While driving a few weeks ago, my 1985 928S 5.0 died at the carwash, before it even got wet, cranked but wouldn't start. Finally had a chance to start troubleshooting today, so this is my method-

Hooked up a timing light to confirm spark - checked several leads from each distributor to be sure, and all were fine, even as it cranked and wouldn't try to start.

When leaving the car for a while it would attempt to catch. If you keep at it, it wouldn't attempt anymore. My first thought was clogged fuel filter, but I did get a rich fuel smell. So I could get it to catch after sitting, and if I pulled Fuse #42 to the fuel pump, I could get it to try to catch again, but never start.

Pulled the plastic intake off and checked the fuel pressure regulator and the other two dampers... the diaphragms all held vacuum, but I discovered a rotted vacuum line coming off the MAF, so I replaced the three remaining original lines there with new vacuum hose. Still no fire.

Finally I went and disconnected the three sensors across the front of the engine, and after that, it would ALWAYS start and then immediately die.

Temp 2 is the only one I recognize, because it has the injector plug looking fitment, I believe I've replaced it within the last couple thousand miles and it looks brand new still. The other two look to be original but I need help identifying.

They are all three within a few inches of each other, temp two, then two to the right. One I think goes into the intake, the other appears to go into the water pump bridge. Both have two small spade connectors attached.

Any ideas to troubleshoot, identify, source these parts?

Thank you!

 

Time to locate and scrub/replace all of your grounds, primarily the grounds for your computer.

When you say it catches, I think you mean the engine lights off a few times from fuel being sprayed by the cold start injector, which runs off battery power. It sounds like your injectors are not being commanded to open/close. This is a computer problem, and is likely caused by a bad/broken/loose ground terminal, which may be located on your passenger-side valve cover, outboard side, above the letters "O" or "H". The one above the "O"' may be concealed by a valve located above it. That's where my trouble was. Screw backed out and I lost the very most important ground in the car (besides batt ground).

I decided to relocate that ground to an accessible and viewable spot, plus added other grounds. I even added one to my distributor, which honestly seems to have helped smooth out the engine vibrations.

So, verify/scrub/replace your grounds, then move forward.

My new mantra is "Verify all grounds before doing any troubleshooting".

 

There are no Vacuum lines going to the MAF... Only a El. connector which you have to verify...........

While you are there, verify the connector to the CPS which is right below the MAF, sort of.

Another idea could be to jump your LH / EZK / Fuel pump relay to see if it starts then.

Check grounds like advised above, however be aware that the locations he is refering to are for a 16v motor, not a 32v like you have.

 

I had this issue and it ended up being the Fuel Pressure Regulator.

 

Check the Temp II and connector. Then check the MAF connector (under the boot on both) and see if you have broken wires.

 

I'm suspecting Temp II, but the connectors look fine. The MAF is either new or replaced less than 5k miles ago. Any chance it's one of the other two sensors up there by Temp II?

 

Did you pull the boots back and look at the wires inside?

 

It seems the noid light is a tool that every 928 owner should have in his tool box. Use it to see if your injectors are being commanded open via the pulsing light bulb. Will tell you so much about what's going on, and in no time at all.

 

A 928 is part ventriloquist

Mechanically or electrically, they trick us into thinking its one thing, when its another.

I'd be looking back under that air box at what Sean suggested and a few more things, mainly grounds at valley base and cps.

Noid light helps a lot, too.

But that said, our 85 and 86 cars have occasionally flooded. Needed to put foot on floor an crank for a long time before they caught and cleared. Never really figured out why.

 

Intermittent failure of fuel pressure regulator diaphragm allowing loads of raw fuel into the intake randomly?

 

Check the Temp II resistance at the LH connector, pins 2 and 25 - (EZF_LH22_32V.pdf)

Could be a bad sensor (even if new), or a broken wire.

 

Sorry it's taken so long to get back - we're in the middle of trying to move houses and I spent a lot of my free time on the roof painting and replacing the trim around my chimney. (I'm not so thrilled with heights, but I'm not going to spend $600 instead of $50 just because it makes me a little nervous).

2 to 25 on the LH measures 1088-1100 ohms. pin 2 to the mounting stud on the computers is 00.0-00.1 ohms to ground, so it doesn't seem as though engine grounding the culprit. (I cleaned and sealed them just a few thousand miles ago, but it's been over a year, too.)

 

For reference, I imposed on Roger, and the other two sensors are the fan switch and the sender for the temperature gauge, so they don't feed the computers at all, just in case the information is useful to anyone else in the future.

 

Maybe, but let me clarify - if I don't disconnect Temp 2, it never catches at all no matter how long it sits. From this description it sounds like if this were the case, if I left the key on for a minute, I should be able to get it to try to start when cranking - then if I leave the key on again, should be able to do it again. That's not the case here.

 

Disconnecting MAF has no effect, and pins 3 and 4 to 6 on MAF come in at 386 ohms.
All the grounds on the LH connector are at or below 00.2 ohms

But I did discover I'm not seeing any voltage on pin 9 with the key on, which should be 12V hot from the relay. Gonna go swap those out now and see if that sorts it.