Philes' Forum by Vic Lucariello

THANK YOU!

This month I have an interesting consultation to report on. [Those of you who read Philes' Forum simply to see how much I pick on Big Al Drugos can stop here, as I am giving him a one month respite. This in acknowledgement of his re-upping for a third term as our Chapter VP. Thanks, Al. Great Job!]

Before I begin, let me remind you that I still wish to receive copies of your vehicle inspection reports from the new, enhanced NJ DMV inspection I have written so much about this year. You can either mail a photocopy of your report or simply e-mail the test data. If you choose the latter, please include both the test results for your Bimmer and the pass/fail criteria for each pollutant. The criteria depend on vehicle model, year, and, I have been told, odometer reading. Also, please indicate what, if any modifications have been made to your motor [aftermarket chip or software download, etc.]. Thanks to Bimmerphiles John Dilzell, Hank Farber, Trip Lee, Larry Martz, Renata Melnitschenko, Karl Rentler, and Supermario Sousa for sending in their test results. The majority of the folks who have sent in test results so far have passed with no problems. One trend which may be emerging is that DMV may not be dyno testing some 5 and 7 Series Bimmers with automatic transmissions. These cars may be receiving the 2500 RPM idle emissions test instead. As I have previously reported, the idle tests do not check for oxides of nitrogen [NOx], the most difficult pollutant to control.

Erstwhile Chapter President, Treasurer, Lime Rock School Registrar, Summit School Event Chair & Registrar [Phew!] Fred Farber had an interesting problem with his E28 535i. The problem and its diagnosis apply to all electronically fuel-injected Bimmers [Indeed, to virtually all fuel-injected, gasoline vehicles on the road today]. Quoth Fred:

The car starts only with application of full throttle and will not idle or reasonably respond to modulation of the throttle pedal. Raw gas is pouring out of the exhaust system and I mean pouring. I am going to check the cold start system but somehow I think it can't pour enough gas to create this problem. Do you have any other suggestions?

There are two culprits likely to cause this great an excess of fuel: The fuel pressure regulator and the engine coolant temperature sensor. The pressure regulator, located under the hood at the end of the fuel rail on the intake manifold, is supposed to maintain a constant pressure drop across the fuel injectors. Provided that this pressure drop remains constant, the fuel injection computer can regulate the amount of fuel injected per 'squirt' by simply changing the amount of time the injectors are kept open. The regulator senses intake manifold pressure [which varies with engine load, RPM, and throttle opening] and maintains the fuel pressure to the injectors about 45 psi or so higher by returning excess fuel to the tank. The coolant temp. sensor, located in the engine water outlet , is immersed in the coolant leaving the engine. The sensor is really just a variable resistance device [Technoids: With a negative temperature coefficient, or NTC] which tells the computer when the engine is cold or warming up so additional fuel can be injected [IE: Injectors held open a bit longer] to compensate for the poor fuel vaporization in a cold engine.

To be sure, there can be other causes of excess fuel delivery. Some are the air flow meter, xygen sensor, cold-start system, leaking injectors, and the intake air temperature sensor. However, these items have a lesser effect on fuel delivery than the 'prime suspects' mentioned above.

My first response to Fred was that he check the coolant temperature sensor. This is done by comparing the sensor's resistance at known temperatures with published specs. This test is performed with the ignition key removed, of course. Fred reported that the sensor checked OK, so he moved on to the fuel pressure regulator. Another diagnostic hint is that a failed or failing temperature sensor may allow the car to start and run normally while cold, but then exhibit Fred's symptoms when it warms up.

Although the proper way to test the regulator is with a test gauge installed in the fuel supply line to the injectors, there is a quick test to do first. Simply remove the regulator's sensing hose. This hose runs from the regulator to the intake manifold. There should be no sign of gasoline in this hose. If there is [as it was in Fred's case], it is indicative of a ruptured diaphragm in the regulator allowing the intake vacuum to suck large quantities of extra fuel into the engine. If there is no evidence of fuel in the hose, reconnect the hose and start the engine [provided of course it will start and idle]. With the engine at or near idle, remove the hose and pinch the end closed. This should increase the fuel pressure and cause a noticeable difference in the engine. If not, the regulator is probably faulty. Incidentally, the sensing hose is the smallest hose connected to the regulator, the other[s] contain fuel, possibly under pressure, so be sure you get the right one! In any case, be sure you do all this testing in a well-ventilated area with your eye protecion on and your fire extinguisher handy. [Fire extinguisher? Yeah, the one you carry in your Bimmer at all times.] Quoth Fred:

I took the vacuum hose off of the fuel pressure regulator and gasoline came out. This seems like prima facie evidence of a failure to me and probably would result in way too much gas being sucked into the intake system. I am going to replace it as much as I hate to replace things as a diagnostic tool.

Running an engine with all that extra fuel, even for a short time, results in dilution of the oil in the crankcase. This is because some of the excess fuel gets by the piston rings, effectively reducing their seal. This allows even more fuel to sneak by. An oil and filter change should be done after the excess fuel problem has been resolved. The spark plugs should be checked/replaced as well.

After things were fixed, Fred was wondering why, with the bad fuel pressure regulator, his motor would only start with the throttle pedal fully depressed. The answer is that most electronic fuel injection systems have a so-called 'clear flood' mode which they execute when the throttle position sensor [Technoids: A switch on Fred's E28; a potentiometer on later models] indicates full throttle while the engine is cranking on the starter. Under these conditions, the fuel injectors are turned off and the ignition is maintained.

See you next month. Happy Thanksgiving, Everyone! [Yeah, Big Al, too!]

Anyone wishing to contribute to Philes' Forum should contact me via the info on the masthead. I'm interested in tech tips, repair /maintenance questions and/or tips, dealer horror stories, product evaluations, etc. Please call before 8 PM, and if you leave a message, PLEASE INCLUDE A PHONE NUMBER WHERE I CAN REACH YOU ON WEEKDAYS. Also, please be patient, I try to return every phone call, but it sometimes takes a while. If you don't hear back from me within a few days, please call again. The best way to contact me is via e-mail.

Copyright 2000 - V.M. Lucariello, PE