Ambient Temp Stalling Issue
 

Chasing this issue for 2 years now with my 190e. When the car is in closed loop and the ambient temp is low-70s and below the car will randomly stall when braking. Honestly it does not happen mid-70s and above which is super weird to me and makes it really hard to track down, can only test in the fall and winter. I can't really reproduce it either, just completely random.

Anyone have any ideas, IACV has been replaced?

Interesting issue. For a car to stall you need to either lose fuel or spark or both.

What I would do:

Wire up one volt meter to the fuel pump and the other one to the ignition coil and put them in the interior. When the stall happens just observe what they say.

Then I would start diagnosing further

Also - Year? 2.3? 2.6?

Sounds like a manifold vacuum leak. Cold weather exacerbates vacuum leaks and CIS systems are very sensitive to them.

I doubt it. That would show up as a hard start/rough idle

His car stalls when braking and when the engine is warm. More like an intermittent bad contact.

Doubt all you want, I went through exactly this behavior with a manifold vacuum leak on a BMW. When the engine is cold, it runs open loop (rich). When it warms up, the ECU tries to control fuel trims. The EHA has a limited degree that it can control the fueling to override the MAF plate on the fuel distributor, if there's enough of a vacuum leak, that can be enough to make the engine run lean enough when coasting or braking to stall. Cold air (especially if damp) makes the problem worse since it is denser than hot, dry air.

89 2.6

I have had an intermittent hard start the whole time as well, hot and cold weather, always starts on the second turn, but hard starts seem to be the norm on these. No idle issues though.

I've been meaning to try a smoke test regardless so maybe I'll start there. I also haven't tested the fuel pressures which is also on my list.

I haven't had a chance to do a smoke test yet, but I had another stall today which is an oddity. Backing into the garage, using the brake to regulate speed annnnnd stall. Would a vacuum leak still be a likely candidate here?

Consider an investigation of the vacuum line to the brake booster, the check valve in that line, and the control valve in the booster.

Booster line seems to be in good condition, definitely not original, nothing obvious that I can see.

That said, this only happens once the car is warmed up and in sub-mid 70s temps. That engine is HOT by the time it hits operating temp. What possible effect could the cold air outside the car have on the vacuum lines. Even if there were leaks, the engine bay temp is the same in August as it is in December when the cars warmed up.

I am going with the vacuum leak theory, and there may be associated other faults as well such as brake booster leak, O2 sensor, air temp sensor. The brake booster "consumes" vacuum when you press the brakes. Just as a test, I would try pressing the brakes multiple times (when you are standing still, not driving) when the engine is warm (and running obviously) on a "warm" day. See if you can recreate the stall without the air being "cold", just by using the brakes more. It might provide a clue. Or maybe not.

The air flow potentiometer can cause stalling when braking to a stop. Check the voltage on it as deflecting the plate. An open spot near the zero position will do it.

Would the dead spot cause an issue whenever idling, not just when braking?

I’ve never been able to replicate it consistently. Has never done it while idling. It has happened when when parking at a high steering angle, not sure if that’s coincidence or the extra load on the power steering has caused an increased chance of it happening, however it’s happened with no load on the power steering as well.

Pouring down rain today so I’ll do some potentiometer testing tomorrow.

Only when braking. It actually happens coming out of decel, when the ecu is looking at the potentiometer voltage to set the idle valve position. If potentiometer voltage drops below the lower threshold (somewhere around .4-.5V IIRC) it assumes a stalled engine and doesn't even try to catch and idle it.

When checking the potentiometer voltage, its this lower voltage range that must be smooth and linear. A glitch at 2V isn't going to cause this problem, but if it goes open somewhere around .5V-1V you've found your smoking gun

That sounds really likely, any idea if it would happen in cool/cold weather only?

Since its basically an air mass metering device, colder air could put it in a slightly different range than warm air

What testing procedure should I do? I've found about 6 different ways to test this thing.

Neg on PIN1 and Pos on PIN3 gave me a steady 4.125amps.

Pos on PIN1 and Neg on PIN2 was all over the place with a very obvious dead spot about 10% into air flow meter, could that be my issue? Sometimes at rest it was at 2.XX amps and sometimes 20+ amps.

You need to check voltage on the signal pin, pin2 IIRC. Key on not running slowly depress the plate on the flow meter. Voltage should rise steadily, if you have a dead spot around 10% thats your problem.

So I went digging in my spare parts bin and came up with a spare. Went ahead and swapped it in and the readings are much more linear and much less erratic. I think I need to adjust it a bit, but I'll do that measurement in the morning and see what I get.

0.3v at close, that should be 0.7v right, so I need some adjustment. That said, voltage rose steadily to around 5v in full open with no dead spots or erratic readings.

On a hot engine, no accessories on, I set it .7-.75 at idle.

How do you set it at idle? I have to remove the FPR line in order to get to the screws, can I set it with engine off ignition on?

Better to set it with it running as that's when it needs to read correct. Get it loose where it can be adjusted, then reassemble enough to run, lock down the screws that you can, then disassemble enough to finalize. Its a pain, but it gets a proper adjustment.

Welp, cars doing exactly what it did before but now its every time I touch the brakes. So I think I have my culprit. Decided if I'm going to adjust it correctly I'm going to order the Bosch unit instead of the cheapo chinese one I have.

You have a Chinese airflow sensor?

The spare I had was one of the cheap made in China ones.

Bosch unit came today and got it installed correctly. Adjusted to 7.27v idling, after tightening and rerunning it I was at 7.5v which I know is in spec and I'm leaving it.

That said, the top two bolts wouldnt torque down fully which is a bit concerning. They were fine the last time but at the wrong adjustment. I pressed on the potentiometer when idling after getting them all as tight as I could and the value didnt change so its tight enough I guess.