OldCadTech

The Transmission selesct switch must be out of adjustment. I would check the battery and all the cables as @rockfangd suggested also.

If it shipped dry, I would remove the compressor, verify that it is dry, and add some ( 50ml or so ) oil and spin the compressor as rockfangd suggested. I wouldn't run it dry...those compressors weren't that tough. Safe to drive as long as the compressor stays off...

Good, I was thinking the heater was switching from hot to cold rapidly. If this heater core plugged in a year I would replace the radiator. GM instructed the pellets to be added to the radiator,,,,

Still sounds like an air mix door problem. I went back through the post, but, since I don't subscribe to photobucket I couldn't see the pictures. The Deville used the air mix door to control the amount of air through the heater core. Even if the actuator is moving, that doesn't mean the door is operating correctly. True, the tabs would eventually clog radiators and heater cores but that was over a long period of time. One year to plug a heater core is very extreme...especially since the coolant is in pristine condition.

Did you check to make sure the valve/orifice is not plugged? You can do a temp check... If the inlet is HOT and the others aren't as hot pull it out and look. Back flush it if everything else checks fine before you pull the core, or pull it since you had fun last time... JK Is the coolant level correct? What is the coolant temp?

I would check the heater control valve first, not uncommon for those to restrict coolant flow. Let me know how it checks out, There are a couple of other things you can do

LOL, I agree, the wife may NOT be too happy if it happens again ( only a guess )

Okay, it's great that you have it running again but the tap test on the PCM is a test for a defective PCM. Unless you cleaned a bunch of corrosion from the contacts I would replace the PCM.

I think the minimum fuel pressure is 45psi, but I could be wrong. Fist thing that comes to mind is the fuel pressure regulator. Since your update, no injector pulse, I'm leaning towards a cam or crank position sensor. Check for codes with a scan tool. The PCM may not be seeing the engine cranking

@Jesse B Switch the relay with one of the other relays of the same kind. Does everything operate normally? Did the noise stop? If you still have a noise I would suspect an A/C programmer and/or a temperature blend door actuator as @rockfangd suggested. Check ALL the fuses!! Something caused the wire to burn, a short to ground is the most common. Trace the wiring issue to it's source.

Sorry for the delay getting back to you... RL gets in the way.....

Okay, now that you straightend me out on the optispark system, I was still thinking distributor at the rear of the engine, sorry. The optispark was not as reliable as the regular shaft driven distributor and earned several infamous names. SO, in short, replace the optispark and do the tune up. I think you'll get the smile back!! I'd go reputable, quality aftermarket for the optijunk (oops) Opti-spark......

Sounds a lot like the fuel pressure regulator is defective or there is a restriction in the sending unit/fuel pump assembly. To me, the foaming of the fuel would indicate the pump is sucking air. Since the fuel pressure regulator is non serviceable, replaceable yes, were you disassembling the fuel rail? When you replaced the fuel pump did you replace the screen/filter at the pump inlet (commonly referred to as a sock)?

I guess the first question I have is - has the vehicle ever had a tuneup ? The mileage would indicate a tuneup is past due. Plugs, Cap, Rotor, wires. Coil if it is burnt or the button is bad. Replacing the distributor seems a little extreme. You say you went from a tip-in hesitation to a "crumble", does that mean miss? There is a base timing, and the computer "controls" the timing from there. I doubt very much that the timing is off if it has been unmolested.

Glad you're drivin & smilin!! Thanks for the follow up report 👍

I've always taken them out the top. Mainly because I never liked pulling transmissions. And it seemed to me like waayyy to much work to drop em out the bottom. I don't remember seeing the procedure in the 98 FSM (that I gave away) either, but I had been taking the engines out since '85, so I never really looked either. I guess the question is ... do you have access to a lift? - twin post works the best IMO but center lifts can be used... Do you have access to a "cherry picker"? The engine comes out over the rt fender. It is going to take some sepecial extensions and swivel sockets and it requires lifting and lowering the vehicle several times.

Usually a tip in hesitation is caused by a throttle position sensor. Not uncommon and relatively inexpensive to repair. Personally, I would clean the throttle plates and bore and see if that improves the condition.

The vacuum pump may not have been on the 1990. The 1988 or 1989 with the 4.1 may have been the last year for the pump.

I'm thinking that is the vacuum pump.

@TedK This is one of the hardest codes to diagnose. We nicknamed it the 800 line cuz ALL the modules use it. As you probably know it can be caused by ANY module on the 800 line or any combination of modules. You have started down the right path, as checking grounds is the first step to perform. The parts department at the dealership I worked at would let us plug and play modules as this was the only way to determine the defective module. The codes you have are of no help as the rule of starting with the lowest code first does not apply with this 800 line malady... The ground I would start with is behind the drivers kick panel. Look in the FSM at the grounds section and the locations section to determine locations. Personally, I would start with the pass key/security module as most of the others (except the PCM) would allow the vehicle to start but they would set a hard code and turn the light on.

Do a OBD code check first... Black smoke from exhaust? Sounds like too much EGR maybe. @BodybyFisher

Sounds like the ignition module is getting hot if it is a crank-no-start condition... If it isn't cranking/turning over refer to what barczy01 suggested. The ticking in the distributor area could be a worn distributor shaft, not common but it can happen...

Rock, I sold the 98 and the FSM with it. I had a preliminary edition, I will look for it,

@BodybyFisher Thank you ! I think I did another post on the inlet removal a while back???????

The A/c does not have to be evacuated. Disconnect the mounting bolts and let it hang. Clamp and/or hose removal is pretty easy after that. Most of the time it is the inlet leaking but you may be the exception.