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.