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740 Stalled - Wont Start

colgate41

colgate41

Active member
Joined
Jul 31, 2008
Location
New York
Car in question is a 1991 744 Turbo with 185K & stock.

I haven't been driving the car so I went out today to start it up and let it run for a bit and then I was going to take it around town.

Car started right up and idled smooth enough. Gave it some gas and everything sounded fine and the car "felt" fine. I let it idle and run for about 10 minutes and then it just died with no warning. Since then I've tried to get it to start but have had no luck. It was a humid day today, about 85 and sunny.

In doing some research on the forums and the rest of the internet, I've got some ideas but can't be sure just yet.

Fuel pump relay works and pump kicks on when i turn the key.

The radio suppression relay also seems to kick on when I turn the key. I was able to get a finger on it and turn the key at the same time.

The crank position sensor was replaced a few years ago (over 5) and it looks in ok shape. I thought it might be that due to the car not starting when hot but I left for the day and when I came back in the evening when everything had cooled down I still can't get the car to start.

The car cranks and cranks but just won't even try to turn over.

Next up will be to check spark at the plugs.

After some cranking I do smell some gas at the tailpipe.

My tach doesn't move when I try to start (not sure if that makes any difference)

Any help or ideas would be great. I did order a new crank position sensor just incase. I figure I can install the new part and if it still won't start I'll just return it.
 
i Tested for spark. I pulled the second plug and tried to arc it on some metal in the engine bay and I saw nothing.

I?m going to pull and inspect the ignition module tomorrow. Maybe the contacts are in bad shape or maybe it just failed.
 
Assuming I have no spark. Like I said, I did test it yesterday but it was in a rush. We have a newborn and had to run. Once my wife is able i'm going to test for spark again but I'm pretty sure there is none.

I had a extra coil that I swapped in. Nothing.

I pulled the ignition module and the contacts looked good but I blew them off and cleaned what I could. Re-installed and still nothing.

Could the crank position sensor really give the problem of no spark/start?

That new part should be delivered today just so I can test it.

I also ordred a new ignition module to test out. If these new parts don't work, I'll just return them.
 
Do diag.

Check the signal into the powerstage. A LED test light will work. It's a 5v square wave. I wanna develop the specs on what should be measured with a meter.

Then confirm output of powerstage by test light to coil negative to ground. Should blink.

No spark on these will likely end up being powerstage or crank sensor.
 
Timing belt wouldn't cause a no spark on a 1991 740 if tested from the king lead (wire from coil)
 
ZVOLV said:
Timing belt wouldn't cause a no spark on a 1991 740 if tested from the king lead (wire from coil)
Click to expand...

Right. I know these are non-interference motors but I'd expect a little more commotion if a timing belt snapped.

Anyway, timing belt is good.

Coil seemed good and when a working one was swapped in there was still no start.

Ordered new crank position sensor but I replaced that a few years ago so I really didn't think it was that. Never got to installing the new one.

Ordered a Bosch (now made in Taiwan, wtf?) ignition module and swapped that in.

Went to start the car and it stumbled for a second but then started right up!

I let it run for about 10 min so it could get up to temp and it ran smooth (as smooth as it can run anyway). Shut the car off. Swapped in old module and no start. So I'm fairly confident that was my problem. The old one may be original to the car, but who knows. It did look old.

Anyway, thanks for the tips and I hope this thread can help someone in the future.

I'm very much in the camp of trying not to throw parts at problems because in the end, you want to get to the root of it. Also, wasting money sucks. But for certain parts like sensors and electronics, I really don't mind for the simple fact that you can just return the parts if that's not the problem.

:)
 
Glad you got it fixed.

I had similar symptoms with the powerstage being the culprit.

Did you apply thermal paste between the module and the heat sink? Under normal operation they generate some heat and need to have a good connection to the heat sink to prevent the module from overheating.
 
oldschoolvolvo said:
Glad you got it fixed.

I had similar symptoms with the powerstage being the culprit.

Did you apply thermal paste between the module and the heat sink? Under normal operation they generate some heat and need to have a good connection to the heat sink to prevent the module from overheating.
Click to expand...

Did not but will do!
 
Hey, my 1990 240 lost spark and I made a very educated guess that it was the powerstage and was right. I tossed one in on a 10 minute break and it fired right up. It's about speed and convenience sometimes. I work on cars where sometimes I can guess the problem correctly before it even makes it into the work bay.
 
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