Rough running engine with no explanation

[Av8ferg]

I thought I start a special thread on a recent development on my Z.   Absolutely beautiful day today and perfect for driving the Z.   Pulled it out of the garage and I noticed it was running very rough.  It didn't really hit me until I tried to accelerate away from my house.   Engine was weak, and running rough as if a cylinder was down.   Hmmm....ran perfect last time I drove it.  The only thing Ive don't since last drive was to pull the cold start injector and test it ton make sure it wasn't the cause of my running rich condition.   I put it all back together as it was but when a problem developes its prudent to inspect the last item you touched.  

So I drove back to the house and began going through each possibility one by one and I still cannot figure it out.

Checked:

1. Spark...strong on all wires.  Got about a 1" spark from each plug wire.

2. Timing.. 11 Deg before TDC at idle

3. Full Pressure- 30 PSI at idle 

4. Checked each injector with a screw driver, all were firing.

5. Extra Fuel filter I installed coming from tank is clear of debris

6. Pulled the valve cover off.  Valve train looks normal.  Checked valve lash, all were good. 

7. Swapped to my backup E12 ignition unit and no change 

8. Replaced the ECU

9. Checked for vacuum leaks, all looked normal

10. Checked Temp sending unit...unplugged and car died.  Acting normal

what else could I be missing. 

 

[siteunseen]

Low battery maybe? Could be a vacuum line fell off? Check your vacuum. Hope it's nothing! Good luck

[Patcon]

Is your cold start valve still plumbed in? Do you have a way to measure AFR? Maybe the cold start is dumping fuel in? Maybe blank off the line?

Have you checked the plugs themselves for fouling?

[Av8ferg]

Battery is good, vacuum is good.   CSV is plumbed and functioning properly.  
 

next step is a compression check.   

[Zed Head]

2 hours ago, Av8ferg said:

running very rough.  It didn't really hit me until I tried to accelerate away from my house.   Engine was weak, and running rough as if a cylinder was down.

Is it running bad rich or running bad lean?  Bad lean would give you some intake popping and bucking.

You might check the FPR vacuum reference hose for fuel.  That is something that could go bad suddenly.

Also, if you still have EGR might be worth checking that.  Maybe a channel opened up in the EGR passage.

And, of course, the old thrown rocker arm.  Fairly common after an engine has been sitting.  You can eyeball that under the valve cover.

[Patcon]

Have you tried pulling the plug wires one at time while it's running to see if one cylinder doesn't change (ie isn't firing)? Use a leather glove; getting tagged by the coil is very unpleasant and not very healthy!!

[Zed Head]

I'd pull injector plugs.  Electronic ignition systems don't like it when their spark can't get to its intended destination.  It will go somewhere though.  I damaged an HEI module when I left two plug wires off.

[Av8ferg]

Good ideas,  So yes I have checked the FPR.  When idling I pulled the vacuum line off the FPR and the fuel pressure increased to around 40 psi +/-2.  
I already shocked the heck out of myself when pulling each spark plug wire of the plug when running, didn’t feel to good.  The spark is strong, and jumps over an inch I’d say.

Rockers are are good,  pulled the valve cover and inspected the entire valve train.  

Here is a data point when I pull either the #6 plug wire or the #6 injector wire there is no charge in engine idle condition.  When I pulled any others I do notice the engine running worse.   I suspect #6 could be the culprit but don’t want to assume just yet.  I pulled the plugs to look at them and they look terrible.  They have maybe 400 miles on the and are pitch black and heavy soot.  See pics below.  This may be the problem.  It verified what I already knew…..that I’m running very rich.   Maybe it finally started to manifest in fouling.
In my quest to solve the rich condition I’ve done several things.  Replaced the AFM and ECU with good known tested combo from a car that didn’t run rich when installed.  I bench tested the AFM per the FSM and it checked good.  I removed the Throttle Position Sensor, opened and cleaned and checked contacts with multi meter, I replaced the AAR with one I bench tested and checked the opening warm and cold to confirm.  The car ran well after all those replacements albeit still same rich condition…that didn’t change at all.  My next suspect is the Water Temperature Sensor.  I have a P79 head so I opted to use the 82’ head temp sensor with my car not one that goes in the thermostat housing.  This one is slightly different in that it’s not for “wet” applications as the 280z ones are, but maybe it’s specs are slightly off because of that. I’m guessing the temp at the head between Cylinder #6 and #5 is hotter that the engine water temp….maybe? Or a ZX ECU has a different temp band curve?  So I need to confirm the resistance at specific temps.   I had to add 2 feet of wiring to run that temp wire around the back of the engine.  Could be extra resistance in the circuit or a bad sensor.  Might just go back to the putting the right 280z sensor in thermostat housing.  
I don’t have a compression tester so I’ll get a loaner from AutoZone maybe tomorrow.  
The first pic is plugs 5&6. 

[Patcon]

4 of those are pretty dark

[S30Driver]

Since you are using the CHTS instead of the water temp sensor, warm up the motor, shut it off, and use your multimeter to check the resistance at the big ecu connector.  The fuel injection bible has the chart for the temperature reference.   If I remember correctly, it is pin 13 measured to ground. 

[Zed Head]

9 hours ago, Av8ferg said:

I’m running very rich.   Maybe it finally started to manifest in fouling.

This seems most likely.  I remember way back in my early car days spending a lot of time and effort trying to figure out a similar problem, only to find that it just needed a tuneup.  Really, just new clean plugs.  The carbon will absorb moisture while sitting and get more conductive.  Then you get no spark.

40 psi is high on the fuel pressure.  Many new aftermarket regulators seem to run high.  4/36.3 = 11%.  That's a lot of extra fuel.  Plus, if you have a new aftermarket AFM it might be running rich also.  My MSA reman did, bought back in 2009/2010.  And, I've also read that there is a lot of variability in the coolant temperature sensors.

An adjustable FPR like Hussein Holland and I were talking about might help solve the running rich problem simply.  Watch out for the shiny aluminum adjustable FPR's, they almost all leak pressure rapidly when the pump shuts off.

 

[siteunseen]

I bought an aftermarket adjustable fpr when I put my aluminum fuel rail on, it was crap. Ended up using the 1978 OE fpr. One in, one out plus the return and vacuum nipple.