I have a 1991 740 that wont start after the engine is warm. If I add starting fluid or a few ounces if gas into the intake it will start. I've changed the fuel pressure regulator and fuel filter with no results. I was thinking it may be the fuel pump check valve. Was wondering if it is located with the external fuel pump? and If I'm on the right track? Thanks.

 

I'd check the ECT (Engine Coolant Temp Sensor). that tells the ICU and ECU how cold/warm the engine is, so it can use the right mixture. you can measure its resistance at the connectors to the ECU and ICU, there's a table in the 7xx FAQ saying what the resistance should be at various temperatures.

have you confirmed the fuel pumps come on when you turn the key (they should go off a second later) and then come on again when you crank the car? even without the check valve, it should still start, just maybe require 2-3 seconds of cranking instead of 1-2 seconds.

 

Thanks for the advice! I dont have a meter to check the resistances but was willing to change the sensor so, I looked and dont see the engine temp sensor. I can clearly see the water temp sensor ( to gauge ) and the knock sensor, but not any others. Where might I look for it?

 

the CTS is right in that row with the temp gauge sensor and knock sensors... it has 2 pins, one goes to the ECU and the other to the ICU.

any cheap digital multimeter should have a 'ohms' resistance setting.

 

I have seen pictures on the web and mine seems different layout. I will attach a picture of mine where i think it should be. and thanks again.

 

Here is a bigger picture. Sorry Im new to this site and posting.

 

I believe but can't vouch for it, that the engine coolant temp sensor ("ECT") is below and somewhat to the rear of the #3 intake manifold runner. its pretty crowded back there, what with the EGR stuff (if you have that) and the oil seperator and flame trap plumbing, never mind the idle air controller plumbing.

I've always tested mine by unplugging the ECU, which is in the right side of the passenger footwell, just forward of the lower door hinge, and using my digital multimeter to read its resistance directly. Doing this tests the wiring path which is as likely to be a problem as the sensor itself. I'll pull off the panels to get access to the ECU and ICU, then drive the car til its hot, park it back in my work area, and do the measurement immediately to get the hot measurement, then wait an hour for everything to get cold, and measure it again. if these are too far out of whack, THEN I'd consider pulling the sensor and testing it directly.

I do not like throwing new parts at problems, as a lot of aftermarket parts are WORSE than what came on the car, and even more likely to cause problems down the line.

 

Im thinking the ecu has at least 30 wires to it Im not sure how I would know which one to check. But thanks anyway for your time and advice.

 

Bosch LH 2.4, its pin 13. Hey, Regina has it on pin 13 too!

on the EZ116K or Rex-1 ICU's, its pin 2.


btw, the wires on the ECT are red-black (to ICU) and grey-white (to ECU)

 

OK, Thanks Pierce. Im tring not to get frustrated. I will get my hands on a meter and check it out tomarrow. I may need your expert help again. Thanks for your patience. Know you are appreciated

 

did I mention the pin numbers are embossed on the inside of the plug at the ICU/ECU sides? i needed a small bright LED flashlight to read them