> Main > Gas Index > DIY Repair > Miss diagnosis

Troubleshooting?  Use the Starter.

General tips for locating the cause of a engine miss.
Visit Geno's Garage
for Truck accessories.
Geno's Garage Truck Accessories

Subject:    Troubleshooting?  Use the Starter.
Date:         Fri, 27 Mar 1998 06:51:19 -0500
From: (Henry LaViers)

  Here is an article you may or may not like, but is kinda interesting.

------ Forwarded Article <01bd5757$e0b261c0$8158cccf@default>
------ From "John Young" <>

                   -- Tips from the Old timer --

Troubleshooting?  Use the Starter.

Good mechanics can tell a lot about the condition of an engine just by listening to the sound the starter makes as it turns the engine over.

The procedures listed below will help you correctly diagnose a low speed miss caused by a vacuum leak, burned intake or
exhaust valve, or worn piston rings with no test equipment other than your practiced ear, the vehicle's starter, and a
length of rubber vacuum hose. These tests assume the engine has a carburetor, not fuel injection.
{NOTE: many of these tests will also work with FI engines as well - Dave}

First, disable the vehicle's ignition system by removing the coil wire or disconnecting the primary (+) wire at the ignition coil. (On cars with high energy ignition, pull the distributor end of the coil wire and lay it against the engine block to harmlessly dissipate the high voltage.)

Now spin the engine with the starter and listen carefully. An engine in good condition (equal compression across all cylinders) will sound like this:


An engine with low compression on one or more cylinders will sound like this:


The drag on the starter will be less on cylinders with low compression and the sound produced will be uneven.

This technique takes a bit of practice to use effectively but the time saved on diagnosing common engine problems makes the
effort well worthwhile.

Now that we know the general condition of the engine, locating the cause of the low speed miss is quick and easy.

If compression is ok and the miss goes away at higher engine speeds, suspect a vacuum leak. At idle, intake manifold
vacuum is high and each cylinder receives a rarified air\fuel charge. A vacuum leak will lean out the mixture to the point
where the nearby cylinders will skip and misfire, but run ok at wider throttle openings.

Here's the easy way to determine if a vacuum leak is causing the problem. Remove the air cleaner. Start the engine and let
it idle.  Slowly and carefully place your hands over the carburetor throat. The idea is to choke the engine slightly.

If the engine picks up speed, you've got a vacuum leak.

If the engine slows down or dies, the trouble is elsewhere.

Once you've determined a vacuum leak is present, here's the easy way to locate it. Get a piece of 3/16" rubber vacuum line several feet long. Carefully place one end to your ear and move the other end around in the intake manifold area. The hissing sound of the leak can be clearly heard through the tubing and will guide you right to it.

No leaks around the intake manifold? The next most likely suspect is the power brake booster. Pull the hose off with the engine idling and plug it with your finger. If the miss goes away, the diaphragm in the booster is leaking or the vacuum hose connection to the booster is faulty.

The air conditioning system is the next most likely culprit. Remove the line that provides the A/C vacuum from the intake manifold and plug the port with your finger. If the miss goes away, use the 3/16" tubing trick to listen under the dash board to locate the source of the leak.

Still no luck? Check the remaining vacuum lines that connect to the intake manifold. Pinch each line with needle nosed pliers. The engine will smooth out when you've pinched the right one. If the hose has several branches, pinch close to the intake manifold first, then work your way down the branches to locate the one that's leaking. If pinching cracks the rubber hose, it has reached the end of its service life and should be replaced anyway.

(Note. Pinching the PCV hose will cause a change in engine rhythm even if the PCV system is working properly.)

Here's the procedure to check for a burned valve or bad piston rings.

BURNED INTAKE VALVE: Remove the air cleaner and disable the ignition as described above. Use the 3/16" tubing trick again and place the end near the carburetor throat. Have someone open the throttle and spin the engine with the starter. If an intake valve is leaking, you will hear a hiss from the carb throat each time that cylinder goes over.

BURNED EXHAUST VALVE: Same procedure except use the tubing to listen in the vehicle's tail pipe. If an exhaust valve is leaking, you will hear the hiss in the tailpipe each time that cylinder comes up on compression stroke.

No burned valves? Next, determine which cylinder is missing. Start the engine and let it idle. Get a set of jumper cables and connect one end to the negative battery terminal. Using the clamp on the other end of the same cable, pull off and re-install the spark plug wires, one at a time. The grounded cable will prevent you from being shocked. Work carefully to avoid damaging the spark plug wires and boots.

Pulling the plug wire off a good cylinder will result in a change in engine rhythm. Keep going until you find the one that doesn't effect engine speed when removed.

When the missing cylinder is located, remove the spark plug, squirt some motor oil in the spark plug hole, and replace the spark plug.  If oil in the cylinder improves compression (run the starter test again) worn piston rings are the problem. If compression doesn't improve, all that's left is broken rings, cracked piston, blown head gasket, cracked head, cracked block, worn cam lobe, collapsed valve lifter, bent push rod, or broken valve spring.

The procedures described above should performed only by persons experienced with engine work and aware of the dangers involved. If you do not feel comfortable with any listed procedure, please seek out the help of a competent mechanic.

John Young      HTTP:// (Pow-R-Lube site)

------ End of Forwarded Article