Engine Management and Driveability

This section contains special information on engine management and driveability that is not found in the Service or the Bentley manuals.

All information contained in this FAQ is provided by BMW enthusiasts who are not typically fully trained in the art of BMW maintenance. As such, all information in this FAQ is provided "as-is". Any use of this information is strictly the responsibility of the using party. The supplier of the information and the Webmeister assume no liability for incorrect information or use of this information.


Warm Starting Problem

Ignition Upgrade for Idle Improvements

Idle Improvement - Engine Tuning

SRS Diagnostics

Idle Control Valve Cleaning

Warm Starting Problem - Douglas Wyman <dwyman@houston.geoquest.slb.com>

My '84 E23 acted like this:

- - when cold it started on the first turn

- - when hot it would start immediately after being shut off but....

- - if it had soaked for 1/2 hour or more when hot it would require 4 to 8 seconds of cranking... but would always start, eventually


The check valve in the main fuel pump was leaking allowing the fuel injector rail residual pressure to bleed off resulting in formation of a huge fuel vapour bubble in the fuel rail and back down the fuel line toward the pump. The extra cranking time was needed by the fuel pump to recompress and eliminate the fuel vapour bubble in the line and repressurize the fuel injector rail.

I could have replaced the fuel pump for $200 but I fixed the problem by putting a $10 brass check valve (3/8") in the fuel line near the pump. I bought the check valve from a Dayton Electric outlet. I can get the part number if you're interested. I also bought a pair of small brass hose barbs, some teflon joint tape and two small stainless hose clamps to complete the installation. Total cost was about $15 and 1/2 hour of my labor.

Now the engine starts on first turn, hot or cold, overnight or after standing for a while ....ie it always starts easily and immediately, even after soaking in Houston sunshine while hot.

Ignition Upgrade for Idle Improvements

Ignition Upgrade for Idle Improvements - "Gene M." <MClan@worldnet.att.net>

Your 1982-1984 U.S. model 633 (also applies to 533 and 733) may have had an upgrade performed by the dealer to address idle problems. Technical Service Bulletin TSB 13 07 86 (1162) discuss these changes.
The major components of the upgrade were to:

(1) Replace the ignition system (coil, cap, rotor, wires) with the 30 kv components (the type in the 635) and adding a rotor adaptor;

(2) Install a 35 degree C thermotime switch;

(3) Replace the idle control unit with the green unit (or may be a black unit with green tape);

(4) Install a purge control module;

(5) install an external resistor in the wire for the temperature sensor that feeds the Motronics (should be the one with the light grey connector) Resistor type is 270 ohm, 1/2 watt, "Gold Band" metal film resistor for Motronics Control Units with a Bosch code date of 342 or higher (production date after 9/83) and for Motronics 341 and lower (9/81-9/83 production dates), it is a 560 ohm, 1/2 watt "Gold Band" metal film resistor .

This upgrade was performed at no charge, but may not have been done on your vehicle. If you have the slip on rotor instead of the bolt on type, the full upgrade was not done, but you may still have the external resistor installed under TSB 11 11 84 (920), which was only added to address a complaint of "hesitation/poor throttle response" during several minutes of operation after starting a cold or warm engine at an ambient temp above 75 degrees F.
A few things on these model years because of these changes.

First, when replacing ignition components, make sure you are getting the correct coil, cap, rotor and wires, depending on whether your car has the upgraded components. If the upgrade was not done, a rotor adaptor (PN 11 31 1 718 761) is still available if you choose to upgrade to the 30 kv components.

Second, if your car is not running right and you have questions, make sure to specifiy whether your car has had the upgrades.

Third, the external resistor may be the source of a "no run" condition, if the solder connection breaks from handling the temperature sensor connector. This resistor is usually soldered just behind the connector and may be concealed under the rubber boot. A broken or bad solder connection will mean that your car will not start or run, and this could be an intermittent problem because of engine vibration. In an emergency, you can disconnect a wire to the short 2 prong temperature sensor that feeds the idle control module, and the car will run, but the idle will surge hard. It may be better to carry a short wire with small insulated alligator clips on both ends in your tool kit to do a roadside repair.


