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Express van won't start in cold

My 2006 Chevy Express 3500 (cutaway) 6.0L won’t start when the outside temperature is below 35 degrees. Warmer than 35, it starts right up. Seems to be a fuel problem, as it fires, but doesn’t keep running. I can start it by fooling around with the position of the gas pedal, but usually it takes a few minutes of cranking. My mechanic thought it was low fuel pressure and replaced the fuel pump- a thousand dollars later, it is no different. Any thoughts?

The problem might be with the coolant temp sensor for the computer.

The computer uses the information from this sensor to determine the air/fuel ratio when the engine is started cold. So if it’s 30 degree’s out but the sensor is telling the computer the coolant is at 40 degrees the engine will run too lean at a cold start.

Tester

Yes, your mechanic owns a boat or a cabin.

Once you get it running, does it eventually smoothen out to the point where it runs without fiddling with the gas pedal?
Edit: I think tester is onto something.

I think @Tester has the right idea on this one. The ECM isn’t correcting the fuel/air ratio for temperature. Likely either the coolant temp sensor, or (less likely) the intake air temp sensor if this engine has one of those. But this sympton can be caused by a dozen different problems. So if there’s no check engine light and no codes pending, and if this van hasn’t had all of it’s owner manual suggested routing maintenance brought up to date, that’s probably where I’d start first.

Did your mechanic perform any meaningful diagnosis?
Or did he just make an assumption?

BTW van has 30k miles on it. Far as I know the scheduled maintenance is up to date- I didn’t see much that needs to be done in the owner’s manual. The check engine light has never come on, so I suspect it hasn’t thrown off any codes.

As for meaningful diagnosis, I’d have to say no. I thought it might be the intake air sensor, but it doesn’t seem to have one. My hope is to avoid taking this to the Chevy dealer, for a variety of reasons.

@RemcoW: once it starts, it runs just fine, no fiddling with the gas at all.

@RaymondShwalb, the intake air sensor may be built into the mass air flow sensor.

You don’t have to bring the vehicle to the dealer to figure this out. An Indy shop could figure this out.

Leave the vehicle overnight when the temperature drops. Then in the morning hook a scanner up and see what the coolant temp sensor reads relative to the ambient temperature.

Tester

@Tester is absolutely right. Both sensors should read the same thing.

These are good words. Thanks @Tester. I have a plan.

We did tested the coolant and ambient temp with the scanner first thing in the morning, and they read the same, then after getting the van started, watched the coolant temp rise, and the ambient temp stay more or less the same. Further suggestions? Thanks

@RaymondSchwalb so your ECT and IAT sensors are fine. Good.
Still no fault codes, right?
You said your mechanic replaced the fuel pump. Did he check the fuel pressure afterwards to verify that the pressures at idle are correct, and that the pressure holds after shutting it off?
Have you checked/cleaned the throttle body? A lot of weird starting/idling/stalling complaints are caused by a dirty throttle body.
Have you checked for pending fault codes? Mind you, these are not considered history or stored codes. These are problems that haven’t yet occurred often enough to set a hard code.

@db4690 Had the truck at the mechanic again, and satisfyingly it wouldn’t start for him, and he couldn’t get it out of his bay. After a while, he mentioned that the security light on the dash was flashing. I’d noticed that before too, but had no idea what it might mean. Any thoughts? Mechanic says it is holding fuel pressure, and there are no pending fault codes. I don’t know about the throttle body. Perhaps that is the next step.

@RaymondSchwalb this may be your problem. Don’t ask me why, but GMs are known to occasionally have problems with the passlock sensor.

http://dormanproducts.com/p-48259-924-714.aspx?year=2006&make=CHEVROLET%20TRUCK&model=EXPRESS%203500&engine=V8%20%2D%206%2E0L%20%28364%20CID%29%3A%20GAS%3A%20FI%3A%20N%3A%20U&parttype=Ignition%20Lock%20Housing

@db4690 Thanks. We’ll give it a shot.