all although I could not replicate this random startup.

Ran to work, grabbed the regulator and everything to do a tune up (it needed it anyway), changed the regulator, and this was not the issue... Nice. Later that night, the fact that I had a code 14 crossed my mind (I had been ignoring it since it ran fine with the code last it ran). I did some research, and figured the module was bad. I tested it using a method I read on an old forum here on YotaTech, and it told me that my ignition module was in fact bad. I then pulled the wire from coil to dizzy and tried to get it arc to the body, and no spark. I then plugged in an old wire to the dizzy and an old plug, and still no spark. Great! I know the issue now! (Don't make fun of me for not checking for spark earlier, I had no reason to believe it was, I'll agree I still should have checked, but whatever) I then went through PO records and found the module was replaced for this exact no start issue six month prior with a NAPA part. I pulled the module out, brought it to work, warrantied it out, got a new one, and a new coil because why not, the old one was 400K miles old. Installed everything, and nothing. Still no spark.

Today, I pulled everything apart, found a crack in intake hose, got a new one, nothing. Tested VAF using FSM a little more thoroughly than last time, and I did get some funny readings, but other than that, everything checks out good everywhere. I've tested nearly every possible issue with FSM standards, pulled 16 pin on ECU, made sure rotor spins, checked grounds, EFI Relay, Fuses, COR, several circuits for shorts, everything. i'm so lost as to what this could be. Coworker is bringing in a spare VAF tomorrow to test. I have a theory that the code 24 is hinting towards the fact my VAF is bad even through I've watched the door open, and most of it checked out multimeter testing-wise, except for one open circuit on one of the pins (I can't remember which).

My dad thinks it is the ECU at this point, or the Crank Position Sensor, although I doubt the ECU is bad being that I have a check engine light, everything seems to be talking properly, and they rarely fail. I'm thinking of opening it up, and visually inspecting to see if theres any burnt components or anything to rule that out. Does anyone have any ideas? I've properly diagnosed many things I have not yet mentioned, and spent hours trying to figure this thing out. It's so weird that it just dies out of nowhere like this after running perfect, and none of the fixes are the common ones. I'm so lost and at this point ready to throw a 3.4 in there, but this engine runs so smooth, and has so much life left. I'd greatly appreciate any ideas or feedback other than "Did you check your battery connections and fuel level?"... Yes... I have checked all the obvious. Thanks for taking the time to read this.

scope103

Hmmm. You've "properly" diagnosed "everything" (but you don't tell us the results of your diagnosis). You want ideas, but you don't want anyone to tell you to diagnose anything in particular (because you've already done it). Well, let's see. How about a witch doctor?

You need to be systematic; it sounds like your attempts to fix something random is causing a problem elsewhere. All ICEs require just three things: fuel, spark, air (all at the right times). Do you have spark? Don't pull off coil wires; you'll just damage them. Put the inductive pickup of your timing light on each plug wire one at a time. If the light flashes, you've got spark. While you're at it, you should be able to tell if the ignition timing is approximately correct.

Do you have fuel? What happened after you got it running with the fuel return disconnected? Replace the fuel return line with a 1/4" (6mm) clear vinyl tube to a suitable container. (For God's sake, don't just let the fuel spray all over the place!) Using the jumper, run the fuel pump. There is no spec, but I get about 1/2 liter/min. Repeat without the jumper, but instead pushing the VAF vane with your finger (remove the big induction pipe).

You said that it would run with the fuel return disconnected. If the fuel return is blocked, that will raise the rail pressure (a lot!), and flood the engine. Don't try to run the engine with the fuel return off (even with a suitable container, it's just too dangerous). Instead, get a fuel pressure gauge. This one comes with the adapter to connect near the FPR.

Code 24 (open/short in the intake air temp sensor) won't keep the truck from starting. But what causes that code? Could be a bad sensor (unlikely, there isn't much to them), but it's wired through that bundle of wires going to the VAF. If that wire bundle/connector is damaged, it could open the FC line, and that will keep it from running (an open FC line causes the iconic starts-and-quits https://www.yotatech.com/forums/f116...-quits-312747/) You haven't been fruitlessly unplugging and plugging the VAF, have you?

Last, be more careful with the advice you're getting (especially here!) Your friend who knew a ton about Toyotas thought running-with-the-jumper pointed to the FPR (you found out that was way off). Your Dad thinks it's the crankshaft position sensor (sorry, you don't have one).

Last, Good luck. Your problem may take a bit of head-scratching to fix, but "it simply can be solved."