1992 Cadillac Deville Engine misfire

Hi Kinyurd. Welcome to the forum. Unplug the generator and see what happens. There's no way to sugar-coat it. GM went from the second best charging system to the world's worst pile starting with the '87 models. Even when they appear to be charging properly, they are known to develop large voltage spikes that interfere with engine sensor readings and other computers.

It was also very common to go through four to six generators in the life of the vehicle. What many professionals are finding out is the repeat failures can be greatly reduced by replacing the battery at the same time. The old battery will work fine in older cars but they lose their ability with age to absorb those spikes.

Caradiodoc

Try unplugging the small connector anyhow just to eliminate that as a possibility. Also check that no wiring harnesses were rerouted to where they didn't go originally. Based on your description of the symptoms, I'm assuming the problem is related to the extra current being demanded from the generator.

Caradiodoc

Well, at least we can rule out the generator. I guess that's a good thing. One thing that comes to mind is a high resistance connection in part of the circuit that is common to the Engine Computer, heater fan, and power windows. A likely suspect is the contacts and connector pins for the ignition switch. The radio might be on the same circuit too so you might see the display dim.

What I would do next would be to use an inexpensive digital voltmeter to measure the voltage between the two battery terminals while the engine is running. You should have between 13.75 and 14.75 volts. If you watch that voltage while running the windows or fan, I'm betting it will stay fairly steady. That would prove the generator is easily keeping up its part of the job.

Next you will need to measure some point in the system after the ignition switch. This is where I won't be much help because GM does things differently than what I'm familiar with. The windows may be run through a Body Computer that has its own power wire rather than directly through heavy wires and contacts in the ignition switch. The heater fan is probably also run through its own computer module, so now that I think of it, look for corroded fuses under the hood first. Most manufacturers are using large ones in a box that is prone to water getting in. The ultimate goal is see if the supply voltage to the Engine Computer is less than battery voltage, or if battery voltage is less than 13.75 volts.

If possible, see if you can find the electrical connector for the fan motor and take a voltage reading on the feed wire when it's on the highest speed. Normally I would say you should have full battery voltage there, but if a computer is involved, it might be normal to find a little less than full battery voltage.

Caradiodoc

Using the heater fan or power window will draw lots of current which will lower system voltage and lower the spark voltage.

A different ignition coil might stop the symptoms while it might not actually be the cause of them. GM generators since 1987 develop a pile of huge voltage spikes and it takes a good battery to help absorb them so the spikes don't radiate electromagnetically into sensor wiring. Instead of the ignition coil, you might also stop the symptoms with a new battery and generator. (Think of drowning in the flooded basement of your house! You might overcome the problem with a step ladder to stand on, but fixing the busted pipe is the real solution). The ignition coil might be the step ladder, not the pipe.

If the generator is the problem, one of two things will happen when you unplug it. If the engine runs fine when using the window or heater fan, suspect voltage spikes. Start with a new battery. If the misfire is worse, suspect low system voltage. Replace the generator and the battery.

It is real common to go through four to six GM generators in the life of the car. What many professionals are finding out is the number of repeat failures can be reduced by replacing the perfectly good battery at the same time. The old battery will work fine in older, pre-1987 cars. As they age, their "internal resistance" increases and they lose their ability to absorb those voltage spikes.

Caradiodoc

What I would recommend that no one has mentioned, is checking and cleaning grounds. Particularly the engine ground down by the starter. There is a service bulletin that depicts the symptoms you are describing. There are updated parts that are referenced. Here it is.

Bulletin No.
T-92-76
File in Group
6E
Number
97
Corp. Ref. No.
016511R
Date
August, 1992

SUBJECT:
INTERMITTENT DRIVEABILITY CONDITIONS

MODELS AFFECTED:
1985-1992 DE VILLES AND FLEETWOODS 1986-1992 ELDORADOS AND SEVILLES

This bulletin supersedes Service bulletin T-90-55 which should be discarded. This bulletin is being revised to include additional symptoms, change corrections, and update model years.

Some 1985-1992 DeVilles or Fleetwoods and 1986-1992 Eldorados or Sevilles may experience one or more of the following driveability symptoms:

Engine Surge/Miss at Idle with High Electrical Load or A/C Load
Erratic Secondary Scope Patterns
Intermittent Backfires
Intermittent Loss of Power
Black Smoke/Rotten Egg Odor From Tail Pipe
Detonation
Dieseling
Intermittent No Start
Poor Fuel Economy
Intermittent PCM Code 91

These conditions may be caused by a poor engine ground or a poor sheet metal ground. To repair this condition it is important to remove and clean these grounds.

The addition of star washers is not necessary with the new cadmium-plated fasteners. The new cadmium-plated parts provide a superior ground path and over time don't promote corrosion or looseness. Use of star washers with these parts may remove protective coatings and cause corrosion.

