1992 Deville 4.9l stumble upon medium-hard acceleration

[sixtyfiveford]

I have a 1992 Cadillac Deville that stumbles/vibrates on medium-hard acceleration. If I'm cruising and I dump the gas pedal it will stumble but if the tranny down shifts it will act very smooth and peppy. So it seams to just have a misfire at the mid rpm range. Assumtion was misfire so I pulled the plugs and #1 & #8 were souty black. So they shold be my culprite. Here is what has been done:

-New Plugs

-New Wires

-New Cap & Rotor

-New Fuel injectors out of a Crown Vic (yellow top)

-New Fuel Filter

-High octane premium fuel

-Fuel Pump tested good (around 45-50psi if I recall)

-EGR tested good

No Check Engine codes are present but there are some reoccurring Service Vehicle Soon Codes: E91(PRND123 switch) , E61(Cruise Control), E98(P-N D-R issue(only once)).

All of these things made a very minor difference and maybe no differnce at all and it's just in my head. But still acts the same.

Where should I go from here?

[KHE]

Remove the air inlet piping to the engine and hold the throttle plate wide open by the linkage. There will be two EGR tubes about 1/2" in diameter. If they are plugged up with carbon, take a screwdriver and rod them out and vacuum out as much of the crud as you can.

[barczy01]

ECM had problems with those cars and would cause a misfire. Fuel injector harness was an issue, loss of injector pulse.

Edited December 8, 2014 by barczy01

[KHE]

The fuel injectors themselves could also be an issue on the 4.9 engine.

[sixtyfiveford]

Remove the air inlet piping to the engine and hold the throttle plate wide open by the linkage. There will be two EGR tubes about 1/2" in diameter. If they are plugged up with carbon, take a screwdriver and rod them out and vacuum out as much of the crud as you can.

I'll take a look at this.

Edited December 8, 2014 by sixtyfiveford

[sixtyfiveford]

The two EGR ports: One closest to front of car was spotless and the other had mabye 1/32" of light carbon that I cleaned off.

I pulled the plugs in one of the known misfire cylinders #8. It is new(15minutes of running) and it is already getting sooty black, signifiing misfire. I decided to do a compression test on this cylinder to rule that out. It jumped right up to 160psi, compression is great.

I threw my timing light onto the #8 lead so I could watch for spark pulses. It seams good but I wasn't able to test it under load.

[barczy01]

unplug the egr valve and drive to see if it helps.

[KHE]

Have you checked the fuel pressure upon acceleration? It should not drop off.

[sixtyfiveford]

The fuel pressure stays steady with acceration(just under 50psi).

Just thinking: If the spark plugs are blackening with a rich mixture, wouldn't this signify no spark or an injector that is injecting to much fuel? The same cylinders have fouling with the different injectors and all new wires/plugs/rotor/cap. So installing new different items made no difference. Testing spark to the cylinder tests good so could the injector be recieving signals to fire excessively or when it's not its turn?

[KHE]

Based on the fuel pressure readings, you can conclude the fuel pump is good.