Bad Vibrations...

[GMC Guy]

so i took my truck on a 3 hour road trip this weekend to see a brad paisley concert. other than it being over 100 degrees out, the truck cruised fine at 75 the whole way and back. but as i got off the free way coming home, i noticed it was running pretty rough. so i got it and checked everything out, but the past few days it has been shaking at idle and acceleration up to about 30 mph. all the wheels are balanced, tires inflated, all the obvious causes are ok. any ideas of what it could be? i hope i don't start replacing parts that aren't causing it...
any help is always appreciated.

[Storm Chaser]

i noticed it was running pretty rough

it has been shaking at idle

Well, that pretty much rules out unbalanced tires and even driveshaft/U-joint problems. Shaking at idle is likely something engine related. I'd probably start with the fuel filter. Even if that's not the problem, 90% of trucks out there hasn't had it changed in a loooong time, if ever. Has it had a good tune-up lately?

[old Rusty C10]

check the plugs and the cap in the area of firing order where the bad plugs are
any change in mileage lately?

[GMC Guy]

i have replaced the plugs within the last month, i have not replaced the fuel filter yet. i just pulled the cap off, no signs of extreme wear, the quadrajunk does need rebuilding, but i don't think that would cause shaking
i did run it in the dark to check the wires, but couldn't see any sparks. i'll start by replacing the fuel filter, and possibly wires (they're pretty old), but where is the filter located on the quadrajet?

also, i forgot to mention that i can hear definite change in exhaust tone, from when it was running strong up until now. so could i have a problem with my cat or muffler? oh, and i noticed a definite loss of power from idle to about 35 mph.

[storked82]

HI GMC

i would check vacuum lines and then I would do a fuel pressure check just my 2 cents

Take care
Jeff M

[1985-GMC]

The quadrajet has a filter right inside the carb where the fuel line enters the carb. Unscrew the line (mine was a b***h to not break the line) and replace the filter, I think there are two different length filters so get the right one. hope this helps

[jobberone]

If it's running fine at speed but not at idle then its probably your carb or fuel pressure at idle maybe vacuum.

Check the manifold for leaks and check all your vacuum lines. Like someone else said check your fuel pressure.

Does anyone know if old floats can cause that problem? Seems to be I remember that can cause problems.

[GMC Guy]

so the plan for today is to replace the fuel filter and the line that runs from the pump to the carb. yesterday when i tried to unscrew it from the carb, i noticed someone before me had rounded it off and pinched the line a little, so since i need to rebuild the carb anyway, i'm just going to cut it and replace it with a new one.

also i can hear a leak coming out of the top of the carb, and when i spray carb cleaner in it, the engine picks up. so i guess i have a definite vacuum leak in there somewhere.

*note: checked each wire last night with a timing light i borrowed from a neighbor, and found there is spark to every wire.

Oh, i was thinking about doing a rubber fuel line into the carb, that way i can put in an in-line filter. and also where might i be able to put a fuel pressure gauge?

[GMC Guy]

[old Rusty C10]

id be careful using an inline plastic filter ive seen several of these cause fires... is the gasket between the carb and manifold good? loose?

if the carb is dumping too much gas into the exhaust youll have black smoke and if the cat is jammed it will prolly glow when hot plus you usually smell rotten eggs

[GMC Guy]

i'm pulling the carb off today, and i do have a new gasket for it. and what are better to use, glass or plastic in-line filters?

[BusinessProfessor]

I prefer the all enclosed metal fuel filters. I don't trust the plastic or glass ones. Sometimes, if you can find them, the old style glass ones are great. The glass is heaving duty and won't crack. Just my .02 worth.

[GMC Guy]

UPDATE: so today after realizing that my valve cover is leaking, i pulled the oil cap off and found it caked with milky water....yay...so i pulled the whole valve cover and found more milky water...

soo, tomorrow i'll go pick up new valve cover gaskets, 5 quarts, new oil filter, and maybe a new engine if they have any...lol

[KQQL IT]

Sounds like a barfed head gasket. Cracked head nothing good

[GMC Guy]

ya i really hope it's not a cracked head...
is there any way to determine what exactly it is without pulling the head?

[silvereagle07]

You could check the oil dipstick to see if the oil is milky. Check your coolant level and pressure test cooling system. There is combustion or head gasket leak detectors available at NAPA part #700-1006. It's about $50. It's real simple to use. You could also perform a cylinder leak down test with radiator cap off and look for bubbles in the coolant to indicate the faulty cylinder.

[GMC Guy]

so is it possible that a cracked head, or blown gasket may not be the case? i checked the dip stick and it looks ok, but i'll know for sure when i change the oil.

[billnorman]

For a Quick and Dirty, loosen the plug wires on the distributor cap, and pull them off one at a time at idle. If the RPM doesn't drop on one or more, it may not be firing properly. Pull the plug of the indicated cylinder. If it's sparkly clean, you may have a blown gasket or head crack. Sometimes a compression test won't find a cracked head but if it's cold outside, it should blow white smoke rings.
FYI, I once had a distributor cap short out between two cylinders, it was a bear to find. It was one of the caps with a jumper system so the wires would all come out on the same side of the cap as the plugs.

[GMC Guy]

UPDATE: drained the oil today, not even a trip of water. i mean it looked exactly how oil should look after a couple thousand miles. so is it possible that the valve covers got some moisture inside, like condensation, that mixed with the oil and gunked it up under the valve covers?

[GMC Guy]

so with clean valve covers, new oil, and filters, i took the truck around the block today and notice it smoothed out a little. still a little shaky on immediate acceleration from a stop, but evens out rather quickly. seems like i'm still losing some power though. and other than rebuilding the carb, and redoing the exhaust, i'm out of ideas on how to get this motor running how it was before all this mess...

[GMC Guy]

tore into the carb today...big mistake...got it broken down into the three sections, and proceeded to clean with carb cleaner, toothbrush, and compressed air. now i'm trying to put the ******* thing back together...not going well. has anyone else rebuilt a quadrajet before? i know it's not the popular thing to do, and as soon as i get the money i'm dropping on a new intake and carb. i just thought i'd rebuild this one for temporary, and maybe help the engine run a little better.

[1985-GMC]

Here you go this should help some.
http://www.spike.com/full-episode/qu...-rebuild/33035
All the commercials are a pain in the *** though.

[GMC Guy]

Originally Posted by 1985-GMC

Here you go this should help some.
http://www.spike.com/full-episode/qu...-rebuild/33035
All the commercials are a pain in the *** though.

thanks man, i forgot about that episode.