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post #1 of (permalink) Old 07-03-2009, 10:31 AM Thread Starter
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misfire diagnosis w/ vacuum gauge

Hi!

i was reading the junkyardgenius page on using a vacuum gauge to tune my carb, and some questions came up.

I've been using a vacuum gauge to try and get my mc2100, HEI 258 to run a bit smoother for me. She's got a random misfire that's been plaguing me for a couple of years.

Just at idle for example: it'll idle smooth, but every 5-10 seconds it'll have slight miss that I can hear in the exhuast....crawling in first gear gives me a rough, stumbling, surging....the same thing at most rpms, though its less pronounced at higher RPMS...

fuel pump is giving me 3 psi.
plugs are only a month old
HEI dist. is new
Idle is about 800rpm or so...
Timing set 8-9*

So heres my vacuum gauge questions:

1. when my gauge is not connected, the needle rests at at 7in HG, not 0. is that normal?

2. when i've got the gauge hooked up to the port on the rear of the intake manifold, i get about 24in HG at idle, but it fluctuates(vibrates) between 22.5" Hg and 24" Hg
Isn't this vacuum too high? what to do next? Retard the timing?

THANKS!
Tim
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post #2 of (permalink) Old 07-03-2009, 11:01 AM
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Out of the box your vacuum gauge should read zero. Return it.

Is this the only hammer you've got?
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post #3 of (permalink) Old 07-03-2009, 07:08 PM
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YUP! GAUGE BROKEN!

Time for a new 'Cheap' one!

And where you hook up depends on WHAT you are doing.

To determine what the valves and such are doing,
Hook up as close to the cylinders you wan to check...

If you want to set your carb up,
Hook up to the baseplate of the carb...

If you are charting the spark ported vacuum,
Then you will have to pick up that vacuum signal separate from the others.

So Many Cats, So Few Recipes...

Last edited by TeamRush; 07-03-2009 at 07:11 PM.
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post #4 of (permalink) Old 07-04-2009, 10:23 AM Thread Starter
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im gonna go get another cheap gauge tonight....this one was only a few weeks old...

Quote:
Originally Posted by TeamRush View Post

If you want to set your carb up,
Hook up to the baseplate of the carb...
where is the hook up on the base plate?
i've got the stock intake manifold, and a mr. gasket adapter plate for the mc2100

on the intake manifold, below the throttle linkage there is cube shaped brass fitting, it feeds vacuum to the brake booster. There is some serious vacuum coming out the other side of that fitting. I've had it capped off since i've owned the rig. Should i take the vacuum reading there?

Thanks again for contributing on this forum!

Tim
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post #5 of (permalink) Old 07-04-2009, 11:22 AM
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That's the place to start.

EVERYTHING's easy for the guy who doesn't have to do it.
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post #6 of (permalink) Old 07-05-2009, 11:30 AM
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Quote:
where is the hook up on the base plate?
i've got the stock intake manifold, and a mr. gasket adapter plate for the mc2100
If you need to tune the carb, just drill/tap the base plate adapter for base plate vacuum signal.

Quote:
on the intake manifold, below the throttle linkage there is cube shaped brass fitting, it feeds vacuum to the brake booster. There is some serious vacuum coming out the other side of that fitting. I've had it capped off since i've owned the rig. Should i take the vacuum reading there?
This would be Intake Manifold 'Plenum' vacuum signal,
and it's almost as good as baseplate vacuum.
The 'Plenum' is simply the transition area in the manifold between carb throttle bores and the intake runners.

For determining if you have problems with valves and such, this is a good place to start.

If you want to know WHICH valve is causing problems,
Then you will have to tap into individual runners and sample vacuum as close to the cylinder head runners as you can.

