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Compression Test questions

Contemplating doing a compression test on my V6 pushrod (1996) Explorer engine. I've never done one before, so I started reading up about it. The procedure appears to be:-

1) remove all spark plugs. 2) Block throttle wide open 3) Disconnect primary wires from the coil. 4) Insert gauge and crank 4 strokes 5) repeat 4) for each cylinder.

OK, so here's my questions.

a) whats the best way to block the throttle 'wide open'. And when they say 'wide open', is that like pressing the pedal all the way to the floor. Do this in the engine, or at the pedal?

b) Disconnecting the primary wires.... I'm familiar with the coil, as I replaced it recently. I recall that it has 6 wires coming off of it, and an electrical connector going into the side of it. I assume that the primary wire is presaumably the electrical connector, at least on this car. The secondary wires are those going from the coil bank to the spark plugs? So, can I just yank the electrical connector, or should I do more?

c) cranking the strokes - do you do this by cranking with the ignition key? One manual suggested that the best way to do it is to use a remote starter switch and turn the ignition to off (to disable the electronic ignition). How would you do that? Is it necessary?

d) What do you do with the spark plug wires for the cylinders you aren't testing, leave them lying near the engine?

Thats about it really..thanks in advance for any info.

Paul

Reply to
Paul Scrutton
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Floor the gas pedal for wide open throttle, disconnect the electric connection from the coil (or use a remote starter switch), don't worry about the plug wires, no power will be going to them anyway, and crank till you get a steady reading, the gage is not going up any more. 4 complete revolutions should do it. A remote starter switch is the best way, since with the key off the fuel injection won't be pumping fuel into the engine. Do this for each cylinder. Jack

Reply to
Jack

OK. My car has a manual transmission, which means I don't have a remote starter installed, and after reading about defeating the clutch switch to install one, or having to install some neutral detector sensor, I think I'll pass on installing one.

If the goal is not pump fuel into the engine, can I just detach the fuel pump switch up in the passenger area, and idle the car until it runs out of fuel, to achieve the same end?

Paul

Reply to
Paul Scrutton

Mmmmn, ok. I figured it out, I can get one of those cheap trigger control remote switches just for this job. That makes sense. Installing a whole remote starter system is a different bag.

Got one on order.

Paul

Reply to
Paul Scrutton

This brings up an interesting question concerning new vehicles with throttle by wire. Is there any way to hold the throttle wide open? Even with the key on and the pedal floored, will the computer open the throttle to wide open throttle? There has to be some way. An external mechanical point maybe. Please don't say you have to have a scan tool to do a compression test now!

Have fun with that 96 BTW.

Reply to
HerkyJerky

In all honesty, it is rare to perform a mechanical compression test. Electronically, we can perform many tests that will leads us towards or away from deciding a concern is compression related or not. If the diagnosis leads towards a compression concern, we can perform a cyinder leak test on that hole to determine the cause of the concern.

If the motor runs smooth, there is no real need to perform a compression test since, in the truest sense, we are only looking for even readings rather than any finite number. Now, with these "fly by wire" motors, we have even more data we can monitor to localize the possible causes of a concern.

If I am acertaining the condition of a motor for a customer considering a purchase, we are just as far ahead to look for indicators showing regular maintenance, disabling the PCV system and watching for blow-by, performing a relative compression test using the scan tool, performing a power balance test using the scan tool as well as other tests.

The scan tools we use at the dealership are very powerful tools but, like our PC, we need to learn how to make use of their capabilities.

Reply to
Jim Warman

Well Jim. As I recall when I started this thread over 6 months ago, you mentioned that you'd like to see compression numbers,

The history of this issue is a misfire on cylinder 6 code. Currently I'm getting some serious amount of stumbling at 50+ mph going up hills.

4 of the 6 plugs are colored white on last plug change, which to me indiactes a lean condition, possibly caused by an intake manifold problem. I retorqued the intake manifold around cylinder 6, which didn't help.

I plan to replace the manifold gaskets, then get compression numbers. The engine has 247K on it now with no work done on it (the engine) so far.

Paul

Reply to
Paul Scrutton

I believe I'd mentioned at that time that we are more concerned with even than finite readings. For most of us here, running older OBD1 vehicles, we can use compression numbers (within reason) to determine if we can expect a motor to run smooth. For those without access to a capable scan tool, a compression test can aid in diagnosing cylinder miss but there are shortcomings when we come to drive by wire throttles. We can induce a massive vacuum leak (or as big a vacuum leak as possible) to overcome pumping losses but chamges in technology bring changes in how we do things.

For motors that don't offer modern diagnostic processes, the compression test is a valuable indicator but it is still only part of the picture. What I probably failed to touch on would be that, if the motor is running smoothly and I am performing a maintenance tune up, I have little that needs diagnosing and a compression test would be an unexcessary expense for my customer.

Compression numbers do tell us lots but there is more than one way to skin a cat. Also, remember that I was trying to answer a question regarding the direction that running concern diagnosis is taking. Without access to expensive diagnostic instruments, we revert to tried and true "old timey" stuff.

Reply to
Jim Warman

OK, thanks for the explanation in your troubleshooting techniques.

Yes, this vehicle is OBD-II. About the only code that does come up when I floor the engine when it is bogging down is a misfire on cylinder 6.

I'll keep the group informed on how the intake manifold gasket replacement / injector check helps things, and compression numbers when I get round to doing these tasks.

Paul

Reply to
Paul Scrutton

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