Vehicle: 1988 740 GLE Turbo

Hi all. I've searched for days at this forum and others for a possible solution to my issue. My issue is very similar to many, many others I've found, but with a few strange test results that are leaving me baffled.

The other day I was out and my car stalled at a red light, and I could not restart the car. I pushed it to the side of the road and about 10 minutes later I was able to restart it. Then the car stalled again after idling perfectly for about a minute. The car idles perfectly, and starts instantly, when it does start. However, once it starts, it will only idle for about a minute or two at most before it dies. This problem occurs without fail whether the engine is hot or cold. However, as a test, once when I was able to start it I kept it pegged at ~2000 RPM and the car ran just fine for 7 or 8 minutes at which point I let it go back down to idle, at which point it reliably stalled within a minute or two.

I have replaced the fuel pump/injector relay, and have even verified visually that both relays in the switch are engaging when I turn on the car and try to crank the vehicle. I've jumpered the relay connectors with the car off to make sure I hear a healthy whir/whine from both the main pump and the in-tank pump. I also removed the fuel line at the rail and put it in a bottle and cranked to make sure I was getting fuel at the rail. There is a healthy-looking jet of fuel coming out of the fuel line at the rail. I checked the fuel pressure regulator vacuum line for fuel... nothing and seems okay. With ignition on, I checked all injectors for ~12V... check. After the car has stalled I have cracked the fuel line at the rail and sure enough a spurt of pressure/gas comes out indicating to me that there is pressure in the line, even after the car is off. In short, it sort of seems to not be a fuel issue.

My immediate thoughts were then to the ignition control module or the Hall sensor in the distributor. I did a few tests I found online for the ICM and it *seems* to be operating normally. All the pins test out as expected, and a pulse is showing up at the coil, as evidenced by a test light. I also measured the voltage on the low volt side of the coil while cranking (can't remember how now, and it was within specs) and this further led me to think the ICM was working okay. I then thought: this must be the Hall sensor. However, during a period when the car was cranking but not starting, I removed the coil wire from the distributor cap, set it close to a good ground (strut bolt) and found that there was a healthy looking spark there (well, it wasn't blue, but it was a fairly good looking orange spark). My understanding is that if the Hall sensor is bad, then you won't get spark at all. So I plugged the coil cable back into the distributor cap and cranked... still no start. I unplugged it and checked for a spark, and sure enough there was still a spark.

So, this all leaves me very confused as to whether the issue is a fuel or ignition issue. For good measure, I also removed the throttle body and cleaned it thoroughly. I've checked around for obvious vacuum leaks and can find none. I've tried umplugging the mass air flow sensor to try to get the car into "limp" mode, as I've read about, but unplugging does not help the no-start condition. I'm not inclined to think it is the MAF or the IAC or a dirty throttle body, since the car starts instantly and idles perfectly, until it dies.

Does anyone have any suggestions of further things to test, or something I may have overlooked.

Thanks!

Nathan

 

Part 4 -1988 Volvo 740 No Start Diagnostic Case Study.

 

Thanks for the link. I had come across that case study before while looking for possible solutions to my issue. However, I had discounted it, since I thought I was getting a spark from the cable to the distributor. But being confounded by the problem, I went and bought a proper spark tester, as well as a fuel injector test light for a Bosch PFI system. With those in hand, I went back through that case study from page one, running all the tests he ran, in the same order. As it turns out, my results were the same as his. I bought a new ignition control module (for about $95 at Advance Auto Parts), installed it and the problem is solved.

This really demonstrates the hard thing about intermittent issues. I'm sure that what happened to me is that I had the car running, it stalled, I tried to crank it and it wouldn't start, I disconnected the cable from the coil at the distributor and set it close to a ground, cranked and saw a spark, plugged the cable back in and cranked and still got no start. Based on that I assumed/guessed that I was getting a spark, and that the problem must lie elsewhere. Unfortunately, what probably happened was that the ICM randomly didn't work when it stalled and when I tried to start the car, but sadly did randomly work when I happened to disconnect the coil wire and check for a spark.

In any case, I'm responding here for the benefit of anyone else who might run into a similar issue and find this thread somehow.

 

Oh, I should note one other thing. In the case study diagnostics he mentions using either an oscilloscope or a multimeter capable of measuring hertz. I had neither. Based on something I found from another document (http://www.stepbystepvolvo.com/Resou...20No-Start.pdf), I decided to try to test for the proper switching signal at the ICM by using the A/C volts setting on my multimeter. I attached the red lead from my multimeter to the back of pin 5 on the ICM (with the ICM plugged in), and then putting the black lead of my multimeter on the ground terminal of the battery. With the car running (during the few minutes it would run) I noted a voltage reading of about 2.5.V. Assuming this was the correct reading with the car functioning, I did the same test while the car was in a no-start condition. When cranking the engine my multimeter read about 2.4V. I took this to mean that the switching signal was arriving properly, since the reading was essentially the same as when the car was running normally. Whether this assumption is true or not or has any validity, I'm not sure. But I wanted to mention it here in case it does have some validity and is useful to anyone else.

 

re those idle valve measurements... one side of the idle valve, the Blue-Yellow wire (at least on the 1992 who's schematics I'm looking at) is wired to car power via the fuel injection main relay. the OTHER side (red-black on the 1992) is intermittently grounded by the ECU to pulse the idle air controller.

I don't have prints on a 1988 740 so I can't confirm the wire colors are identical.




edit, ooops, wait. read your post again, by ICM, do you mean the ignition power module? ignore my blather above, then.

 

Cool you figured it out. Your 1 of maybe a few people on here that can actually work on cars and contribute information to the site I've seen thus far.

I came accross this link and it helped me too, but my distributor/crank sensor is bad, just got a new dizzy from ipd, slapping it in as soon as I get a day off. My Igniter/ICM/power stage has already been replaced by the previous own.

I learn more working on my own cars. Time to think and do proper diag.

PS: Intermittent problems are a nightmare, as well as noise complaints