hard miss on #6 only notice at idle

[Dave WM]

while on a long road trip I pulled into a stop and with the  AC off (rarely around here) I pick up on a hard miss fire. AC on idle up goes largely away, Idle up take it to about 1.1k while AC is on. I do this to maintain battery voltage under the heavy load while sitting in traffic with AC on. Helps with the rad cooling as well.

thoughts

leaking fuel injector, result in rich idle but not enough to cause issue when more air flow at higher rates.

poor spark (new plug same deal) could see a spark but did not compare to a good running plug to see if it was weak. (cap/HT lead)

Compression loss (valve lash checked ok)

Place your bets...

My diag is going to start with a cold startup to see if the miss is on a hot engine and not on a cold (fuel mix issue, leaking injector not a prob due to eng needing more fuel anyway when cold).

Will read plug (new installed 100 miles ago)

Will install color tune to see combustion process.

Install fuel pressure gauge if it looks like an injector issue, watch for bleed down.

IF FI is a likely problem may try to pinch off the rubber line on the #6 injector with some bent needle nose while looking at the color tune to see if it comes alive. This is operating under the idea that the mix is too rich and pinching the line will lean it out. I can hear the injector clicking so I know its not stuck open. Not sure if that rubber line is long enough to pinch though.

will prob swap the cap and wires with known good before diving too deep as its pretty quick. Same with ECU.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SteveJ]

This is where it would be great to have the ability to measure spark voltage on a scope while driving. 

Great! Now you have me looking at portable 2 channel oscilloscopes! Dang it, I'm spending enough money on tools already.  

[SteveJ]

By the way, on my 240Z, I was trying to re-learn how to use an oscilloscope and was measuring the voltage at the coil, correlating it to an inductive pickup on the coil wire and #1 plug wire. The waveform of the coil voltage had irregular peaks. When I reviewed my results against some Google research, I started looking for extra gaps in the high voltage circuit. Sure enough I found a corroded high voltage terminal on the coil. I plugged in a new coil, and the volume of voltage spikes decreased precipitously.

[Dave WM]

hmm did not think to check the gap on #6 reluctor, will add that to the list. Maybe pull two plugs and crank to compare spark visually as well.

[Zed Head]

How do you know it's #6?

The brake booster supply hose is back there, and the heater/AC valve vacuum supply.  I've noticed that the intake air supply across all six is not even.  #1 tends to run lean (whiter plugs, more air), and 5 and 6 tend to run rich (dark plugs, less).  Could be too lean from a vacuum leak or too rich because of design.

[Dave WM]

1 hour ago, Zed Head said:

How do you know it's #6?

The brake booster supply hose is back there, and the heater/AC valve vacuum supply.  I've noticed that the intake air supply across all six is not even.  #1 tends to run lean (whiter plugs, more air), and 5 and 6 tend to run rich (dark plugs, less).  Could be too lean from a vacuum leak or too rich because of design.

Power test, remove the plug wire from #6, no noticeable drop in rpm. the rest of the plugs had a drop when HT lead pulled.

[Dave WM]

well the verdict is in.....

 

80 psi on #6 the rest were about 155-160.

using a leak down test I found it to be in the exhaust (lots of blow by coming from the tail pipe, intake and crank case were negligible.

Looked with a bore scope, cant see the valve itself as its only a look down type, but the piston crown was very clean (I used some redline injector cleaner for the trip).

starting to wonder if its possible the carbon could be knocked loose and is fouling the valve. I tried tapping the exhaust valve from the top while under pressure to see if it would seal better on rebound, no joy. The lash pads are in place and the lash is correct at .010 for the exhaust.

Before diving into pulling the head I am going to try an Italian tune up. I drive like a little old lady so will give it a go and see if there is any change. 

worse case I have a backup head N42 head and a MN42 head. I know the MN is ok, the N42 may need some work as IIRC it was just a tad out of spec for warpage.

Videos will be posted soon...

 

 

 

 

 

[Zed Head]

Is this your 75 with original engine?  They have the poor valve seat material.  Needs lead.

[SteveJ]

Just now, Zed Head said:

Is this your 75 with original engine?  They have the poor valve seat material.  Needs lead.

Great point. Unless the valve seats have been replaced with hardened seats, one could have become hammered.

[Dave WM]

it was a California car, has use unleaded gas on the gauge, I assumed they would have accounted for that, maybe not.

[Dave WM]

 

[Dave WM]

Italian tune up did the trick! #6 at 155, but now #3 is at 125, hmmm seem odd as it was fine. I suspect I just need to do some more hard driving. Got video of the results (otherwise it did not happen). Same deal on #3 Exhaust valve (wet test no change, blowing smoke from smoldering rag out tail pipe at 90psi from leak down test). I am a beliver in the Italian tune up now. I would take it up to 5500-6000 and some hard pulls thru the gears maybe 15 miles out on the Tpike. So the head will stay on for a while.

also managed to break my antenna mast coming into the garage, not sure how. will be posting in the classified for a donor...