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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: SUX2BU99 on June 14, 2010, 05:19:10 pm
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My truck has developed a sudden misfire issue. I filled up with gas the other day after it being quite low and since then it hasn’t been the same at all. I put in some 92-grade fuel, did a little driving around town and went home. On my way home, going up the hill we live on and having the bed loaded with bark mulch, I could feel it missing and it sounded a little different. As soon as I put it under any kind of load like pushing on the gas, a bit or a lot, it struggles like it doesn’t want to go. If I back out of it, it picks up a little. Sometimes I have to stab at the gas a couple times for it to come around. It drives fine at steady state but is all doggy when I push on it. Then I got mad and nailed it in first at low speed and it was all boggy and stumbling but going up through the rev range, than at 4000 it was like Vtech kicked in (lol sorry) and it ripped it up to 5500 when I shifted. I’m thinking (hoping) it’s just a dirty fuel filter perhaps due to the tank being low when I put gas in it and it kicked up some crap. Or there being crap in the fuel tanks where I got some gas that’s now stuck in the filter. I have put some different gas in since then and it’s still the same.
I’m hoping it’s not a distributor issue but I do have my stock HEI unit as a back-up in case I have to make my Mallory out. Could it be my vacuum advance malfunctioning? My dizzy has 2 advance springs, maybe the lower-speed one broke? The plug wires I have are good-quality Auroras but are also 10 years old so maybe one is getting bad, I don’t know. I put some new plugs in about a month ago, and they are gapped to .040. It was running nicely until this happened.
Any help or ideas is appreciated.
Thanks,
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What have you checked so far? If you've ruled out bad gas and there's no crap in the carburetor take a look at the ignition system.
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I found that my distributor had rotated on it's own and was retarded a little below 0 degrees, if 0 is sitting perfectly in line with the motor (ie. 12 o'clock right in the middle of the firewall). I rotated it a little back to being advanced and it drove much better but I could still feel something missing. Wondering if I might have a fouled plug. I had rotated it too much and it was stumbling and bogging bad. I bumped it back a bit and it was much much better but I have to drive it a little more to see if it's back to normal or what. ANd then I tightened the hold-down nut too. lol
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Counter clockwise will advance it.
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Right. It had rotated itself clock-wise into the retarded position. Drove it some more again and I think that's all that it was! Gotta love the simple and FREE fixes.
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hey thanks for putting up this post and your solultion, my truck does this too, hope timing will fix it.
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Well I of course spoke too soon it seems :-\ I'm still having the same problem but it's erratic. When I drive the truck, it sometimes feels fine and other times it can be sputtering. Driving it around last night and it was doing whatever it wanted. I adjusted the timing a couple of times and I don't think that's necessarily it anymore. While adjusting it did improve driveability a LOT because it was so out, I'm still having another issue.
I'm thinking it must be my distributor. I can't see a whole lot why it might be my carb. I'm thinking of the following:
1. faulty vacuum canister? I'm not sure what can 'stick' in these to make it act erratic but when it was being funky, going WOT it was sputtering but again at 4000 RPM it's like a switch turned on and away it went
2. faulty advance springs? It seems like I'm having the most issues with acceleration under load. When it's running better, it seems to not like steady-state cruising. When it's being funky, it doesn't like part-load acceleration.
3. bad coil? I have a stock HEI with an MSD cap and coil that I took off to replace with the then-new Mallory.
4. if I replace the dizzy, it's just a matter of taking off the wires, unbolting the hold-down and lifting it out and then putting the new one back in the same place? Obviously the rotor will have to be in the same exact spot as the previous one. I was thinking of rotating the motor so the rotor is at the 12 o'clock position because I thinking finding #1 TDC might be an excercise in frustration for me working solo and with limited tools.
Thanks!
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what fuel pump do you have?
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Stock-style, usual parts store kind of pump.
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so its a manual pump
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You mean mechanical pump?
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lol yeah its late and i was in a rush please forgive me
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I would rule out the simple stuff before you swap dizzy's. 10 years is a long time to run a set of wires. Did you replace your fuel filter? If you're worried about the advance springs, they are easy to visually inspect if you remove the rotor and a new spring kit is cheap. +1 for checking the fuel pump
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I removed the Edelbrock/Russell fuel filter and backwashed it out with carb cleaner and reinstalled it. It's a stainless steel mesh filter. No change.
