Earlier today I went through and did some maintenance stuff I'd been putting off on my '84 240 (non turbo). I replaced the Fuel Pressure Regulator (old one was starting the issue of not restarting), pulled out the air idle control valve and cleaned it since it hasn't been done in at least the 6 years I've had the car, replaced a cracked vacuum line, and cleaned the throttle body.

Put it all back together, made sure everything was connected and started it up. It started with no problem, but the engine was idling at about 1600 RPM. Let it run for a few to reprogram the system, shut it off a couple times and restarted it, and same thing every time. Tried adjust the idle valve, and if I turn it in all the way the idle only drops to about 1400.

I've used a voltmeter to test the Throttle Position Switch, and it was good. Used the method in my Bentley's manual. Tried using an old Craftsman engine analyzer on the dwell setting to check air/fuel ratio, and something is up with that. Gave me a steady reading at I think was 10 degree's and if it reved more dropped to zero. Its supposed to be 20-70, but i do think there is something wrong with that test, so I don't put too much stock in the result. I did try adjusting the screw back and forth about a full turn, and got no change of any kind, so I reset it back where it was.

Prior to this the car ran fine, no problems. Maybe a little rough at low idle, but nothing that bad, and then the not restarting thing, that I think was the Fuel Pressure Regulator.

I am totally at a loss as what to do now. HELP!!

 

Quick update- tried pinching off one of the hoses going to the air idle valve, no change. Tried unblocking the hose, no change. Tried unplugging the valve, no change.

Pulled off the intake hose and made sure that nothing was blocking the throttle plate, and it seems to be closing completely. Tried backing out the idle screw a bunch and running it back in, no change. Although There is still about 3/8 inch of thread showing, it feels like its bottomed out. I didn't take that out or move it when I was cleaning, so I don't think thats the issue.

I did try unplugging the MAF and the idle dropped to about 1k, but it ran really rough, and surged between about 950-1050. Plugged it back in and the idle speed back up. I did notice that if I give it any throttle right afterwards it drops down below where its idling at for a second or so, then picks back up to the same idle speed.

All I can figure is that its something with me cleaning out the throttle body, like something was adjusted for it being all dirty and now its not. But I can't for the life of me figure out what it is.

 

Think about how the IAC works but also you should be able to set idle with it disconnected. Disconnect an plug the IAC hose. Start the engine and adjust the idle screw on the throttle body. Oh disconnect the throttle cable. If the idle can't be adjusted then you have a large vacuum leak somewhere. You may have damaged the IAC valve and it is leaking air into the manifold side of the TB.

 

Ok, update time-

It looks like I've gotten it solved- in a nutshell apparently the ECU wasn't resetting correctly and closing the IAC off. Here's all the details:

Back when I was trying to test everything, when I tried clamping off the inlet hose for the IAC I don't think I clamped it all the way. I was worrying about damaging the hose, so I didn't clamp it too hard. I finally got irritated, and since I have a spare hose I went ahead and tried clamping the hose off really hard and the idle finally dropped. So I pulled out the IAC again, checked all the hoses, and they were all fine. The valve in the IAC seemed to move freely, so I tried running power to it to make sure it worked. On the bench it opened and closed just fine, so I knew it was ok.

Reinstalled everything and while it was in place, used the leads to make sure the IAC was opening and closing, and each time I could hear it click, so I knew it was. Made sure the valve was closed, and tried starting it and the same thing happened, high idle. Next I unplugged the IAC and used my test leads and, while it was running, closed the valve. The idle dropped right down to where it should be. Just for curiosities sake, opened it by using the leads and the idle kicked back up. Closed it again and it dropped off. Then I plugged the IAC back in, and ever since then its been doing just fine.

I was wondering if the issue maybe a bad signal from the ECU or MAF, but once I closed the valve with the system on, its been working fine. I don't know if the issue was that the ECU didn't reset right or if there is an actual fault with the ECU or MAF. I have read here and on other forums a few times where people got high idles they could never figure out and eventually gave up, so I don't know if was the same kind of issue or what. It seems like either the ECU or MAF sent a signal to open the valve and then never sent it a close signal.

The thing that kept driving me up the wall was that when I ran whatever test I could on all the various components (TPS, IAC, MAF, O2 sensor), they all tested out good. I know some of them, like the MAF, don't have really conclusive tests unless you swap them out, but given the fact that everything had been working fine before I cleaned the throttle body and with the test results looking good, all the components seemed to be working. Anyway, its been a couple days now, and its been working fine since. Runs smoother, and doesn't have the Fuel Pressure Regulator issues. Sorry for the delay in responding, I always hate it when there's a thread and the solution isn't posted once its fixed. In my defense I've been busy playing catch up since I lost two days I wasn't expecting to, and haven't had a chance to follow up on this here. Anyway, thanks for the help, and I hope this helps someone else out too.