runs rough and shakes when car is warm

The age of the wires doesn't guarantee anything. The wire could be damaged or burnt and the plug could have been cracked during installation. The first thing that needs to be done is to isolate which cylinder is causing the problem.

How do you isolate which cylinder is causing the problem?

That will require equipment and experience which I suspect you don't have either of. You're probably going to need a shop to help you with that.

Is it bad to use Bosch Platinum +4 in a 1990 mercury grand marquis?

Yes, very bad. Use Motorcraft only

could you tell me why they are bad

They are nothing but gimmick that the car was not designed for and they do not work in a Ford engine. NEVER put ANY Bosch plugs in a Ford.

Ok, replaced plugs with autolites, wires, distributor, map sensor, coil.
idle is rough almost missing. car bucks wildly when plugging in spout connector for electronic timing advance

So, it only runs bad when the spout connector is in place?

Yes the problem only occurs with the spout connector in place.
new distributor timing advances about 20 degrees, old distributor timing advanced 4 degrees when plugging in spout connector.
New distributor much worse than old due to incease in advance timing I assume. What computer sensors could be feeding the wrong signal to the computer.
Throttle position sensor?
Coolant temp sensor?
Problem amplified when engine is hot after driving at least 7 miles

Did the new distributor include an ignition module?
Was the problem the same before and after the distributor?

yes, worse after the new, not remanufactured distributor

It seems obvious that there is some kind of issue with the new distributor. If you still have the old one, try swapping the ignition modules.

We already tried that

its doing the same with the old one too

Then I suggest you let a professional look at it. there is only so much that can be done without seeing the car.

My question for PERTPAT is did you ever resolve this issue? I have 140,000 miles on my 1990 Mercury Grand Marquis with similar issues that I cannot diagnose myself. I am down to inspecting the wiring harness by individual wires. Nothing else has worked.

Hello,

I have seen the engine coolant temperature sensor cause this issue. here is a guide to help walk you through the steps:

https://www.2carpros.com/articles/coolant-temperature-sensor-cts-replacement

Please run this test and get back to us.

Thank you, even though the coolant temperature sensor fell apart in my hands and I replaced it. the problem was still not resolved. I have an update though. There seems to be some issues with the aftermarket distributors for these engines. After two attempts with 2 car quest distributors I’m trying a Cardone distributor when it arrives tomorrow.

Okay, sounds good. that can cause the issue, please let us know what happens with the new distributor.

Today I replaced the distributor with a Cordone distributor and the problem appears to be resolved. I want to caution anyone and everyone with the code 18 on these Ford or Mercury cars and Ford truck’s equipped with this 5.0 to be highly suspicious of inferior after market distributors.

Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.