LT5 Misfiring at All Speeds

[Big D]

Ok everyone, Got the plenum off with no problem. Way easier then what I had imagined with all the good info on the Registry. Took about 40 minutes and broke nothing. However, I tried a suggested method for anti-freeze removal, just drain the plenum, and I would not advise that method. After releasing the plenum, antifreeze began bubbling up from the two holes in the injector housing. Plugged the holes quickly and then drained the radiator. Saving ten minutes at first cost me hours in cleanup. Dang, I hate antifreeze. The valley was extremely clean, prior to the antifreeze dump and all connectors and hoses are in very good shape. Well, at least I know the valley drain works just fine.Tested the wires and they come in at 7.5-9 K ohms for all, well within spec. Also checked the coils, all 4 coil resistance within spec. Also did a visual of the coils and they all look good, no signs of heat deterioration or cracking so I am going to let them go and just change out the injectors. I may end up regretting this decision but I was going to go with stock coils anyway so I don't see the advantage at this point. This is a very low mileage car. Looking forward to installing the new FIC injectors and getting her back on the road.

Big D
69 L46, M21
90 ZR1
95 Suzuki 1200 Bandit
2014 Triumph Thruxton

[Dynomite]

Never heard of draining the coolant from the plenum

I would suggest blocking those two coolant passages in injector housings. That would then be an end to any messing with coolant when removing Plenum.

Oh. Just make sure when you get Plenum back on and coolant refilled that the two Injector Housing Coolant Manifolds are getting warm within a minute of firing up the engine

[IMZZ]

I'll be doing the same thing this week. Let us know how everything turns out. Let me know what you think of the FIC injectors.

[Big D]

got the new FIC injectors in. So far so good. They look high quality and I think they will do the trick. Ordered them on Tues, got them Weds. Easy install. I released the old injectors from the fuel rails first and then removed them from the injector housings. Watch for fuel leaks. Have rags available. Seemed to be the easier way. Then installed the new injectors to the fuel rails and then installed them into the injector housings. All went together real easy. Taking your time is the best advice and follow all of the good info on this site. Helped tons. Took it out for a test beat and it screamed to 6500. Like a whole different car. I do have one problem, I have a slight miss when at idle or just idling down the road. Anything off idle up to floored and it doesn't miss at all. Wasn't able to get new plugs for it on Sunday so I had to re-install the old ones. Hoping maybe it is a bad plug. Didn't have time to troubleshoot any further either. All coils and wires checked out within specs. Won't be able to get back to it until next weekend.

[Big D]

hi everyone, couldn't figure out the miss so i put in new coil packs, wires and plugs. Still missing at low idle and while idling down the road. Also, get a bad hesitation off idle. Still screams to 7 grand though. So tonight, after putting the new plugs in, i have isolated the miss to number 6. Pulled the wires on all other plugs and the idle changes noticeably except for number 6. Pulled the #6 plug and it is bone dry, never saw any fuel in the short time it was running. The injector ohms out correct. Fuel pressure is good also, holds after shutdown. Could the new primary injector at # 6 be questionable?

[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.

[Paul Workman]

Curious about the current draw/limits for (FIC) injectors, I spoke with Jon (at FIC) briefly about your problem, and he mentioned that there have been cases where customers have inadvertently swapped the primary and secondary injector connectors.

Thinking about that, if your LT5 is NOT modified for secondary deletion, then I would expect what you are experiencing to be the case:

• Primary injector not running at idle or little throttle causing a constant miss at lower throttle settings
• Bogging when you "step on it"*
• WOT works normal (aka "Screams, as you said!)

*With the primary injector in the secondary runner, fuel would not be getting to the cylinder at low throttle due to the SPT plate being closed. However, fuel would back up and pool in the injector housing runner; possibly flooding the runner, depending on how tight the SPT plate seals (and possibly causing other low speed issues too, should the runner fill and run over with fuel to back-fill and spill into the plenum where it would get drawn in by other cylinders...not good!

However, when the secondaries are signaled ON and the SPTs open, initially a slug of raw fuel drops into the cylinder, temporarily flooding it (hence a bog/stutter/miss/blue smoke, etc would likely occur. In a few seconds, the cylinder would pump out the excess fuel and with both injectors now running the engine would perform in normal WOT fashion.

Note: For LT5s modified to delete the SPTs, it shouldn't matter if the injectors are swapped, as they both run full time except at idle, but not having SPT restrictions, there will be no fuel pooling regardless, and it should idle fine, I would think.

If having the two injectors swapped is NOT thee problem, you will need to wring out the wiring/connector to the injector.

