Help with warm start problem please

[DocV]

I have a ’91 318is that has been my daily driver for the past 2 years. Recently, it began having warm start trouble. When I start the car cold in the am, it always fires up on the first attempt. Once it has been driven, if turned off and turned back on within a few minutes, it usually does just fine. If ~5 minutes elapses, it will stumble or not start on the first try. The second attempt then usually does fine. Once started it runs just fine, with normal power delivery.

There has not been any check engine code flashed. The fuel pump has been replaced in the past year with OEM new VDO unit. I have removed the rear seat and access hatch, and it sounds like it is running fine. I replaced the fuel relay and fuel filter because I had spares on hand and this has not been the problem.

I checked the coolant temperature sensor (ECT / “blue” sensor) and it seems the values are within spec (1700 ohms at 70 degrees ambient, then 230 ohms with engine at operating temp).

My fuel mileage has been relatively poor lately (~23 mpg city driving), and exhaust has smelled fairly rich, so I replaced the O2 sensor (had not been done since the car was in my possession). I believe the mpg has improved somewhat but the warm start issue remains.


What am I missing? What should I be checking? Check my logic here — I have not formally tested fuel delivery/pressure (don’t have gauges, t-piece, etc), but I don’t think it is likely a pressure problem —- if there was a leak-down issue with leaky fuel pressure regulator/leaky injector/fuel pump check valve, it would have the same starting issues in the morning would it not? Similarly, if this was an issue with cam or crank sensor, it would happen regardless of hot/cold, wouldn’t it?

I am likely going to let an independent shop look at it, just wondering if I am missing something obvious before taking it in. Any help is greatly appreciated.

Thank you.

[TobyB]

My first guess (if you're not losing coolant) WOULD be a leaking injector(s).

They fill the intake, and flood it. Then 1 or 2 cylinders don't spark until
they blow out all the raw gas. In the morning, the gas has drained down
into the crankcase... have you been gaining oil?

Easy to test- do the 'flood mode' start when it's hot- floor it before you crank.
If it starts easier, then it's pretty likely that's what's going on.

A fuel pressure gauge is really easy to do, too, and helps diagnose things like this.

t

[DocV]

Hi,
Flooring the throttle prior to and/or while cranking didn't seem to make a lot of difference.

In reference to the above, what would you say if the car was losing coolant? I don't think it is losing coolant in a meaningful way (lost ~ one inch of coolant in the expansion fill tank below the "cold/kalt" line in 6 months of driving), just curious.

Thanks for the help.

[uturn]

I'm going to guess fuel injectors too because a new fuel pump would have a new check valve. Install a pressure gauge and see how fast it leaks down after you shut off the car. After you get the injectors cleaned then you can test the fuel regulator to see if it leaks.

[TobyB]

If you're losing much coolant at all, it can be leaking into one or more cylinders when hot.
It doesn't take too much to extinguish a cylinder or 2, and then it takes 20 seconds
or so for it to start lighting again.

But you'd find a little bit in the oil, probably from that, as it leaked by the rings
and into the crankcase.

Flood mode is- floor it, crank till it catches, release the throttle. It should improve hot
starts with leaky injectors, but it won't completely cure it, as in if it usually
runs rough for 20 seconds, it might now run rough for 5-10.

If none of this helps, try a fuel pressure gauge. If it holds fuel pressure after you shut down,
that completely rules out the injectors and the pump.

Then we get to start chasing harder things like coils, coil drivers, plugs, plug wires...

hth

t

[Boris3]

Has anyone been working under the hood lately? There is a classic warm start issue that happens when someone plugs together the small single wire connector (black and white lead) on the engine wire harness on the fire wall. Folks see it and think it should be plugged in but it should only be plugged in if you have no cat.

If you're looking at the harness on the firewall (under the long plastic cover) from the front of the car, to your left you'll see the small wire and a plug extending out from the wrapped harness wires. Make sure it's unplugged. May not be your issue but it's a good place to start.

[Boris3]

[DocV]

Thanks for all the suggestions -- I've got some fuel pressure gauge stuff on the way and I will report back once I get this figured out