Hey guys! I have a question and before I start I want to say that I really appreciate the help. Ok I have a LT1 that I've I've just put in my single cab chevy pickup,it's a 95 modle LT1,but it has a 92-93 computer...the truck us running pretty good,and when I say good I mean I just drove it probably 45 to 50 miles and it seems reliable and all but the #3 and #5 cylinders have a missfire.it starts up great I've put new coil,new ac/delco opti,new plugs,new wires,new fuel pump 40 psi idle,compression is good,injectors are good,it's just something is making it misfire and I can't figure it out. Like I said,it's a 95 motor with 92-93 PCM.no mass air sensor. The wiring harness was built by street and performance and has been checked thoroughly. Thanks again!

 

Take the ICM moduel off and have it tested.It could be on its way out causing the miss fire.Did you have the Pcm tuned after the install?

 

Thanks my1994z28! Well you see this is my first LT1 so I'm not familiar with what the ICM is. That may have been a dumb statement on my part but I do apologize ha! What exactly is the ICM? And no the PCM was kinda pre tuned for the motor according to the harness and what was said over a phone call but it hasn't been tuned while running in the truck.

 

ICM is ignition control module,it is located above the coil on the driver side head,just unplug it and remove the 2 screws holding it to the coil bracket..If you have a Autozone or O'rileys auto parts by you they can check it for free.Dont forget when you reinstall it to use Di-electric on the back side of it.

 

...it's not dielectric grease you use-it's heat sink compound...

89teroy: It's kind of doubtful that the ICM would affect only two certain cylinders, but I guess anything is possible. Have you tested the plug wires with a meter to ensure continuity? Are they snapped in good on both ends? Are the valves adjusted properly?

If you need some help with basic LT1 stuff, there is a lot of info on my Tech Page (link in sig).

 

O ok! Yeah I was wonder what that part was when I replaced the coil. I will have it checked just in case. Um I haven't checked the plug wires with a meter but I am shure they are snapped in good because that was actually one of the first things I thought it might be. The valves I haven't checked,maybe something else I need to look into to I just thought with this motor being so low miles on it that I wouldn't have to worry about that kind of stuff ya know? This engine came out of a a wrecked car with under 20,000 miles. But I'm not saying it couldn't be the valves. So you don't think the PCM year and the engine year would have something to do with it?

 

There needs to be a coma in that sentence. Either after "reliable" or after "all". Let's assume it's after the word "all" and you mean that #3 and #5 have misfires.

Ok let's attack this one question at a time.

1) Have you triple checked the plug wires are on the right plugs? After all 3 and 5 are on the same side of the opti. More than just a few people here have crossed wires before. Even after swearing they checked them.

2) How are you sure they are the cylinders misfiring?

3) Assuming you are correct about the cylinders involved. Remember it takes fuel, spark and compression to fire a cylinder. With that in mind, you claim spark is good, compression is good and the injectors are good. We'll take the "injectors" to mean fuel to the cylinder. So if you have a misfire, you obviously missed something. So, how do you know the compression on the 2 cylinders in question is good?

4) How do you know they are getting fuel?

I say if compression fuel and spark appears to be there, the wires are crossed. Courtesy of Shoe have a look at this:
http://shbox.com/1/opti.jpg

 

True! It could be the wires! Correct me if I'm wrong but would it not run at all if it had two wires crossed? I checked the compression and it is good. How I know those two cylinders are missing is I pulled the injector plugs off one by one and there was an obvious misfire on all the cylinders when I unplugged them except #3 and #5. and also there is a obvious heat difference on those two header pipes. I have changed those two injectors but could it be that the regulator is bad?

 

It will run with a couple of plug wires crossed, but not smoothly.

What "regulator"? The fuel pressure regulator? If so, no, that would affect all 8 cylinders.

 

Yeah true just a thought. And yeah I mean like I said I actually drove the truck about 40 miles to a good friend and mechanic that has a scanner and other than a lack of power and an obvious missfire it had no major problems,like I mean it didn't have me pulling over on the side of the road from missing and backfiring so bad. I think what ever it is that it's something small I just can't put my hand on it. I have talked to mark at street&performance where the harness was built and he said that if a scanner doesn't find it that he has a truck with the same engine swap and he would take my PCM and hook it to that truck and see what it does.

 

It is RARELY the ECM (93 is an "engine control module", not a PCM = powertrain control module).

You will not get any codes for problems on the high voltage side of the ignition - cap & rotor, coil, coil/plug wires, or spark plugs. There are no misfire codes in OBD-I. You will get codes for problems on the control side of the ignition, including optical cam position sensor in the Opti, and the IC module.

 

So you checked compression and you swapped the injectors AND you checked to see that they are getting pulsed with a noid light right?

So the only thing left is spark and you triple checked the plug wires are not crossed.

BTW, from experience, a SBC will not run with 3 or more cylinders out. 2 will still run, albeit crappy.

 

Some things you can do:

-look at plugs to see if they are fuel fouled/wet

-use an inductive timing light clipped on the plugs wires in question to see if they are getting spark (or put a spare plug on the end of the wire with it's body grounded to look for spark)

-ohm the plug wires

 

I don't like the spare plug method cause it misses the fact that the plug in the head may be defective.

One other note worth considering. If you are continuing to run the engine and the 2 cylinders are out cause of spark. That would mean they are still getting fuel. That can over time destroy the cylinders from "souping". The unburned fuel washes the cylinder of the oil film and the rings gall the cylinder. Not a pretty prospect. You should stop running it for any length of time until you get the cylinders back running.

 

Easily done by taking out the plug and putting the wire back on it. I was mainly pointing out ways to test the wires.