Engine Misfires When Driving

[dirtyoldman]

The last few days the engine on my burb has been misfiring and is getting progressively worse.Before I do a tune up I thought I'd connect a vacuum gauge to see what's up.At idle the engine reads good....even while driving it reads good???I detect no misses at idle but when driving it misses like crazy but the gauge doesn't show it....it should be jumping alll over the place,shouldn't it?When I step on the skinny pedal it takes off and goes normally,when maintaining speed and cruising it misses.
Anyone have any ideas?
I wonder if my gauge is bad and not giving me good readings
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[Cowboy Cadillac]

By your vac reading( at idle) it should be all good. I would check cap,rotor,wires and plugs. You'll at least be able to tell which cylinder is missing if its a single slug issue or resolve the whole problem.
You could still check for vac leaks, a properly running engine should idle between 18-22 IIRC and drop 10-12 under power. I could be (completely) wrong but thats how I think it goes.

 

[Texas Jim]

If you are using platinum plugs.. that sounds like one of the platinum tips came off of a plug...

 

[dirtyoldman]

By your vac reading( at idle) it should be all good. I would check cap,rotor,wires and plugs. You'll at least be able to tell which cylinder is missing if its a single slug issue or resolve the whole problem.
You could still check for vac leaks, a properly running engine should idle between 18-22 IIRC and drop 10-12 under power. I could be (completely) wrong but thats how I think it goes.

A basic tune up is next...I have to look at my records to see when I tuned it up last...

If you are using platinum plugs.. that sounds like one of the platinum tips came off of a plug...

Pretty sure I used ACs,but I'm not sure.

 

[Black02Silverado]

It sounds like a plug issue. At night while it is running do you see any sparks jumping form the wires, which would indicate a bad wire?

 

[OldCracker29]

Sounds like it's time for a tune up.
When was the last time you changed the fuel filter?
You've already checked for vacuum leaks.

 

[dirtyoldman]

At 223,500 miles I changed oil,replaced plugs,wires,cap & rotor...the burb has a little over 235,000 on it now,certainly shouldn't need a tune up already...there's gotta be something else going on..

I changed the fuel filter about three years ago,maybe at something like 205,000miles,summer of '08 IIRC.When I bought the burb mar.'08 it had 202,000 on it.So in almost four years I've put a little over 33,000 miles on it.The fuel filter shouldn't be going bad that soon should it?

 

[OldCracker29]

At 223,500 miles I changed oil,replaced plugs,wires,cap & rotor...the burb has a little over 235,000 on it now,certainly shouldn't need a tune up already...there's gotta be something else going on..

I changed the fuel filter about three years ago,maybe at something like 205,000miles,summer of '08 IIRC.When I bought the burb mar.'08 it had 202,000 on it.So in almost four years I've put a little over 33,000 miles on it.The fuel filter shouldn't be going bad that soon should it?

It's a cheap fix. I change my fuel filter every 15k-20k. Cheap insurance.

I wouldn't think it would need a tune-up within 12k miles. Have you cleaned the TB?

 

[dirtyoldman]

A fuel filter would probably be a good idea,it can't hurt.Keep in mind I have the old TBI setup...no cleaning.I use Chevron gas so the two injectors should be pretty clean.The air filter is recent also.........

 

[Zembonez]

Pull the cap and have a look inside Gary. I've seen aftermarket caps get pretty corroded over a short time. What you describe sounds like a plug wire breaking down. Does it get worse on damp mornings?