1989 M20 Warm Start Issue

[ChairFaceChippendale]

I'm gradually working my way through an 89 325is that spent years sitting. I've replaced quite a few parts with the intent to be able to put some miles on the thing, but I'm encountering a warm start issue that has me puzzled. The car starts right up when it's either cold or hot, but if I let it sit for roughly 30 minutes when it's warm outside, then attempt to start again, it will either not turn over (cranks strong) or won't start without me giving it a little gas on the second attempt.

Yesterday I pulled the plugs when I adjusted the rockers and I have no indications of it running rich, nor does it smell like there's excessive fuel in the oil. I really don't think it's fuel related, but am going to put a gauge on it to verify. Again, the car runs great once it's started and has a stable idle. I've seen similar threads from over the years but nothing that either has a resolution posted, or where the person had quite as many new parts as me.

Below is the list of things I've replaced that would be likely causes or related since I've started working on the car.

-New fuel tank, pump, filter and all new lines. I also installed an inline anti-return valve at the pump. No change here before or after.
-Fuel pressure regulator
-Injectors have been cleaned, rebuilt and leak tested
-Timing belt
-Crank position sensor
-Plugs, wires, cap, rotor and ignition coil
-All vacuum lines and intake boot
-Intake manifold gaskets
-Oil return tube o-ring
-Valve cover gasket
-Blue top coolant sensor
-Throttle switch
-EVAP canister has been removed and ports are capped off
-Zero difference between two good working (and clean) IAC valves
-Battery less than one year old
-New alternator to starter cable as well as fresh ground straps

I've also smoke tested the engine after all of the work above. Found and fixed one small leak I had, but that made no difference.

Assuming the fuel does check out OK I'm going to do a compression test as well. The problem isn't big enough that I wanted to be sure to have all my other work done before I started really focusing on figuring this out.

Anything I'm missing here?

[ADEN]

Starter weaken when hot and draw high current with weaker rotation, test it and fix it to save battery and starter harness.

[Tinkerer007]

You say "it will either not turn over (cranks strong) ". This sounds like a contradiction. Turn over means it is rotating (to me).

My initial thought is that the injector(s) is/are leaking. How did you leak test the injector? You say giving it a little gas will help it start. What if you hold the throttle open (to give it more air, if it is flooded)?

Unplugging the AFM would rule out ECU related issues... (I think).

[ChairFaceChippendale]

Starter is also relatively new, (not as new as the rest of the parts on the list above) and it works great when cranking when hot even when it's 90+ degrees after a long drive or stop and go. The injectors were done by Injector Rehab. They provide a flow report as well as leak down check check after cleaning. Doesn't make it impossible there's still an issue, but pretty unlikely. If I do find out they are leaking I won't be using them again . Plus, it starts right up on cold start even after sitting for a week. If I was loosing pressure I would imagine I'd also have cold start issues if the system wasn't holding pressure.

It doesn't take much gas to get it started on the second attempt. Quarter throttle will pretty much do it. I wonder if that would point to dead spot on the AFM? The AFM probably needs to be looked at either way. It looks like it's original, but I didn't want to mess with it if at all possible given the car runs so well in general aside from this-- I just don't know when that comes into play on start up compared with the crank sensor, etc.

Yeah that is a little redundant on my part with turning over/cranking. It cranks strong in either situation, just doesn't seem like it's getting fuel to me. I should have time to put a pressure gauge on it tomorrow or Sunday, so I'll update when I do. I'm a little skeptical of the pump despite it being new.

Thanks for the ideas so far!

[Tinkerer007]

I think the thought on warm start issues, is that the fuel leaks into the cylinders and makes it too rich to ignite. And then if you wait longer, the fuel in the cylinders dissipates.
Stepping on the throttle opens the throttle valve and lets more air in. It also takes the ECU off the "idle" map.

I don't think a bad spot on the AFM would cause the car to not start. It would cause a hard start at a certain airflow position.

I don't think it is fuel pressure, because it would have low pressure when cold as well.

I assume your air filter is clean?

[ChairFaceChippendale]

Yup, air filter is new also. Finally got time to get a fuel pressure gauge on the car this morning. Seems to be running at about 32-34 psi consistently. Spec is 43.5 +/- 0.9. Snapping the throttle got me to just under 40psi, but that still seems a little low No real change by pinching the return line. Just for good measure I pulled the fuel rail and injectors with the gauge still hooked to the supply line with the car off, and didn't see a drop of fuel over the course of about 45 minutes.

I didn't test volume, but since I've been a little skeptical about this pump from the beginning I feel better just replacing it anyway. Only thing I can think that makes sense is the pump must be out of tolerance enough that it's not building enough pressure right away in that warm start period, but is tight enough when cold to let the engine fire up on the first try. Ordered a new VDO pump, so we'll see how it does with that installed. The one in there now is a Spectra that came with the new tank.

[jeffnhiscars]

I had a no hot start problem that got progressively worse over a matter of months until it would not start at all when hot. Long story short is I hooked up a fuel pressure gauge, did a cold start and once it reach OT fuel pressure went to zero. Turned out to be a bad fuel pump (not the in tank). $106 later and the problem is gone

[D.Martijn]

I've also had trouble starting the car when it was warm/hot, giving it some throttle also made it start quicker. (few few seconds the engine ran poorly)
Turned out my FPR was leaking fuel trough the vacuum line into the manifold, making the mixture to rich. giving it some revs burned the excess fuel away.

I think it might be a good idea to check fuel pressure as well.