Hard Warm Start

[528i-1981]

Had a very hard warm start situation. Replaced the ignition trigger coil in the dizzy. Runs a bit nicer especially at higher RPM, so I thought I was done.

Car starts fine cold. Runs fine. Still a hard start when warm. Verified cold start valve operation and replaced thermo-time switch. Still a hard warm start, disconnected cold start valve and eventually restarted after a few minutes. I smelled gas. Noticed a little stumbling and roughness while driving home but only once, mostly running fine.

After it failed to start, I removed the no. 6 plug using the trunk tool kit and it was dry.

Fuel pressure at the rail normal.

I'm going to start tracing everything and measuring all the electricals, but if anything jumps out please let me know. I need to attempt a warm start with the cold-start valve disconnected immediately to see if that helps.

[tlake]

Had a very hard warm start situation. Replaced the ignition trigger coil in the dizzy. Runs a bit nicer especially at higher RPM, so I thought I was done.

Car starts fine cold. Runs fine. Still a hard start when warm. Verified cold start valve operation and replaced thermo-time switch. Still a hard warm start, disconnected cold start valve and eventually restarted after a few minutes. I smelled gas. Noticed a little stumbling and roughness while driving home but only once, mostly running fine.

After it failed to start, I removed the no. 6 plug using the trunk tool kit and it was dry.

Fuel pressure at the rail normal.

I'm going to start tracing everything and measuring all the electricals, but if anything jumps out please let me know. I need to attempt a warm start with the cold-start valve disconnected immediately to see if that helps.

Perhaps a bad cat converter oxygen sensor, making it go lean (hence the dry spark plug) I wonder if all the spark plugs are dry

[528i-1981]

O2 sensor checks OK, but you may be on the right track. Coolant temp sensor is out of spec, returning low resistances (which would lean out the mixture). I've ordered one and will report back.

Residual fuel pressure at the fuel rail held up well, and there's good vacuum at the FPR and a climb in rail pressure when vacuum is removed. The fuel system is evidently fine.

[528i-1981]

I've replaced the engine temp sensor. Warm starts have improved (i.e., it starts) but still requires some cranking - maybe 5 seconds. Not terrible, but not perfect. I tried "key priming" (turning the engine switch to ON several times for a couple of seconds without cranking to get the fuel system primed and no difference. This isn't surprising based on the previous good fuel-rail diagnostics. I am currently experimenting with "feathering" the accelerator to about one-half while cranking and also trying WOT. I've read conflicting things about what the ECM might do during warm starts based on throttle postion, so I'm just going to experiment.

[Robert Bondi]

Did you ever try your warm start with the cold start injector unplugged? I'm also wondering if you've determined whether the issue is actually a lean problem or a flooding situation?

A few other thoughts on your problem:

1. For fuel priming in a troubleshoot situation, I like to jump AFM pins 36 and 39 using a small segment of solder. You'll run the pump with the key in ON. This is a safety bypass, so don't leave it that way forever.
2. Related to the cold start injector, I was wondering if you might have issues with the thermotime switch. Its purpose is cold starting, but if it's failing in the direction to give more gas than it should, that could be relevant.
3. I frequently see a warm start issue, but the situation is probably more extreme. My scenario is a spirited drive up a mountain grade that includes an abrupt pull-off stop....something like 10000 ft elevation. My 530i often struggles with that start about 10-15 minutes after the stop. In this case, I think the situation is tilting toward vapor lock....this isn't a carburetor, but the situation of hot fuel, low ambient pressure, and I don't have a pre-pump in the 530i all point in that direction. You might check your pre pump in the 528i. Under normal driving, most of the fuel just keeps circulating between tank and engine. Shutting off the hot engine cooks all the coolant and fuel sitting in the engine as the heat radiates away from the cylinders. I often see my temp gauge drop a full quarter when the engine starts....that's just because you've started circulating the fluids again. I try to counter this hot start situation by letting the car idle a few minutes before the shutoff. I'm not usually in troubleshoot mode on a long road trip, but this is another situation where I often wish I could run the fuel pump like (1) to avoid flirting with vapor lock.


Robert

[528i-1981]

Thanks Robert. So, yes on the warm start with the cold start valve unplugged. It behaves as it now does with the cold start valve plugged in, I.e., about 5-7 seconds of cranking. As luck would have it I replaced the in-tank pump a few months ago when I replaced the fuel tank. The thermo-time switch is brand new (it was my first suspect and i was too impatient to test it fully). The fuel lines are also all new and there are no leaks.

I really don’t know if it’s lean or flooded at this point but I agree that’s a critical piece of data. The time I smelled gas I was cranking a lot, but also had one dry plug. Some fuller testing is in order.

I never knew about all this fuel vapor lock stuff until this situation. The E12 always has something to teach.

[528i-1981]

I did confirm proper cold-start valve operation using an LED. The problem appears to be an undercharged battery. My trips in the E12 are all short and sporadic, with sometimes weeks between. My battery today was showing 12V on a voltmeter, but my smartcharger said 80%. After it was topped, I took a drive to warm up the engine and the hard warm start disappeared. It's possible the ECU was voltage starved and grumpy.

I'll keep it on a trickle charger moving forward and see if the problem reappears.

I always knew cold starts were affected by a weak battery, and cold starts draw more current. But a weak battery might be causing my problems during a lean start.

[Robert Bondi]

I did confirm proper cold-start valve operation using an LED. The problem appears to be an undercharged battery. My trips in the E12 are all short and sporadic, with sometimes weeks between. My battery today was showing 12V on a voltmeter, but my smartcharger said 80%. After it was topped, I took a drive to warm up the engine and the hard warm start disappeared. It's possible the ECU was voltage starved and grumpy.

I'll keep it on a trickle charger moving forward and see if the problem reappears.

I always knew cold starts were affected by a weak battery, and cold starts draw more current. But a weak battery might be causing my problems during a lean start.

Interesting. Glad you made some progress on that... I'm still a bit confused that you had a warm start problem without a cold start problem if it's the battery, since every warm start follows a cold start a bit earlier.

For the cold start valve, I'd personally like to see it work. Just going off the LED light, you prove some voltage is going to the CSV; however, most LEDs usually don't need 12V to work, the valve could be stuck closed, and/or maybe the fuel line feeding the valve is blocked or empty. Years ago, I did the blue book test of just disconnecting it and dropping it into a glass jar to see it spray. This was also a good way to time what you're getting out of the thermotime switch.