My truck is in my sig. It has developed a shake at an idle (I think at the rest of the RPMs). I have been trying to find a vibration in my drivetrain for months but I am now thinking it is attributed to something in my engine being out of balance.

So the vibration is pretty apparent at idle, and happens in a constant pattern. At idle it will just be .........bu-bump.........bu-bump..........bu-bump......... You get the idea.

What might be causing this? I changed my spark plugs/wires/cap/rotor a few months ago and I changed the fuel filter the other day. Also replaced the ignition coil with an MSD blaster coil. Any ideas?

 

Have you checked the engine mounts to see if one of them broke

 

I was worried about that... But I don't know how to check them... Any suggestions?

 

It is really just a visual inspection make sure that they are clean and inspect for cracks and brakes it may help to use a mirror and high powered flashlight. if they are broke don't worry they are not very expensive

 

If it is not in the motor mounts or any part of the support system of the motor, trans etc. then you need to consider the possibility of a missfire. This means checking for a lack of fuel, spark and compression for each cylinder. There are quick, easy and 100% accurate ways to do this and then there are pain-in-the-*** methods and how to go about getting to the bottom of this all depends on what you have available for tools and equipment.

 

Too much hot-rodding with the new rear-end me thinks.... j/k...

 

Negatory on the hot rodding, I seldom do that. I really treat my truck like a baby... Plus I have had this vibration before I was doing performance mods, and hence, before I was ever ABLE to do hot rodding.

Eco, I do have lots of tools and equipment at my disposal. What exactly is the best method for checking for misfire?

 

Another way to check the motor mounts is to open the hood so that you can see the motor from inside. Then start the truck, and with your foot on the brake put it in reverse and gas it up a little and see how much the engine moves. and do the same in drive. You'l definetly see if it a motor mount and which one it is.

 

When I check plugd I take them out and hold them to something, while someone cranks the engine. I do that because you see if the plug (not just the wire) is getting fire, and you see what kind of shape the plug and spark are in.

 

If I were doing this, I would first measure the HC content of the exhaust...high HC = probable spark missfire. To do this you need an exhaust gas analyzer. Acceptable HC readings upstream of the cat is ~ 300ppm and downstream of the cat is ~30-50ppm.

I would then look at the O2 content of the exhaust. For this you will also need an exhaust gas analyzer and acceptable levels of O2 are 0%-2%. Oxygen is a leanness indicator and thus high levels of O2 in the exhaust would indicate a probable lean air/fuel mixture, fuel missfire, vacuum leak and/or an exhaust leak. Prior to taking this reading, you should first check for an exhaust leak with a diagnostic smoke machine and then the induction system and vacuum lines for leaks (also with a smoke machine) and fix accordingly that way you know that an exhaust leak and/or vacuum leak is not throwing off your O2 reading.

Another rule of thumb is that if the motor is running right CO will equal O2. If one is higher than the other, you have a problem. CO is a rich indicator, O2 is a lean indicator and if both are equal that means that lambda is 1.0 and the ingnition system is properly set up.

If the tests indicate a spark missfire I would hook up a Rotunda TFI-IV Ignition Analyzer...assuming that all the obvious things have been looked over (cap, rotor, plugs, plug wires etc).

If the tests indicate a fuel misfire I would hook up a breakout box and watch each fuel injector in real time with a scopemeter.

The smoke tests, exhaust gas tests, breakout box and TFI-IV Analyzer are quick, easy and nonintrusive. If either of these tests throw a red flag (fuel or spark missfire) diagnose accordingly. If not, go on to a compression test...the only other remaining variable if your problem is indeed a missfire.

 

do a compression test ..... its not hard . Rule out a burnt valve .

 

The bu-bump.................bu-bump..........................bu-bump steady miss thing is the #1 symptom of a slightly bent or warped (possibly burn't valve.

Each time a valve is lifted, it is turned slightly. Each time around it gets to the bad (worst) spot and misses.

You might try disconnecting plug wires one at a time. It might not show on a compression test if the valve isn't in the bad spot.

 

a vacuum gauge will diagnose engine problems.

 

yup ..... if the needle flutters like mad on a vac guage .... you need to do a compression check for sure .... because a valve will make it flutter .

 

I second the vacuum gauge Idea.