My 91 z34 is bogging down and misfiring?

[AnthonyD07]

My alternator went so I changed it and when I started it up, it started to bog down and misfire.

so I changed the EGR valve and the plugs,that still didn’t work, still misfiring and what should I change next ??

[Schurkey]

Change your procedure.  Stop replacing parts, start diagnosing.  The only parts you should be replacing are the sort of "tune-up" items that would be expected to fail after a normal service life.

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. TBI systems don't hold pressure when the pump stops running, but the other fuel injection styles should. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)

2. How old are the usual “tune-up” parts and procedures? Inspect/replace plug wires, spark plugs. Use quality parts sourced from an authorized seller (NO COUNTERFEIT PARTS FROM AMAZON, EBAY, OR OTHER SKETCHY SELLERS!) Be sure the ignition coils will reliably fire a spark-tester calibrated for HEI when the coils are fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper electronic spark advance. Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.

3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle.

 

 

[bluecalais79]

Watch those rear 3 spark plug wires, they need to be securely popped into place. I had one loose when I first bought my 94 3.4 vert and it ran like crap until I popped in the loose #1 plug. 

[Black92GS]

Not a 3.4….but fuel and ignition wise they are very similar in how they operate. Not a bad idea to watch these to get some troubleshooting ideas   

 

[Black92GS]

And if it’s a constant misfire, try something as simple as a cylinder drop test first. Unplugging each cylinder individually at the coil to see which are contributing and which are not will be very helpful as to which direction to go. 
 

 

[Schurkey]

On 12/9/2024 at 12:31 PM, Black92GS said:

And if it’s a constant misfire, try something as simple as a cylinder drop test first. Unplugging each cylinder individually at the coil to see which are contributing and which are not will be very helpful as to which direction to go.

Right idea...wrong procedure.

Removing the plug wire from the coil creates an open circuit on the ignition secondary side.  An open circuit drives the voltage sky-high, which can cause insulation failure including inside the ignition coil.

Find a way to GROUND the spark, which drops the voltage to zero, and causes no insulation stress.  In this case, if you remove the plug wire from the coil, connect a jumper wire from the coil terminal to engine ground.  THEN start the engine to see if that cylinder is contributing.

 

It's easier on a distributor than on a coil-pack.  A few small nails, blunted, and dabbed with silicone dielectric grease, slid BETWEEN the plug wire and plug boot (Don't puncture the insulation) so that the nails touch the metal spark-plug wire metal terminal; and a grounded INCANDESCENT test-light or jumper wire works wonders.  Start engine, touch test light or jumper wire to each nail in turn.

Maybe you can sneak small nails into the coil-pack end of the plug wires.  I wouldn't be able to on my 3.4L Luminas, there's no room to work with the coil packs bolted to the front of the engine.

 

 

For the record...since you had an alternator failure, did you ever re-charge the battery?  Low battery voltage can cause all sorts of problems.

Edited December 18 by Schurkey