Idle Improvement - Engine Tuning - Don Eilenberger <deilenberger@monmouth.com>

1. Throw out the Bosch Platinum plugs - they're basically not compatible with this car. Spend about $4/each for the Silver ones that are ESPECIALLY made for just this engine (check a BOSCH cross-reference book - guess what? The only engine using these plugs is the big 6!).

2. Set them to 0.032" minimum gap. Even to 0.035-38" won't hurt, the ignition system can easily handle it.

3. Install and TORQUE them in with no anti-seeze. 20 fl/lbs is about right.

4. Adjust valves to minimum of +0.002 over spec. +0.004 isn't even noisy and will help even more.

5. Run mega-dose of techron full strength stuff (not the watered down k-mart stuff) [mega-dose = 2 16oz can's in the last 1/4 tank. Run until almost empty. Fill up. Change oil!!).

6. Run the heck out of it. It will like it.

Problems with the '86-88 big six engine idle is related to exceeding (vastly) the EPA emissions standards. The things run VERY lean at idle (CO is almost unmeasureable at the tailpipe). None of them idle great - but the above can make it tolerable - and not embarrassing at a stop light (car rocking and rolling). The biggest change is made by opening up the valve adjustment, followed by the correct plugs and a wide gap. You'll notice as these open up with normal wear, the car runs better and better - I'm often disappointed after I tune BOHICA up, 'cause it runs WORSE - and end up resetting the new plugs to the gap the ones that came out were at - and doing the valves again. I'm currently running the valves at 0.014 +.001 - .000 - this seems to work well, and with BOHICA gonna reach 100,000 miles TOMORROW - the cam and lifters look JUST LIKE NEW - the car uses no oil, and the compression is excellent, and very even. It won't hurt the engine to run these wider - and will greatly improve the idle.

What do I set the valve clearance (lash) to on my '87 535i? The answer is 'usually 0.02" over specs' - which are 0.012" cold. So the answer is 0.014" clearance.

The reason - it helps the crappy idle these engines are afflicted with due to BMW's attempts to meet emission regs from around 1985-88 (they actually FAR exceeded them.. there is NO measureable CO at my tailpipe - have to go back to the exhaust manifold to see a reading).

If the car IS tuned to specs - it idles very poorly and tends to intermittently stall. Not all of this may be due to the valve clearance and plug gap - but using the wider settings sure helps.

Don Eilenberger says: What do I set the plug gap to on my '87 535i? I also set the plug gaps oversize - same reason again - in this case, spec is 0.028" - I typically use 0.032" If the car IS tuned to specs - it idles very poorly and tends to intermittently stall. Not all of this may be due to the valve clearance and plug gap - but using the wider settings sure helps.


Idle Control Valve Cleaning - Rick Sparks <sparksr@juno.com>

Have had a couple of requests to add this procedure to the FAQ, so herewith is what I did to clean the idle control valve on my '86 to solve idle surging from normal to 1500rpm on warm-engine startup.

The valve on my '86 is a round metallic cylinder about 1" long toward the back center of the top of the engine. The valve plugs into about the center of the black fuel injection boot, firewall side, and in turn has a black Bosch electronic unit plugged into it. Touch the valve when the ignition is on without starting the engine, and you should feel it humming.

1. To remove the valve, loosen the hose clamp under the valve that attaches it to a small rubber hose.
2. Remove the little nut that attaches the valve's metal support to the engine valve cover.
3. Unplug the black Bosch unit from the rear of the valve by squeezing the Bosch unit's wire lock together with your thumb & forefinger.
4. Remove the valve with the support attached, and then remove the support by squeezing in on one of the two little rubber plugs attaching the valve to the support.
5. Spray carb cleaner or penetrant liberally into one of the valve's two inlets, and it will run out the other. Spray into both inlets, and then let the spray all run out before reinstalling the valve. It may take a few minutes running the engine for the valve to fully loosen so it won't surge, but either carb cleaner or a good penitrant (not WD40 - one I like is Popular Mechanics "All Purpose Lubricant" available at Walmart) should do the job.
6. Reattach valve.

This procedure might prevent or postpone having to buy a new valve. It worked for me. Good luck.