Video: Please Help Diagnose My Vacuum Gauge Reading

Discussion in 'Fox 5.0 Mustang Tech' started by JasinC19, Jun 19, 2013.

  1. '93 GT
    Built, assembled by yours truly

    Symptom - runs rough, bad shaking at idle

    Please watch the video below. The needle on the gauge is violently shaking back and forth. I think it's supposed to be steady.

    I ran a cold compression check a few minutes ago. All cylinders were between 150 and 155 psi.

  2. Vacuum should remain steady at 1000 RPM with 18"-20 " vacuum. A sweep such as shown in your video suggests that there are valve train problems. They could range from a bent pushrod to a broken valve spring.

    Do a Cylinder Balance test to spot the problem cylinder and then pull the valve cover for that cylinder . Then visually check the pushrod, rockers and springs for that cylinder for damage.

    Cylinder balance test: use this to find dead or weak cylinders:

    Revised 25 March 2012 to add necessity allowing the KOEO tests to finish before starting the engine and the need for a properly functioning IAB/IAC to run the cylinder balance test.

    The computer has a cylinder balance test that helps locate cylinders with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.

    Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

    Be sure to turn off the A/C clutch depressed to the floor, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

    Here's how to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.



    If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

    What to expect:
    You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

    Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

    Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

    Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

    Cylinder balance test

    If you have idle or IAC/IAB problems and the engine will not idle on its own without mechanically adjusting the base idle speed above 625-750 RPM, this test will fail with random cylinders pointed out every time it runs. The IAC/IAB must be capable of controlling the engine speed to run in the 1400-1600 RPM range. Playing with the base idle speed by adjusting it upwards will not work, the computer has to be able to control the engine speed using the IAC/IAB.

    Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Let it finish the Key On Engine Off (KOEO) code dump. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
    Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

    Do a compression test on all the cylinders.
    Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

    How to do a compression test:
    Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

    With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

    I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

    A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.

    See the link to my site for details on how to build your own blow down type compression tester.
  3. Thanks J. I'll do the balance test tomorrow.

    2 quick questions:
    How likely is it that valvetrain problems cause the vacuum gauge to behave like that? Could it be something else?

    Is the compression check not a fail safe way to diagnose valvetrain problems? All my cylinders read fine.
  4. Update: The balance test came back fine.

    Based on the chart below, I may have a leaking intake manifold gasket (sucking unmetered air) or some sort of valve issue. I am going to try to test for lower intake vacuum leaks tonight then take the valve covers off to readjust valves and do a visual inspection.

    Before I do that, does anyone have any advice they can give me for what I should be looking for? I am going to keep an eye out for valves that aren't shutting, bent or misguided push rods, and spring problems, but I'm not sure the best method to test these things other than watching them go up and down as I turn the engine by hand. :)

    I have been reading that the valvetrain components that come with the TFS heads aren't wonderful. Does anyone have anything to say about that?

    I am going to try to check for an intake manifold
  5. I have the valve covers off. All of the springs look great. Without taking anything apart, everything looks to operate correctly.

    With each cylinder at TDC compression, the rockers wiggle just a little bit on the valve side. This eliminates the possibility of improperly adjusted valves causing the vacuum reading fluctuation right?

    Is it possible my Trick Flow Stage 1 cam is just that lopey to cause this fluctuation?

    Should I be looking at my plugs/wires/coil? I know they are all firing. Cylinder balance test came back ok. Fuel delivery is not that high at idle speeds...

    I'm starting to run out of ideas again.
  6. I have a similar issue, how does this apply to a non-stock motor?

    331 Stroker, EFI intake, AFR185s, .565"/.574" lift - 112 Lobe cam. Currently pulling 8-10" vac at idle
  7. Do a ghetto compression test: turn the ignition on, floor the pedal and hold it there to invoke Clear Flood Mode that shuts off fuel delivery, then listen while you crank the car for 5 seconds. If it cranks nice and even, like r-r-r-r-r-r-r-r-r, then you can be fairly sure the compression is at least even from cylinder to cylinder. But if it sounds louder as it passes certain cylinders, like r-r-r-r-R-r-r-r-r-R-r-r-r then you know you have a weak one. If you do, coupled with the vacuum readings, it might indicate too much preload on a rocker, or a broken spring, or a bent valve, etc.
  8. Hey thanks,

    I have done a compression and a leakdown test. The cylinders were all between 150 and 155, and the leakdown was basically consistent between 8%-12% leakage. (I think the rings were gapped for a little extra blowby when the engine was built)

    I really don't want to take the lower intake off lol... but I guess eventually I will so I can check the lifter retainers and actually see if the pushrods are all ok etc etc.

    I was going to look more into ignition. I was reading that a rapid sweep of 2" on the vacuum gauge indicates a possible ignition problem. I have the stock coil & distributor and MSD wires. I'll test the wires best I can tonight and maybe put new plugs in her just in case one is weak and I don't know it.

    From what I understand if the distributor PIP goes bad, the car just won't run. I wish I had 2 of every part on the car so I could test it the "easy" way. :)
  9. If the engine is running rough the vacuum will be unsteady, but this doesn't automatically mean the problem is with the vacuum system.

    The old school way to quickly check for vacuum leaks is to run an (unlit) butane torch or similar combustible spray over the intake seals & vacuum joints & listen for a a change in engine sound if it starts sucking in that "extra" fuel. This isn't a sure thing, but is a quick & easy test to help diagnose if you have a vacuum leak.

    Note, do not use chlorinated brake cleaner since it turns into very serious poison when ignited.
  10. Propane leak check? Brings to mind the words of black comedian Richard Pryor " Fire is a wonderful depilatory" after he roasted himself while freebasing cocaine.

    Finding vacuum leaks

    Revised 04-Aug-2011 to add pintle cap, PCV grommet & power brake check valve grommet to checklist.

    There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and every hose and connection.

    Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

    Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

    The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

    The PCV grommet and the power brake booster check valve grommet are two places that often get overlooked when checking for vacuum leaks. The rubber grommets get hard and lose their ability to seal properly. The PVC grommet is difficult to see if it is correctly seated and fitting snugly.

    Fuel injector O rings can get old and hard. When they do, they are prone to leaking once the engine warms up. This can be difficult to troubleshoot, since it is almost impossible to get to the injectors to squirt oil into the fuel injector mounting bosses. If the plastic caps on the fuel injectors (pintle caps) are missing, the O rings will slide off the injectors and fall into the intake manifold.

    Fuel injector seal kits with 2 O rings and a pintle cap (Borg-Warner P/N 274081) are available at Pep Boys auto parts. Cost is about $3-$4 per kit. The following are listed at the Borg-Warner site ( ) as being resellers of Borg-Warner parts: or or or

    Most of the links above have store locators for find a store in your area.

    Use motor oil on the O rings when you re-assemble them & everything will slide into place. The gasoline will wash away any excess oil that gets in the wrong places and it will burn up in the combustion chamber. Heat the pintle caps in boiling water to soften them to make them easier to install.

    Diagram courtesy of Tmoss & Stang&2birds

    Vacuum leak due to slipped lower intake manifold gasket...

    Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.


    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Everyone should bookmark this site.

    Ignition switch wiring

    Fuel, alternator, A/C and ignition wiring

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

    Vacuum diagram 89-93 Mustangs

    HVAC vacuum diagram

    TFI module differences & pinout

    Fuse box layout