Intermittent Stumbling -- super lean

[Tom]

So my 951 just did a weird thing. It's been running fine recently, but I just took it to the post office and on the way back, I lost all power and the motor was stumbling and could barely hold an idle. I was able to get it off the road to a safe spot and restart it, but the motor barely ran and the wideband showed the car was super lean -- maxed out the gauge actually at 20-21 to 1. For some reason, the motor was getting nearly no fuel. While it was doing this, I could see the AFR and idle go hand-in-hand, with the motor running a smidge better at 20:1 than 21:1, etc. This lasted about 5 minutes until it just went away and the motor was back to normal. Nothing obvious under the hood, and rattling harnesses did nothing. I'm guessing it's an electrical glitch somewhere, who knows where. Not a fan of intermittent problems like this. Might be time for that Kroon harness sooner rather than later.

[Belgian951]

Yeah this does sound like an electrical issue. I would guess one of the sensors is on its last legs. My knowledge is too limited to identify the sensor, but maybe the AFM or the TPS?

[icb]

I would suspect something is up with the pump or the circuit. DME relay contacts, pump terminals, ground, broken wires entering a crimp connector, etc. If it was a clog I don't think it would just go away so easily and not come right back.

[Tom]

My hunch is that there's a bad harness connection somewhere. The symptoms were identical to having a disconnected AFM, so I'm thinking the AFM signal on DME pin 7 is disconnecting or shorting to ground somewhere. Could be low fuel pressure, but I have the f9 dme relay, heavy duty wiring to the pump, and a one year old AEM 044-clone pump. Never say never, but in contrast to all that, my harness has been endlessly hacked over the years for one project after another so seems like the more likely culprit. I'm going to add a wire to pin 7 of the DME so I can see if that's the source vs. the fuel delivery stuff. If the AFM signal looks ok when the car glitches, then I start looking at the fuel pressure/delivery. Of course, tracking it down will require the issue to come up again, and who knows when that might happen. In the meantime, I'm going to get a Kroon harness since I've wanted one since I first saw them anyway.

[cda951]

Hi Tom,

Intermittent drivability issues such as these on old cars are the bane of my existence at work. With an OBD-II car, one can drive the car while logging/graphing the appropriate channels using a modern scan tool with a decent sample rate, and can often deduce what the problem might be, or at least narrow it down to a few potential culprits.

I once installed one of the F9 DMEs with OBD capability in a customer 951, it was a very early version that the customer had on the shelf for years, and part of the reason for installing it was to chase down an intermittent lack of throttle response (which I think was eventually attributed to the TPS itself, can't remember). Driving with a laptop while monitoring live data was helpful---the early F9 DME had to be replaced due to stalling issues (the old BMW E30 trick of a boot to the passenger floor would bring it back to life). It was replaced immediately by F9 with the later version at no cost.

For most '80s Bosch DME cars, I use a breakout box similar to the one linked below, plugged in between the DME and the on-car connector. I use our older Snap-On Vantage oscilloscope, which has only two channels, but is portable and I can quickly switch leads between different DME pins to scope different channels. This is obviously not done while driving, but the car is driven while monitoring the suspected circuit until the symptom happens, and assess from there.

https://www.baumtools.com/shop/volvo-to ... in-w-case/

[Tom]

Hi Tom,

Intermittent drivability issues such as these on old cars are the bane of my existence at work. With an OBD-II car, one can drive the car while logging/graphing the appropriate channels using a modern scan tool with a decent sample rate, and can often deduce what the problem might be, or at least narrow it down to a few potential culprits.

I once installed one of the F9 DMEs with OBD capability in a customer 951, it was a very early version that the customer had on the shelf for years, and part of the reason for installing it was to chase down an intermittent lack of throttle response (which I think was eventually attributed to the TPS itself, can't remember). Driving with a laptop while monitoring live data was helpful---the early F9 DME had to be replaced due to stalling issues (the old BMW E30 trick of a boot to the passenger floor would bring it back to life). It was replaced immediately by F9 with the later version at no cost.

For most '80s Bosch DME cars, I use a breakout box similar to the one linked below, plugged in between the DME and the on-car connector. I use our older Snap-On Vantage oscilloscope, which has only two channels, but is portable and I can quickly switch leads between different DME pins to scope different channels. This is obviously not done while driving, but the car is driven while monitoring the suspected circuit until the symptom happens, and assess from there.

https://www.baumtools.com/shop/volvo-to ... in-w-case/

Super helpful, thanks! I've been in the process of making the car less of a hot-rod lately, so running some old parts like the AFM and FPR that I haven't used in a while, along with a different DME and KLR and chips. So logic would suggest it's one of those things, but equally likely it's something in my perma-hacked harness. I have a Zeitronix installed that can monitor TPS and a few other things, and I've tapped into the AFM signal at the DME to monitor it with a multimeter. It's possible there's something going on with the fuel deliver (pump, regulator, etc.), but my guess is it's in the harness or AFM. Hopefully it happens when I'm all hooked up and logging. I've mulled for years the idea of making a diagnostic break out board with LED's and read outs for all the important signals. Maybe this will motivate me to finally make that.

But I'm curious about the F9 OBDII DME. Is that fully developed and ready for prime time now?

[Thom]

A friend of mine with a similar hardware set up went through random engine cutouts which were traced down to a faulty KLR unit. I lent him my old one which seems to have cured the issue.

[Tom]

A friend of mine with a similar hardware set up went through random engine cutouts which were traced down to a faulty KLR unit. I lent him my old one which seems to have cured the issue.

Yes, a similar thing happened to Crazy Eddie, which turned out to be a cracked solder joint in the KLR. Once we found it, the fix was free, but finding a once-a-week electrical problem is an exercise in patience for sure.