e30 rough idling. what can cause ?

[bleeeeech]

Looking at purchasing a 1989 320i.



body is nice and straight, everything else is fine. however it was rough idling. What are possible causes for this ?



Seller also mentioned that over 100 km/h occasionally the engine will "missfire" or incur loss of power in a quick burst before settling. Suggested it could be possible fuel injector issue ?



Should i just walk away or could these be quick fixes ?



cheers,



Jazz.

[jlevie]

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
Pann gasket or oil level sensor gasket

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.

[bleeeeech]

I honestly cant say thank you enough for that reply. I definitely owe you one. I will look into all you mentioned

[jalopi]

You never mentioned the price... is it good? From what I understand, it's getting harder and harder to find clean e30's.

If you're mechanically inclined and don't mind figuring out what these issues are AND the price is good... I'd say go for it.

[bleeeeech]

3k NZD. maybe around 2.6k USD. No cracks in dash, had a respray at beginning of year and no structual rust i could see. Everything is sitting at stock (so rides and looks like a boat) apart for a head unit.

[Harvest14]

Hey jlevie, or anyone... you seem to give some very thorough advice. May I ask an Idle Control Valve question?

My idle speed remains perfect and virtually everything electrical is new, plus a recent valve adjustment. At idle though, after about 30 seconds from revving down, the engine stumbles. You can feel it in the car but the rpm remains unchanged. Motor mounts are new, new cap/rotor, wires, plugs, coil, no vacuum leaks, new throttle body gasket as well as intake manifold gasket...new injectors, 191 plug cleaned and filled with dielectric grease....etc. etc. The stumble won't go away. I presumed the idle control valve was good because it hums when powered up. It's clean enough to eat out of! But, I saw on Pelican's website how to test the ICV with an ohm meter. They called for 40 ohms between outer two pins and 20 ohms from center to either outer. I got around 60 ohms at ANY combination of pins. Before I spend a couple hundred dollars, is this sufficient evidence that the ICV needs replacing?

I'm new to forums, so if this is "hijacking" or any other faux pas please forgive me.

Thanks in advance for the help!

[rcsoundn1]

I would say that is evidence enough, though I am no expert, so wait for others with more expirence to chime in.

[jalopi]

All the ICV does is let air into your engine when the throttle plate is closed (aka when idling). This translates to controlling the rpms... which you said are fine and normal. Based off your description it's not the ICV. Sounds like it's something either fuel or spark related.

[Harvest14]

That makes sense, but the coil,wires,cap,rotor and plugs are new as are the fuel injectors. The fuel pressure tests out good and the valves are adjusted. The only thing not new is the fuel pressure regulator. I guess I could replace that, but I'm told they rarely ever go bad.

Is it possible that the ICV is moving inside when it's not suppose to, possibly briefly cutting off air supply during idle. Briefly enough not to noticeably effect idle, but causing the stumble instead? If the ohm readings are off, which they are, could it be misinterpreting signals from the ecu and moving erratically? Just speculating here. Thanks

[agent]

When the ICV is off the car can you hear it opening and closing when you rotate it quickly by hand? Sticky ones can be cleaned out with TB cleaner or brake cleaner.

You say you have no vacuum leaks, but a cracked breather hose caused a similar idle issue for me. It's the one that runs under the intake manifold and connects to the VC, commonly called the "bitch tube" for the degree of difficulty replacing it (without removing the entire manifold assembly).

[Harvest14]

I agree that a small vacuum leak seems like the most logical culprit. I have replaced the breather hose (because it WAS cracked), along with the large one on the throttle body. I have spayed tons of carb cleaner everywhere and can't make the rpm change. Perhaps I should pay for a smoke test before I buy more new parts?

[agent]

I'd say that's a good idea. The carb cleaner leak check didn't reveal the cracked bitch tube for me. I'm pretty sure I emptied an entire can looking for it.

[jalopi]

It's not your ICV

[Harvest14]

Thanks for the help guys. Thursday is my day off, I will try to get the smoke test done then. In the meantime I've held off on ordering any new parts (ICV in particular). Thanks again.

[Harvest14]

Smoke test revealed a tight vacuum system....no leaks. Oh well,the search continues.