"Fixed" 1988 A/T 4.0 missing, backfiring, almost zero throttle response

[Shoveldead]

Let me start by saying, the truck was running beautifully until 2 days ago.  Engine died at a stop light.  When trying to restart, it stumbles, idles rough, back fires.   Won't build rpm.   Will run with erratic idle speed and then shut down.  Easily restarts with same conditions.   I performed all the Cruiser54 tips except for upgrading the sensor ground (those were checked).  It does have upgraded grounds and positive cables everywhere else.   Things I have tried that did not work, tested and replaced with known good parts the following: ECU, CPS, TPS, IAC, MAP sensor, Coil and Ignition module, distributor, fuel pressure regulator, injectors.  Setup distributor per Cruisers instructions.  Fuel pressure is 40 psi at the rail and it holds a good long time after you turn it off.  Pushed, pulled and twisted all engine wiring looking for a problem, no change.  I am out of ideas.   Any body have anything else that I havent checked already?   Smells extremely rich when its running.    

[Ωhm]

Bypass the fuel pump ballast resistor just to make sure it's not part of this problem.

[Shoveldead]

Thanks!  I will check that now.  I have a known good one if those as well.   What exactly does the ballast resistor do in the system?  

[Ωhm]

98% of the time, fuel pump current flows through the ballast resistor causing a voltage drop across the resistor. This lower voltage is applied to the fuel pump motor for the purpose of quieting down the motor noise. People complained about fuel pump noise.

[Shoveldead]

Just tried the new resistor.  No joy.   

[Ωhm]

2 hours ago, Shoveldead said:

Smells extremely rich when its running

 

Check Fuel Pressure Regulator (FPR) hose to manifold for fuel. I know you said it new. Check hose to MAP sensor from manifold.

 

 

2 hours ago, Shoveldead said:

Setup distributor per Cruisers instructions.

 

Might need to double check this. Symptoms sounds like misfire in the distributor cap.

[Shoveldead]

No fuel in the regulator vacuum line.  2 distributors tried, both have been indexed per cruisers instruction, the one that was in it when it started having the problem was indexed exactly the same.   Noticed lots of wet fuel on #1 spark plug.    

[Ωhm]

If your holding fuel pump rail pressure (40psi) with the KEY OFF a leaky injector would show that pressure bleeding down. Maybe check this again. No mention of new secondary wires for the ignition.

[Shoveldead]

Sorry.  Tried new cap, wires, plugs.  Swapped in new injectors as well.  Leaning toward a compression test now

[Ωhm]

Starting to sound more mechanical now. Anyone else here, chime in.

[Shoveldead]

Performed a compression test.   Most plugs were very wet with fuel when I pulled them out.  #1 cylinder at 80 psi.  All others sitting right around 100 psi on my guage.  Seems really low across the board according to the specs I could find.  Suppose it could be my tester as well.  

[Shoveldead]

Found the issue.   Rotor in the distributor had a invisible to the naked eye hole in it that was allowing spark to ground out from the center contact in the distributor cap to the distributor shaft.   That rotor probably had 150 to 200 miles on it.   I think compression test was low because of all the gas the injectors had washed the cylinders with.   Never seen this problem with any vehicle before.   The rotor still looked brand new with no arcing on any of the contacts 

[Pete M]

wow.  that's certainly a new one to me.   but glad you caught it!   

[Ωhm]

Proof that firing the 'parts cannon' will sometimes solve your vehicle problems. Suggestion: Buy a Powerball ticket.  

[cruiser54]

Strange for sure. 

[Shoveldead]

There was a tiny burn mark inside the distributor shaft cavity on the underside of the rotor where the spark had been travelling through it and grounding on the distributor shaft.   New rotor and all problems disappeared immediately.    Strange is an understatement!