cabin taking in engine fumes during cool day warmup

Hi, I'm new here! Not a mechanic, but not an idiot either. I'm having a cabin fumes engine problem. It's a classic Ranger 2001 4-cylinder 2.1 liter with standard transmission (good condition, runs great, I'll never part with it!). On cool days for the 1st start of the day and during warmup only (that's all key), the cabin is getting flooded with awful exhaust - maybe exhaust plus fuel mixture. It's getting a lot worse now. At times now, I just have to walk away from the truck till it warms up. All air is off, and once the engine warms up, the problem's over. It's not from the exhaust pipe. The smell is flooding all in the engine compartment. What do I look for for this? What's active just during cool weather warmup - like a particular hose that's only active during that time and needs to be replaced, or fuel mixture problems just at that time, anything. With fall approaching, I want to finally nail this one this year. (I'm central Texas, by the way. Cool weather is 60 and below.)

And a 2nd question on this - with all air off, why is it getting into the cabin in the first place? Does that air intake box (I think it is, on passenger side just under the hood) need to be taken apart and cleaned? That's my single complaint about my Ranger - is the cabin has never been sealed with all air off, and it picks up heavy exhaust from other traffic at times (or at least mine does). When I turn the air off, I can hear the air panels somewhere closing. But it's not sealing. Or is it not meant to?

Any thoughts here highly appreciated.
MikeA.

Welcome to TRS

Not that it matters, but you will have a 2.5l SOHC Lima or 2.3l DOHC Duratec 4-cyl engine in a 2001 Ranger, 2.3l replaced the 2.5l mid-year 2001 which is why it could be either

The Cab's Fresh air intake is indeed on passenger side but just in front of the windshield in the cowl, so between windshield and hood, Rangers never used a Cab Filter, so there is no access to the vent from the outside.

First thing I would check would be the Hood Gasket at the rear, if it is not sealing when hood is closed then engine compartment air could be coming out and is being pulled inside, fan on or off wouldn't matter.

Also, in the engine bay you can see the back of the Blower motor, passenger side, and a rubber tube elbow on that motor.
This equalizes air pressure in the motor but gets the air from INSIDE the cab.
If this rubber tube is cracked or broken then engine air can come into the cab

Next fix, the oil leak, the exhaust manifold has oil leaking on to it, so at cold start the oil is being burned off, causing that smell.

Replace PCV Valve and clean its hoses, PCV system creates a negative pressure inside the oil pan and valve covers, this keeps "engine air/blow-by" inside engine, non-functioning PCV system will mean that "air" will be pushed out by blow-by pressure, and it smells like unburned fuel and exhaust both.
Working PCV system also helps prevent oil leaks from valve covers and oil pans, pulls oil away from leak points because of negative pressure

Fresh air vent can only be closed by MAX A/C setting, so if you have that setting turn it to MAX AC and leave fan off during warm up, see if the smell goes away
If you don't have Factory AC then there is no way to close the fresh air vent

If you are still get the smell then check manual trans shift boot, could be getting smell from torn boot.

If you put your hand over the tail pipe with engine idling you can often hear/find any exhaust leaks that may not be noticeable except by smell

As said above I think the source of the smell is oil leaking onto exhaust when engine is off, so if you eliminate that most of the problem will be solved

Thanks! Lots of useful info here. I'll start probing around in those areas.

By the way, I do have A/C, and that's the time I can hear the vent close like you said (but only on the max-A/C setting). But there's no fan-off setting on this model! Just fan speeds starting at low. The only off setting is on the air mode knob instead of the speed knob, and the vent opens back up on that one!

Also by the way, the problem isn't always. It's very hard to narrow it down to when it triggers, but I suspect higher humidity is also a factor on 1st start of the cool/cold day. Sounds like PVC system is the best starting point.