RealDealZ28

I'm working on a '95 Z28. When I bought it, the motor was locked down. I had the block bored .030, had the crank turned .010 under, and had the OEM cylinder heads rebuilt to stock. I put in new pistons, rings, hydraulic lifters, etc. The only deviation from stock is a Comp Cams CC306 cam and Comp's recommended valvesprings.

When I cranked the rebuilt motor for the first time, it ran horribly. I had adjusted the valves on the engine stand without the motor running and thought maybe I had done it wrong. I did a compression test and had no compression in cylinders 1 and 3. All other cylinders were at 150 psi. I pulled the driver's side valve cover back off and backed off the valves for cylinders 1 and 3. When I redid my compression test, cylinder 1 came up to 150 psi and cylinder 3 came up to 148. At this point, all 7 cylinders were at 150 psi, except for cylinder 3, which was at 148 psi, which seemed normal. With the valves adjusted, the motor fired and ran normally, except for a consistent misfire at all RPMs.

Here's what I've done to try and troubleshoot.

• Verified that all 8 plugs are getting spark - used a spark test light and ran it in series between the plug wire and the spark plug and let the car run.
• Double checked that spark plug wires were connected and that I had the firing order correct (plug wires are new AC Delcos)
• Checked fuel pressure using the schrader valve on the back of the fuel rail. Pressure was in spec.
• Pulled the fuel rail and tested each injector - used a Power Probe and manually triggered each injector so I could look at the spray pattern
• Tested the fuel injector wiring to make sure each injector was getting 12V with the key on
• Checked everywhere for vacuum leaks
• Changed out the Optispark with another OEM one I had on hand
• Pulled the valve covers and re-adjusted the valves with the car running. I backed each rocker off until it clattered, tightened it back until the noise went away, and then tightened it another half turn.

I'm hoping it's not a bent valve, but if it was, wouldn't the affected cylinder have noticeably less compression?

Thanks in advance for any advice/suggestions.

Injuneer

Did you pull the plugs and check them for cracks, incorrect gap, fouling, etc.? Did you try misting the plug wires in the dark and looking for arcing? Did you use a noid light to verify the PCM is supplying the ground to fire each injector?

Has the PCM been tuned for the cam? By whom?

RealDealZ28

I did pull the plugs before I did my compression test--that's when I realized I probably had a valve hung open on two cylinders, as two of the plugs clearly hadn't been firing. However, I haven't pulled the plugs again. I will do that again. On the plug wires, I tested them yesterday in low light so I could see if any were arcing. I did use a noid light on the injectors, but will do that again just to be sure I haven't overlooked anything.

The PCM was tuned by Keith at PCMforless.com. I sent him the cam specs and told him it was an otherwise stock engine besides the CC306 cam.

Injuneer

Have you tried pulling the injector wires one at a time, to see if you can pin down which cylinder? Does it have headers? Might facilitate checking each primary with an infrared gun to see if one cylinder is cooler than the others?

Another way to find if the misfire is specific to one bank is a PCM data log that will allow us to look at the long term fuel corrections for signs if the misfire.

RealDealZ28

I have tried pulling the injector wires. To me, it sounds like cylinder 5 definitely and possibly cylinder 7 have an issue. I don't see or hear a noticeable difference in how the motor runs when either of those injector wires are pulled individually. I have purchased headers but have not installed them yet. The car currently has the OEM manifolds installed.

I did hook the car up to our shop's Snap On Solus Pro scanner, hoping it might give some insight. It didn't tell me anything other than all of the sensors seem to be reading within spec. It's not setting a code for a misfire, and the check engine light never comes on.

Injuneer

There is no misfire detection on a 95. That was added in 96 as part of OBD-2.

Can the scanner read the long term fuel trims (LTFT)? At idle should be operating in Cell 16, and show individual LTFT readings for Bank 1 and Bank2. A misfire dumps unburned fuel and air into the exhaust. The O2 sensor sees the oxygen, interprets it as lean, and the PCM adds more fuel to correct. Now you have 3 cylinders running rich and one still misfiring. The Bank with the misfire is likely to show an elevated (greater than 128) LTFT on the Bank that has the misfire. None of this would set a code.

An alternative to using the scanner would be the free download of Scan9495 software, specifically for the 93-95 LT1. Written by a member here who can help with install and setup, if any problems. I review the data logs, and sometimes can find the problem.

https://www.camaroz28.com/forums/com...95-lt1-874306/

i also have a guide to scanning the LT1:

https://www.camaroz28.com/forums/com...-guide-886891/

RealDealZ28

Just pulled the plugs on the driver's side bank. Cylinder 5's not firing, as the spark plug is way too clean. I went ahead and put a new spark plug in, but it's still skipping.

Injuneer

Check the plug wire. Check both ends. Make sure the part that sticks into the Opti is not corroded of damaged. Make sure the wire isn't cracked or damaged at the plug boot, Are you using a factory metal shield on that plug boot? Sometimes that can cause problems. The fact that both 5 and 7 were identified as possibly misfiring could indicate the wires for 5 and 7 are cross-firing.

RealDealZ28

I've checked the wires, both ends. When I run the car with a test light plugged into cyl 5's plug wire, it indicates that the wire is firing correctly. I am using the metal shield on cyl 5 and 7's plug wires.

RealDealZ28

I went back and pulled the driver's side plugs. Looks like cylinder 5 isn't firing at all, and cylinder 7 is really rich.

RealDealZ28

Well, I may have found something. And I take full responsibility for being a dumbass. I was having trouble reinstalling the spark plugs on the driver's side bank from the top of the engine. I decided to do cylinders 5 and 7 from underneath. While I was under the car, I physically traced the plug wires for 5 and 7. I had them swapped on the optispark. My working theory is that cylinder 5 was firing all along, just at the wrong time. That explains why I never could find a problem with the valves, injectors, etc. As showing in the picture, cylinder 7's spark plug was very fouled. That's because it was firing in the wrong sequence.

I corrected the spark plug wires and took it for a short test drive. I ran out of time to drive it much, but from what I can tell, the misfire is gone. I do still have some slight hesitation that I'm going to have to look into, but the car is at least drivable now.

Injuneer, thank you so much for your help with this! And again, it's completely my fault that I didn't triple check the Opti. I checked it one time from the top of the car, and from then on assumed it was good.

Injuneer

Assuming you cleaned those plugs. Hesitation could be in the tune. Hard to find a good mail-order tuber these days.

RealDealZ28

Who do you recommend?

Injuneer

Moe Bailey in Texas.

MoeHorsepower@peoplepc.com

254-644-2656