1977 Cylinder #1 Mystery

[Reptoid Overlords]

Jeff G and Zed Head provided good advice when I was exploring stock EFI and more cam.  Ended up just going straight to aftermarket EFI.  Fast EFI is what I have and it works fine, but kind of wish I had more tuneability. 

Although your problem sounds like it's somewhere in the harness, and possibly not the ECM/ECU. Just something else to read.

https://www.classiczcars.com/forums/topic/63898-lift-480-480-duration-274274-cam-with-oe-fuel-injection/

 

[racer47201]

I had the bucking on my 82; turned out it was my O2 sensor. If you have one check it out.

[Jeff G 78]

The injectors are batch fired, so the order of the injector plugs doesn't matter.  I can't remember if you said if you tried swapping the injector wiring between injectors #1 and #2, but if not, that's the next thing I'd try.  If the issue follows the plug, you know it's a wiring issue in the injector harness.  If the problem stays in cylinder #1, you can rule out the fuel system since you already changed the injector.  At that point, you can start looking elsewhere.

Assuming the issue stays in #1, I'd recheck the cylinder itself next.  If you have a reasonably decent air compressor, go to Harbor Freight and buy their cheapo leakdown checker.  It's similar to a compression gauge, but you put compressed air into the cylinder rather than using the starter to build pressure.  With the piston at TDC, you add pressure and listen for air leakage.  It can leak from several places and each tells you what's wrong.  If you hear air in the airbox, you have a leaky intake valve.  Air from the exhaust is a leaky exhaust valve.  Leakage out of the adjacent spark plug hole or if the coolant in the radiator bubbles, it points to a bad head gasket.  Finally, air coming out the timing chain cavity means the rings aren't sealing.

The tester itself has gauges on it that tell you the percentage of leakage.  Less is better and under 10% is good.  Again, cylinder to cylinder variation will tell you if #1 is unlike the others.

If the fuel injector test and leakdown don't give you the answer, then I'm at a bit of a loss without more details.

Good luck and let us know.

[ckurtz2]

@Jeff G 78

Thank you! So yes, the #1 cylinder didn't experience a change when swapping connector from #1 to #2. Only sometimes would the #1 connector work on the #2 connector. I felt like I was getting close with figuring it out, due to having no voltage at the ECM from pin 15(cylinder 1). However,  I then messed around with the dropping resistor connector by pushing it in farther (it didn't seem like it wanted to go farther, but it was barely on). I then started the car again and noticed the noid light I had on the #1 injector connector was flashing. So I reconnected the connector to the injector and magically the #1 cylinder began firing again. However, the car ran even worse than before oddly enough, so I looked if all the cylinders were firing. Low and behold I noticed the #5 cylinder was barely firing or just plain misfiring (it would make the motor drop rpm a lot more before. It also makes noticably less difference than any other cylinder).. I tested this many times to confirm I wasn't loosing my mind. I did this by pulling the plug wire while it was running and would barely noticed a change. I want to mention when the motor was cold it made no difference, and when warm is when I could tell it was firing, just really weakly.

So all the cylinders work now, it just bucks like hell and is definitely missing. I pulled the #5 spark plug and it looked wet with fuel after running (probs from pulling spark a few times). So here is a question, have you ever worked on a Z that ran really poorly due to carbon fouled spark plugs? Mine are still kind of fouled, but they are new. They fouled be me toying with the AFM while diagnosing the #1 cylinder issue. 

Thanks for the info! I definitely need to do a leak down test at some point.

[ckurtz2]

Also for reference, the cam I am running is the Comp Cams 260s. https://zcardepot.com/products/performance-camshaft-comp-racing-cam-240z-260z-280z?variant=19279549825137

Less lift and duration than the one @Reptoid Overlordswas considering and less lift than what you had in the past that didn't like the EFI. Don't know if that matters. 

[Jeff G 78]

1 minute ago, ckurtz2 said:

Also for reference, the cam I am running is the Comp Cams 260s. https://zcardepot.com/products/performance-camshaft-comp-racing-cam-240z-260z-280z?variant=19279549825137

Less lift and duration than the one @Reptoid Overlordswas considering and less lift than what you had in the past that didn't like the EFI. Don't know if that matters. 

The cam in question sat on my shelf from 2006 until 2013 when I put it in my SU carbed L28 race car.  I loved the cam in that engine.  It pulled strong up to 7,000 RPM.  It was a night and day difference between how it ran with the EFI and the SU's.  I still have the EFI car with a stock cam and it runs just fine.  It just didn't like the duration of the cam and the low vacuum that came with it.

