You can troubleshoot and address many/most of the possible issues without taking things apart. If you had everything working properly and the components like the EGR Vacuum Modulator, EGR Valve and lines are relatively new then my first assumption is that you've got blockage somewhere in the system. There are a few choke points that tend to block repeatedly until no more loose carbon is in the system. This loose carbon could be residual from when the 80 was driven with failed components that introduced gunk, possibly from the vacuum canister, etc.
A quick way to address what I think are the most common vacuum choke points follows. First with the two ports on the throttle body:
- Grab a can of relatively mild aerosol solvent/cleaner. Perhaps an electronics cleaner? Brake clean, carb clean, etc. could all work but may be harder on any rubber/etc. so keep that in mind.
- Start the 80 and let it idle.
- One at a time pull the rubber hoses off of the two ports that are directly on top of the throttle body. These rubber hoses are slid onto 90 degree copper/brass elbows.
- One at a time, with the straw on your spray cleaner, put the straw into the brass/copper elbows and squirt a bit of cleaner into the port.
- If the port is blocked there will be no impact on the engine idle speed. Clean as necessary. If you can't clean the port with just solvent you can gently turn/remove the elbow with pliers and then use a small wire to clear the port in the TB itself.
- If/once the idle speed changes when the cleaner is sprayed switch the the next port.
Next up the vacuum modulator which I think is the most common clog point:
- If this is your first time working on the EGR Vaccum Modulator just go ahead and remove it. In the future you can check/do the following with it in place.
- Get a short length of vacuum hose (6" to 1') and connect it to one of the three small nipples at a time on the VM.
- Blow into the vacuum hose to confirm that air passes through each of the ports. You can use your fingers on the other two ports that don't have the line on them to confirm that all is air tight and to confirm the airflow/etc.
- The most likely clog is in the straight in line ports and it's the TB side that clogs. This is port P to Q I believe. I've taken an old VM apart to inspect and the passage inside P is reduced to a small size that likes to clog.
- If you have a clogged port squirt it with solvent to blow it out. This usually means that you squirt into port P. I enlarged the inside diameter on my VM to make the spray straw fit into it a bit so that this is easier.
- Flow air through the VM to get the solvent out asap so that it doesn't damage the rubber.
The above have been repeating issues on my 80 and I'm slowly troubleshooting why. I've added an inline, clear fuel filter between the charcoal canister and vacuum system to see if that stops the flow of carbon chunks but will wait 6+ months before deciding on that. I've blown the system out, replaced components, etc. While I've applied other relevant fixes on my 80, the above clogged ports are what I've encountered after months of operating without a po401 so they are my best guess as what you may be experiencing. I've never had to clean my intake, temp sensor, etc. a 2nd time and my VSV valve has only been replaced initially, etc.
Beyond the above, which I've not seen documented online much or at all there is a lot of good information on this system available so I won't recreate it. If the above 10 minute clearing of ports doesn't resolve your issue you can move through broader tests to learn the system and identify your problem(s). Try searching the following in Google and note that a few of the videos show nice troubleshooting processes that are easy/quick and don't require taking much apart beyond pulling vac. lines:
Additionally I found it really helpful when I was learning how the system works to use a vacuum gauge connected at different points in the system. For instance you can remove the VSV from the system (leads to a po402 if done long-term) etc. to simplify testing by blocking the line from the EGR modulator to the intake pass-through. Add your vacuum gauge to the system after the EGR valve and you can watch when it should open. I have done this with the gauge mounted to the wiper arm so that I can watch the gauge in real time as I drive. Very informative.
Also keep in mind that after a fix and reset you have to drive to speed (45mph+ ?) and stop then drive to speed again before the computer will throw the code again. As you troubleshoot keep this in mind to help you get a sense of success or not.
In any case, as long as your components are functional I'm sure that you can troubleshoot and resolve this issue without a lot of disassembly or time invested.
