Weird CFI starting issue

[Billy]

So I took my 1985 into town today for the first time since switching to 15w40 Rotella T. The car behaved pretty well for most of the day today, until right before I was about to go home. My girlfriend and I stopped in a shop for about 10 minutes, and when I came back out, the car absolutely refused to start. The electronics were all on, nothing was dim, the fuel pump primed, and the starter turned over hard and then just winded down instead of starting the engine. It had done something somewhat similar earlier but it started after a couple tries then. I tried starting it several times, and was about to give up, when my girlfriend suggested giving it a little throttle and turning the key. I did that and it started right away.
I have no idea what the issue could be. The car could be not liking the 15w40 very much, but its weird that this issue only seems to happen when the engine is still hot. The car does not ping (not that I can hear at least), and doesn't misfire much, so I don't think it's an ignition problem and don't think it's the EGR or an electrical issue. Though on a side note, I have noticed that the car was kind of humming/whining today and it seemed vacuum/electrical load dependent. No clue if that's related in any way but I figured I'd mention it.

If anybody knows what could cause this, I'd really appreciate the help. It's pretty much got me stumped. I'd be willing to chalk it up to CFI weirdness if nobody has a better answer.

[a_d_a_m]

My Firebird hated hot starts. Turned out the battery was pissed off. I'd have to pop the hood and let it cool down for a while.
Presto change-o, all good with a new battery.

I don't know if that's your problem, but...hmm. I doubt it's the Rotella but why in the hell did you switch over? That seems excessive.

[Billy]

Zach suggested I put in the thicker oil because the car had low low oil pressure at hot idle. This seems to have almost completely fixed that.

[85crownHPP]

If it started up fine when you gave it some throttle, I'd doubt its anything serious. It could be as simple as a vacuum leak, or you could try cleaning the throttle plate. I'd also test the TPS sensor to make sure it reads close to 1 volt with the throttle closed.

[Billy]

If it started up fine when you gave it some throttle, I'd doubt its anything serious. It could be as simple as a vacuum leak, or you could try cleaning the throttle plate. I'd also test the TPS sensor to make sure it reads close to 1 volt with the throttle closed.

That makes the most sense out of anything. I'll probably replace as many vacuum lines as I can, which I was planning on doing anyway. TPS sensor issue also makes sense, and that gets me to wondering, do these cars have an IAC valve? If so, is it possible for it to get sticky at startup? The thing that stumps me the most is that I don't have any problems with idling really, besides a dip in rpms sometimes, but that's to be expected on a worn out 302.

[85crownHPP]

No IAC on a CFI car - it has a vacuum operated throttle plunger that will tweak the idle when the A/C is on. When my 85 was CFI, I wound up hooking up the vacuum solenoid that controls this with a switch so I could manually control it when the idle would act up.
CFI also has a choke-like mechanism, but it usually causes hard starts and have-to-keep-foot-on-gas to warm up until it will idle.
If you notice it 'miss' a little when you give it a little gas while coasting or while barely on the throttle, Id suspect the TPS even more.

[pantera77]

Even the owners manual on my 85 says to give it 1/4 throttle when starting warm. It will not start warm otherwise, just crank and crank, give it a touch of gas, and fires right up.

[gadget73]

yeah, they work like a halfass carb. Sometimes you have to pump the gas to set the high idle cam, or just manually give it just a wee bit of extra pedal to make it go.

[Billy]

Even the owners manual on my 85 says to give it 1/4 throttle when starting warm. It will not start warm otherwise, just crank and crank, give it a touch of gas, and fires right up.

While I'll still check on the TPS when I get a chance, this seems to exactly describe my issue.
I'm still getting used to CFI and all of its "quirks".

Er... is there anything else I should know? I might be taking the car on a 700mi trip this week, and I'm hoping for it to be incident free.

[Billy]

I need to give the car throttle every time I start it now, bit it has no throttle issues while running. I've decided that this is probably an issue with the choke. The choke on these cars is vacuum controlled, right? This car is a whole mess of vacuum stuff, so can somebody point me to where the line for the choke is?

[sly]

the choke is mechanical in nature. You should need to press the gas about half way to set the choke before starting anyhow. If it runs fine after that without pressing the gas further, that's the issue... just getting use to the older car's way of working. If it still needs you to hold the gas a while for it to idle, you may need to adjust the choke.

[Billy]

Cfi has a mechanical choke? If so, something must be wrong with it, because this issue is new.

[gadget73]

yes, it has an electric choke just like a carb, though without the top plate. The linkages get gummy sometimes and the high idle cam won't drop into position. All that stuff is on the passenger side of the throttle body.

[sly]

by mechanical I mean all the linkages and crap. It's "controlled" electrically... so to speak.

[gadget73]

well, not so much electronically. The high idle cam is all mechanical with an electric choke coil like you'd find on any carb. There is also a high idle stepper solenoid used for the AC, but cold start high idle is mechanical.