quigman

Hey guys I am new so please be gentle. I have an 05 lq4 I put in a 52 chevy pickup. Cylinder #4 is dead I am stumped. I have swapped coils, plug wires, plugs and no change. I did a compression test and it was within the 10% margin. I have tested the spark and have spark at the plug, the spark color is the same as cylinder#2. I used a noid light and it pulses the same as the other cylinders. I did have a problem with an injector stuck open and in this cylinder so I have swapped the injector for a reman one. The cylinder was misfiring before I swapped the injector. The plug comes out wet with fuel when removed from the cylinder. The wiring is from Howell and the computer has there perfomance tune for a stock engine.
Please help!!!

kossuth

iTrader: (12)

Ok, so it's getting fuel but no spark. It can't be getting spark at all or at the right time or else that cylinder would be running. So what I would do here is double check the pin-out on the PCM and make sure that the coil output for that coil is going to the correct spot. You need a good multimeter for this. Also ensure you have a good ground on your motor to the vehicle chassis and where all your grounds for your coils and PCM are tied into. Make sure you have less than an Ohm of resistance from your coil connector ground to the negative post on the battery. Either you have a miswired connector (possible but I wouldn't think it's likely if the manufacture is reputable which I have no idea if they are or aren't) or you have a poor ground someplace and your coil isn't building up enough of a charge and is "blowing out" under compression. Also ensure that you have a good 12V + feed going into the coil. It's probiably something super simple.

quigman

Update: So I checked the ground for the coil. All the coils share the same ground wire. I had no resisitence in the #4 ground wire so the wire is fine. I checked the pinouts for both the injector and the coil and found everything is as it should be. My compression test yeilds 160 in both 2 and 4 and 175 in both 6 and 8. I swapped the injector from a known good one and it changed nothing. Does anyone have any ideas. I have checked everything I can think of. I am not very good at the wiring and electronics stuff so if you have a suggestion any tutorials would be appreciated.

kossuth

iTrader: (12)

If you got compression and you've double checked your plugs and coil packs then it has got to be the wiring. Could be in the injector or the coil. Someplace you have either miswired something or a pin is off in the PCM body or something. It's going to be something simple, just gotta track it down.

jake 03

iTrader: (5)

I just had a similar problem on a completely stock '10 silverado. Misfire on #8. Compression was ok,swapped plugs,wires,coils,injectors to no avail.
Took valve cover and rocker arms off,compressed valve springs and found I had a sticking exhaust valve. Weird thing was it would only miss at just above idle,made it a real pain to track down.

itsslow98

iTrader: (18)

Do a leakdown test, could be something in the top end of the motor.

JaYZeig

iTrader: (11)

id pull the valve cover and see if your rockers are moving
while cranking..obviously disable fuel..

quigman

Sorry for the long delay, I have been busy with the new baby. I pulled the valve cover and everything seems to move perfectly. The rockers move the same rate and distance as all the other cylinders. I did pull the plugs and with the key off noticed I have a small amount of fuel that blows out the plug hole while I crank the motor over and only on #4. # 2 had no fuel coming out at all. Is it possible for the injector to be putting so much fuel in that it is drowning out the combustion process. I have looked over everything and am stummped. How do i check for a stuck valve, and if it were stuck wouldn't it backfire some how.

5w20

iTrader: (21)

How did you swap the coil over? Swapped it between cylinders or with another "known good" one?

Since you say it has raw fuel, I would check pin tension on the ignition coil connector because it definitely sounds like you are getting fuel but nothing is firing on it. Try bypassing the wire and running your own as well.

quigman

I swapped the coil a few times with coils from the other cylinders. I have spark at crank and while the truck is running. I have verified this with 2 different spark plugs. I will check the pin tension. It certainly can't hurt.

quigman

Well the problem is solved. It turns out on the truck intake the fuel pump regulator gets its vacuum signal from the #4 intake runner. The regulator diaphragm had failed and the #4 runner was sucking a ton of fuel into the cylinder drowning out the combustion. It sucks to have taken so long to figure this out but better late than never. Thanks to all who had input.