I need help here guys. I recently purchased a 90 k1500 pickup with a high mileage 4.3l engine (odometer stopped at 218k). When I bought this truck, it ran rather poorly but I figured that I could get it to run a little better and that it would be a good beater. First thing that I did was replace the power steering pump so that I could actually hear the motor run, and a tune up with seafoam in the tank.

Now it has a miss that acts like a vacuum leak, however I can't for the life of me find one. The really strange thing is that it has a miss at idle that is definitely ignition related that is making me pull out what little hair that I have left. The truck did not miss like this when I bought it. At or slightly above idle it has a miss that you can actually hear "click" in the distributor as well as witness a stumble in the engine. The hotter it gets, the worse it gets. It appears that cylinder six is firing double time when hooked to a timing light. Not at all steady, but the rest seem fine.

Timing has been set to 0 deg, replaced distributor, plugs, cap and rotor twice, third set of wires, 3 ignition modules, new coil, new ecm ground with cable to negative terminal, swapped ecm's (no change), new knock sensor, fuel pressure is 13 running and driving, closed plug gap to .030 to eliminate crossfire possibility, good coil voltage, swapped map sensor, swapped esc, injector pattern appears good.

What in the world could cause the ignition miss that I have? Should I try rewiring from the ecm to the distributor? Something is giving a false trigger to one possibly more cylinders but I am out of ideas at this point. I have tried everything that I can think of. HELP!!! The miss is exactly the same on both distributors and all other components changed. I'm getting ready to scrap this truck!

 

Maybe the intake gasket is leaking on the bottom side so you cannot find it with the good ol carb spray method?

 

I absolutely thought about leaking intake, but that does not explain the ignition miss. After I solve the mysterious ignition problem, I will move on to any other issues that may arise. At this time, ignition is first priority.

 

Do the injectors work properly? IDK if what you have has 6 injectors or the TBI, but I know that if an injector isnt working right or if its dead, then it can sound like a miss in the ignition. (solved this problem on a 90 Firebird 3.1 V6 TPI)

 

As I stated earlier, injector pattern appears fine. It is a tbi so a miss that bad would be visible. The problem that I am fighting is definitely ignition. I can hear the out of time firing in the cap and witness it on a timing light.

 

Have you pulled the distributor out and checked the teeth on the gear drive? the cam gear is solid steel, but the stock gear on the dist. is bronze. Check the gear and the bushing cus that could mess up the timing accuracy. Also check your timing chain, if thats worn out and full of slack then it wont be accurate either. If your ODO stopped at that high of a mileage then... wow, original parts Im pretty sure are going bad. lol

 

Ooops, missed that, I thought you said you replaced the distributor cap! Sorry bout that, then all thats left is to check the timing chain... yea, its alot of work. But It can be done in a day.

 

I'm not saying that my timing chain doesn't have some slop in it, but I don't think that a loose chain could cause the multiple spark that I am seeing on cylinder 6 (correct me if I'm wrong). Plus, it did not miss like this when I bought it and I haven't put a hundred miles on the truck yet. I'm pretty confident that something is giving a false trigger to the coil but what could it be? All cylinders appear steady on a timing light except 6 and it appears to be firing almost double at times. Now I know that the ecm controls timing but can it cause a double fire on one cylinder? If not, what else could? Bad wiring maybe?

 

Could be, Im not very experience with this problem, I was just giving info that I remember seeing before about loose chains and worn distributor gears. Those could cause a similar problem, the miss, but your double sparking issue I never heard of that before, It must be a rare problem. I was just spit balling a few since no one else was coming up with anything :S

 

I am up for any advice at this point and I do thank everyone for their input. I played around with it a little bit tonight and you may be onto it with the timing chain. The double spark turns out to be enough bleed over from the other parallel running spark plug wires to trigger a timing light but it is not true spark. Timing light is steady when hooked to 6 near the distributor cap but is picking up extra fires down near the plug. The crossfiring that I can hear in the distributor diminishes greatly when I set base timing to about 6 degrees and the truck runs better but still bad. I spent some time with the timing light on it today and when it stumbles, it seems that the timing is momentarily very advanced. What I don't know if this is just coincidence with the ecm trying to recover or if the timing advance is causing the stumble. As much as I hate to, I may just have to throw a timing chain at it to see what happens. The only thing that bothers me is that this all seems to be new. The truck did not run good when I bought it, but a miss like this could not have been overlooked. Maybe now with all good ignition components it is just more pronounced. We'll see.

 

I'd get some of those spark plug wire holders to stop the crossfiring. Put dielectric grease in the boots too.

 

I have most of them but there are one or two of the wire holders that are broken and I just used wire ties like I have many times in the past but this one is giving me trouble. I also used dielectric grease when I installed the wires. The "crossfire" that I am getting is happening in the cap and you can actually hear it good when it's idling. I definitely have a rogue spark here and there and it may very well be caused by a stretched timing chain. I just hate to have to tear into a beater that much but I will probably have to.

 

I see alot of problems w/ aftermarket cap & rotors, esp. on vortec chevys. I'd make sure the terminals are brass.
Test the timing chain by lining up the timing mark on the crank balancer, remove the cap & watch the rotor as you turn the crank back & forth. You see the slack in the chain when the rotor doesn't move. I think anything ove 8 degrees at the crank is signs of a worn timing set.

 

I will definitely check the timing chain in the way you described before tearing into it. I actually pulled a new brass terminal cap and put a new Delco cap on it thinking that it might be a bad cap. The Delco cap is not brass much to my surprise. I will cover all bases before doing a timing chain.

 

Checked the slop in the timing chain last night and only had about 4-5 degrees of slack in it. Now what? I didn't really think that this was timing chain related due to how fast the miss appeared but everything ignition related that I can think of has been replaced most of it at least twice. What else could possibly cause this miss? Maybe try yet a third cap and rotor?

 

i know you changed the knock sensor. maybe defective? or maybe the ESC control module? its reading wrong and possibly retarding the timing..i dunno.

 

No change between the new knock sensor and the old, also have swapped esc with a used one. No change no matter what I do. I even ran down the road with the knock sensor unhooked to see if that was what is what it was. This miss is driving me crazy. The strange thing is that when it misses and I have a timing light on it, it always seems that it is too far advanced. I even once again swapped in my backup ecm and still no change. If this truck was my daily driver, I'd be screwed. I've always loved tbi trucks because I could always troubleshoot them, but this one is kicking my butt.

 

Did you do a compression test on the engine especially the that specific cylinder? maybe you have a burnt valve, flat cam lobe or something.

 

I have not done compression test yet, but I will. However the miss that I am chasing is definitely ignition related appears to be timing related and intermittent at idle. Can hear the spark jump in the distributor cap and see it momentarily advance timing on a light.

 

spark jumping in the cap? i dunno. You changed the cap numerous times already. It's really hard to say. good luck with it.

 

Try a HEI distributor for a moment to see if it cures the spark jump. HEI wont be the right one for this engine, but its a much larger cap. Try a vacuum advance distributor, and just have the power wire hooked to the coil. If its running deep to the computer somehow thats one way to start trouble shooting I guess

 

The ecm gets a signal from the dist. to fire the injectors.

 

Oh yea.. sorry my mind is still in carburetor mode..