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Engine runs like crap after it's warmed up

LilWes278

LilWes278

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Ok, truck is a 1978 K10 with a 350ci. Here's the problem: When you first start the truck up, it runs fine. But after it warms up, it starts running very rough, almost like it's got a dead cylinder, and sometimes like it's trying to backfire through the carb. If you give it WOT, it smooths out somewhat. This started happening immediately after changing out the spark plug wires, so we went back and checked all the wires (they all looked good), so then we checked the cap and rotor (they looked good too). I also checked to see if the choke was stuck, but it seemed to be working just fine. I'm kind of wondering if maybe the timing chain jumped a tooth, or if maybe we should just replace the cap and rotor to be sure that's not the problem. Anyone have any ideas? It's late and my brain's not working too well...
 
If it's only after the engine warms up, I'd be looking at choke, (again) and anything that is emissions related (if present) as a cause.

Also, doublecheck for a vacuum leak, as the choke and/or a high idle can mask that problem until it is put into gear.
 
Make sure you reconnected all your vacuum hoses. Check your PVC hookup. Make sure nothings loose or got knocked loose. Is it a temp controlled vac choke or mechanical?
 
It's an electric choke carb. And it runs fine even after it has kicked down from high idle. But after you've been driving it around for about 2 or 3 minutes, that's when it starts acting up. Since it started happening immediately after installing the wires, I figured it would have more to do with that area. And no vacuum lines were tampered with during the plug wire replacement. Everything is just the way it was before (except new wires). I'm gonna try to throw a timing light on there this weekend and see if it still looks good. If that's ok, then I have another set of plug wires I can swap out and see if it's still acting up. I'll probably put a new cap and rotor on while I'm at it. If that still doesn't work, I guess I'll have to tear into the carb. Damn I hate working on other people's junk!
 
LilWes278 said:
It's an electric choke carb. And it runs fine even after it has kicked down from high idle. But after you've been driving it around for about 2 or 3 minutes, that's when it starts acting up. Since it started happening immediately after installing the wires, I figured it would have more to do with that area. And no vacuum lines were tampered with during the plug wire replacement. Everything is just the way it was before (except new wires). I'm gonna try to throw a timing light on there this weekend and see if it still looks good. If that's ok, then I have another set of plug wires I can swap out and see if it's still acting up. I'll probably put a new cap and rotor on while I'm at it. If that still doesn't work, I guess I'll have to tear into the carb. Damn I hate working on other people's junk!
Click to expand...

Okay - here is what the troubleshooting section has to say regarding the specific items you mentioned. There are other possibilities but I will stick with the areas you worked on.

IDLES ROUGH OR STALLS; Faulty Distributor, Incorrect Timing or Secondary Wiring. "Check for loose distributor or incorrect timing - use a timing light to check for abnormal timing fluctution; check for correct sparkplug wire routing;

LOW SECONDARY VOLTAGE OR FAULTY SPARKPLUGS; check sparkplugs for broken insulators, loose or damaged plug wire boots or fouling; check sparkplug wires for "breaking down" under load. test for adequate coil output voltage, check sparking condition.

Hope that helps you.
 
Whats the vacuum line to the distributor look like?

If its on some sort of vacuum switch, and it was disconnected, maybe thats it?
 
Ignition module. I'd bet money on it. Pull it out and go to your local parts store and have them test it. Make sure they test it 5-6 times to get it warm.
 
Yes, good point, I've been reading up on this, heat seems to really make those things act goofy if they are marginal.

Supposedly GM modules (and some aftermarkets) have an indicator "dot" that raises up or something when they fail, apparently in some cases even if the module still works, but not correctly. I've never seen this, but I'd like to, so I know what to look for.
 
Heres what I would do...warm the engine up for about 30 minutes from cold, set the choke and just let it run and see if it starts to miss. If it starts to miss start pulling plug wires one at a time and see if its one or two cylinders missing. If the miss jumps from cylinder to cylinder check the module. If the same cylinder keeps missing then check the plug,wire, cap, and rotor. If its more than one cylinder then check the firing order. Good luck.
George
 
LilWes278 said:
Ok, truck is a 1978 K10 with a 350ci. Here's the problem: When you first start the truck up, it runs fine. But after it warms up, it starts running very rough, almost like it's got a dead cylinder, and sometimes like it's trying to backfire through the carb. If you give it WOT, it smooths out somewhat. This started happening immediately after changing out the spark plug wires, so we went back and checked all the wires (they all looked good), so then we checked the cap and rotor (they looked good too). I also checked to see if the choke was stuck, but it seemed to be working just fine. I'm kind of wondering if maybe the timing chain jumped a tooth, or if maybe we should just replace the cap and rotor to be sure that's not the problem. Anyone have any ideas? It's late and my brain's not working too well...
Click to expand...
Based on your first post, I am assuming your rig is non-computer controlled. I have never seen a '78 with a factory ECM in it yet.

I think you answered your own question. go back and check the plug wires. Not the location (if one was off it'd run like crap all the time, not just when warmed up). You prolly have a bad plug wire. Start it up, take it for a drive to warm it up and get it to misfire for you. Then start pulling plug wires to determine which one is the culprit. You prolly got a bad one right out of the box.
 
Thanks for all the advice guys. I haven't had a chance to crawl under the hood yet. I'll work on it this weekend and let you know what I find.
 
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