Shakes @ idle in D or R

[farmerfred]

I'm all but convinced that it's the valve guides now. I found a very helpful vacuum gauge site http://www.secondchancegarage.com/public/186.cfm. They describe my situation (scenario 8), including the fact that the vac. gauge needle becomes more steady as the RPMs increase. Other things I did:

• I checked for vacuum leaks but couldn't find any.
• Checked for bubbles in radiator, but found none. (I did this just to rule out a blown head gasket).
• I set the idle accounting to the sticker (600 RPM in neutral), but that became 400RPM in drive. So I set the idle for 600 RPM in drive, which became 900 RPM in neutral. The vacuum fluctuated rapidly from 7-10 in. Hg @ 600 RPM, 12-18 in. Hg. @ 900 RPM, and 18-20 in Hg. @ 2000 RPM.
• I set the carb idle mixture by vacuum instead of tach., and that sure helped.

I'm hoping to do a compression test tomorrow.

[farmerfred]

I'm all but convinced that it's the valve guides now. I found a very helpful vacuum gauge site http://www.secondchancegarage.com/public/186.cfm. They describe my situation (scenario 8), including the fact that the vac. gauge needle becomes more steady as the RPMs increase. Other things I did:

• I checked for vacuum leaks but couldn't find any.
• Checked for bubbles in radiator, but found none. (I did this just to rule out a blown head gasket).
• I set the idle accounting to the sticker (600 RPM in neutral), but that became 400RPM in drive. So I set the idle for 600 RPM in drive, which became 900 RPM in neutral. The vacuum fluctuated rapidly from 7-10 in. Hg @ 600 RPM, 12-18 in. Hg. @ 900 RPM, and 18-20 in Hg. @ 2000 RPM.
• I set the carb idle mixture by vacuum instead of tach., and that sure helped.

I'm hoping to do a compression test tomorrow.

[clay68c10]

Have you checked if you have a spark-caused misfire? That would also give you the rough idle. An easy way to check is to put your timing light lead on each plug wire one at a time and listen to the light as it blinks. You can hear the light clicking internally. If it's nice and even, no miss. If the ticking is uneven or real weak, you found your miss. This only works for an electrically caused misfire, not a mechanical or fuel caused miss.
You will get some funny looks trying this, but it really works well. This is the first thing I do if I'm chasing a rough idle on an older car at work.

[farmerfred]

I'll have to try you timing light trick, Clay68c10. I did hook the timing light up to each wire just to make sure there was something going on, but I didn't think to check whether the clicking (or flashing I imagine) was uneven.

[farmerfred]

I did some more diagnostic work today. I checked the ignition wires; all were in spec (resistance under 10K). I changed the spark plugs. #1 and #6 were oil fouled. #5 had some ash. #8 looked newer, almost as if it wasn't getting as much use as the others. The other 4 plugs looked normal. I also replaced the fuel filter, and shot some carb cleaner in the vacuum advance port. No improvement.

One thing that I still do not understand is why putting the truck in gear has such a bad effect, even if it's not moving. Any ideas?

[jhow66]

What do you have hooked up to this port? With engine running disconnect #8 and see if it slows engine down.

[farmerfred]

Jhow66: behind the carb, there are two vacuum ports. One is in the block and the other, I think, is in the carb. One goes to the brake booster; the other, the tranny. The manifold vacuum port on the carb (not the ported one) is capped.

I have an HEI. I didn't think it was safe to pull a wire while the engine is running on those systems. Did I make that up?

[cableguy0]

seperate all of your ignition wires when you get them apart look for streaks of white or burnt looking spots. if there are any spots that look odd you can probably bet that the wires are crossfiring to each other which will cause a huge issue under load.cheapie parts store replacement wires are known for not lasting very long and having crappy insulation which allows voltage to leak and then arc to whatever is closest to it. you can pull off one wire at a time and check things but with crappy old wires i wouldnt reccomend it because chances are the hei will shock the sh!t outta you through the boots and its not comfortable.get yourself a spray bottle of water and when it gets dark tonight have a friend or your wife or whoever sit in the truck with it running in gear and spray the spark plug wires with water if you see spark arcing off the wires or if it starts really acting up you found your problem. also pull the cap off and check the condition of the cap and rotor. if theres a lot of buildup on the terminals they need replaced.

[farmerfred]

Thanks, cableguy0. I checked the wires visually yesterday, and didn't notice any of the problems you mentioned. I'll try the squirt bottle trick tonight. I think I might chock the wheels. Having my wife behind the wheel with me in front of the truck might be more of a temptation than she can resist.

[jhow66]

Check the one going to trans-it maybe off-if so this will kill the fuel signal to #8. To check # 8 to see if it has an effect on engine speed you can pull plug wire off before starting and insert an old plug in wire and lay on engine so it will have a path to ground (that is what will sometimes damage an HEI if it has no way to ground). Then start engine and see if it had any effect on engine speed.

[farmerfred]

Jhow66: Sorry, I'm not following you. What's the one going to the trans that I should check? I like your suggestion b/c I've been thinking that the transmission was involved here. Do you mean to check the vacuum line to the trans. for leaks?

[Blazer1970]

I think he means the vacuum line to the trans. It really sounds to me like it is running on seven cylinders. I would invest in new plugs/wires/cap/rotor if you have any question about any of them. What does a compression test show?

[farmerfred]

Thanks Blazer1970. It's probably too dark to check anything now, except for the ignition wires. I just put in new plugs, and the wires/cap/rotor seem okay. I checked the wires yeterday and they all had 8-9K Ohms resistance (limit is 15K).

[jhow66]

Originally Posted by farmerfred

Jhow66: Sorry, I'm not following you. What's the one going to the trans that I should check? I like your suggestion b/c I've been thinking that the transmission was involved here. Do you mean to check the vacuum line to the trans. for leaks?

Yes. If the line to the trans runs off the one in the intake that I marked and has a leak or is off the modulator, the # 8 cylinder will not have a suction on the intake stroke (will not pull any fuel and air into cylinder). The reason I mention this is that you said # 8 plug looked clean (no fuel being burned in this cylinder would cause it to be clean). Do you have a tach on the truck?

[farmerfred]

Originally Posted by jhow66

Yes. If the line to the trans runs off the one in the intake that I marked and has a leak or is off the modulator, the # 8 cylinder will not have a suction on the intake stroke (will not pull any fuel and air into cylinder). The reason I mention this is that you said # 8 plug looked clean (no fuel being burned in this cylinder would cause it to be clean).

I'll have to check this out. Thanks.

Originally Posted by jhow66

Do you have a tach on the truck?

No. I have to hook one up when I do a tune-up. What would I look for?

[farmerfred]

I had a mechanic check the compression. He said it was "good." I didn't ask him for numbers 'cause he wasn't charging me. I think that solves both problems--the rough idle and confirms the vacuum gauge reading. Now the only problem is to figure out what to do. I had started another thread threadon that, and I don't want to cross post. I really appreciate everyone's help.

[jhow66]

If you hook up a tach and pull one plug wire at a time it will show which cylinder is not working. ( the one not working will not change the rpm)

[Boog]

You don't have to buy and install a tach to find which plug isn't firing. Just pull the plug wires off one plug at a time until you find one that doesn't make a change in engine rpm. I highly recommend using a pair of plug wire pliers when doing so. Or the brave can pull them with their fingers.