1985 bmw 735i engine stalls once warmed up

[DenButkevych]

Hey everyone.
Have a problem. My e23 735 does not accelerate/runs rough under acceleration/stalls but only when the engine is hot. I was pretty sure it is fuel pump related so replaced both fuel pumps and filter but the problem is still there. Engine starts ok, runs and accelerates fine until normal temperature is reached. Once it's warmed up, it starts hiccup when pressing gas pedal, and then eventually stalls. Starts up fine again every time, and then dies again. Must have it cooled down to continue. Any guesses? I'm completely frustrated (

[bmw4dayz]

I'd say check your cold start injector. Mine does the opposite of what you are experiencing and I'm pretty sure it's because my cold start linjector is bad and doesn't add the correct amount of fuel. That's my best guess for now

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[DenButkevych]

I'm not even sure if I have the CSV, will check that tomorrow. Anyways, it doesn't seem to be an issue. As I said I've replaced both fuel pumps and fuel filter. Actually, first time it happened at traffic light, I've heard some whining noise from the rear, then it start hiccup. After a while, when I got home I figured out that the noise was coming from the external fuel pump, but not all the time. When it was cold, it was quiet. So I ended up replacing both pumps and filter. The filter was SUPER dirty, full of rust from the fuel tank (the tank itself was pretty clean though, previous owner had it cleaned, at least that's what I was told). I've never seen filters THAT dirty honestly. So I'm guessing if it could clog the injectors? But why then it runs fine when cold? If they were clogged the engine would run rough regardless temperature IMHO. Or may be don't I understand something?

[Hit Man X]

So you are throwing parts at it aside from a much needed fuel fuel filter...a CSV does not operate past a few seconds upon a stone cold start. 40C sounds right.

What else have you done to baseline this thing? Stalling while hot how...immediately die? Slowly fizzle out? Any fuel pressure when dies? How does the engine sound when dies?

More info provides more data. My guess at this point with incomplete data is that it is flooding with fuel, ergo a sensor is out of whack. CTS. O2. Who knows. Further diag required.

[DenButkevych]

Symptoms... It doesn't die immediately. It gradually getting worse. Starts with sputtering under load, then runs kinda ok if I release the accelerator. RPM won't go higher than 3500-4000, gets worse when going uphill, almost dies in drive but once it's in neutral it runs ok for a while, but then eventually stops running. Let it sit for a couple minutes and it starts right up, put it in drive - same story. Right before it dies it feels like it's running out of fuel even though the fuel tank is full. Put some Liqui Moly gas system cleaner BEFORE this issue appeared, not sure if it's related or not. I've bought this vehicle in March but barely drove it (literally couple of times). Replaced camshaft, rocker arms, head gasket, starter, timing and oil pump chains and tensioner and a lot more including transmission rebuilt. Finally got it finished, test drove it for about maybe 40-50 miles, everything seemed to be fine, and then this happened. I'm wondering if these symptoms are common for a faulty AFM? It might be either flooding with fuel or suffocating.

[Hit Man X]

Well, I would start by testing the CTS. Figure somewhere around 275ohm when hot and unsure cold, maybe 3000? Been a while since I tested. Ljet and Motronic sensors are different by color. I feel Motronic specs blue and Ljet is white.

Disconnect the O2 sensor, underhood is the green wire on passenger side. This could just be feeding junk info to the ECU to compensate.

Check fuel pressure, pop in a gauge and see. 3.0 bar is spec for your FPR, should be about 45psi. What is your idle vac and RPM? Should be around 850rpm and around 17" in park with no load.

There is a hose from ICV to the intake plenum, this could have a split. How about the rest of your small vac lines? AFM is the last thing to tweak unless has been dicked with before.

[DenButkevych]

Idle is about 850-950 rpm, fluctuates within 50-100 rpm when hot. All the vac lines were replaced with brand new, I don't think it is an issue. About temp sensor - what if I simply disconnect it? What are the default readings sent to DME? Hot or cold?

[DenButkevych]

And I have 3 temp sensors in the thermostat housing, at least they all look like temp sensors. So one of them is for cluster gauge, the other is for DME. What is the third one for?

[Hit Man X]

Test the CTS, no clue how the 1.0A will react unplugged. Been nearly a decade since I messed with an -059 car. The -013 turbo cars usually stall and die.

Two pins separate is for cluster. Big is for cold start. Smallest guy is CTS.

I pull CTS pop in freezer, check for high resistance. Put in hot water, should be low. It is a plain NTC sensor.

[DenButkevych]

So, yesterday I've tried to swap fuel pump relay and O2 sensor relay, with not so much hope. Drove it for about half an hour, not even a hiccup. Today I've replaced both relays with nice used Bosch relays from my parts car, drove it for a while, engine goes all the way up to the redline with no issues. I'll keep monitoring but so far it looks like the relay was the issue. One more thing, since I swapped the relay it idles at 950-1000 rpm, higher than before (about 900 rpm) for some reason.

[DenButkevych]

Keep driving it, no problems so far since I've replaced the relay.

[Hit Man X]

Good news there!