No or low spark #4 and #7 plugs

[DTZ ZR1]

I have a 90 ZR-1 and I was having a problem with my engine running rough and also having SYS flash on my display. I changed the battery because I did have problems with it and that took care of the SYS. At first it seemed to clear the engine problem. Took the car on a 400 mile trip over the 4th, she performed flawlessly. This week she started to idle real rough and felt like a cylinder was miss firing. I changed the easiest thing the plugs, I put AC 41-602s in. I took 1,3,5, & 7 out and replaced them. I started the motor and it still was idling rough. I then took out 2,4,6, & 8, when I took #4 out it was wet. I replaced those four and started the motor. It idled better, but still not what I liked. I then took #4 wire off, idle did not change. Took all the other wires off, I could hear the spark jump and engine idled rougher except for #7, there was no change. I looked at my service manual and found that they are off the same coil pack so I figured that I would replace all four. The question is while I have the plenum off what else should I change or look at? Also is there anything I need to be aware of when removing it? What would be a good coil pack to replace them with? Thanks for the help.

[tomtom72]

Great diagnostics! Use the stock GM coils, they give the least trouble. D-555 P/N 10472401 ( I think is the P/N) and the part numbers are here in a thread do a search. Remember the intake is wet, so drain the rad!

I hate to say this, but if the injectors are OEM on your 90 then, well, you know about the alcohol & what it does to our OE injectors....not wanting to spend your money for ya...

Other than the coils, I would just check all the vacuum lines, the whole 2* vacuum system in fact while I had it opened up...it's just easier. I'd maybe think about the two temp senders in the left I/H, Starter is okay so just maybe do some clean up on the valley floor to make sure the drain hole is free & open, maybe at least check the CCVB cover bolts ( they were finger tight on my 90, man what a mess they made!) and the hoses to the I/H from the box. Have you had the plenum off prior to this, as I maybe telling you stuff you have dunn already... If this is the first time let us know so we know where you're at with your Z.


Tom

[DTZ ZR1]

Tom, this is the first time I have attempted this. It is not like my 85. Thank you for the info. Let me know if there is anything else I might need to do. One thing I have read somewhere that there sometimes a problem with refilling coolant and airpockets.

DT

[A26B]

I can provide gaskets & seals if you need them. Use link below my signature.

[XfireZ51]

I have a set of stock ignition coils If you're interested.

[flyin ryan]

DT, when filling the coolant just take your time & be patient, fills rather slowly. Doesn't hurt to jack up the RH front (passenger) side a little to keep it higher than the rest of the system too.

[ZR1Vette]

Since you will be in the area covered by the plenum tighten (torque) every bolt you can see and get to. In particular the crank case vent valve cover (19 lb ft), inspect crank case vent hoses and tighten the hose clamps. Good time to make sure the valley under the starter is clean and the drain hole is not clogged (I just poured a bit of isopropyl alcohol into the valley and should drain out under the engine). Also inspect the secondary actuators and lines. When installing the plenum back follow the tightening sequence shown here>>


I used painters tape to cover over any openings...do not want to drop something down those holes!


When you first lift up the plenum you would be seeing this...front green arrow points to vacuum hose (disconnect) and the red arrows point to the front 2 ignition module connectors (disconnect)


Here are the 2 rear ignition module electrical connectors (red arrows)>>


A handy 'tool' is a wooden hatchet handle ($2.50 at Home Depot) to pry up the fuel rail assembly


and to pry up to plenum too>>


Glad to provide more pointers and pics if you need..
Michael

[tomtom72]

DT, as you see you have come to the right address! We Brothers of the Beast will leave no man ( or women, to be P/C ) out in the cold with a broken Z! Okay enough about our character....but it's true!

First, as this is your first plenum pull you will need a few things. Chief among them would be the use of a FSM = Factory Seervice Manual, a T/Q wrench, Torx sockets (T-45 long), a hand vacuum pump, gaskets ( see Jerry's posted link! ), Mike has given you photos, and a bit of skill from wrenching on cars in general.

If you have most of that, then this is actually alot easier than it looks. Trust me as when I had to do this the first time I think I was scared chitless...

Plenum pulls require no more skill than what you used when you did your first sbc intake swap, truth! This is different, and there are a lot more parts, but in actuality you are just pulling an intake manifold. At this forum there are Mike's thread, and a coupla of others. I'm going to list some links for you to look over to gain the needed background info. Look'em over and you'll see it is a lot of detail work but nothing a new Z owner need to farm out. However you will need a place to work when you do this for the first time, after you will pull the plenum in less than an hour round trip.
http://www.pnwzr1.com/
http://www.pnwzr1.net/
http://www.bcvettes.org/?http://www....o/zr-1/plenum/
http://www.zr1specialist.com/
http://www.corvetteactioncenter.com/...diagnosis.html

You look and then tell us what you need more detail on. You can do this, even by yourself with the coaching that you can get here.


Tom

P.S. if you lived close to me I'd have been knocking on your door already, tool kit & all!

[ZR1Vette]

As Tom pointed out...some basic tools you will need:


Also recommend getting new gaskets from Jerry (aka A26B) and if you not obtain FIs yet, many of us have selected the Bosch units from Fuel Injector Connection ( http://fuelinjectorconnection.com/shop/ )...just speak to Jon (great guy and tremendous knowledge on FIs.

Just make sure you install the injectors in the fuel rail correctly... should look like this>>

[vettn71]

Just as an FYI, when I pulled my plenum (first time) it took me 4 hours and I discovered:
1. It's necessary to flush the fuel rails. If you don't a bit of trash may find its way into an injector (ask me how I know)
2. The second time I pulled the plenum (to change the injectors) I installed a set of Delphi injectors Jon Banner had set up for me. My car is a NCRS car and appearance is important. Second pull took 45 minutes. I also put in new coil paks, as I found number 4 and 7 not firing at hat point.
3. Third plenum pull took me 30 mionutes to replace a brand new GM coil. The other three were fine. If you put in coils, check the resistance of the primary and secondary windings before you install them. Could have saved me another pull if I had done that.

Pulling the plenum is pretty easy after the first time. Just watch where things go and read all the information recommended on this site.

Good luck

Jim