Bricksters,
Here is my exact symptoms, cold start is OK, idle jumps to around 1600 and drops quickly back to about 750RPM (normal operation of IAC right), runs very smooth but shudders trying to get off idle in response to throttle and will ocasionally stall when I put into gear (Auto). As the engine warms up idle speed slowly increases to about 1000RPM and the engine begins surgeing rythmically 2 seconds fine 1 second shuddering with loss of RPM. Drives OK, seems a little down on power.
This is what I have done. Cleaned throttle body, adjusted throttle stop, adjusted throttle position switch, checked for vac leaks, replaced injector seals, dist cap, rotor, leads and plugs, cleaned IAC, cleaned PCV, checked O2 sensor, FPR. BTW Cat is rattling and will be replaced soon. Wiring harness is suspect.
Before I start randomly replacing harness, AMM, O2 sens, FPR, IAC etc I once again throw myself at the mercy of the collective wisdom. I thought it may have been the FI temp sensor, when I unclip it RPM's rise and the surge almost disappears. I have the details on how to check it at the ECU from the FAQ but how do I do this? it says: with the control unit connected check pin x to pin x? I can not get to the pins with the connector on? Do I have to take it appart? how does the pin numbering work?
Thanks All
Karl
86 245 B230F

Thanks for the sugestions!
I will investigate.
The ongoing search for perfection continues....or perhaps just smooth running at this stage.
Karl

I have experianced the same problem recently (see post 'Surging idle' on May 1). My problem seems to have been solved by adjustment of the basic idle mixture at the AMM. It did not take much change of the mixture screw to smooth out the idle. I found a vacuum gauge helpful in setting the mixture. When the mixture was off the vacuum was low. Also, ever surge cycle caused the gauge needle to sweep. I also checked the mixture using a digital voltmeter to get the voltage to cycle from .3 to .6 volts. I have read other posts saying the reading should cycle from .1 to .9 volts, but I was unable to get this large a span.

Had the same problem. Took forever to find reason and fix. It turned out to be the plunger in the air/fuel sensor unit. Where the air filter is located. The service mechanic that I really trust and didn't take to initially, found it within three hours. No one else could fix it. Take the cover off. Clean with carburetor cleaner extensively. the plunger is stuck. Look at a service manual like Bentley's. I will come back on Monday and give you a more detailed description. sorry, I'm at work and can't remember everything.

I replaced and adjusted everything and still could not fix it. Even the idle control valve. It turned out to be something that a simple spray with carburetor cleaner took care of.

Have fun.

Oh, on the cat rattling: there is a heat shield above it that tends to rattle. It is a sleave of metal that is tacked to the pipe. I just cut mine off with a Dremel. That's a lot less expensive. But maybe I misunderstand your problem with it. Maybe you thought of the heat shield. Just an FYI.

-JSBB
--
'Beware of the man who works hard to learn something, learns it, and finds himself no wiser than before... He is full of murderous resentment of people who are ignorant without having come by their ignorance the hard way.' -- Kurt Vonnegut

You should really invest in a Bentley manual for your 240. This will help for diagnostics. When I got my '86 240, I had the same problems. I did the ECU tests and found at least one problem. More importantly, I excluded many other theoretical problems. This will REALLY help you narrow it down to have the info in that book.

It really sounds like a dramatic vacuum leak to me. Try spraying brake cleaner all around the intake manifold and, of course, watch for a response. Also, the intake hose, the fat plastic one that goes from the AMM to the throttle body, should be removed and inspected for leaks. These rub against the body and wear through.

Also, bad idle control valve also seems very plausible. This is supposed to balance out the idle, which is obviously not happening. But, then again, the racing could be the result of the idle control valve attempting to compensate for another problem. Again, the ECU tests will help with this.

-JSBB
P.S. I am not a pro by any means. I just happen to have experience with the same car.
--
'Beware of the man who works hard to learn something, learns it, and finds himself no wiser than before... He is full of murderous resentment of people who are ignorant without having come by their ignorance the hard way.' -- Kurt Vonnegut

Hi Karl,

In your old post I suggested you clean the IAC, and obviously that didn't help... or did your symptoms change. Couldn't idle below 900?

Regarding the ECT, I believe you can break one that is old and brittle just by working in the area, bumping its connector, the stiff wiring harness, and so forth. The "in-ciruit" measurement in the FAQS is good because it checks both the voltage supplied by the ECU and the resistance of the sensor, but, yes you need to take the housing off of the ECU connector to probe the pins.

Best tool is a strong light -- at least that is the advice from an old presbyope. The pin numbers are molded into the connector body, though from memory (also old) I can't tell you which portion.

Remember the sensor or its wiring can be intermittent (broken) so that its failure may be temporarily hidden from your tests.
--
Art Benstein near Baltimore