LT5 Misfiring at All Speeds

[Big D]

Hi all, my 90 ZR-1 is missing/rough running at all speeds especially when it reaches operating temp. Turning the power key on and off made no difference. I get spark at all plugs but when I pulled the #3 wire the idle didn't change much. Let her cool down and pulled the plug and the plug showed little evidence of firing. Didn't smell much of fuel either. Pulled a couple of others and they looked good. Next step was to check injector resistance and it was as follows: 1:15.3, 2;14.5, 3:5.7, 4:14.7, 5:13.0. 6:17.8, 7:15.1 and 8:17.3. I replaced the #3 plug and still the same. this time I checked the injector resistance when it was still hot and got the following:1:18.2, 2;19.8, 3:8.0, 4:19.8, 5:11.0. 6:20.6, 7:18.8 and 8:20.4. Now I'm thinking maybe a bad injector at #3 and maybe #5. If so, looks like I'll be pulling the plenum for the first time, what fun. Any suggestions would be appreciated
Big D
'90 ZR1
'69 L46/M21

[TomZR1]

Change your coil packs, I had same problem. I did all the tests, fuel pressure new plugs wires, even thought bad gas.
(01/24/25: Fuel injectors turned out to be Op's problem, not bad ignition coils)



___

[Dynomite]

Originally Posted by Big D

Hi all, my 90 ZR1 is missing/rough running at all speeds especially when it reaches operating temp. Big D
'90 ZR1
'69 L46/M21

Welcome to the Forum

Injectors need replacing

[tomcat]

I agree with all the above posts. Take your time, their are folks in Oregon & Washington that can help you.

Manfred E.......
91 #906

[Paul Workman]

Jon at FIC can set you up with a new set of stainless injectors and gaskets and seals (Gaskets c/o Jerry's Gaskets, BTW). I use the Accel #150821s (from Summit Racing) with no complaints for 4 years running now.

Note: As for coil packs, I installed fresh coils with GM coil (direct replacement) "while I was in there" too. (When in doubt of what the readings should be, I measured resistance on both the primary and secondary windings, looking for consistency. However, one of the coils measured "open" between output terminals, so I replaced the all. All the readings then were the same across the lot, and the car runs great!) That was 6 years and 20k miles ago w/ no issues with them since.

[Big D]

thanks everyone for your inputs. Looks like I will be going under the plenum for the first time. Should be a good time....... Looking forward to getting her running back the way she should.

[Paul Workman]

Entering under the plenum is a "right of passage" for LT5 owners, it seems.

Might consider R&R'ing the started solenoid high-current contacts; set ya back around $10 or so.

Plug wires?

Vacuum lines, check valves, rubber unions = OK?

Secondary actuators and linkage works OK?

[Big D]

Looks like I'm going to go with the injectors from FIC. Can anyone suggest which brand of coils I should go with and where to get them.

thanks again everyone,
Big D

[XfireZ51]

Originally Posted by Big D

Looks like I'm going to go with the injectors from FIC. Can anyone suggest which brand of coils I should go with and where to get them.

thanks again everyone,
Big D

My MSDs have been running the motor for 7 years.
If you are diving under the plenum, may as well swap out the old stuff as a precaution.
Also clean the valley up and drill out the drain hole under the starter. Seal vacuum lines too.

[IMZZ]

You're making me nervous I'm doing the same thing and ordering injectors from FIC this week. Did you call them for advice about the injector? I know it would be a pain but you could swap injectors to see if the problem changes too.