Is the icv my idle problem?

[Shurik]

Ive been having an Idle problem ever since it started getting cold. When I start the car, it stays at 2700 rpm for 1-3 minutes. And even when the weather is warm, and the car is all warmed up (after 15 minutes of driving or so), the idle tends to stick at 27 hundred for about 5-10 seconds and then come back down. And every time I let off the gas, if the idle does not get stuck somewhere, it does not go down to normal idle level right away, but slowly goes down.

This is an 86 325es btw.

So figuring it was the icv just sticking, I stuck an icv on that i got from a member here, and when I started the car it worked fine (car was already warm) but when I rev'd it, the idle got stuck at 27 hundred-ish (tachometer not currently working) and didn't come down at all. even after turning the car on and off. I ended up putting my old one back on.

Is the icv that i tried to put on not properly working, or do I have some other kind of problem?

[jlevie]

Here you go. Start at the top and work to the bottom without skipping any steps and you will fix the problem.


Idle or hard starting problems are most commonly caused by intake leaks and/or
a sticky or defective Idle Control Valve (ICV). The only reliable method of
locating intake leaks is to have a smoke test run on the intake and crank case
and to test the brake booster with a gage and vacuum pump. The complete list
of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hoses, valve, and expansion tank
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings

While leaks in some of those can be found by inspection or by spraying carb
cleaner on suspect areas, not finding leaks that way doesn't eliminate the
possibility. Only a smoke test will really work.

Once the possibility of intake leaks is eliminated, the ICV needs to be
removed and cleaned with carb cleaner until the vane inside moves freely. When
the ignition is switched on you should be able to feel vibration from the
ICV. If no vibration the ICV is bad, there's a problem with its wiring or
connector, there's a problem with the TPS, or the DME (or Idle Control Module
(ICM) on an ETA car) is faulty.

For the DME (or ICM) to control idle, the idle switch in the TPS must work
correctly. The switch should close when the throttle stop is 0.030-0.060" off
the idle stop screw. Hearing a click doesn't mean the TPS is working. Check
with a meter or test light.

The fuel system should be tested via the suite of tests in the Bentley manual
as invalid rail pressure can be a contributor to idle and starting problems. A
simple injector check is to pull the injectors, jumper the fuel relay to run
the pump, and see if the injectors are leaking. You can also point the
injectors into a towel, remove the coil wire, and crank the engine to see if
all of the injectors appear to be spraying in a similar fashion. The best
approach to possible injector problems is to have the injectors cleaned and
flow tested. Since raw fuel can or will be released in these tests, have a
fire extinguisher handy.

While a bad check valve in the high pressure pump can result in longer than
normal cranking, if the fuel system is working as it is supposed to the rail
will reach normal pressure in a few turns of the engine. A weak pump, clogged
filter or leaking FPR in conjunction with a failed check valve can result in
longer cranking and/or idle poroblems.

The O2 sensor can be a contributor to idle problems. The O2 sensor is a
scheduled maintenance item with a useful life of no more than 100k. If the
sensor has that mileage or more (or is of unknown age), replace it.

The AFM can be a contributor. If the vane doesn't move freely or the
resistance track is worn the DME may be receiving invalid data from the
AFM. And if someone has fiddled with the bypass air adjustment the DME may be
unable to stabilize idle. The bypass air adjustment should only be adjusted
per the procedure in the Bentley and with an exhaust gas analyzer. And even
then everything else associated with engine management has to first be
operating properly. If the AFM becomes a suspect, replacement with a good used
unit is the best approach.

Improperly adjusted or malfunctioning valves will affect idle and starting. As
can compression issues from ring or cylinder wear. A valve adjustment is
called for every 15k. A useful diagnostic is to run compression (dry and wet)
and leak down tests on the engine. Aged ignition wires, plugs, distributor
cap, or rotor can cause problems. Insulation does break down with time and
heat. And since the youngest E30 is going on 20 years old, if the ignition
system is original or the plugs are old replacement is indicated.

Although not usually a problem, a bad DME temp sensor is a possibility. That
generally won't cause an unstable idle, but can cause hard cold or hard hot
starts and/or a rough idle. As can problems with the timing reference
sensors. Although not commonly encountered, a bad harmonic balancer on an
M20B25 or M30 engine will cause problems.

When all other possibilities have been eliminated and idle or starting
problems persist, replacement of the DME, or if applicable the ICM, is
indicated.

In many cases the cause of problems will be a combination of factors. So it is
important to test and repair all of the possible causes.

[bloodsnot]

Here you go. Start at the top and work to the bottom without skipping any steps and you will fix the problem.

Awesome response, jlevie! I've got an M20B23 with its own fuel injection issues and will probably need to run through all or most of these things as well. I've got hard warm starting + awful fuel economy.

I've already replaced the fuel filter and ran a bunch of Seafoam snake oil through it. Many vacuum lines have been renewed along with a new(er) AFM boot and AFM. Aside from those things, I've read that a failed temperature sensor, a sticking ICV, a failed Cold Start Valve, or leaky injectors are likely culprits.

Would you rearrange the above procedure or pick a different starting place? I don't want to hijack this thread, maybe just piggyback a bit..

[jlevie]

While you have replaced some parts, use the list as is. It is in order of likely hood.