It might be easier to pull the plenum than mess with the relay and ECM connector pins, and thereby gaining direct access to the injector itself. Grounding the (black/yellow wire) side of the connector and providing an instantaneous on-off connection to the other side to power the injector, you should hear a audible "click" which would indicate the injector should be working normally...should be.

Page 6E3-A-5 (1990 FSM) is the scematic to help guide your troubleshooting.

Update when you can...

[Big D]

Hi Paul, I also talked to Jon this morning and he said that you had called him. Thank you. He was helpful and gave me the same information. I find it hard to believe that the injector would be bad but i have to check it. So, the plenum is off, again, gettting pretty good at this. The wire bundle is tight and would not allow mis-connection but i checked it anyway, per Jons instructions, and the connectors are to the correct injectors. Jon said to use a 9 volt battery to check the injector and it fired just fine, just like the others that I checked next to the questionable one.
The wire diagram in the manual is not all that helpful, i'll need greater detail now to figure out how to wring out the wiring. Since the other primary injectors appear to be working correctly i'll need to shoot the wire from the injector to the direct ignition housing. The manual just shows all injector wires going to what appears to be one splice or terminal.
and then on to the ignition and the Direct Ignition Housing. It would be helpful if i knew what fired the primary injectors, the manual diagrams don't show that or explain it.

Ken, don't be alarmed, I'm sure it is a vehicle problem and not the injector. Jon is very helpful and understanding. He even offered up a replacement injector before I even verified this one was bad, but i declined the offer. So after my checks above, i feel confident the injector isn't bad and that it is a vehicle problem. The top end run is great, better then it ever was and i contribute that to the new inlectors. But, if i can't find any wiring problems i will have to do the injector swap and see if the problem follows the injector.

[Paul Workman]

Originally Posted by Big D

Hi Paul, I also talked to Jon this morning and he said that you had called him. Thank you. He was helpful and gave me the same information. I find it hard to believe that the injector would be bad but i have to check it. So, the plenum is off, again, gettting pretty good at this. The wire bundle is tight and would not allow mis-connection but i checked it anyway, per Jons instructions, and the connectors are to the correct injectors. Jon said to use a 9 volt battery to check the injector and it fired just fine, just like the others that I checked next to the questionable one.
The wire diagram in the manual is not all that helpful, i'll need greater detail now to figure out how to wring out the wiring. Since the other primary injectors appear to be working correctly i'll need to shoot the wire from the injector to the direct ignition housing. The manual just shows all injector wires going to what appears to be one splice or terminal.
and then on to the ignition and the Direct Ignition Housing. It would be helpful if i knew what fired the primary injectors, the manual diagrams don't show that or explain it.

Ken, don't be alarmed, I'm sure it is a vehicle problem and not the injector. Jon is very helpful and understanding. He even offered up a replacement injector before I even verified this one was bad, but i declined the offer. So after my checks above, i feel confident the injector isn't bad and that it is a vehicle problem. The top end run is great, better then it ever was and i contribute that to the new inlectors. But, if i can't find any wiring problems i will have to do the injector swap and see if the problem follows the injector.

Do you have an FSM? What year Z is it? I have a 90 FSM, and it shows (referenced page) how the injector is controlled. I'll send the schematic to you, if that will help.

Far as the 9 volt battery goes, Jon said he uses 9 volts instead of 12 to reduce the heating/burning of the little coils inside the injector. However, a 5Ω resistor in series with a lead from your handy 12v battery will do exactly the same thing w/o having to round up a special 9 volt battery. And, that said, I can tell your I've done that test on my injectors "full strength" but just quickly tapping the probe to the injector and pulling the probe immediately away. There's no doubt about it, you can hear that injector's little "click" w/ NO issue, far as damaging the little coil. HOWEVER, using a 5Ω resistor will accomplish the same thing and reduce the current by approx 24% - same current as you'd expect using a 9 volt battery (which as I said: for the short one or two 'taps' to verify the "click" should do no harm either.

Schematic (for a 1990, but essentially the same for other years)



Essentially, the ECM provides a ground for the pair of relays for each cylinder. However the secondary injectors require further signal from the ECM, via two separate relays #1 (inj. 1+8, & 4+3) and relay #2 (inj 6+5, & 7+2) each controlling two secondary injector pairs.

If the #6 injector checked out (by direct application of voltage) then there is a connection somewhere. Hopefully the schematic will help. (I have another diagram too, and if I can find it, I'll send it too. It might help locate the various connectors and pins for further testing.

Update when you can...

Paul.