ISFFUN

iTrader: (4)

If anyone is having trouble with a CEL 71 / 401 for EGR issues and you've been through that multi-step diagnostic nightmare, like @KahnBB6 outlined when you get to the y-pipe you have limited options due to the freeze plug. @Blkexcoupe had a great idea to get in there by popping the plug and tapping a bolt hole for future access, but if you can't do that here's another option.

My grandfather showed me this trick when the actual EGR valve small pipe was clogged years back.

Remove the y-pipe and take a propane torch, and apply heat along the air passage inside to the bottom. The EGR passage passes though the side and exits in the center of the y-pipe on a horizontal bar.

Mine was COMPLETELY blocked, allowing no air to pass. I found this out by applying 60PSI of compressed air into the side of the y-pipe EGR port and nothing happened - the air did not exit inside the horizontal tube.

I sprayed carb cleaner, tried a brush, then got the propane torch (bernzomatic attachment)

Here's a video. You can see the carbon has nowhere to go from the high heat and will push itself out of the tube like one of those snake fireworks. Spray some more cleaner and repeat. You can also apply some heat to the port side if there's excess carbon and then chip it away with a small screwdriver or pick.

This will not completely remove all carbon that's burnt into the walls of the inside passage but will clear it enough for airflow. After doing this my CEL went off in 2 drive cycles and I'm ready for smog.


doing the same thing (sorry, very old low quality video from years ago but same concept)

ISFFUN

iTrader: (4)

By the way, does anyone know what triggers an insufficient flow code if the EGR system is functioning normally except the clogged y-pipe? There's no sensors in the y-pipe or lower intake runner. I can't understand how the ECU is detecting low flow - is it the O2 sensors detecting a rich condition based on the expected map of the EGR working? I can understand the insufficient flow code if the valve isn't working because you have the temp sensor and the VSV, but not the 'last mile' trigger in the y-pipe. We don't have a MAP sensor so there's no trigger there either.

KahnBB6

iTrader: (5)

Nice tip above! Burning off the caked on carbon buildup definitely works to clear the passages enough.

Just FYI, those OEM freeze plugs are removable and should be possible to tap back in. I've done it. At least one Dorman generic replacement plug that I listed in my DIY thread should suffice to replace an OEM plug that cannot be recovered. They're steel from the factory I believe. Though Toyota/Lexus published no procedure for the cleaning of the EGR passages the engineers certainly put those freeze plugs there so that cleaning with a router/grinder flex tool could be done.

As far as how the OBD1 ECU detects "insufficient EGR flow or malfunction"... well... it's extremely simple and not a very smart system as far as narrowing down a culprit.

You'll get a Code 71 from the ECU if:

--If your EGR VSV fails

---If your EGR valve isn't functioning correctly and thus creates a flow blockage

--If your EGR vacuum modulator diaphragm is failing and creates a flow blockage by not allowing the EGR valve to operate correctly when the computer wants it to

--If your EGR gas temp sensor (1993-1997 models. 1992's do not have this sensor if I recall correctly) has failed or if it simply detects excessive EGR gas pressure buildup (in the form of too much gas temperature)

--If your EGR Y-pipe has too much EGR passage blockage from carbon buildup or if the steel EGR pipe going to from the EGR valve to the cylinder head is too blocked up with carbon then the ECU might detect an issue via the EGR gas temperature sensor reading too cool and out of the expected operating range when the engine is in open loop mode at full operating temperature.

As far as pinpointing any of these things specificaly... the OBD1 ECU cannot do that. It's not sophisticated enough.

If I'm wrong about any of these points someone please correct me but this has been my understanding for the OBD1 EGR equipped JZ cars.

ISFFUN

iTrader: (4)

I have an ODB2 but I'm guessing the core EGR system elements are the (mostly) same between the years.

Hmm, most of those possibilities are all attributed to flow conditions, but I still don't quite get what is exactly the mechanism for insufficient flow triggering a CEL flag. I understand the possible causes but not the ECU mechanism for that code. This is all just me being curious and wanting to understand the concept haha.

I'm going to guess/assume it's either a mapping to EGR open/close state temperature range, or a combination of that and O2 readings. I'm just thinking about the sensors specifically. There's a 2-wire connector to the VSV solenoid, an EGR temperature sensor, and eventually down the line the O2 sensors. That's it. The VSV check must be a binary condition, and the EGT/O2 a map of ranges.

Technically the exhaust cooler plate tube flow is clear, the valve plunger is actuating and clear. So exhaust airflow is expected to be a certain temperature right out of the head EGR cooler plate port (I forget the routing here been about 8 years since I had the head off) - it loops through the EGR plate first then to the tube?) but will vary on load and numerous other factors.

So if the final exit point (y-pipe internal tube) is blocked, the air/gas temperature should be higher, but what doesn't make sense is that the EGR temp sensor is below the plunger, so this condition could only be checked when the valve is open. This leads me to think it does some sort of combination of O2 mixture and EGR temp sensor checking.

I wish the repair manuals were more clear about the sensor/ECU logic just for curious minds but I guess that would be too close to giving away design secrets or something haha.

KahnBB6

iTrader: (5)

You are onto something with the ECU taking into account the O2 sensor reading in addition to the EGR temp sensor reading. Plus a lot of other sensor readings. An ASE trained old school smog mechanic could probably explain it better. The TSRM does cover a blow by blow of the operation and troubleshooting matrix but you're right in that it doesn't exactly go into great detail as to how the ECU determines every minute decision it makes. Maybe that went into Toyota trade secret territory But more likely it just had to do with the TSRM being for troubleshooting primarily.

The VSV indeed is just an old style (to us now but not in the 80's and 90's) electrical pressure solenoid for the ECU to control the vacuum operated part of the EGR mechanism. It's pretty simple in what it does. Some Toyota engines of that era are full of VSVs all witholding/allowing vacuum or withholding/allowing pressure. These days most of this is replaced by direct actuators that the ECU acts on rather than a vacuum or pressure operated system with VSVs... but Toyota still uses a few in new engines even today.

I think a good deal of what you are looking for comes down to how the Toyota engineers fine tuned the engine to operate the EGR system well within the boundaries of the emission standards they had to comply with for the time. Those minute reasons probably would never have been published (by them anyway).