1980s 1984 Eldorado 4.1- Transmission Overdrive or Ignition issue

[limikep]

Hello

When I accelerate on the highway and the car goes into overdrive all is ok up to that point.  Now when I try to maintain speed in overdrive or I try to accelerate in overdrive, the car has a slight vibration (like the car has a misfire).  I changed the distributor cap, rotor, ignition coil & ignition control module (all NOS GM parts).  I did not change the plugs or wires.  Could this still be an ignition issue or a transmission issue.

Thank You
Mike

[Cadman-iac]

  Because the transmission converter goes into lock-up mode at highway speeds, any type of misfire is felt more keenly, as your drive train is positively coupled with the engine.
 I would definitely start with a new set of plugs and wires if they have any more than 50K miles on what you have now.

[Dave Shepherd]

Agreed on all points.

[TJ Hopland]

First thing I would check based on recent experience is fuel pressure. I had a friend with an 84 and similar symptoms.  Hooked up a pressure gauge and best it would go is about 5 psi.  Its supposed to be like 13.  I was amazed it ran pretty good with less than half the pressure.  Luckily Cadillac included a test port, the other GM's that used this system didn't.  The cause in this case and the most likely cause with some age is there is a short piece of hose between the in tank fuel pump and the sending unit it mounts to.  This hose was just sort of a blob, no idea how it was getting any fuel to the engine.  Its inside the tank so even though it was a massive leak it was just squirting around inside the tank.     

May also be worth going through the whole idle speed timing setup procedure.  It has to be done in the correct order and set to the factory specs.  If you skip any part of the procedure or just eyeball settings it tends to mess things up because the computer doesn't know where it started so it can never find the results its expecting.  I bought more than a couple vehicles with this system that were full of new or junkyard parts and didn't run worth a darn that just needed to be set up properly.   

[bcroe]

If the pressure is low, do check all rubber fuel hoses. 
My 79 pump was removed from the tank in 2017, the
special submersible hose was completely sheared. 
Bruce Roe

[limikep]

Thank you all for all your input.  I checked my records and found the following:

plugs installed 15 years ago and only put 8,711 miles on them
wires installed 27 years ago and only put 17,015 miles on them
Fuel pump changed 6 years ago and only put 3,588 miles on it.

With that being said, I will change the plugs and wires first as this is the least expensive and I can do this.
And now I will have a complete tune up as the ignition components were already changed.

Again, thanks for all your input & I will keep you posted so others can learn what solved the problem.

[Cadman-iac]

  I hope that fixes the issue. I've had similar problems in the past. I haven't had any issues with the fuel pressure causing any missing or misfires, but I can definitely see how that would also cause the same symptoms.
 Good luck with it, and definitely let us know how it goes.

   Rick

[limikep]

Hello all

I finally have an update on my issue.  As stated, the only two items left for me to do was the spark plugs & wires.  Installed them and now I have a complete tune up with all tune up components changed at the same time.  Unfortunately, the plugs & wires did not solve the issue.  The fuel pump pressure was my next check but a friend of mine suggested I put a can of "Berryman B-12 Fuel Injector Cleaner" in.  I put in a bottle of Techron 15 months ago and only drove 600 miles.  Didn't think it was necessary but I had 1/4 tank of fuel so I added the Berryman and filled up the car.  Took the car on the highway and after 10 miles the car started to smooth out in overdrive, which is when I felt the problem.  I have driven 50 miles since the Berryman and appears to be 90 percent better.  I hope at my next refill, the car will run even better.  I only drive 500 miles a year so maybe I got some bad gas and fouled the injectors?  I hope this helps others to apply this simple inexpensive fix.  All the best.