With an 'Engine Miss',
You would be better off pulling the spark plug wires off the DISTRIBUTOR CAP one at a time to determine where the 'Miss' is...
(Before you go pulling plug wires off the cap, Strip about 5" of electrical wire, and make a circle out of it.
Ground the other end of that wire,
And then pull the coil wire, and lay that circle around the coil wire terminal on the top of your distributor cap.
this will give a reasonable ground to the spark energy while you are pulling spark plug wires)

Remember, an exhaust leak at the head will sound just like a 'Miss' to someone that isn't experienced with this!
Leaks should show up with a black soot trace where they are leaking,
And you can use a feather around exhaust gaskets,
Flanges where tubes connect to header plates,
And up and down tubes to find a pinhole leak that will make a 'Miss' sound when that tube is being used.

Also, an infrared heat gun (about $20 now, dirt cheap many places) aimed at each exhaust tube will tell you which tube is 'COLD'...
'Cold' tube is the 'Miss'!

So Many Cats, So Few Recipes...
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post #7 of (permalink) Old 07-05-2009, 10:45 PM
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TR, to gain proper baseplate vac signal when setting up a carb, since there are two barrels on the carb, is it suggested to drill/tap two holes, one for each barrel, on the adapter? This may make setting a carb properly a difficult task since the two throttle bores do not mix together until the plenum. A vac reading in each would be necessary for setting each needle, unless the two ports were to be tee'd together...

For determining valve issues, is drilling and tapping a port at each runner close to the cylinder head recommended? I would believe that to be a good place for cracks to develop due to the faster heating/cooling of the alluminum manifold compared to the cast steel head and their close proximity to eachother. But then again, multi port injectors are commonly placed around that same area.

'88 YJ
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post #8 of (permalink) Old 07-06-2009, 07:24 AM
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Quote:
Originally Posted by USMCYJ View Post
TR, to gain proper baseplate vac signal when setting up a carb, since there are two barrels on the carb, is it suggested to drill/tap two holes, one for each barrel, on the adapter? This may make setting a carb properly a difficult task since the two throttle bores do not mix together until the plenum.
No, not at all.
Pressure differentials (Vacuum) mixes fairly quickly in the plenum so there really isn't a reason to drill the spacer unless you don't have access to the plenum vacuum.

Most carbs have baseplate vacuum sources that sample BOTH Venturis so they are mixed before you ever see the nipple on the carb base plate.

Quote:
A vac reading in each would be necessary for setting each needle, unless the two ports were to be tee'd together...
Nope, always move the needles TOGETHER, exactly the same amount.
The ONLY TIME the needles shouldn't be moved together is if you have split plenum (Dual Plane) intake and you have a air/fuel ratio indicator or exhaust 'Sniffer' probe in each side of the exhaust.

Always move the idle mixture needles TOGETHER when you are tuning with a vacuum gauge.

Quote:
For determining valve issues, is drilling and tapping a port at each runner close to the cylinder head recommended?
Not really, since there are many other ways to see if each cylinder is doing it's job or not...
And taking an intake off to drill/tap is a ROYAL PAIN IN THE AZZ!

A $20 temp gun will tell you if all cylinders are exhausting the same temperature,
A leak down tester or compression gauge will tell you if you have leaking valves or piston rings,
Vacuum gauge will tell you if you have bad valve guides (and you never just pull ONE cylinder... you always do all cylinders when you do a valve job!)

Although pretty expensive (around $1,000), we have 'Bore Snakes' that can curve right down the intake and have a look at the back sides of the valves to see if there is buildup of deposits on the valve, gaps, leaking gaskets, ect...

The only time I tap intake runners is to install mass airflow sensor or individual pressure sensors when we are doing dyno testing.
It's MUCH easier than it used to be, since most intakes have bosses for fuel injectors now! No welding up our own bosses anymore!

Quote:
I would believe that to be a good place for cracks to develop due to the faster heating/cooling of the alluminum manifold compared to the cast steel head and their close proximity to eachother. But then again, multi port injectors are commonly placed around that same area.
Yup, depends on if you build up a boss for support there before you drill...
Cast iron intakes are MUCH more prone to cracking than aluminum when you drill them!

Most of the time you are just checking for leaking valve guides...
If you find a leaking valve guide with a vacuum gauge,
You are more than ready for a valve job and head rebuild!