Does this look right? My wifes cousin and her husband came out to visit on Sunday and he is a heavy-duty mechanic. He brought his Snap-on dial-back timing light and we got my timing set to 12 base and advanced at 3000 rpm turned out to be 44, which I know is high. This is where the distro ended up sitting with those settings. To me, that looks awfully far retarded and not advanced. Usually my distro is much more counter-clockwise than that. If you mirroed where it is sitting now, that's how it has usually been. And it has ran okay like that. It was boggy at throttle tip-in but didn't miss and ran fine otherwise. Driving it like this today was unpleasant. Just like when the problems first showed up. Idled just fine. It's okay under it's own power but as soon you press at all on the pedal and put any load under it, it stumbles, bogs, and just doesn't like it. LIke it's being choked big time. Funny thing though is when I drove it cold, it felt good. Just as it warmed up it got worse. I'm going to mark this setting and turn it counter-clockwise (advance) to see if it improves. It was feeling better before this change when I was messing around with the distro position.
I'm wondering if my distro is actually a tooth off? And if the setting that it's at now is supposed to be 'correct' but if it's a tooth off and I had it mega-advanced to make it run okay, that was masking the off-tooth problem?
I don't see how a mechanical fuel pump problem would show up like this. It's okay when cold and worse when warmed up. If the pump diaphragm had a hole in it, that should show up at all times and not inconsistently like I have now and not be affected by the distro position.
When an ignition module in an HEI goes bad, doesn't the motor just not fire altogether?
I could possibly have a fouled plug. I have Autolite 26 plugs, gapped at .040 and I run 89 octane fuel. I have zero detonation issues except when I turned the distro fairly far advanced and then I had some rattle at 7/8 throttle, which I expect after having put in a hotter cam and heads.
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K, it's not letting me Edit and then Save my changes for some reason. Anyway, I wanted to add that it looks like #1 is at the 6:00 position and usually I have the distro twisted to where it's at the 4:30-5:00 position.
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Its supposed to be a 4 or so
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4 degrees? Or 4:00 position? I know stock timing is about 4 degrees but usually it gets bumped to at least 10 for some better response and bottom-end.
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I just gave it a twist and took it for a spin. Running so much better now. So I'm not sure what the deal is then with my timing. I know when my cam and heads were put in, my mechanic said he wasn't sure which of the 2 positions that the timing tab was supposed to go back on. Maybe he got the wrong one? He said he figured it out but maybe not. My tab is at about the 2:00 spot on the timing cover. Fingers crossed that I won't experience the missing and shuddering that seemed to come and go, but it's behaving much better with the distro twisted somewhere back to where it used to be.
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4:00 is about the area I would think that the tab should go about 2 o’clock on the balancer
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Okay, I see. The twist made it run better but it's still missing and doing some of it's thing (sigh). I think I'll have to pull all the plugs and see what's up.
What gets me is the thing were if I stomp on it, it will be all funny until about 4000 RPM and then rip like usual. So I'm thinking either:
a) a fuel blockage is being bypassed by the pressure of the pump, which increases with RPM and eventually just builds enough pressure (or suction) to overcome whatever is impeding flow. But I would think this would show up at all times at lower RPM's and not intermittently
b) a fouling plug, failing plug wire or corroded/burnt contact inside the distributor is provding erratic spark delivery at lower RPMs (and lower spark voltages) but at higher RPM, the voltage increase is enough to blow past whatever the problem is. I know electronics and heat do not mix so often an erratic electrical or spark issue can be heat-triggered and it's hard to know when or where it's going to act up
??? ???
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10 years is a long time to run a set of wires.
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I know that sounds like a very long time, and in a daily driver it would be. These wires are quality pieces and weren't used all that terribly much. The truck was rebuilt as a nice day driver and a toy so it wasn't used and abused daily. Even still though, I guess things like wires do deteriorate over time.
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I know that sounds like a very long time, and in a daily driver it would be. These wires are quality pieces and weren't used all that terribly much. The truck was rebuilt as a nice day driver and a toy so it wasn't used and abused daily. Even still though, I guess things like wires do deteriorate over time.
The age alone indicates dried out insulation, amongst other things with wires that old. EVERY wire manufacturer would tell you they should be changed well before 10 years REGARDLESS of usage.
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I dunno. I've replaced wires older than that before and it was running fine. Parts store wires are only like $20, worth a try?
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When all else fails, back to basics.
Cap, rotor, plugs, wires....
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I may try replacing the fuel pump. It's old, and stock. Just wondering if I should stick with stock capacity or go wtih an 80 GPH.