Engine cranks, but only starts after letting off key? - I'm perplexed

[Grit dog]

I was thinking heat soak possibility as well, but that doesn’t make sense with the ignition switch since it’s in the cab and doesn’t see heat from the engine.
Guessing they didn’t actually hot test it at the shop, if you immediately have the same repeatable condition.
I would have bet dead spot on the ign switch as well, but maybe some adjustment to the ign switch will get it right. May have been that all along? Spitballing at this point.

 

[creich68]

Covered in post 12,he needs to do as in post 4 and eliminate all possible wiring and switch problems.

It's stored at the shop this weekend but I'll be sure to mention it to the shop first thing Monday morning so they can try that. The good thing is it's starting every time and running normal, just acting a little odd with the switch when up to temp.

Battery is older, but voltage is good and cranking normal

 

[Grit dog]

Now leaning towards maybe something heat soak related under the hood. IGN module or even coil.
Coil windings expand as they heat up. Could need just slightly more power when warm/hot to do coil things and it gets that power as the draw is released from the starter motor.
Don’t recall, if you said dist coil has been replaced recently or not.

 

[Grit dog]

Even though you have a “modern” squarebody, lol, it’s still going to me more subject to experienced diagnosis rather than most shops/mechanics now who are frequently only having to do computer diagnostics and replace parts.
Idk how old school your shop is, but finding someone who is experienced in old cars may be more beneficial.

 

[creich68]

Now leaning towards maybe something heat soak related under the hood. IGN module or even coil.
Coil windings expand as they heat up. Could need just slightly more power when warm/hot to do coil things and it gets that power as the draw is released from the starter motor.
Don’t recall, if you said dist coil has been replaced recently or not.

The whole distributor assembly was replaced which I assume includes the HEI coil. I’ll have to double check that.

The switch issue seems to have developed after they fixed the vacuum leaks and replaced the distributor. Makes me wonder if putting the original distributor back in would shed any light.

The lead tech is an older gentleman, along with the shop owner. They seem to be familiar with TBIs and older vehicles in general. They’re always super busy and have a good reputation locally. They’re definitely being tested this time.

 

[Grit dog]

The whole distributor assembly was replaced which I assume includes the HEI coil. I’ll have to double check that.

The switch issue seems to have developed after they fixed the vacuum leaks and replaced the distributor. Makes me wonder if putting the original distributor back in would shed any light.

The lead tech is an older gentleman, along with the shop owner. They seem to be familiar with TBIs and older vehicles in general. They’re always super busy and have a good reputation locally. They’re definitely being tested this time.

Did they put a whole new distributor in it or just a cap n rotor or?
Sometimes it’s the oddest thing. That’s why electrical gremlins are called gremlins. For instance son’s mustang was throwing cam sensor codes and running poorly. New cam sensors didn’t fix it. It was alternator diodes causing it. Was totally stumped and read one thing on the web that pointed me to that.

 

[creich68]

Did they put a whole new distributor in it or just a cap n rotor or?
Sometimes it’s the oddest thing. That’s why electrical gremlins are called gremlins. For instance son’s mustang was throwing cam sensor codes and running poorly. New cam sensors didn’t fix it. It was alternator diodes causing it. Was totally stumped and read one thing on the web that pointed me to that.

Whole new distributor top to bottom.

 

[cksquaredtrucks]

Well....the ignition switch has been replaced. The shop drove it to test cold and hot starts. Everything seemed to be back to normal according to them.

I picked it up this afternoon and stopped at the store on the way home. Seems to run normal. Came out from the store, same issue...cranks while holding key and starts immediately after letting off.

Called the shop and owner said come back. Was able to replicate in front of him. He said there is some adjustment in the switch for the contact points (I'm taking his word for now) and that he will pull the switch back out Monday morning and check eveything again. He's convinced it has to be the switch. I'll have my popcorn ready for Monday.

Bummed I'll miss the local 80s-90s car show tomorrow morning but there's always next year!

I have had mis adjusted ignition switches do the same thing. When you install a new switch you have to test it a few times to get it to work correctly. The mounting holes are slotted. I have admittedly never owned a TBI truck so I can't help you if that system is causing the problem.

 

[SUBURBIANV30]

I had the same issue with my 88 suburban . … I actually changed to a different column with tilt and a new key tumbler and problem solved starts right up every time as it should

 

[OBSbuilder2021]

Battery wire to distributor (ignition feed while cranking)
Pull that wire off, put positive lead of voltmeter on that terminal, ground your other voltmeter lead, look for 12v (this wire is a constant "hot"),
Once confirmed then turn key on, again confirm 12v, then while cranking...that wire should have 12v WHILE CRANKING....if not, and when you release key it goes back to 12v then that's why it's firing as you release key
Knowing mouse chewed wires and everything butt connected....probably source of issue
Plus, not much room back there and its dark so wire harness could've been spliced incorrectly.