PhantomCS

Trying to track down a no start issue in my '92 SC400 I just recently brought home. Prior to this, the car would start, run like ***, and backfire through the intake. All signs and symptoms pointed to bad caps on the ECU. Pulled the ECU and discovered it is a reman unit, and also the boards and caps look mint, so I don't believe it to be the issue. Only error code I get is 24, but I can't find the info for testing the MAF assembly anywhere. It's getting fuel, but I can't confirm spark as I can't observe and crank at the same time when it's not dark outside. It has been a hot minute since I messed with an OBD1 car, so this has me scratching my head a bit.

Any help?

LeX2K

Is the temp sensor in the MAF? If it's bad would explain the poor running. FYI code 24 is bad intake temp sensor.

PhantomCS

The temp sensor is indeed in the MAF. I can't confirm if it's in spec or not, since I can't find any test procedures for it. However, would an IAT sensor prevent the motor from starting at all? All she does is crank over and make compression noises, but won't fire up. I can smell fuel coming from the exhaust as well.

LeX2K

Not sure on the SC400. I've worked on other cars (Toyota's) with a bad MAF the engine was hard to start wouldn't idle properly pressing the accelerator didn't do anything. Is 24 the only code? The thing is since the engine won't run the computer might not be able to trip any other codes.

PhantomCS

At the moment, that's the only code. I'm almost positive I tripped that code trying to start the car with it unplugged. Any other codes were likely erased when I removed the ECU for inspection.

LeX2K

Well you're dealing with OBD-I so the EFI system is fairly primitive but the fact the engine has a MAF instead of a mechanical AFM is a weird combination.

Testing shouldn't be hard I assume 4 wires
black - ground
red - supply 12volts
purple? - MAF signal
white? - temp signal

Would be nice if you had a known good MAF to swap in.

PhantomCS

That it would. To test the IAT, I would reference white to black pin?

Also, checked the timing marks on crank vs cover, and the cams seem to be in time, so that scratches that one off my list.

However, I noticed something funky while poring over the engine bay. One, a rodent took residence under the hood for a short time, so wiring is always a possibility. Two, on the driver's side of the fuel system, there appears to be a pressure regulator or possibly fuel damper, and the screw on it was backed out to the point where if these engines didn't run so smooth, it would have fallen out. I tightened it back down.

Decided to pull a plug... and it's no wonder it currently won't fire. The fuel on this plug REEKS. So, I think I might be in for some injector diagnosis if this continues.


Now to clean these plugs and start from the beginning trying to get this thing to run now.

LeX2K

Hope you don't have rodent damage. For testing the MAF find 12 volts and ground, the other two wires are airflow output and temperature. You can test the MAF with compressed air you'll have a voltage change as you blow air across the MAF. But be careful don't blast it with air 1 inch away!

Have you verified spark?

PhantomCS

Have not yet verified spark. Don't have a spark tester, or an assistant. I hope I don't have rodent damage either because this harness looks like a nightmare to replace.

LeX2K

Use one of the spark plugs wrap a wire around the threads and ground it. Crank and look for spark. Good thing is you know there is fuel, you're not sure about spark or compression.

PhantomCS

Compression I believe is good, or at the very least sufficient. Pulsations coming from the engine while cranking are all strong and even. In the event that the MAF is actually good, and I have spark, then it must be a faulty cold start injector I think. It smoked like a damn train the one time I had it running. It was dark grey smoke.

LeX2K

Hard to say without actually being there but you're having some odd problems that I tend to see when the ECU is bad. But you don't have any parts to swap in/out so makes diagnosing harder especially since OBD-I is limited.

PhantomCS

Yes, in a situation like this, it's hard to diagnose without proper service documentation or known good parts to test against. I'm a field service tech, and I hate "parts changers" who just throw a bunch of parts at a problem in hopes of solving it.

KahnBB6

iTrader: (5)

I am mostly familiar with the OBD1 SC300 more than the OBD1 SC400 but to confirm, yes, your IAT sensor is actually located in the Karmann-Vortex style MAF and it is actually very easy to remove. Something to note: NEVER clean those IAT sensors with anything other than a rag or towel. DO NOT EVER spray any type of cleaning agent or water into those plastic sensors. It will damage them and create the same issue you have right now.

From the sound of things yours may have gone bad as a matter of natural course and age. You can pick up a used OEM one off ebay for about $20-$30 or get aftermarket replacements off Rockauto.

The aluminum MAF housing itself isn't going to be replaced, just the little sensor that bolts into it with 2-3 screws. That sensor has a neck that extends down into the aluminum MAF housing and that is what actually measures the air for the ECU.

Also, just because an ECU was labeled as a reman from Toyota doesn't necessarily mean that Toyota replaced the capacitors inside it. I've bought a reman ECU from a dealer for my SC300 before and I still sent it out to get its capacitors replaced anyway just for good measure.

Your plugs probably need replacing if they were run hard with a lot of overfueling fouling them up before the IAT sensor failed (likely failed).

That's what happens when you have an IAT failure or if someone cleans one of those sensors when they absolutely shouldn't: the ECU sees little or no air coming into the engine and assumes it needs to compensate with even more fuel. Unfortunately it basically causes the car to sputter mostly if it starts at all and maintains revs for even a few seconds before the engine won't stay running.

Replace your IAT sensor first (again, eBay for a used Denso OEM unit that works... no need for the entire MAF assembly, just the sensor... or you can get aftermarket versions off Rockauto. I last fixed mine in my SC300 with a good working OEM used one from eBay).

And of course check your plugs and O2 sensors since those are the two main things that tend to get really fouled by too much over-fueling condition.

After that if there are still issues starting you may need to send your ECU out for capacitor repair which hopefully will solve the issue (cap repair usually does if the board traces are fine).

PhantomCS

Perhaps some yahoo damaged the IAT/MAF by doing as you just described. There's definitely been some running/drivability issues since October of 2017, based upon receipts from a shop where she'd had work done on the car. Curiously, she was quoted and charged for only six spark plugs. Wires, rotors, and caps were changed, as well as air filter (Doubt, because there's a short ram intake on it), fuel filter, PCV, and 2 oxygen sensors, as well as a used MAF and fuel pressure regulator. I'll definitely replace that before going too much further with the car. Regarding bad capacitors, like I said earlier, when I took the ECU covers off and looked inside, there wasn't a single leaking capacitor. Wouldn't that usually point to the ECU not being the issue?

Before I send the ECU out, I have my suspicions about what's going on. The plugs get absolutely rinsed with fuel even after only a few revolutions trying to start the car... is it possible my cold start injector is stuck open and flooding the motor out? Tomorrow I'm grabbing a spark tester to verify that I'm getting ignition because I couldn't really see due to the location of everything when I tried last night, and I don't have anyone to help me. After that, I'm going through all the harnesses with a fine tooth comb just to be sure I don't have any rodent damage screwing me up. If that all passes, I wanna do a fuel pressure leakdown test to make sure I don't have stuck open injectors or anything of the sort.

I just find it really suspect that the car ran, terribly, then backfired through the intake and now it won't even pop or try to fire or anything. And after cranking for just a second or two, it REEKS of fuel at the exhaust. Damn daylight saving time, I wanna tinker with this car, but I don't get home from work until after dark.