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e30 Vacuum Leak

Couple things I learned from my "low idle/CE light" problem...

My e30 started to have some problems - if I let it idle for more than a few minutes the idle would drop and the "Check Engine" light would come on. Blipping the throttle raise the idle and put the light out, but after a few minutes it would return. The problem got progressively worse over a few months to the point where the idle would drop quite a bit and the CE light would come on after only 30-seconds or so.

I began hunting the Internet and have listed below the things I found. One of the immediate things you will find is that there are lots (and lots) of things it could be. Fortunately for me my issue was relatively easy to find (crack in the intake boot), inexpensive and easy to remedy. It was also a good opportunity to get in and clean out 100k miles of residue in the intake system.

First, here is a picture of the engine bay with particular things marked for reference.

1. First step is diagnosing the CE Light fault. You should get a 4-digit fault code, here is a good explanation of how to get it and what it means:

Motronic 1.3 - The fault codes for these systems will appear on the check engine light when you turn the ignition key to the On position (but don't start it), then: fully depress the accelerator pedal, then fully release it. Repeat the depress, release cycle 5 times fairly quickly, but not too fast. It should be completed in a few seconds. What you are doing here is clicking the wide open throttle switch then the idle switch each 5 times. This signals the Motronic unit to send the fault codes by flashing the Check Engine light. You should see the light flash once then it will start flashing the 4 digit codes. If there are no faults, you should see the 1444 code (and 2444 if you have a 12 cylinder). The codes appear as a series of flashes for each digit. The flashes indicating one digit are about 1 second apart, the next digit appears after a couple second interval. The rest of the fault codes appear in Table 1 below - more in-depth info here.

Table 1. The Motronic Fault Codes.

Malfunctioning System.Fault Code
DME Control Unit1211
Air Mass/Volume Sensor1215
Throttle Potentiometer1216
Output Stage, Group 11218
Output Stage, Group 21219
EGO(O2) Sensor 11221
EGO(O2) Sensor 21212
Lambda Control 11222
Lambda Control 21213
Coolant Temp. Sensor1223
Intake Air Temp. Sensor1224
Knock Sensor 11225
Knock Sensor 21226
Knock Sensor 31227
Knock Sensor 41228
Battery Voltage/DME Main Relay1231
Throttle Idle Switch1232
Throttle WOT Switch1233
Speedometer A Signal1234
A/C Compressor cut off1237
A/C Compressor1242
Crankshaft Pulse Sensor 1243
Camshaft Sensor 1244
Intervention AEGS 1245
Ignition Secondary Monitor1247
Fuel Injector 1 (or group 1)1251
Fuel Injector 2 (or group 2)1252
Fuel Injector 3 1253
Fuel Injector 4 1254
Fuel Injector 5 1255
Fuel Injector 6 1256
Fuel Injector 7 1257
Fuel Injector 8 1258
Fuel Pump Relay Control 1261
Idle Speed Actuator 1262
Purge Valve 1263
EGO Heater 1264
Fault Lamp (check engine)1265
VANOS 1266
Air Pump Relay Control1267
Ignition Coil 11271
Ignition Coil 2 1272
Ignition Coil 3 1273
Ignition Coil 4 1274
Ignition Coil 5 1275
Ignition Coil 6 1276
Ignition Coil 7 1277
Ignition Coil 8 1278
Control Unit Memory Supply1281
Fault Code Memory 1282
Fuel Injector Output Stage 1283
Knock Control test Pulse 1286
No Failures 1444

You will probably get the "1222" code, if you get a different code the following will probably not help you.

2. The most common cause of a idle drop/CE light and the "1222" code is a vacuum leak. You can test for leaks with a can of Fi-safe carb cleaner or a spray bottle filled with rubbing alcohol by spraying various areas while the engine is running and listen for a change in idle speed. This can be dangerous however and should only be done outside and sparingly. I tested the vacuum hoses, the throttle, the intake manifold, oil fill cap and dipstick, and the valve cover with no luck.

3. One of the most common fault areas for vacuum leaks on cars with age/miles is the Air Intake Boot (the big rubber air bellows between the Air cleaner Box and the Intake.) On a car with high miles you should probably put a new one in anyway as they get brittle with age and the swap out takes all of about 30 minutes.

Recommendation: Replace Air Intake Boot. You'll need a screwdriver, new intake boot and new hoseclamps (check your particular intake configuration carefully before ordering parts.) You should also have a can of Fi-safe carb cleaner and several clean shop rags to thoroughly clean the intake and ICV. It is also an excellent opportunity to replace the short ICV to intake elbow hose and the crankcase to intake hose, as well as put in a new air filter. As a side note, I used half a can of Fi-safe carb cleaner looking for a leak and never found it, replacing the bellows and cleaning the ICV and Intake solved the problem - turns out the leak was a small crack inside the accordian section of the intake bellows.

3. There are several other common sources for air leaks, including the crankcase vent hose & the ICV elbow hose. These two hoses are inexpensive and simple to replace. Again, on a high mileage car you might want to replace them anyway. Less common but still easy to fix are leaks at the valve cover gasket, at the oil fill cap, at the oil dipstick, the hose that runs from the intake manifold to the fuel pressure regulator, and at the vacuum hoses that run to the brake booster and the charcol canister. The hoses are all relatively easy to replace and you might want to replace them as a preventative measure. Less common and harder to repair include leaks at the intake manifold, at the metal fitting from the throttle to the ICV elbow hose, and at the intake butterfly spindle.

4. The recommended change interval for the O2 sensor is 60k miles, but many report a much longer lifespan. If yours has more than 60k miles on it you might consider changing it anyway. It is a relatively easy procedure and should take no more than an hour. For more information on O2 Sensors and how they work and how to test them, please see the e30 O2 Sensor FAQ.

5. ICV fault - when the ICV goes the idle will generally hunt, drop and rise. It is easy to clean, just remove it, spray it full of fi-safe carb cleaner and hold it up like a torch while slowly making a circular motion. Empty and repeat. You may need a few q-tips to very carefully make sure the slider valve can freely open and close (they get quite munged up with gunk with age.) A new ICV is not cheap, but easy to replace.

6. A more rare but often troublesome failure is fouling of the C191 Multiplug. This connector, directly under the Throttlebody, can often become contaminated with moisture and crud and cause all kinds of idle-related issues. Please see the C191 Connector FAQ.

7. Other - less common problems can also be faulty injectors and injector o-rings, clogged or faulty injectors, low fuel pressure, oil return tube o-ring failure, faulty coolant temperature sensor or switch, misadjusted ICV, front or rear crankshaft oil seal leaking or damaged, oil pan gasket leaking or damaged.


e30 Idle Faq - extremely thorough!


Couldn't have done it without the advice of Don Eilenberger and the very knowledgeable folks on the Roadfly e30 Forums.


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