I have a 1990 740 GL. This is a non turbo car. It started giving me problems starting. If I got is started it ran fine. I have changed the fuel pump relay, pressure regulator, fuel filter, and the fuel pump. This is the single pump regina system. It seemed to work fine for a couple of days, now my has the car at college and had to have it towed back to campus. It would not start. Someone mentioned the ECT sensor. Is this difficult to change and how expensive is it. I am about ready to take to someone to diagnos before a spend more money. I had the relay and regulator, but had to buy the filter and fuel pump. Any other suggestions would be appreciated. You can email me at science@bytheshore.com or post.
Thanks
Lou

1) Does the car turnover fine when it isn't starting (sounds normal when turning, but not starting at all)
2) When it does start, does it sometimes cut out again after a few seconds?
3) Will it stay running only when you give it plenty of gas and RPMs?

If 3 is correct, then it might be your AMM (if there is one in a 740)

If 1 and/or 2 is correct, then you might want to check your connection at the ECU. In my 240, it is a black box just below the windshield wiper fluid resevoir. There will be a plug with a bunch of colored wires going into the ECU. Next time your car doesn't start, go and wiggle this plug and then try ot start the car. Try this several times.

After having my car towed several times and endless mechanics not finding the problem (of course, my car would magically start once off the tow truck), I finally found the problem at the ECU connection. However, I haven't found a fix for it (see my post) yet. But at least I can get the car going again when it does fail.

Because the car started on a shot of ether this is a long shot but it won't cost you anything...

The radio suppression relay on your car is mounted on the shock tower on the passengers's side. There are two identical black box relays about 1 x 1 x 3. Swap those two for each other and see if the car starts. If it does you can probably solve the problem by soldering both relays as John has suggested.

Randy

Thanks for all the suggestions. I am going to pick the car up next week and try all the suggestions and see if I can get it to go. Otherwise it will have to go to the professionals.

It will run with a bad Engine Coolant Temperature Sensor, but not well. Catastrophic failure of this sensor is unlikely. I would check the RPM sensor. The early ones were marked with yellow, and the updated ones with white. It could also be the Power Stage (ignition amplifier). Have you had any of the wiring harness connectors disconnected form the engine?

No nothing has been disconnected. This problem just started with no warning. Drove the car in the morning, came out in the afternoon and it would not start. Iwas able to start with a shot of ether, then it ran fine. But when turned off it would not start again. After changing the fuel pump it seemed ok, then acted up again. I put in another fuel pump relay and all seemed fine for a couple of days. Now back to where I started.

Does the tach needle bounce when you try to start the car? I don't trust used relays unless they have been re-soldered and then they are better than new relays.

I will check when I get the car home. Will have to trailer it in the next couple of days. When you resolder do you just reheat each of the contact points. Can you usually see the problem with the solder, because the one I took out looked fine.

You don't touch the contact points. To re-solder a relay, carefully pry the cap off and inspect the soldered connections on the printed circuit boards. If you have a hand lens, use that. Usually you can see bad solder joints on the Bosch relays. There will probably be cracks in the solder around the metal tabs protruding through the printed circuit board. Get a 25 to 35 watt soldering iron and some electronic solder. Tin the tip of the hot iron and then touch the tip of the hot iron to the joint to be re-soldered. When the solder melts, add just enough solder to provide a good joint. The local Volvo Master Tech has brough me a few relays to re-solder. I could find no visible defects with the overdrive relay from his own car, but it did not work. After re-soldering it worked fine.

The Radio Suppression Relay (which actually provides power to the fuel injectors) should also be done. I have not had trouble with one of those yet, but several brickboarders including Don Foster have reported trouble with these.

What error codes are you getting?
Look in the 700/900 FAQ's for instructions on retrieving and reading error codes

I don't have the car at home, and have not been able to check anything. I may go get it in a couple of days.