Speaking of which, what is your vacuum reading at idle?

Your last post tells me that there is a problem with the connector and/or wiring between the dropping resistor and injectors.  You went from #1 not firing to #5 not firing.  If I recall correctly, there are two resistors.  One for 1-3 and the other for 4-6.  Somebody correct me if I'm wrong.  It has been a decade or two since I did any work on a EFI Z.  The fact that you went from 1 to 5 is a bit odd since they are on different resistors (if I remembered right).  Use DeOxit on both sides of the dropping resistor connection and then take resistance measurements from one end to the other for each wire going between the dropping resistors and the injectors.

[Mark Maras]

9 hours ago, ckurtz2 said:

@Jeff G 78

 

 So here is a question, have you ever worked on a Z that ran really poorly due to carbon fouled spark plugs? Mine are still kind of fouled, but they are new. They fouled be me toying with the AFM while diagnosing the #1 cylinder issue. 

 

 I have many times and I'm sure others have too. I'd suggest buying a couple of sets of new plugs (NGK BP6ES is recommended) and replace the fouled ones. It can make a difference.

[Zed Head]

Are you running the 1977 ignition system?  1977 was still using the weak electronic system.  Your cam is going to cause variation in the gasoline/air ratio, especially if you try to maintain a low idle RPM, plus the batch fire nature of the old EFI system also causes variation across the cylinder.  A better ignition system might help.

[ckurtz2]

To answer everyone's questions

@Jeff G 78 My vacuum reading at idle when cold is around 11hg which is terrible, and when warmed up it climbs to around 15.5hg. Still below what it should be with that cam and well below stock numbers. Yeah, there are two resistors, and both cylinder 1 and cylinder 5 are on different ones, so I am confused as well. It could have been mere coincidence, but I am not quite sure. It will all be looked at this weekend. 

I will make sure to do spark plugs again then. would you recommend cleaning them, or just new ones. 

@Zed Head yes it is still the stock ignition system with a petronix flame thrower coil and rebuilt distributor. What would you recommend?

 

[Jeff G 78]

8 minutes ago, ckurtz2 said:

To answer everyone's questions

@Jeff G 78 My vacuum reading at idle when cold is around 11hg which is terrible, and when warmed up it climbs to around 15.5hg. Still below what it should be with that cam and well below stock numbers. Yeah, there are two resistors, and both cylinder 1 and cylinder 5 are on different ones, so I am confused as well. It could have been mere coincidence, but I am not quite sure. It will all be looked at this weekend. 

I will make sure to do spark plugs again then. would you recommend cleaning them, or just new ones. 

@Zed Head yes it is still the stock ignition system with a petronix flame thrower coil and rebuilt distributor. What would you recommend?

 

Sounds a lot like when I tried the performance cam in mine 15 years ago.  I was getting around 11 and it ran like crap.  Like I mentioned a few posts back, I'd clean contacts in the connectors and then check resistance on each wire to see if the harness has an issue.  You never know if it got pinched at some point and has broken strands that are barely flowing current.

You can clean plugs, but for a few bucks each, I wouldn't try.  I've hit them with a torch to burn off oil and gas and then wire brush them clean.  I used to do that all the time on my old 2-stroke dirt bike plugs when they would foul.

Edited April 7, 2022 by Jeff G 78

[Zed Head]

Multiple spark discharge ignition was kind of developed for race cams so they could get cleaner low RPM performance.  MSD built their company on it.  It might help.  And raising your idle RPM will probably increase your vacuum reading.  These are the small things the guys that get theirs to run just fine don't tell you.

You could swap to a GM HEI module really easily for cheap and you'll get a stronger spark.  Get rid of the ballast resistor and keep the Flamethrower.  When you say rebuilt distributor you don't mean a ZX distributor, right?  You're still using the module down by the fuse box.

 

Here's some MSD history.

https://en.wikipedia.org/wiki/MSD_Ignition

[ckurtz2]

Ok, just got new plugs. I will install them before the car goes to the shop as well. 

Interesting, I am looking into it:) Does the GM HEI or Petronix HEI modules really make that much of a difference in the strength and consistency of the spark? Trying to save some dollars and cents. Yes, I am still running the stock dizzy with the transistor ignition module inside the car as well.