454 shake and noise troubleshooting

[texdave]

I had my 76 C20 running really well not too long ago. Took it on a 800 mile trip and it idled, cruised, and accelerated very well. Smooth 650rpm idle and no bogs. Smooth speedo and tach. Recently I had also refurbished and tuned the quadrajet.

I sent it in to my shop to fix an oil leak. They said it was the intake manifold so they replaced the gaskets and seals. When they gave it back, I noticed right away that: the tach was bouncing at all rpms but more so at idle, the engine was shaking at idle, and there was an intermittent noise coming from the engine.

Take a look and listen.

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I've made sure that the timing is right (13btc), I was planning on changing all the spark wires so I did that too, and I made sure the idle and mixture screws were setup right. It helped a bit but the shake and noise is still there. Up next, thinking of doing a compression check.

Any thoughts on what it could be and what I can do to isolate the issue? I'm thinking 1) shop dropped something in the valley while the manifold was off, 2) a rocker/valve/lifter issue, or a bottom end issue maybe due to increased crank pressure after sealing up the intake better?

 

[Bronze Knight]

My first guess is that the shop didn't do the intake manifold seals properly OR they left a vacuum hose disconnected somewhere. I'd get a can of brake clean or engine starting fluid and spray around the top of the engine and see if the RPMs pick up. If they do that means there is a vacuum leak somewhere.

 

[Edelbrock]

The shop screwed up big time. Either by accident or on purpose. They should fix that for free, but may be trying to juice you for more cash.

The shaking is caused by a steady cylinder misfire, or maybe 2.

 

[cadillac_al]

That's a sad story. When people are learning on your vehicle you never know how it will turn out. I wonder if the float is acting up. I have a friend that is ANAL about never turning a carb upside down because he is afraid of the float hanging up. I have done it quite a few times but ya never know, one time out of 100 the float needle could get cockeyed. Some people do like to turn them upside down to look at the throttle plates or clean the throttle plates.

You might have to start from square one and check the spark plugs to see how fouled they are getting, distributor cap for age or possible crack, check and verify everything is working like it should. Absolutely check for vacuum leaks as as already mentioned. I hope they didn't mess with the valve adjustment while they had it apart. I don't trust many people to adjust valves but that's just me. good luck.

 

[Ricko1966]

The shop screwed up big time. Either by accident or on purpose. They should fix that for free, but may be trying to juice you for more cash.

The shaking is caused by a steady cylinder misfire, or maybe 2.

Yea they probably did it on purpose because all us techs and the shops we work for,that's how we earn our checks. And what's really strange is that the word never gets out and people keep bringing us cars to screw up intentionally.

 

[Ricko1966]

You need to take it back to the shop and talk to them. Quit Messing with it yourself before you get blamed for the problem. I would check plug wire routing it's real easy to get 5 and 7 swapped. Sealing the intake leak is not going to cause extra pressure and a problem,unless your crankcase ventilation is screwed up,it also is not a carbureator that got turned upside down. There is absolutely no reason they should have messed with Valve adjustment. Did they change the oil? Look at the bill.

 

[texdave]

My first guess is that the shop didn't do the intake manifold seals properly OR they left a vacuum hose disconnected somewhere. I'd get a can of brake clean or engine starting fluid and spray around the top of the engine and see if the RPMs pick up. If they do that means there is a vacuum leak somewhere.

I do need to hook my vacuum gauge and get a feel for where I'm at now. I had it up to 19" of mercury before. There's not that many vacuum hoses on this setup (manifold to AC/cruise/transmission, brake booster, pcv, choke pull-off, and vacuum advance) and they all seem fine. I don't hear any leaks but I'll hit the sides of the intake with some brake clean just in case.

The shop screwed up big time. Either by accident or on purpose. They should fix that for free, but may be trying to juice you for more cash.

The shaking is caused by a steady cylinder misfire, or maybe 2.

They've worked on this truck before and by this point, any work they do has been and will be covered under their warranty. Case in point, this intake gasket was covered work. They've done good work before, just takes a while to get the car back sometimes. With that said, a less experienced tech may have worked on it this time.

If it is a cylinder misfire, couldn't I address that by looking closer at the ignition components? I could pull the plugs and make sure they're still gapped right and not cracked. I was also thinking of replacing the dizzy cap/rotor/coil if needed.

You need to take it back to the shop and talk to them. Quit Messing with it yourself before you get blamed for the problem. I would check plug wire routing it's real easy to get 5 and 7 swapped. Sealing the intake leak is not going to cause extra pressure and a problem,unless your crankcase ventilation is screwed up,it also is not a carbureator that got turned upside down. There is absolutely no reason they should have messed with Valve adjustment. Did they change the oil? Look at the bill.

No oil change on the work order. I wouldn't imagine they would have needed to for an intake gasket replacement. Yeah, this is likely what I'll do, it's the right first step even if my hopes aren't high. There is a more old school focused shop nearby that I could also take it to to get a second opinion.

 

[Ricko1966]

Don't drive until the oil is changed,don't change the oil until you call the shop. Coolant and oil do not mix neither does the fibers from roloc disc's. I find it unlikely they were able to pull that manifold without getting any antifreeze in the valley.

 

[edgephoto]

I am sure they screwed it up intentionally. Shops do this because they love when customers come in all angry and upset. Much more fun to argue than to make money.

The oil should have been changed as part of the job. The old gasket material, coolant and cleaning debris ends up in the pan.

I bet the firing order is off.

 

[texdave]

First thing I did was check the firing order but it came back right (1-8-4-3-6-5-7-2). #1 on the cap was pointing toward #1 cylinder as before.

 

[Ricko1966]

First thing I did was check the firing order but it came back right (1-8-4-3-6-5-7-2). #1 on the cap was pointing toward #1 cylinder as before.

Did you double and triple check 5 and 7,they get crossed all the time.