My new square has a miss

[lcann25]

Hello everyone, First post here!

I just got my 1984 C10 with a 305( I think) a few days ago, and I have to admit that I basically bought the truck sight unseen from a common acquaintance. Well upon driving the truck home I knew something wasn't right and it didn't seem to be running on all cylinders. Well, I've checked all the wires for spark with the screwdriver ground method and they all seem to be working fine, then I checked all the plugs. The plugs look decent and appear fairly new, but they are Champion RV15YC4's
Well from quick research these are 350 plugs, not 305 plugs. Now who knows if this engine has been changed to a 350 at some point in its life, but it looks pretty original to me.

I guess my first question here is, could the RV15YC4's be causing the truck to have a miss? This is really just going to be a garbage truck and sit around 95% of the time, so I don't want to sink a ton of Money into it, but I'd like to at least run properly.. Any advise I can get from you guys would be appreciated. Thanks!

 

[1987 GMC Jimmy]

If the spark plug gap is different on those plugs than it would be on Delco plugs, then I'd say yes. I haven't heard of someone using Champion plugs in a GM. Your emissions sticker will tell you the right gap spec, and you can get a gap tool and pull the plugs and see what that does. If it were me, I'd switch to the regular AC Delco copper plugs and do an ignition tuneup: plugs plus wires (if needed - check contacts on both ends for charring or scoring and make sure the insulation isn't burned or chewed), distributor cap, and rotor with all AC Delco parts.

 

[MrMarty51]

What 87 Jimmy said.
A plug wire may appear oksy and put out a ton of spark, until it has to push that spark against compression.
If You can not confirm when it was last tuned up, replace it all and use AC/Delco plugs, wires too if they are available, distributor cap and rotor.
You might also want to "rent" a compression tester set and see what its running for compression.

 

[1987 GMC Jimmy]

You might also want to "rent" a compression tester set and see what its running for compression.

Yes, this. I'd do that while I was messing with the plugs. Kill two birds with one plug.

 

[spanky55amg]

Yes, this. I'd do that while I was messing with the plugs. Kill two birds with one plug.

And while we are doing this, just ohm out the wires and verify 100% if they are good. Once again, you dont know till you put a load on something if they are good. A 22 gauge wire could jump a small spark.

Hell, I just found a battery-start cable was bad and didnt know till we put a load on it.

 

[lcann25]

Good advise guys. I frowned as soon as I saw Champion on the first plug I pulled out, knew it wasn't right but didn't think they could possibly cause a miss, Ive had SBC's with all different kinds of varieties of plugs and never really had a problem. I think you are all right to do plugs/wires and possibly cap and rotor as well. I'll do this at some point and report back with what happens.

 

[theBIGnaud]

If you compression checks out, check your distributor for advancment actuation and shaft free movement. If it is stiff or binds replace the distributor.

 

[AzGeo]

After doing all the things above .

Most 305's are "closed chamber" and most 350's are "open chamber heads .
Take a piece of wire (like a coat hanger), bend one end into a 90 degree, small arm on the end . Small enough to slide into a spark plug hole, to "drag along the threads" . Trying to determine if the threads are for "long or short threaded spark plugs" .

If the cylinder that misses is #8, look for vacuum leaks from the intake manifold up the hoses to the A/C and other places . PINCH OFF the hoses close to the metal fitting on the intake and see if the miss stops .

If #6 has the miss (ay carumba), it may have "overheated valves" because the EGR VALVE is close to that cylinder . A LEAK DOWN TEST will give you the answer without disassembly .

Good luck .

 

[lcann25]

Well guys, I went out today and installed new rotor, cap, wires and plugs. All AC stuff. And unfortunately, truck runs exactly the same as before, I can't visibly see or hear a vacuum leak anywhere. I didn't do a compression test on it as I don't have one and didn't have a second person around, but I assume that will be the likely next step. I'm a bit disheartened here as I was really hoping this stuff would solve the problem. My luck that it didn't.

 

[1987 GMC Jimmy]

Regardless, you made the right move doing the tuneup. Autozone and O'Reilly have compression testers you can borrow. A vacuum gauge is the best tool to diagnose vacuum issues because sometimes you can't see or hear them. It'll tell you if your idle vacuum is nominal, and it can diagnose all kinds of things. I'll post a picture. If the compression test goes well, I'd also be checking the fuel filter and accelerator pump. The fuel filter in a Quadrajet is behind the inlet. Just unscrew it, and be careful with the spring that's in there. You check the accelerator pump by working the throttle after it's been ran recently and seeing if you get two healthy shots of fuel down the throat of the carb as soon as you start moving the throttle. If you only see it on one side, not at all, or it's just sloppy, the carb could use a rebuild. I'd do the compression test first, though, since that won't cost you anything. The bottom right condition on that chart may also reaffirm a bad compression test if you end up getting a vacuum gauge.

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[lcann25]

Thanks for that awesome diagram Jimmy. I've got a buddy that's got a compression tester and a vacuum gauge, we are going to try that next time he's available and if all check out I'm going to then look for a distributor that distributor we can drop in there just to see what happens. Hopefully we can chase this down.

 

[Oldrider]

How does it idle? Does the miss get worse as you speed up or place a load on the engine?
If you go through the ignition system and don't find anything and the compression test is fairly good, then suspect the carb. A rebuild might get
you up and running better. it probably needs a rebuild anyway.

 

[lcann25]

I may be starting to narrow this down a bit... Started pulling wires off last night with the truck running and some make a noticeable difference to the idle, some hardly at all. #3 makes very little difference when yanked off, and the spark is very weak. Boot almost has to be over top of the plug to get the spark to jump. But yet pulling #4 off, had a noticeable idle drop and a good 3" spark jump. #4 obviously is a way shorter wire than #3.. I'm starting to suspect either bad engine ground, low input voltage at distributor, or coil failing..

 

[lcann25]

#3 cylinder has 40PSI of compression. Didn't bother to test it any further than that. Case closed. Thanks for all your help everyone! I really appreciate it.

 

[MrMarty51]

#3 cylinder has 40PSI of compression. Didn't bother to test it any further than that. Case closed. Thanks for all your help everyone! I really appreciate it.

Add some oil to that cylinder then redo the compression test and see if the compression comes up.
If it does then it will be the rings, if not then suspect a valve job is in order.