BRM 'stumbly' start

Hey fellas. I've seen this issue on two BRM's now. This one is mine. I bought an '06 early this year. I replaced the cam and timing belt along with a few other maintenance items that are common areas. I've seen this issue once before on a BRM that I worked on before it was wrecked. I was never able to figure out what is causing it but I'd really like to now since this is an every day car.

When you start the car hot it stumbles during the start. You have to hold the key through a couple stumbles and it fires up. When it's cold it starts very nice and smooth. I have the torsion set at -0.5. The injector balance is very good for the miles and I have no engine codes stored. Everything is 100% factory. No tune, no aftermarket injectors, etc...

It almost acts like it's starving for fuel in one or more of the cylinders during hot/warm starts. Cold starts are GREAT as I said. If you were to let off the key as soon as you heard it fire, like you would most cars, it would die on it's face right NOW and you'd have to attempt another start. If you hold the starter engaged through this process it starts right up and idles perfectly immediately after. It's really weird.

A video is worth a thousand words, so I uploaded a video to youtube of what it does. I've see it stumble 3-4 times like this during a start.

Any ideas? I really don't want to throw any parts at it as I like to properly diagnose things rather than firing the parts cannon at it and wasting money. I thought perhaps the a couple of the glowplugs were weak but wouldn't it start bad cold too? If it is glow plugs perhaps the heat after running makes a couple of them work poorly. The glow plug harness was replaced at the dealer by the previous owner not long ago but I think the plugs are original.

https://youtu.be/wD7B3aiITzg

A quick check you can do is to look at the temp values with a scan tool when you know it will act up. Make sure the intake air temp, coolant temp, and fuel temp is all where you'd think it should be. You'd almost have to compare it to another car just like it that is NOT acting up in your case, as I am sure those values will be different when warm. I am usually looking at that stuff when cold, where they should all be close to identical.

You could potentially have an injector or tandem pump that is losing fuel prime somewhere, but it is so minor it only happens when the fuel is thin, so hot.

Are there any puffs of white smoke during this extended cranking process coming out the tail pipe?

Maybe while it is running hot, wiggle the harness around especially by the starter, and see if there is any changes in idle quality. Maybe a crank or cam sensor isn't working 100% once it is warm and sending an off signal.

oilhammer said:

A quick check you can do is to look at the temp values with a scan tool when you know it will act up. Make sure the intake air temp, coolant temp, and fuel temp is all where you'd think it should be. You'd almost have to compare it to another car just like it that is NOT acting up in your case, as I am sure those values will be different when warm. I am usually looking at that stuff when cold, where they should all be close to identical.
You could potentially have an injector or tandem pump that is losing fuel prime somewhere, but it is so minor it only happens when the fuel is thin, so hot.
Are there any puffs of white smoke during this extended cranking process coming out the tail pipe?
Maybe while it is running hot, wiggle the harness around especially by the starter, and see if there is any changes in idle quality. Maybe a crank or cam sensor isn't working 100% once it is warm and sending an off signal.

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I'll do that. I get a decent puff of black smoke whether cold or warm start. I thought that possibly one or more of the injectors is leaking when off causing an air pocket to form or something. But, it starts great when cold. If I start it cold, shut it off and start it again, it'll start just great. It really only acts up when warm. It's weird. It's almost like a vapor lock back in the old days with carbs.

The cause/solution to this problem was far too simple. The starter motor. I began to realize that the engine started slow when cold and very rough when hot. This had to do with the ambient heat in the starter while the engine was hot. The slow start with a strong battery when cold was the main clue. One Bosch SR0496X later the car explodes to life at all times. I was certainly overthinking this one. The difference is stunning. As I've seen OH state in the past, these starters typically don't just die. They get weak. When the first cylinder fired over on start the starter didn't have the proper strength to handle the acceleration followed by the next hard compression stroke. It all makes sense now that it's over. I don't even get any more smoke out the exhaust at start.

banshee365 said:

The cause/solution to this problem was far too simple. The starter motor. I began to realize that the engine started slow when cold and very rough when hot. This had to do with the ambient heat in the starter while the engine was hot. The slow start with a strong battery when cold was the main clue. One Bosch SR0496X later the car explodes to life at all times. I was certainly overthinking this one. The difference is stunning. As I've seen OH state in the past, these starters typically don't just die. They get weak. When the first cylinder fired over on start the starter didn't have the proper strength to handle the acceleration followed by the next hard compression stroke. It all makes sense now that it's over. I don't even get any more smoke out the exhaust at start.

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On one of my past mk4's I took the starter off and hooked it straight to a battery with jump leads. You'd think it was trying to push an elephant up a hill the way it spooled up!