1985-1989 DeVilles and Fleetwoods:

Grounds near the starter (refer to the appropriate Service Information Manual for location):

1. Remove the starter bracket mounting bolt to the block (or starter bracket nut, ground cable[s], and starter bracket mounting stud if equipped) and discard the bolt (or stud and nut if equipped). Refer to Figure 1.

2. Remove the nut that secures the starter bracket to the starter.

3. Remove the starter bracket and discard.

4. Install a new starter bracket, P/N 3538914, and stud, P/N 3537885. Torque to 38 N-m (27 lbs. Ft.).

5. Reinstall the nut that secures the starter bracket to the starter. Torque to 3.0 N-m (27 lbs. In.).

6. Clean ring terminals for grounds located at the starter bracket (if equipped) and grounds located at the motor mount with a wire brush.

7. Relocate cables that are grounded at the motor mount to the starter bracket stud. Install the ring terminals, largest to smallest, with the largest ring terminal against the stud.

8. Install a cadmium-coated 8 mm flat washer, P/N 25614644.

9. Secure ring terminals to stud using an M8 x 1.25 cadmium-coated nut, P/N 11500760. Torque to 27 N-m (20 lbs. Ft.)

1990-1992 DeVilles and Fleetwoods:

Grounds near the starter (refer to the appropriate Service Information Manual for location):

1. Remove the starter bracket mounting bolt to the block, the nut that secures the starter bracket to the starter, and the starter bracket.

2. Discard the starter bracket and the starter bracket mounting bolt to the block.

3. Remove the ground cable located at the motor mount.

4. Clean both the ring terminal and the engine block in the area where the starter bracket was located with a wire brush.

5. Using a new starter bracket, P/N 3538914, bend the starter bracket as necessary to provide clearance between the block and the bracket for the ring terminal.

6. Install the starter bracket with the ring terminal between the block and the bracket.

7. Install a new starter bracket mounting stud to the block, P/N 3537885. Torque to 38 N-m (27 lbs. Ft.).

8. Reinstall the nut that secures the starter bracket to the starter. Torque to 3.0 N-m (27 lbs. In.).

1985-1992 DeVilles and Fleetwoods:

Sheet metal grounds at the fender (refer to the appropriate Service Information Manual for location):

1. Remove cables and nut(s) from sheet metal ground stud.

2. Clean stud and ring terminals with a wire brush to remove paint, corrosion, or rust.

3. Install cable ring terminals, largest to smallest, with the largest ring terminal against the fender.

4. Reinstall one nut and torque to 9 N-m (6.6 lbs. Ft.).

1986-1989 Eldorados and Sevilles:

Grounds near the starter (refer to the appropriate Service Information Manual for location):

1. Remove the two starter bracket nuts, cable(s), starter bracket mounting stud, and starter bracket. Discard the starter bracket mounting stud, the nut that holds the cables to the stud, and the starter bracket. Refer to Figure 1.

2. Install a new starter bracket, P/N 3538914, and stud, P/N 3537885. Torque to 38 N-m (27 lbs. Ft.).

3. Reinstall the nut that secures the starter bracket to the starter. Torque to 3.0 N-m (27 lbs. In.).

4. Clean ring terminals with a wire brush.

5. Install cable ring terminal(s), largest to smallest, with the largest ring terminal against the stud.

6. Install a cadmium-coated 8 mm flat washer, P/N 25614644.

7. Secure with an M8 x 1.25 cadmium-coated nut, P/N 11500760. Torque to 27 N-m (20 lbs. Ft.).

1990-1992 Eldorados and Sevilles:

Grounds near the starter (refer to the appropriate Service Information Manual for location: 1. Remove the two starter bracket nuts, cable, starter bracket mounting stud, and starter bracket.

2. Discard the starter bracket mounting stud and the starter bracket.

3. Clean both the ring terminal and the engine block in the area where the starter bracket was located with a wire brush.

4. Using a new starter bracket, P/N 3538914, bend the starter bracket as necessary to provide clearance between the block and the bracket for the ring terminal.

5. Install the starter bracket with the ring terminal between the block and the bracket.

6. Install a new starter bracket mounting stud to the block, P/N 3537885. Torque to 38 N-m (27 lbs. Ft.).

7. Reinstall the nut that secures the starter bracket to the starter. Torque to 3.0 N-m (27 lbs. In.).

1986-1992 Eldorados and Sevilles:

Sheet metal grounds at the fender (refer to the appropriate Service Information Manual for location):

1. Remove cables and nut(s) from sheet metal ground stud.

2. Clean stud and ring terminals with a wire brush to remove paint, corrosion, or rust.

3. Install cable ring terminal(s), largest to smallest, with the largest ring terminal against the fender.

4. Reinstall one nut and torque to 9 N-m (6.6 lbs. Ft.).