If you just find an engine misfire with the vacuum gauge,
99 times out of 100 it's a bad spark plug or plug wire.


Spark plug quality was so bad about 20 years ago, we RARELY got a set of 8 good plug in the same box...
Usually picked up 10 plugs for a V-8 engine,
And I'm still seeing a bad plug in about every other tune up we do around here.

People think I'm going overboard by resistance testing the plugs before I put them in,
And checking firing voltages after they are in, but with 1/8 of ALL your power/fuel being lost out the tail pipe,
I think it's a REAL good idea to send that vehicle out the door with all cylinders firing correctly!

NEVER, EVER, UNDER ANY CIRCUMSTANCES PRY ON THE CENTER ELECTRODE OF A SPARK PLUG!

YOU WILL BREAK THE CENTER CARBON RESISTOR IN THE PLUG BODY AND RUIN THE PLUG!


Use a pair of needle nose pliers to open the gap!

NEVER EVER 'TAP' THE GAP CLOSED!

Again, baning the plug can break the center carbon pile in the plug!

IF you drop a plug during install, THROW IT AWAY.
It's not worth $2 to loose 1/6 or 1/8 of all gasoline you burn until the next tune up!

If you get a box of plugs with a crushed corner,
REJECT THE BOX!
You can bet it was dropped before it got to you, and you can also bet the plug it landed on is shot!

REMEMBER TO USE A DAB OF 'NEVER-SEIZE' ON THE SPARK PLUG THREADS!
'Never-Seize' is Zinc or Copper based, and it helps the plug get a proper electrical ground, along with keeping the plug threads in the head where they belong!

Run a dedicated ground wire to your engine head(s)...
Spark plugs are receiving between 10,000 and 45,000 volts!
Make sure they are getting a good DIRECT path to ELECTRICAL 'GROUND'!

So Many Cats, So Few Recipes...

Last edited by TeamRush; 07-06-2009 at 07:35 AM.
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post #9 of (permalink) Old 07-10-2009, 09:16 PM Thread Starter
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back on this subject(AARRRGH!!!!! i'm so sick of this misfire!)
I got a hold of a new vac. gauge.

some background:

1983 258
Timing set around 8*
hei/mc2100
plugs with 1k miles on them
wires about a year old
fuel press: 3psi(plenty right?)
NO EGR valve

here goes:
At the intake manifold plenum, i get a vacuum reading of 17 in hg @ 800 rpm. The needle stays pretty steady, maybe fluctuating .25-.5 in hg. Is this a reasonable vacuum reading for this engine?
I'm still getting a slight misfire at all RPM.
for example, it at its worst @1100 the engine will shake and shudder, and will not stay steady, fluctuating 25-75rpm. But it doesn't sound as bad as plug wire being unplugged, it just sounds like a cylinder isn't firing as strong as it could.
At higher RPM the miss is still there...

I'm leaning toward leaky intake manifold gasket maybe? it was loose when i bought the rig 7 years ago, i tightened it, but maybe it doesn't seal up the same as in the initial install?( I hear a very, very slight whistling sound for a few seconds somewhere in that area when i shut the engine off after it is warm)
Ive sprayed carb cleaner around there and found no leaks. However, i never sprayed from the bottom up...i'm gonna try that tomorrow.
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post #10 of (permalink) Old 07-10-2009, 11:08 PM
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Sounds? Or vibrates?

How's compression? Warm engine, remove ALL the plugs at once - block the throttle open, test, spin it so each cylinder gets the same number of "puffs - 3-5."
Record each as you go. Tell us the results.


While running - pull off plug wires one at a time - see if you can determine which cylinder is low power. I know, some folks that don't understand will say it's dangerous to the ignition, not so. Thousands are done that way each day without consequences. Or have someone do an individual cylinder power test with a scope.

If it's not just sound you are hearing it's one cyl producing low power. First you have to determine which one before "fixing it."

A vacuum leak will affect idle more than at higher speeds. A leak doesn't change size - at idle it's a big part of the total volume - at higher speeds it's less of the total.
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